Vapor Intrusion Pathway Feasibility Study

Transcription

1 Vapor Intrusion Pathway Feasibility Study East Hennepin Avenue Site Minneapolis, Minnesota Prepared for General Mills, Inc. April MarketPointe Drive, Suite 200 Minneapolis, MN Phone: Fax:

2 Vapor Intrusion Pathway Feasibility Study April 2016 Contents Executive Summary Introduction Scope of Feasibility Study Site Characterization Background Central Area Setting Previous Investigations and Response Action Activities Site Conceptual Model Vapor Sources Geology and Hydrogeology Spatial Distribution of Contaminants Potential Receptors Vapor Transport Mechanisms Building Vapor Mitigation Central Area Building Mitigation Status Risk Assessment Regulatory Considerations Remedial Action Objectives ARARs and TBCs Classification of ARARs and TBCs Preliminary Remediation Goals Technology Screening Development of General Response Actions Evaluation and Screening of Technologies Retained Technologies Development of Remedial Alternatives Alternative 1 No Further Action Beyond Previous Response Actions Alternative 2 Monitored Natural Attenuation Alternative 3 Long Term O&M of SSD Systems...22 i

3 5.4 Alternative 4 Enhanced Bioremediation via Injection Events Alternative 5 Enhanced Bioremediation via Recirculating System Detailed Analysis of Alternatives Detailed Analysis Overview Alternative Definition Evaluation Criteria Threshold Criteria Balancing Criteria Modifying Criteria Additional Considerations Comparative Analysis of Alternatives Detailed Analysis of Remedial Alternatives Alternative 1 No Further Action Beyond Previous Response Actions Alternative 1 Threshold Criteria Alternative 1 Balancing Criteria Alternative 1 Additional Considerations Alternative 1 Summary Alternative 2 Monitored Natural Attenuation Alternative 2 Threshold Criteria Alternative 2 Balancing Criteria Alternative 2 Additional Considerations Alternative 2 Summary Alternative 3 Long-Term O&M of SSD Systems Alternative 3 Threshold Criteria Alternative 3 Balancing Criteria Alternative 3 Additional Considerations Alternative 3 Summary Alternative 4 Enhanced Bioremediation via Injection Events Alternative 4 Threshold Criteria Alternative 4 Balancing Criteria Alternative 4 Additional Considerations Alternative 4 Summary Alternative 5 Enhanced Bioremediation via Recirculation Alternative 5 Threshold Criteria...37 ii

4 Alternative 5 Balancing Criteria Alternative 5 Additional Considerations Alternative 5 Summary Recommended Alternative Summary and Conclusions...41 References...43 List of Tables Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Potential Action-Specific ARARs and TBCs Potential Location-Specific ARARs and TBC Potential Chemical-Specific ARARs and TBCs Remedial Technology and Process Option Screening Amendment Sensitivity Analysis Comparative Analysis Summary by Alternatives Alternative 2 Cost Estimate Alternative 3 Cost Estimate Alternative 4 Cost Estimate Alternative 5 Cost Estimate List of Figures Figure 1 Location Map Figure 2 Study Area Figure 3 Potential Off-Site TCE Sources Figure 4 MPCA SA249 Groundwater and Soil Gas Sampling TCE Results Figure 5 Building Mitigation Status April 2016 Figure 6 Glacial Drift Groundwater Monitoring Network Figure 7 Alternative 4 Enhanced Bioremediation via Injection Event(s) Figure 8 Alternative 5 Enhanced Bioremediation via Recirculation List of Appendices Appendix A Appendix B Supporting Cost Details Sustainability Evaluation Summary iii

5 Certifications I hereby certify that this report was prepared by me or under my direct supervision and that I am a duly Licensed Professional Engineer under the laws of the state of Minnesota. Alec Danielson PE #: April 6, 2016 Date iv

6 Executive Summary This report describes the vapor intrusion (VI) pathway feasibility study completed by Barr Engineering Co. (Barr) on behalf of General Mills, Inc. (General Mills) in Minneapolis, Minnesota. Feasibility study activities were informed by the results of numerous soil, soil gas, and groundwater investigations conducted by General Mills, the Minnesota Pollution Control Agency (MPCA), and others to assess volatile organic compound (VOC) concentrations, primarily trichloroethylene (TCE) at and in the vicinity of the property at 2010 East Hennepin Avenue (Site), and a human health risk assessment (HHRA) prepared by Haley & Aldrich, Inc. on behalf of General Mills (H&A, 2015). The Study Area, the larger area where investigations have been conducted, is divided into four geographic regions, including the Site, Northeast Area, Central Area, and Southwest Area. Feasibility study activities focus on the Central Area, only a portion of which may be down-gradient of the Site. Although off-site sources are impacting shallow groundwater in the Central Area and the HHRA concluded that exposure pathways in the Central Area currently are either incomplete or insignificant and are not expected to be significant in the future, this feasibility study was prepared for the Central Area at the direction of MPCA in its letter that provided comments to the Vapor Intrusion Pathway Investigation Report (VIPI Report; Barr, 2015c, MPCA, 2015a). By preparing this study for the Central Area, General Mills does not acknowledge or agree that TCE impacts in the Central Area or elsewhere in the Study Area are associated with its former operations at the Site. A great deal of remedial action work was completed at the Site and in the Study Area prior to beginning this feasibility study. General Mills has completed sub-slab soil gas and/or indoor air sampling at 344 properties and installed sub-slab depressurization (SSD) systems for vapor mitigation at 189 properties in the Study Area since November This work is documented in the Sub-Slab Sampling and Building Mitigation Implementation Report (Implementation Report; Barr, 2015b). In addition to the numerous investigations conducted by General Mills, this feasibility study follows 25 years of groundwater remediation performed by General Mills. The remedial action objective (RAO) developed for the Central Area as part of this feasibility study is to maintain insignificant potential risk to human health from inhalation exposure to TCE in indoor air resulting from TCE concentrations in soil gas and groundwater. As stated in the Remedial Action Plan (RAP) Modification #1, dated March 11, 2014 (MPCA, 2014a) to the Response Order by Consent between General Mills and the MPCA, dated October 23, 1984 (Consent Order; MPCA, 1984), General Mills is responsible only for implementing response actions to address impacts that are due to its former operations at the Site. By preparing this feasibility study and developing this RAO, General Mills does not acknowledge or agree that TCE impacts in the Central Area or elsewhere in the Study Area are associated with its former operations at the Site. The preliminary remediation goal (PRG) selected to evaluate the remedial alternatives developed as part of the feasibility study is the MPCA intrusion screening values (ISVs) for TCE in indoor air. 1

7 Various response action technologies were screened based on their ability to meet the RAO and PRG. Appropriate technologies were then assembled into a range of remedial action alternatives that are evaluated in the detailed analysis phase of the feasibility study. Response action technologies may target the TCE-impacted soil vapor itself and/or the shallow groundwater that may act as a source of the TCE impacts in soil vapor. The response action technologies that were retained following the screening and used to develop comprehensive remedial alternatives for further evaluation included: Installation of additional SSD systems (at residential properties within the Central Area that currently do not have active SSD systems); Operations and maintenance (O&M) of SSD systems; Institutional controls; Monitored natural attenuation; Expanded/modified groundwater extraction; and In-situ groundwater bioremediation. The five remedial alternatives developed as part of the feasibility study were: Alternative 1 No further action beyond previous response actions; Alternative 2 Monitored natural attenuation; Alternative 3 Long-term O&M of SSD systems; Alternative 4 Enhanced groundwater bioremediation via injection events; and Alternative 5 Enhanced groundwater bioremediation via recirculating system. A detailed evaluation of each remedial alternative against criteria set forth in the Comprehensive Environmental Response Compensation and Liability Act (CERCLA) was performed. The CERCLA evaluation criteria include overall protection of human health and the environment, compliance with applicable or relevant and appropriate requirements, long-term effectiveness and permanence, reduction of toxicity, mobility or volume through treatment, short-term effectiveness, implementability, and cost. Additional considerations with the potential to impact the community also were evaluated. Alternative 3 Long-term O&M of SSD systems presented the best balance of tradeoffs of the primary balancing criteria and was selected as the recommended alternative based on the detailed analysis. 2

8 1.0 Introduction This report describes the vapor intrusion (VI) pathway feasibility study activities completed by Barr Engineering Co. (Barr), on behalf of General Mills, Inc. (General Mills). Feasibility study activities were informed by the results of numerous soil, soil gas, and groundwater investigations completed in connection with the property at 2010 East Hennepin Avenue (Site), including the VI pathway investigation conducted in 2014 and 2015, and documented in the Vapor Intrusion Pathway Investigation Report (VIPI Report; Barr, 2015c). The investigations assessed volatile organic compound (VOC) concentrations in soil, soil gas, and groundwater at and in the vicinity of the Site, including impacts from off-site sources where elevated concentrations of trichloroethylene (TCE) are present in groundwater. The VI pathway investigation work was conducted in accordance with the Remedial Action Plan (RAP) Modification #1, dated March 11, 2014 (MPCA, 2014a) to the Response Order by Consent between General Mills and the Minnesota Pollution Control Agency (MPCA), dated October 23, 1984 (Consent Order; MPCA, 1984). This feasibility study is also informed by investigation work MPCA is conducting in the vicinity of the Site at a recently established project area named the Southeast Hennepin Area Groundwater and Vapor Site (SA249). Since November 2013, sub-slab soil gas and/or indoor air sampling has been completed at 344 properties and sub-slab depressurization (SSD) systems for vapor mitigation have been installed at 189 properties in the Study Area. This work was completed in accordance with the RAP Modification #1 and is documented in the Sub-Slab Sampling and Building Mitigation Implementation Report (Implementation Report; Barr, 2015b). In addition to the numerous investigations conducted by General Mills, this feasibility study follows 25 years of groundwater remediation performed by General Mills pursuant to the Consent Order. Thus a great deal of remedial action work was completed at the Site and in the Study Area prior to beginning this feasibility study. This feasibility study also is informed by the results of the human health risk assessment (HHRA) prepared in 2015 by Haley & Aldrich, Inc. on behalf of General Mills. Although not required by the RAP Modification #1, the HHRA was prepared to evaluate exposures to soil, groundwater, and indoor air associated with residential and commercial property uses at and in the vicinity of the Site. The HHRA is documented in the Human Health Risk Assessment Report (HHRA Report; H&A, 2015). The Consent Order uses the term Site to refer to the former General Mills property at 2010 East Hennepin Avenue (Figures 1 and 2). This terminology is retained in this report, and thus references to the Site are intended to refer only to the property at 2010 East Hennepin Avenue. General Mills investigated soil, groundwater, and soil gas at the Site and at locations northeast, south, and southwest of the Site as part of the VI pathway investigation and other investigations. These areas are collectively referred to in this report as the Study Area. The Study Area is divided into four geographical regions, including the Site, the Northeast Area, the Central Area, and the Southwest Area (Figure 2). For the purpose of this feasibility study, the southeast portion of the Site is included in the Central Area as shown on Figure 2. 3

9 This feasibility study was completed for the Central Area for the reasons described in Section 1.1. By preparing this study for the Central Area, General Mills does not acknowledge or agree that TCE impacts in the Central Area or elsewhere in the Study Area are associated with its former operations at the Site. 1.1 Scope of Feasibility Study The RAP Modification #1 provides that a feasibility study be performed, as necessary, to mitigate the VI pathway potential and reduce VOC concentrations in soil, soil gas, and groundwater due to General Mills former operations at the Site. Feasibility study activities are focused on the Central Area, only a portion of which may be down-gradient of the Site. The feasibility study does not include the Northeast Area, which is located hydraulically up-gradient of the Site and where off-site sources have impacted shallow groundwater with TCE, or the Southwest Area, where the evidence indicates that TCE impacts in groundwater are not related to General Mills former operations at the Site. As noted above, by preparing this study for the Central Area, General Mills does not acknowledge or agree that TCE impacts in the Central Area or elsewhere in the Study Area are associated with its former operations at the Site. This feasibility study was completed in general accordance with the United States Environmental Protection Agency (EPA) guidance document for conducting remedial investigations and feasibility studies under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). Based on EPA guidance, a feasibility study should either be scaled down, as appropriate to the site and its potential hazard, or eliminated altogether if a risk assessment indicates that a site poses little or no threat to human health or the environment (EPA, 1988). The HHRA prepared for the Study Area concluded that the only exposure pathway that may be potentially complete and of potential significance is vapor intrusion of VOCs from shallow groundwater in the Northeast Area. The HHRA concluded that exposure pathways in other areas, including the Central Area, were currently either incomplete or insignificant and not expected to be significant in the future. Therefore, the scope of the feasibility study will be substantially informed by CERCLA and associated EPA guidance and includes feasibility study components as appropriate for conditions at the Central Area. Although off-site sources are impacting shallow groundwater in the Central Area and the HHRA concluded that exposure pathways in the Central Area are currently either incomplete or insignificant and not expected to be significant in the future, this feasibility study was prepared for the Central Area at the direction of MPCA in its letter that provided comments to the VIPI Report (MPCA, 2015a). TCE from one or more potential releases up-gradient of the Central Area are the predominant cause of TCE concentrations in groundwater. Groundwater impacts up-gradient of the Central Area and the Site are outside of the scope of this feasibility study. Until the extent and magnitude of the impacts associated with the off-site sources are defined and addressed, remedial action to address groundwater will not be effective at reducing TCE concentrations in groundwater in the Central Area due to continuing recontamination from these up-gradient sources. It is assumed for the purposes of this feasibility study that off-site sources will be investigated and addressed by others. 4

10 The scope of work for the VI pathway feasibility study included the following tasks which are described in the indicated report sections: Summarizing site characterization information Section 2.0; Discussing regulatory considerations Section 3.0; Screening available remedial technologies Section 4.0; Developing remedial alternatives Section 5.0; Evaluating remedial alternatives Section 6.0; and Reporting results and summarizing conclusions Section

11 2.0 Site Characterization This section summarizes the site characterization data collected to date. It provides background information and summarizes investigation and response action work completed from the early 1980s to the present. It presents the current understanding of the site conceptual model as it relates to the Central Area and summarizes the HHRA. 2.1 Background TCE, a widely used industrial and commercial solvent, is present in groundwater throughout the Study Area with the highest concentrations measured up-gradient of the Site in the Northeast Area. The 700- acre Mid-City Industrial neighborhood occupies the northeast portion of the Study Area. The Central Area primarily includes residential properties intermixed with industrial and commercial properties. A limited review of MPCA files in 2014 identified documented TCE releases to groundwater at several off- Site properties and a review of various historic resources including city directories, fire insurance maps, and regulatory database reports identified properties with potential solvent use and/or releases (Barr, 2015c). The approximate locations of these properties are shown on Figure 3. Separately, in recognition that other sources of chlorinated solvents up-gradient and side-gradient of the Site exist, MPCA is conducting soil, soil gas, and groundwater sampling in connection with SA249. In early 2015, MPCA completed a CERCLA pre-screening assessment for the purposes of listing SA249 on Minnesota s Permanent List of Priorities to conduct further investigations. Data collected to date by MPCA at SA249 is summarized in Section 2.2 and the data was evaluated and incorporated into the site conceptual model described in Section 2.3. The nature and extent of contamination from off-site sources in the Northeast Area have not yet been defined Central Area Setting The Central Area is a primarily residential urban neighborhood covering approximately 40 acres. The Central Area was first developed in the late 1800s with mature trees and tight road and alleyway spacing consistent with development from this era. Within the Central Area there are 177 individual properties (including the Site) with buildings covering approximately 8 acres and roads and alleyways covering approximately 5 acres. 2.2 Previous Investigations and Response Action Activities The VIPI Report provides an overview of the investigations and response actions completed by General Mills (Barr, 2015c). Previous investigations and response actions conducted to date are summarized chronologically below: Multiple investigations have been conducted since the early 1980s to characterize soil and groundwater conditions at and near the Site and the Central Area. The early investigations detected VOCs, primarily benzene, toluene, ethyl benzene, and xylenes (BTEX) and, to a lesser extent, chlorinated VOCs (cvocs) including TCE, tetrachloroethylene (PCE), and 1,1,1-6

12 trichloroethane (1,1,1-TCA) in soil and shallow groundwater in the southeastern corner of the Site near a former disposal area. In contrast, TCE was the predominant constituent detected in groundwater off-site in the Central Area. General Mills agreed to install and operate a groundwater extraction and treatment system beginning in 1985 to limit the migration of TCE. The system was installed, operated, and monitored as a groundwater remedial action under the Consent Order until 2010 when MPCA suggested and then approved shutting down the system as the remedial objectives set forth in the Consent Order had been met. Once the remedial action objectives were met and the groundwater extraction and treatment system was shut down, steps toward delisting the Site were initiated, including vapor intrusion, potable water well, and surface water pathway evaluations. From 2011 into 2013, phased investigations of shallow groundwater and soil gas were completed in accordance with MPCAapproved work plans. Sub-slab soil gas sampling and mitigation work began in November 2013 under MPCA oversight. Sub-slab soil gas and/or indoor air sampling has been completed at 344 properties and SSD systems have been installed at 189 properties since November The sub-slab soil gas sampling results pointed to the likely existence of multiple TCE sources unrelated to the Site. This work was completed in accordance with the RAP Modification #1 and is documented in the Implementation Report (Barr, 2015b). The status of SSD systems in the Central Area is discussed in Section In 2014, soil gas and groundwater investigation work was conducted under MPCA-approved work plans at areas northeast, south, and southwest of the Site, and at the former disposal area on the Site. TCE was detected in groundwater and soil gas at multiple locations northeast of the Site, providing further evidence of the presence of off-site TCE sources including sources up-gradient of the Site (Barr, 2014b). No evidence of source material was found at the former disposal area at the Site that could act as a continuing source of TCE to shallow groundwater (Barr, 2014c). The VI pathway investigation was implemented in late 2014 and early 2015 under a work plan approved by MPCA. The results of the investigation indicated that off-site sources were impacting TCE concentrations in groundwater and confirmed that the Site is not an ongoing source of TCE to groundwater at concentrations that would contribute to the VI pathway in the Study Area. The VIPI Report concluded that no current TCE source areas have been found on the Site and that multiple sources in the Northeast Area, up-gradient of the Site, are the predominant cause of TCE concentrations in groundwater in the Study Area (Barr, 2015c). Additionally, the VIPI Report concluded that multiple potential sources of chlorinated solvents, unrelated to the Site, exist throughout the Study Area. MPCA acknowledged multiple potential release sources of TCE in the vicinity of the Site in its letter that provided comments to the VIPI Report and has started investigating those potential sources (MPCA, 2015a). 7

13 A HHRA was prepared in 2015 to evaluate exposures to soil, groundwater, and indoor air associated with residential and commercial property uses in the Study Area (H&A, 2015). The HHRA is summarized in Section 2.4. Sentinel vapor monitoring points and new groundwater monitoring wells were installed at the Study Area in late Quarterly sampling of the sentinel monitoring network was completed in December 2014, March 2015, June 2015, and September The results were documented in the 2015 Sentinel Monitoring Network Report (2015 Sentinel Report; Barr, 2015d). The results showed that the sentinel monitoring network locations are positioned appropriately to measure changes in soil gas or groundwater concentrations at the perimeter of the soil gas monitoring area developed for this project. The sentinel monitoring results collected to date indicate that sources in the Northeast Area continue to impact the Central Area. The highest TCE concentrations in soil gas and groundwater are detected at locations up-gradient and sidegradient of the Site. The 2015 Sentinel Report recommended collecting an additional four quarters of samples from the sentinel monitoring network beginning in the first quarter of MPCA approved the 2015 Sentinel Report and concurred with the recommendation for another four quarters of sampling in their letter, dated January 28, 2016 (MPCA, 2016). In December 2015, Bay West LLC (Bay West), on behalf of MPCA, conducted an investigation associated with SA249. Bay West installed 18 soil borings (SP-01 through SP-18) and six vapor probes (VP-01 through VP-06) for the purposes of collecting soil, groundwater, and soil gas samples for VOC analysis. The majority of the investigation locations were advanced in the Northeast Area with several locations in the Central Area. Investigation results from SA249 indicate that sources in the Northeast Area continue to impact the Central Area and that multiple sources may also be present in the Central Area. The highest TCE concentrations in soil gas and groundwater were measured at locations up-gradient of the Site. TCE groundwater and soil gas sampling results are shown on Figure 4. Additionally, as described in the VIPI Report, several types of institutional controls (ICs) have been implemented for protection of public health and the environment, limiting access to impacted soil and/or groundwater at the Site and the Central Area to assure long-term protectiveness. The controls already in place include a restrictive covenant that limits soil disturbance and groundwater use at the Site, and required notification to the Minnesota Department of Health (MDH) of proposed groundwater well construction, including water supply wells and wells or borings located in Special Well and Boring Construction Areas, per Minnesota statutes and rules. Sampling of the sentinel soil gas and groundwater monitoring network and glacial drift groundwater monitoring network is planned for 2016 (Barr, 2015d; Barr, 2016). 2.3 Site Conceptual Model The site conceptual model (SCM) for vapor intrusion in the Study Area presented in the VIPI Report has been focused on the Central Area for purposes of this feasibility study and is further refined with data collected from MPCA s investigations associated with SA249 and sentinel monitoring conducted by Barr 8

14 on behalf of General Mills. The data collected to date indicates the presence of multiple TCE sources that are impacting soil, groundwater, and soil gas in the Central Area. Information regarding documented releases to groundwater and potential vapor sources, the physical characteristics of the area, the spatial distribution of TCE, vapor transport mechanisms and migration pathways, potential receptors, and mitigation measures installed as part of response actions completed by General Mills has been incorporated into the discussion below Vapor Sources As described previously, TCE was a commonly-used industrial and household solvent between the 1930s and the 1990s and is still in use today. Documented releases of TCE to shallow groundwater are present in the Northeast Area and potential users of TCE are present in the Central Area. The highest TCE concentrations in groundwater in the glacial drift are present in the Northeast Area, which is hydraulically up-gradient from the Central Area. The extensive investigations conducted at the Site found no evidence of DNAPL or other source material at or emanating from the Site. TCE is not detected in unsaturated soils at the Site, and only low concentrations of TCE (less than 1 mg/kg) are reported in the soil below the water table near the former disposal area based on recent investigations. These low concentrations are consistent with the dissolved TCE measured in the shallow groundwater and do not indicate the presence of DNAPL or source material. The Site is not an ongoing source of TCE to groundwater that would contribute to the potential vapor intrusion pathway in the Central Area Geology and Hydrogeology Glacial drift in the Central Area generally consists of heterogeneous fine- to medium-grained sand, with lesser amounts of coarse sand and gravelly sand. In some locations, the sand and gravel deposits are overlain by up to 20 feet of fill or peat. Glacial drift is underlain by discontinuous glacial till and/or shale at the Central Area which together act as a confining unit. Groundwater within the glacial drift is present at depths of 15 to 25 feet below ground surface (bgs). The shallow groundwater flow direction is and has consistently been to the southwest, with little to no seasonal variation, since at least the early 1980s, including the periods before and during the 25-year operating period of the legacy groundwater extraction system. The groundwater flow direction and gradient are influenced locally by the saturated thickness of the glacial drift above the glacial till and/or bedrock and other factors including: hydraulic conductivity distribution, surface topography, and drainage features such as the Mississippi River. Groundwater from the Northeast Area flows to the southwest into the Central Area Spatial Distribution of Contaminants TCE is present in groundwater at various locations and varying concentrations within the Central Area. The varying distribution, presence, and concentrations of TCE within the Central Area indicate multiple sources. The TCE concentrations in groundwater in the Northeast Area are higher than concentrations in the Central Area. The magnitude and extent of TCE in the groundwater in the Northeast Area are undefined and the presence of DNAPL and other continuing sources of TCE to groundwater in the 9

15 Northeast Area are unknown. The down-gradient extent of TCE in groundwater is defined in the Central Area Potential Receptors The land use in the Central Area is primarily single- and multi-family residential development with some interspersed commercial and industrial properties. Commercial and industrial buildings are the primary land use at the Site and at several properties along Como Avenue SE. There are currently 14 buildings present at the Site that are numbered Buildings 1 through 12, 14, and 15. Some buildings at the Site have basements or crawlspaces, while others are slab-on-grade construction. Observed building slab thicknesses at the Site ranged from 4 to 24 inches. The basement floor slabs of the residential properties in the Central Area are typically 10 feet or more above the groundwater table. Based on these land uses the following populations of receptors would be expected to reside or work within the Central Area: Long term residents, including young children, older children, adults and elderly adults; Short term residents, including occupants of rental properties such as college students; Commercial workers such as employees at commercial and industrial establishments; and Visitors and patrons Vapor Transport Mechanisms Based on the results of the various investigations conducted to date, the primary transport mechanism for soil vapor within the Central Area is diffusion of vapors from groundwater into the shallow glacial drift. Diffusion of vapors from groundwater occurs as a result of a concentration gradient between the groundwater and the soil gas in the overlying glacial drift. Vapor migration through preferential pathways may occur via natural and man-made pathways in the subsurface (e.g., buried utilities) such that the feature creates a pathway from a source to a receptor. Although utility plans indicate that sanitary sewers and other utilities are present, this potential pathway is unlikely since the utility bedding materials are likely similar to the native sandy unsaturated zone soils. In addition, no preferential pathways were identified based on the results of the extensive sub-slab soil gas sampling performed throughout the Study Area Building Vapor Mitigation Building mitigation systems, specifically active SSD systems, have been installed at 166 of the 177 properties in the Central Area (94 percent) as part of the sub-slab soil gas sampling and mitigation project. The SSD systems are operating as designed and are considered by MPCA and EPA to be among the most effective vapor intrusion mitigation strategies for existing or new buildings (MPCA, 2010; EPA, 2015). Additionally, post-mitigation indoor air sampling was performed at the 19 properties in the Central Area with sub-slab soil gas TCE results greater than 2,000 g/m 3 (Barr, 2015b). This sampling confirmed 10

16 that the SSD systems were operating effectively at properties with the highest TCE concentrations in subslab soil gas. The current status of building mitigation in the Central Area and the larger Study Area is shown on Figure 5. Information regarding the 11 properties in the Central Area that do not have SSD systems is provided below Central Area Building Mitigation Status Of the 11 properties in the Central Area that do not have active SSD systems, three properties (including the Site) had indoor air sampling results below the applicable MPCA intrusion screening value (ISV), three property owners declined SSD systems, and owners of five properties could not be reached or did not provide access to participate in the sub-slab soil gas sampling and mitigation project. Indoor air sampling was performed at five buildings (Buildings 8, 10, 11, 12, and 14) located at the Site where at least one sub-slab soil gas sample had a reported TCE concentration greater than 60 g/m 3 (10 times the industrial ISV). Reported TCE concentrations in indoor air were below the applicable ISV for all samples collected (Barr, 2015a). Two residential properties in the Central Area had two rounds of indoor air sampling with reported TCE concentrations in indoor air below the residential ISV (2.0 g/m 3 ) for both sampling events. Additionally, the owner of one of the two properties declined the installation of an active SSD system due to the presence of an existing passive SSD system for radon mitigation and because indoor air results were below the residential ISV. Reported sub-slab soil gas TCE concentrations were below 20g/m 3 (10 times the residential ISV), indicating the VI pathway is insignificant, at two of the three properties where owners declined SSD systems. Owners of these two properties were offered SSD systems because they were bounded by properties with reported sub-slab soil gas TCE concentrations greater than 20 g/m 3. TCE was greater than 20 g/m 3 in sub-slab soil gas at the other property where the owner declined a SSD system. A minimum of three, good-faith attempts were made to obtain access to install SSD systems at the three properties and the properties were referred to MPCA when access was not obtained. Owners of five properties in the Central Area could not be reached or did not provide access after a minimum of three good-faith attempts were made to obtain access and entry to each property to collect samples as part of the sub-slab soil gas and mitigation project. These properties were referred to MPCA. 2.4 Risk Assessment MPCA recommended preparation of a comprehensive risk assessment for all exposure pathways in its most recent Five-Year Review for the Site (MPCA, 2014b). The HHRA prepared for the Study Area by Haley & Aldrich, Inc. on behalf of General Mills considered all potential exposure pathways, including soil, groundwater, and vapor intrusion (H&A, 2015). Direct exposure pathways to soil were either incomplete or insignificant based on soil data collected from the Study Area. Similarly, direct exposure pathways to groundwater were incomplete because groundwater in the Study Area is not used as a source of potable water. The vapor intrusion pathway was evaluated further, and the HHRA concluded that the only exposure pathway that may be potentially complete and of potential significance is vapor intrusion of VOCs from shallow groundwater in the Northeast Area. The HHRA concluded that exposure pathways in 11

17 other areas, including the Central Area, were currently either incomplete or insignificant and not expected to be significant in the future. The MPCA provided comments on the HHRA in their letter dated November 3, 2015, and MDH provided comments in a letter from MPCA dated November 12, 2015 (MPCA, 2015a; MPCA, 2015b). Responses to MPCA and MDH/MPCA comments are in the letters from General Mills and Haley & Aldrich, Inc. to MPCA dated April 6, 2016 (GMI, 2016; H&A, 2016). 12

18 3.0 Regulatory Considerations This section presents remedial action objectives (RAOs), identifies potentially applicable or relevant and appropriate requirements (ARARs) and to-be-considered criteria (TBCs), and presents preliminary remediation goals (PRGs) for the vapor intrusion pathway at the Central Area that is the subject of this feasibility study. The remedial action objectives, potential ARARs and TBCs, and PRGs are based on the understanding of Central Area conditions developed during the various investigations and response actions that have been completed and the results of the HHRA. 3.1 Remedial Action Objectives In general, the RAOs provide the goals for protecting human health and the environment. The RAOs address the contaminants of concern (COCs) and potential exposure routes and receptors, and provide an acceptable contaminant level or range of levels for each exposure route that exceeds threshold criteria for protection of human health, as defined by CERCLA and the National Contingency Plan (NCP), to human health or the environment. The RAOs are to be as specific as possible without limiting the range of remedial alternatives that can be developed for screening and detailed analysis. The HHRA prepared for the Study Area considered all current and potential (future) exposure pathways, and focused on the vapor intrusion pathway for further evaluation. The HHRA concluded that exposure pathways in the Central Area were currently either incomplete or insignificant and not expected to be significant in the future. The RAO developed for the Central Area focuses on maintaining protectiveness of human health and the environment. TCE is the primary COC and drives potential human health risk in the Central Area. Accordingly, the remedial action objective (RAO) for the Central Area is to maintain insignificant potential risk to human health from inhalation exposure to TCE in indoor air resulting from TCE concentrations in soil gas and groundwater. As stated in the RAP Modification #1, General Mills is responsible only for implementing response actions to address impacts that are due to its former operations at the Site. By preparing this feasibility study and developing this RAO, General Mills does not acknowledge or agree that TCE impacts in the Central Area or elsewhere in the Study Area are associated with its former operations at the Site. 3.2 ARARs and TBCs The NCP provides that remedial action alternatives be assessed to evaluate whether they attain applicable or relevant and appropriate requirements (ARARs) under federal and state environmental laws or facility siting laws, or provide grounds for obtaining a waiver. In addition to ARARs, the identification and evaluation of remedial action alternatives may consider, as appropriate, other advisories, criteria, or guidelines, collectively called to-be-considered criteria (TBCs). The final acceptable exposure levels should be determined on the basis of the results of the risk assessment and the evaluation of the expected exposures and associated risks for each alternative (EPA, 1988). 13

19 3.2.1 Classification of ARARs and TBCs ARARs and TBCs are classified as action-specific, location-specific, or chemical-specific in the evaluation process: Action-specific ARARs and TBCs are technology- or activity-based requirements or limitations on actions or conditions. They establish performance, design, or other similar specific controls or regulations on actions. Action-specific ARARs and TBCs potentially applicable to the Central Area are identified in Table 1. Location-specific ARARs and TBCs are those requirements that relate to the geographical location of a site and restrict actions or contaminant concentrations in certain environmentally sensitive areas. Examples of areas regulated under such ARARs and TBCs include floodplains, wetlands, and locations where endangered species or historically significant cultural resources are present. Location-specific ARARs and TBCs potentially applicable to the Central Area are identified in Table 2. Chemical-specific ARARs and TBCs are usually health- or risk-based numerical values or methodologies used to calculate acceptable chemical concentrations that may be found in or discharged to the environment. Chemical-specific ARARs and TBCs include those that regulate the release to the environment of specific substances having certain chemical or physical characteristics or materials containing specific chemical compounds. Chemical-specific ARARs and TBCs potentially applicable to the Central Area are identified in Table Preliminary Remediation Goals PRGs are developed on the basis of chemical-specific ARARs, when available, other available information, and site-specific risk-related factors (EPA, 1988). PRGs serve as goals for the remedial action alternatives and typically consist of chemical concentrations that are considered protective of human health and the environment (chemical-specific PRGs). There are two general sources of chemical-specific PRGs: (1) concentrations based on ARARs and TBCs (ARAR/TBC-based PRGs); and (2) concentrations based on a risk assessment (risk-based PRGs). Where chemical-specific ARARs exist for a COC, these ARARs are the basis of the PRG. Where chemical-specific ARARs are not available, risk-based or TBC-based PRGs are applied. There are no chemical-specific ARARs available for TCE that are applicable to the vapor intrusion pathway. Vapor intrusion guidance documents prepared by both EPA and MPCA contain risk-based, generic screening values for TCE concentrations in indoor air (EPA, 2015; MPCA, 2008). MPCA s intrusion screening values (ISVs), which were developed in conjunction with MDH and are based on an incremental cancer risk threshold of one in 100,000 (1 x 10 5 ) and a non-cancer hazard quotient of 1, guided the subslab soil gas sampling and mitigation project. The current TCE ISV for residential property use is 2 g/m 3 and the current TCE ISV for commercial or industrial property use is 6 g/m 3. MDH considers the residential ISV a safe level that protects all people from health effects and the commercial/industrial ISV a safe level for people who may have exposures in the work place over many years (MDH, 2016a). MPCA ISVs for TCE were selected as PRGs for indoor air. 14

20 In general, there is a relationship between VOC-impacted groundwater concentrations and VOC concentrations in overlying soil gas. Many factors can influence the relationship including chemicalspecific characteristics (e.g., Henry s Law Constant) and site-specific conditions such as subsurface geology (e.g., soil type and stratification) and the presence of preferential pathways. EPA and MPCA vapor intrusion guidance provide generic screening values for groundwater that can be useful in initial evaluation stages to help define the magnitude and extent of potential vapor intrusion impacts. However, MPCA guidance states that their groundwater screening values (groundwater ISVs) are designed to be used as an additional line of evidence to help refine the SCM and to help determine the scope of further investigation, and should not be applied alone to screen out vapor intrusion risk at a site or for remedial action levels (MPCA, 2010). EPA guidance states that vapor intrusion screening levels for groundwater are not intended to be used as cleanup levels (EPA, 2015). Accordingly, a specific concentration-based groundwater PRG was not developed for this feasibility study. 15

21 4.0 Technology Screening The objective of developing and screening remedial action technologies is to compile an appropriate range of technologies and processes for the media and location(s) where remedial action(s) may occur. Appropriate technologies and process options are then assembled into a range of remedial action alternatives that are evaluated in the detailed analysis phase of the feasibility study. Appropriate remedial technology processes that meet the RAOs may involve the elimination or destruction of hazardous substances, the reduction of hazardous substance concentrations to acceptable health-based levels, prevention of exposure to hazardous substances via engineering or institutional controls, or a combination of the above. Remedial alternatives are developed by assembling technology processes into alternatives that address impacts for the identified media and locations of concern. 4.1 Development of General Response Actions General response actions are medium-specific actions that satisfy RAOs (EPA, 1988). Response actions may target the TCE-impacted soil vapor itself and/or the shallow groundwater that acts as a source of the TCE impacts in soil vapor. General response actions for mitigating risks posed by soil vapor may be applied individually or in combination. Potential response actions and their ability to achieve the RAO and the PRG are summarized as follows and in Table 4. No Further Action: No further action is evaluated as a basis for comparison during the feasibility study process, even though it typically does not achieve the RAOs and PRGs at sites where no response actions have been implemented. However, as noted previously, General Mills has already implemented significant response actions in the Central Area including operation of a groundwater extraction system for 25 years and installation of SSD systems at 94 percent of the properties. No further action beyond previous response actions is retained for consideration. Vapor Mitigation: Vapor mitigation involves installing and operating building controls, such as SSD systems or control of HVAC systems to maintain positive pressure, to disconnect the VI pathway on a property-specific basis. Currently, buildings at 166 of the 177 properties (94 percent) within the Central Area have active SSD systems. Vapor mitigation is retained as a general response action. Institutional Controls: Institutional Controls (ICs) are legal and administrative tools used to maintain protection of human health and the environment at sites where exposure to contaminated materials could occur. EPA defines institutional controls as non engineered instruments that help minimize the potential for human exposure to contamination and protect the integrity of the remedy (EPA, 2012). ICs are generally consolidated into the following four categories: o Government controls (e.g., zoning, local ordinances) 16

22 o o o Proprietary controls (e.g., easements, restrictive covenants) Enforcement and permit tools (e.g., consent decrees, administrative orders) Informational tools (e.g., notices filed in the land records, advisories) ICs restrict access to contaminated media by notifying property owners, perspective property purchasers, and workers of contamination. ICs help ensure the effectiveness of remedial actions by limiting activities that could degrade the effectiveness of the remedy, by providing access for regulatory agencies to monitor the effectiveness of the remedy, and by requiring periodic reporting. Institutional controls were retained as a general response action. Containment: Containment technologies consist of physical or hydraulic containment to provide a barrier between impacted groundwater and overlying structures to disconnect the VI pathway. Possible containment remedial technologies include installing a buried horizontal grout cap over impacted groundwater and horizontal hydraulic barriers. Containment was not retained as a general response action for the Central Area due to lack of implementability and uncertain effectiveness. Physical Removal: Physical removal involves removing impacted soil, groundwater and/or soil vapor. Remedial technologies for this general response action include soil excavation, soil vapor extraction systems, in-situ thermal treatment, and groundwater extraction and treatment. Physical removal has previously been implemented in the Central Area via operation of the legacy groundwater extraction system and the reported excavation of the disposal area in 1981 (Barr, 2015c). Vapor extraction and in-situ thermal treatment were not retained due to the limited radius of influence and access limitations. Soil excavation was not retained due to access limitations and lack of source material. Expansion or modification of the groundwater extraction system was retained as a general response action. Biological: This general response action involves using biological processes, enhanced or natural, to remediate impacted groundwater (bioremediation). Remedial technologies for biological treatment vary, but may include monitored natural attenuation (MNA), phyto-remediation, and various in-situ amendments to promote/support bioremediation. Several biological processes have been effective for remediating TCE-impacted groundwater under appropriate conditions, including: enhanced reductive dechlorination, aerobic co-metabolism, and biogeochemical processes. Each of the biological processes occurs under different conditions, but the processes can occur simultaneously within an impacted area or at different times with varying subsurface conditions. Biological treatment was retained as a general response action. Chemical: Chemical technologies involve adding chemicals to the subsurface that cause reactions that destroy the chemical of concern. Chemical technologies are primarily composed of in-situ chemical oxidation (ISCO), which uses a strong oxidant or in-situ chemical reduction (ISCR), which uses a strong reducing agent. ISCO and ISCR have been used effectively at locations with high TCE concentrations (e.g., source areas) but have not been used at locations with low-level 17

23 groundwater concentrations over a large area like the Central Area. Chemical treatment was not retained as a general response action due to lack of implementability and unlikely effectiveness. Electrical: Electrical technologies involve applying a voltage via an array (similar to a reactive barrier) to electrically break down contaminants within the groundwater. This technology is still in development and its effectiveness is unknown at locations similar in size to the Central Area. Electrical treatment was not retained as a general response action due to unproven effectiveness and lack of implementability. 4.2 Evaluation and Screening of Technologies In this section, the general response actions that are retained in Section 4.1 for further evaluation are subdivided into remedial technologies and are further screened. Descriptions of the retained remedial technologies and associated process options are discussed in Section 4.3. Remedial technologies are general categories of technologies within a general response action, and process options are specific processes within a technology category. For example, in-situ bioremediation and monitored natural attenuation are two remedial technology categories within the biological general response action. Aerobic co-metabolism via injection wells and enhanced reductive dechlorination via injection wells are two process options within the remedial technology category of in-situ bioremediation. Potentially viable technologies and associated process options for each retained general response action are summarized in Table 4. The potentially viable technologies and associated process options were then screened based on effectiveness, implementability, and relative cost as required by the NCP. These screening criteria are defined as follows: Effectiveness: the ability of the remedial technology or process option to perform adequately to achieve the RAO(s) alone or as part of an overall system. The evaluation considered whether the remedial technology or process option has demonstrated effectiveness at other TCE-impacted sites similar to the Central Area. Implementability: the degree of difficulty expected in implementing a particular remedial technology or process option under practical technical, regulatory, and access limitations and schedule constraints. The evaluation considered whether the remedial technology or process option has been implemented at other locations with similar technical, regulatory, and logistical constraints. Relative Cost: qualitative costs of each remedial technology or process option were developed for comparative purposes and include construction and long-term O&M costs. The comparison of relative cost is used to preclude further evaluation of technologies that are very costly where other choices perform similar functions with comparable effectiveness at a lower cost. Relative cost was assigned to each remedial technology for this evaluation; however, relative cost was not used as a rationale to screen out a remedial technology or process option. 18

24 Screening was based on professional experience and judgment; published sources; knowledge of chemical, geologic, and physical conditions in the Central Area; and experience with the previous implementation of remedial actions in the Central Area. 4.3 Retained Technologies Retained technologies were reviewed based upon effectiveness, implementability, and relative cost as described within Section 4.2. Relative cost was not used to screen out remedial technologies during this evaluation. A short description of each retained technology and its associated process options as applied to the Central Area follows. Install SSD Systems at Un-Mitigated Residences in the Central Area This remedial technology involves installing mitigation systems at the remaining residences in the Central Area without mitigation systems, provided receipt of property owner consent. Process options include active SSD systems or positive pressure HVAC systems designed to provide a pressure barrier for sub-slab vapors to enter the buildings. Active SSD systems are retained as the process option for this technology. Operation and Maintenance (O&M) of SSD Systems This remedial technology requires an O&M plan (process option) to be developed and implemented for SSD systems within the Central Area. The objective of this plan will be to help ensure that the SSD systems continue to disconnect the VI pathway by remaining functional over time. In addition, the SSD systems will remain operational until they are no longer needed (e.g., sub-slab soil gas concentrations are below risk-based levels). This process option is retained for further evaluation. Institutional Controls Pertaining to SSD Systems The process option for this remedial technology is area-wide institutional controls (ICs) pertaining to active SSD systems. The ICs can be developed as a city-wide ordinance or other area-wide control, which will require regulatory cooperation. ICs may be a component of an O&M plan. This process option is retained for further evaluation. Expanded/Modified Groundwater Extraction This remedial technology has been implemented previously at the Central Area. Both process options for this remedial technology involve removal of the contaminated groundwater via submersible pumps. The removed groundwater will be treated and discharged to the storm sewer of via an existing, or separately acquired, National Pollution Discharge Elimination System (NPDES) permit (process option 1) or the contaminated groundwater will be treated and reinjected into the subsurface (process option 2). Both process options are retained for further evaluation. Monitored Natural Attenuation The process option of this remedial technology is to allow the native microorganisms and natural processes to reduce TCE concentrations and associated daughter products and to monitor the progress. 19

25 This process option can be achieved by developing and implementing a groundwater monitoring well network and associated sampling plan. This process option is retained for further evaluation. In Situ Bioremediation Various in situ bioremediation process options exist. Process options using permeable reactive barriers (PRBs) were not retained due to lack of implementability (limited access within the Central Area) and biogeochemical process options were not retained due to uncertain effectiveness (unlikely that the aquifer matrix will support the chemical processes) and lack of implementability (engineered geochemical process alternatives typically involve constructing PRBs). Aerobic co-metabolism via injection wells and enhanced reductive dechlorination via injection wells are the process options retained for further evaluation. Both process options require injection of a carbon substrate into the glacial drift groundwater. Aerobic cometabolism has an additional requirement of adding oxygen to the groundwater along with the carbon substrate. 20

26 5.0 Development of Remedial Alternatives Remedial technologies and associated process options that were retained for alternative development in Section 4.0 are combined to form comprehensive remedial action alternatives. Assembly of alternatives provides a method of identifying and screening the inter-relationships between remedial technologies and process options applicable to the Central Area. Five alternatives are described in the following sections. Natural attenuation of TCE in groundwater will occur during all alternatives which leads to a reduction in TCE concentrations over time. Additionally, the SSD systems previously installed in the Central Area will continue to provide protection from vapor intrusion. These aspects will not be discussed in detail below. 5.1 Alternative 1 No Further Action Beyond Previous Response Actions No further action includes no additional work beyond the past operation of the legacy groundwater extraction system, previous installation of SSD systems, and existing institutional controls (ICs) already implemented. General Mills installed a groundwater extraction and treatment system in 1985 and operated the system until The system was installed, operated, and monitored to limit the migration of TCE in groundwater and reduce concentrations of TCE in groundwater as a remedial action under the Consent Order. Approximately six billion gallons of groundwater were treated and an estimated 7,000 pounds of TCE were removed from the glacial drift and deeper bedrock aquifers (Barr, 2015c). The system was shut down in 2010 with approval from the MPCA. As described in Section 2.3.6, active SSD systems have been installed at 166 of the 177 properties in the Central Area. Additional detail regarding the sub-slab soil gas sampling and building mitigation work can be found in the Implementation Report (Barr, 2015b). Several types of ICs have been implemented for protection of public health and the environment, limiting access to impacted soil and/or groundwater at the Site and the Study Area to assure long-term protectiveness, as described in Section Alternative 2 Monitored Natural Attenuation Monitored natural attenuation (MNA) includes developing and implementing a groundwater monitoring plan that will assess natural degradation of TCE. The groundwater monitoring plan will use the existing groundwater monitoring well network (see Figure 6) and will include sampling for TCE, TCE daughter compounds (e.g. dichloroethene, vinyl chloride, methane), electron acceptors (e.g., iron, manganese, nitrate, sulfate, dissolved oxygen) and general chemistry parameters (e.g., total organic carbon, alkalinity, hardness, ph). The suite of parameters was developed to be consistent with the parameters recommended for evaluating MNA (EPA, 1998). It is assumed that VOC sampling (for TCE and daughter compounds) will be conducted annually at 25 wells and that the full suite of MNA parameters will be 21

27 analyzed at samples collected from 10 wells within the monitoring well network. A report summarizing the results of the monitoring will be provided to MPCA annually. The objective of this alternative is to monitor natural processes within the glacial drift groundwater that act to decrease contaminants to levels that would eliminate the need for SSD systems in the Central Area. 5.3 Alternative 3 Long Term O&M of SSD Systems Alternative 3 includes preparing a long-term O&M plan that would detail routine monitoring and maintenance of the SSD systems in the Central Area. This alternative assumes that residences within the Central Area have, or will have installed, a functioning SSD system (assuming cooperation from property owners). Initial Installation/Sampling Alternative 3 includes installing active SSD systems at the 10 remaining residential properties within the Central Area that do not currently have an active SSD system, provided property owner consent and coordination. SSD system installation will include coordinating access, diagnostic testing, designing the system for each property, installing the system, and evaluating initial performance, consistent with the existing work plan (Barr, 2014a). Alternative 3 also assumes that subsequent indoor air and sub-slab soil gas sampling will be conducted at five commercial buildings (Buildings 8, 10, 11, 12, and 14) at the Site. Based on the indoor air results collected to date, it is assumed that indoor air TCE results will be below the applicable ISV and that installation of SSD systems at the five buildings will be unnecessary. However, if the applicable TCE ISV is exceeded during the additional sampling, and assuming cooperation of the property owner, SSD systems would be installed at one or more of the five buildings, as appropriate. Operations and Monitoring Alternative 3 includes preparing and implementing a long-term O&M plan for the SSD systems within the Central Area. The developed O&M plan would include the methods and procedures to verify fan operation, visually inspect the vent pipe, and visually inspect the manometer pressure gage. For the purposes of this feasibility study, Alternative 3 assumes that the inspections would be completed by the property owners with support provided by a technical resource on operation of SSD systems and support provided for coordination and communication with contractors for routine maintenance. Due to the demonstrated long-term reliability and simple operation of the SSD systems, it was assumed that visual inspections completed by the property owners would be the least intrusive option to monitor the SSD systems. On the rare occasion that maintenance is required, it was assumed that access would be coordinated for an engineer and/or contractor to inspect and complete routine maintenance (e.g., fan replacement). Additionally, a city ordinance could be developed to provide requirements for disclosure of property-specific vapor data and existence of SSD systems in connection with property transactions. 22

28 5.4 Alternative 4 Enhanced Bioremediation via Injection Events Alternative 4 involves installing injection wells within the rights-of-way, delivering carbon substrate and other amendments to promote enhanced biodegradation of TCE in the glacial drift groundwater, and conducting performance monitoring. Access to rights-of-way is contingent upon negotiations with rightof-way administrators (e.g. City of Minneapolis and the railroad). There is uncertainty associated with the ability to gain access for the purposes of installing injection wells and delivering amendment. Subsurface conditions, access limitations, and technology limitations provide significant uncertainty regarding the effectiveness of Alternative 4. The appropriate substrate and amendment dosing would need to be evaluated during the design phase, if selected. Injection Well Construction and Initial Amendment Delivery Alternative 4 includes construction of approximately 40 injection wells as tentatively shown on Figure 7. The injection well locations were designed along transects perpendicular to groundwater flow along the railroad tracks or road rights-of-way that provide the greatest access and space for injection well installation. The injection well spacing was based on the anticipated radius of influence that could be achieved with a fully penetrating well in the glacial drift, estimated hydraulic conductivity ranging from 20 meters per day (m/day) to 100 m/day, injection rates ranging from 25 gallons per minute (gpm) to 50 gpm, and limiting groundwater mounding adjacent to the injection wells below the depth of utilities and basements. There is uncertainty regarding the radius of influence that could be achieved and injection well spacing would be determined during the design phase. There is the potential that the radius of influence varies significantly and delivery will not be successfully applied to significant portions of the Central Area with limited access. It is assumed that reductive dechlorination would be the primary biological process adjacent to the injection wells. The aerobic co-metabolic process would likely occur near the periphery and other biogeochemical processes would occur as conditions are suitable. Many factors influence the amount of carbon substrate required to create reducing conditions in the aquifer supportive of anaerobic reductive dechlorination. Factors that significantly affect the estimated quantity of substrate required include: natural demand from competing electron acceptors within the aquifer (e.g., naturally occurring sulfate), the design period or treatment timeframe, and the design or safety factor employed. The natural demand from the aquifer has been estimated based on site-specific dissolved phase concentrations of major competing electron acceptors using a substrate estimating tool (Parsons, 2010b). The design period and design factor can vary by amendment type and injection program. A sensitivity analysis was performed using a range of recommended design periods and design factor values (Parsons, 2010a). Results of the sensitivity analysis are presented in Table 5. Based on the sensitivity analysis, it is estimated for the purposes of this feasibility study that approximately seven million pounds of carbon substrate would be required for treatment of the Central Area. Emulsified vegetable oil was selected as the carbon substrate based on its longevity in the subsurface for quantity and cost estimating purposes; however, there is significant uncertainty regarding the carbon substrate type and amendments that would perform most effectively. 23

29 Operations and Monitoring For the purposes of this feasibility study, Alternative 4 assumes that the majority of the carbon substrate and amendments will be delivered during initial injections and that the substrate will be distributed across the treatment area via natural advective flow. However, there is significant uncertainty regarding substrate distribution in the subsurface. Thus, the behavior of substrate distribution would need to be evaluated as part of the design stage. Subsequent injections are assumed to occur periodically for three years at the injection well network installed for the initial injection. Performance monitoring is included to identify locations that would need subsequent injection. After three years of injection at the initial injection wells, it is assumed that additional wells would be needed to target specific locations within the Central Area where substrate delivery may have been limited by natural advective flow. For the purposes of this feasibility study, Alternative 4 assumes that 20 new injection wells would be installed at locations to be identified during performance monitoring conducted during the first three years. It is assumed that a higher initial dose would be used at the 20 additional wells, followed by subsequent injections for three additional years consistent with the initial injection procedures. 5.5 Alternative 5 Enhanced Bioremediation via Recirculating System Similar to Alternative 4, Alternative 5 involves installing injection wells within the rights-of-way for carbon substrate delivery to the aquifer to promote biodegradation of TCE in the glacial drift groundwater. In contrast to Alternative 4, Alternative 5 uses extraction wells within the Central Area and conveyance piping to recirculate the amended groundwater within the Central Area. Alternative 5 also includes constructing treatment control buildings where the extracted groundwater would be amended prior to reinjection and assumes that access for treatment control buildings can be obtained within the Central Area. Given the density of residential development in the Central Area, the treatment control buildings are assumed to require two typical lots for construction of each treatment control building. There is significant uncertainty regarding the ability to obtain access for the treatment control buildings. Access to rights-ofway for injection well installation is contingent upon negotiations with right-of-way administrators (City of Minneapolis and the railroad). There is uncertainty associated with the ability to gain access to install the injection wells and piping. Subsurface conditions, access limitations, and technology limitations provide significant uncertainty regarding the effectiveness of Alternative 5. The appropriate substrate and amendment dosing that would be required would need to be evaluated during the design phase. The recirculation system would have the flexibility to deliver high concentration solutions of carbon substrate and other amendments at the beginning of operations while also having the ability to add smaller quantities of amendments during recirculation. This functionality allows for amendment distribution in the subsurface to be more precisely controlled than with Alternative 4; however, there is significant uncertainty regarding the degree to which substrate distribution could be controlled. As with Alternative 4, the behavior of substrate distribution would need to be evaluated during the design phase. The recirculation system would have functionality to switch from promoting anaerobic conditions to promoting aerobic conditions in the aquifer as necessary to promote the reductive dechlorination or the 24

30 aerobic co-metabolic process, respectively. Assumptions and costs for construction of appropriate treatment facilities, maintenance to the system and wells, design phase considerations, performing supplemental amendment delivery, and performance monitoring are all included within Alternative 5 as discussed below. Full Scale Recirculation System Construction and Initial Amendment Delivery For purposes of this feasibility study, Alternative 5 assumes constructing 40 injection wells as conceptually shown on Figure 8, constructing two new extraction wells and connecting to four existing extraction wells, constructing conveyance piping, and constructing treatment control buildings that would contain pumps, piping, valves, instrumentation, and controls. The treatment control buildings are assumed to be 3,500 square feet in size and constructed similar to a warehouse to provide sufficient capacity for tanks, pumps, piping, and controls. Extraction well spacing is based on previous performance of the legacy groundwater extraction system. The injection well locations were designed along transects perpendicular to groundwater flow along the railroad tracks or road rights-of-way that provide the greatest access and space for injection well installation. The injection well spacing is based on the anticipated radius of influence that could be achieved with a fully penetrating well in the glacial drift unit, estimated hydraulic conductivity ranging from 20 m/day to 100 m/day, injection rates ranging from 25 to 50 GPM, and limiting groundwater mounding adjacent to the injection wells to depths below utilities and basements. There is uncertainty regarding the achievable radius of influence; as such injection well spacing and injection rate would need to be evaluated during the design phase. Given the greater control and ability to adjust to conditions observed during implementation, it is unknown what amendments would be added for Alternative 5. The recirculation system for Alternative 5 could be operated to promote either anaerobic reductive dechlorination or aerobic co-metabolic processes. For preparing cost estimates, it was assumed that a carbon substrate of a similar dose to Alternative 4 would initially be applied and that subsequent operations could be adjusted to maintain reducing conditions or modified to promote aerobic co-metabolic degradation. Selection of the dominant biological process and dosing requirements would be determined during the design phase. Operations and Monitoring For the purposes of this feasibility study, Alternative 5 assumes an initial injection of amendments followed by operation of the recirculation system for up to 10 years. The first five years of operations will include performance monitoring and system modifications based on conditions measured at the treatment control buildings and at monitoring wells located within the Central Area. The recirculation system would have the flexibility to adjust amendment type and dosing rates and would be operated by a full-time operator to maximize the effectiveness of treatment. Following five years of treatment, it is assumed that a sufficient amount of amendment would have been delivered throughout the Central Area and the system would operate primarily for recirculation of groundwater to promote uniform treatment across the Central Area. 25

31 6.0 Detailed Analysis of Alternatives This section presents the detailed analysis of the remedial alternatives described in Section 5.0. The purpose of the detailed analysis is to gather sufficient information to adequately compare the alternatives and select a remedy. 6.1 Detailed Analysis Overview The detailed analysis of the five alternatives consists of the following three components: 1) further definition of each alternative with respect to volumes or areas of contaminated media to be addressed, the technologies to be used, and the primary performance requirements associated with those technologies; 2) an assessment and a summary of each alternative against the evaluation criteria; and 3) a comparative analysis among the alternatives to assess the relative performance of each alternative with respect to each evaluation criteria Alternative Definition Each alternative was reviewed to determine if additional definition was required to apply the evaluation criteria consistently and appropriately and to develop order-of-magnitude cost estimates. Information considered to refine the alternatives consisted of preliminary design concepts and an evaluation of the limitations, assumptions, and uncertainties associated with each alternative. An important factor in the decisions was to provide a reasonable set of design assumptions for each alternative sufficient to provide a reasonable detailed analysis Evaluation Criteria EPA has established evaluation criteria to address the technical and policy considerations that have proven to be important for selecting remedial alternatives. These criteria serve as the basis for conducting the detailed analyses of the feasibility study and for subsequently selecting a remedial action. EPA evaluation criteria are divided into the following three groups: threshold criteria, balancing criteria, and modifying criteria. In addition to the EPA evaluation criteria, this analysis evaluates additional considerations with the potential to impact the community Threshold Criteria Threshold criteria are defined as statutory requirements that each alternative must satisfy to be eligible for selection. Overall Protection of Human Health and the Environment This criteria draws on assessments conducted under other criteria, especially long-term effectiveness and permanence, short-term effectiveness, 26

32 compliance with ARARs and consideration of TBCs. It focuses on whether the alternative meets the RAO and PRG and describes how risks are eliminated, reduced, or controlled. As detailed in Section 2.4, a risk assessment was completed to evaluate potential risk to human health (H&A, 2015). The risk assessment concluded that the exposure pathways in the Central Area were currently either incomplete or insignificant and not expected to be significant in the future. Accordingly, overall protection of human health and the environment is assessed by evaluating the additional protection offered by each alternative compared to the no further action alternative (Alternative 1). Radon is a naturally occurring radioactive gas that is present in shallow soils in Minnesota. While not evaluated as part of this feasibility study, the SSD systems installed by General Mills also provides protection to human health from radon. SSD systems are considered by MDH to be an effective technology for mitigating radon and operate for many years with minimal maintenance requirements (MDH, 2016b). Compliance with ARARs This evaluation criterion is used to evaluate whether the alternative complies with federal, state, and local ARARs. It also addresses other information from advisories, criteria, and guidance that is to be considered in the detailed analysis of an alternative Balancing Criteria Balancing criteria are technical criteria upon which the detailed analysis is primarily based. The assessment of the technical balancing criteria informs the assessment of the threshold criteria, modifying criteria, and additional considerations. Long-Term Effectiveness and Permanence This assessment evaluates the long-term effectiveness and permanence of the alternative in maintaining protection of human health and the environment after response objectives have been met. Reduction of Toxicity, Mobility, or Volume through Treatment This assessment evaluates the anticipated performance of the specific treatment technologies an alternative may employ. Short-Term Effectiveness This assessment evaluates the effectiveness of the alternative in protecting human health and the environment during construction and implementation of a remedy until response objectives have been met. Implementability This assessment evaluates the technical and administrative feasibility of the alternative and the availability of required goods and services. Cost This assessment evaluates the capital and O&M costs of the alternative. The cost estimate is prepared to an accuracy of plus 50 to minus 30 percent with the exception of those items noted to have greater uncertainty. Anticipated accuracy range for FS cost estimates is consistent with EPA guidance (EPA, 1988). This accuracy range is associated with the most likely cost of the project based on the level of design that has been completed and the uncertainties in the project as scoped (e.g., quantity uncertainties for wells and other capital cost items, pricing changes for amendments, variability in project 27

33 schedule/phasing, etc.). It does not include costs for future scope changes that are not part of the planned project or risk contingency (e.g., additional media cleanup, change in technologies, new contaminants of concern or expanded treatment areas, etc.). The present worth estimate is calculated assuming a discount rate of 7 percent and a timeframe of 30 years. Supporting cost details for each alternative are provided in Appendix A Modifying Criteria Modifying criteria evaluate state and community acceptance of implemented remedial actions. Assessments of the modifying criteria are not provided in this detailed analysis and will be completed after the public comment period. The assessment of balancing criteria and additional considerations are intended to inform the evaluation of state and community acceptance. State Acceptance This assessment reflects the state s (or support agency s) apparent preferences among or concerns about alternatives. Community Acceptance This assessment reflects the community s apparent preference among or concerns about alternatives Additional Considerations Additional considerations could be threshold or balancing criteria, but are evaluated separately in this feasibility study to provide further details on distinct items with the potential to impact the community. The assessment of threshold and balancing criteria incorporates the assessment of the additional considerations. Incremental Reduction in Vapor Intrusion Risk This assessment provides a comparison of the reduction in vapor intrusion risk associated with implementation of each remedial alternative relative to Alternative 1 (no further action beyond previous response actions). Secondary Impacts This assessment evaluates potential secondary impacts of implementing the remedial alternative. In some cases, the risk of secondary impacts may be greater than the vapor intrusion risk. Secondary impacts are considered during the evaluation of short-term effectiveness. Sustainability This assessment provides a comparison of sustainability impacts (e.g., greenhouse gas impacts, energy footprint, and worker injury risk) associated with implementing each remedial alternative relative to Alternative 1 (no further action beyond previous response actions). SiteWise, a tool developed for green and sustainable remediation, was the assessment method used to estimate sustainability impacts. SiteWise estimates sustainability metrics by considering impacts associated with the production of materials used in the remedy, transportation of personnel and equipment, equipment use (e.g., pumps), and residual handling (e.g., waste disposal) throughout each phase of the remedial action including construction, operation, and long-term monitoring. Further information regarding the sustainability impacts evaluation is in Appendix B. 28

34 Construction Time This assessment provides an estimate of the time required to implement the remedial alternative Comparative Analysis of Alternatives A comparative analysis of alternatives is conducted to evaluate the performance of each alternative relative to the specific evaluation criteria and to identify the relative advantages and disadvantages of each alternative. The comparative analysis of alternatives is summarized in Table Detailed Analysis of Remedial Alternatives The alternatives described in Section 5.0 of this report are evaluated in detail using the evaluation criteria presented in Section Alternative 1 No Further Action Beyond Previous Response Actions Alternative 1 includes no further action beyond the already-completed installation and operation of the legacy groundwater extraction system, installation of the existing SSD systems, and existing institutional controls already implemented. Natural attenuation of TCE in groundwater will occur as part of all alternatives, including Alternative 1, which leads to a reduction in TCE concentrations over time Alternative 1 Threshold Criteria Overall Protection of Human Health and the Environment Operation of the legacy groundwater extraction system reduced TCE concentrations in groundwater in the Central Area. Installation of SSD systems reduced potential exposure to TCE resulting from vapor intrusion. Significant effort has been completed through installation and operation of these systems to protect human health and the environment. The HHRA concluded that the exposure pathways in the Central Area were currently either incomplete or insignificant and not expected to be significant in the future (H&A, 2015). Compliance with ARARs Alternative 1 would comply with chemical-, action-, and location-specific ARARs Alternative 1 Balancing Criteria Long-Term Effectiveness and Permanence SSD systems have a demonstrated track record of reliable long-term performance. Alternative 1 does not include a systematic plan for O&M of the SSD systems to verify and maintain long-term performance. Over the long term, natural attenuation processes will eventually reduce TCE concentrations in groundwater. Reduction of Toxicity, Mobility, or Volume through Treatment The legacy groundwater extraction system provided significant reduction in toxicity, mobility, and volume of TCE through the removal and treatment of approximately six billion gallons of groundwater and removal of approximately 7,000 pounds of TCE. The existing SSD systems significantly reduce the mobility of sub-slab soil gas to migrate into buildings. Short-Term Effectiveness Since Alternative 1 requires no further construction, there are no implementation-related risks to workers, the community, or the environment from Alternative 1. 29

35 Implementability Implementation has already been completed. Alternative 1 does not involve the implementation of further actions. Cost No additional costs are associated with Alternative 1. General Mills has performed extensive investigation and response action work in the Study Area since the early 1980s. General Mills spent over $3.5 million on early investigation work and the installation and operation for 25 years of the legacy groundwater extraction system. General Mills has spent over $11 million since late 2013 on vapor intrusion investigation and installation of SSD systems Alternative 1 Additional Considerations Incremental Reduction in Vapor Intrusion Risk Installation of existing SSD systems reduced potential exposure to TCE resulting from vapor intrusion. Alternative 1 is the baseline for comparison. Secondary Impacts There are no secondary impacts resulting from implementation of Alternative 1. Sustainability Alternative 1 is used as the baseline for comparison for sustainability. Construction Time Alternative 1 does not require construction for further implementation Alternative 1 Summary The RAO has been substantially achieved through installing SSD systems at approximately 94 percent of properties in the Central Area. Alternative 1 does not provide a systematic plan for O&M of the SSD systems or address future VI risk as robustly as Alternative 3, but it has already been implemented and poses no short-term risks due to construction activities or potential secondary impacts from remedial activities like Alternatives 4 and Alternative 2 Monitored Natural Attenuation Alternative 2 relies on naturally occurring degradation of TCE in the groundwater and includes monitoring of the natural attenuation process by collecting groundwater samples from the existing monitoring well network and analyzing the samples for TCE, TCE daughter compounds, and common electron acceptors. Additionally, the SSD systems previously installed in the Central Area will continue to provide protection, although Alternative 2 does not provide a systematic plan for O&M of the SSD systems Alternative 2 Threshold Criteria Overall Protection of Human Health and the Environment The natural degradation rates observed in the Central Area indicate that it will take a long time to reduce groundwater TCE concentrations to a level that would eliminate the need for SSD systems. Alternative 2 evaluates groundwater conditions and assesses the timeframe for natural treatment until it is not necessary to operate SSD systems to maintain protection of human health and the environment. Compliance with ARARs Alternative 2 would comply with chemical-specific, action-specific, and locationspecific ARARs. 30

36 Alternative 2 Balancing Criteria Long-Term Effectiveness and Permanence Similar to Alternative 1, Alternative 2 does not include a systematic plan for long-term O&M of the SSD systems. Over the long-term, natural attenuation processes will eventually reduce groundwater concentrations. Reduction of Toxicity, Mobility, or Volume through Treatment Alternative 2 does not provide additional treatment of soil gas or groundwater beyond naturally occurring degradation, which over time will reduce groundwater concentrations. The existing SSD systems significantly reduce the mobility of sub-slab soil gas to migrate into buildings. Short-Term Effectiveness Since Alternative 2 requires no further construction, there are minimal implementation-related risks to workers, the community, or the environment. There would be a minimal increased presence of workers in the neighborhood for completing groundwater monitoring events. The groundwater monitoring events would involve an environmental professional driving to monitoring wells, collecting groundwater samples, and transporting them to a laboratory for chemical analysis. Implementability Alternative 2 is readily implementable using the existing monitoring well network. The services, materials, and technologies are available and implementation is technically and administratively feasible. Cost The present worth cost to complete Alternative 2 is estimated to be $1.3 million with a range of costs estimated between $0.9 million and $1.9 million. The cost estimate for Alternative 2 is summarized in Table Alternative 2 Additional Considerations Incremental Reduction in Vapor Intrusion Risk The incremental reduction in vapor intrusion risk is negligible in the short-term when compared to Alternative 1. The additional monitoring of natural TCEdegradation processes conducted for Alternative 2 does not directly address vapor intrusion risk. Secondary Impacts There are no secondary impacts resulting from implementation of Alternative 2. Sustainability The greenhouse gas emissions, energy footprint, and worker injury risk for Alternative 2 are similar to Alternative 1. Construction Time Alternative 2 could be implemented in less than a year. The existing monitoring well network is considered sufficient for an effective monitored natural attenuation program Alternative 2 Summary Similar to Alternative 1, the RAO has been substantially achieved through installing SSD systems at approximately 94 percent of properties in the Central Area. There is a long anticipated timeframe for natural processes to degrade TCE to a level that would eliminate the need for SSD systems in the Central Area. However, Alternative 2 provides a plan for monitoring of natural attenuation processes which is not included in Alternative 1. Similar to Alternative 1, Alternative 2 does not provide a systematic plan for 31

37 O&M of the SSD systems or address future VI risk as robustly as Alternative 3, but it is easy to implement and poses no short-term risks due to construction activities or potential secondary impacts from remedial activities like Alternatives 4 and Alternative 3 Long-Term O&M of SSD Systems Alternative 3 includes preparing and implementing a long-term O&M plan that would provide measures for routine monitoring and maintenance of the SSD systems. Elements of the O&M plan would include verifying fan operation, visually inspecting the vent pipe, and visually inspecting the manometer pressure gage. Alternative 3 assumes communication with and cooperation from property owners to conduct visual inspections and coordinate routine maintenance needs. Alternative 3 also includes installing active SSD systems at the remaining residential buildings within the Central Area where active SSD systems have not been installed. This assumes cooperation from property owners for access and installation of the SSD systems. Additionally, natural attenuation of TCE in groundwater will occur as part of all alternatives, including Alternative 3, which leads to a reduction in TCE concentrations over time. Monitoring of natural attenuation processes is not included in Alternative Alternative 3 Threshold Criteria Overall Protection of Human Health and the Environment Long-term O&M provides added assurance that the existing SSD systems will continue to operate safely and reliably. Alternative 3 provides controllable protection of human health and the environment regardless of TCE concentrations in groundwater and of the level of reduction in future TCE groundwater concentrations needed to eliminate the vapor intrusion pathway. It provides confidence that the vapor intrusion pathway is disconnected, and therefore, the SSD systems remain protective of human health and the environment. Compliance with ARARs Alternative 3 would comply with chemical-, action-, and location-specific ARARs. Additionally, Alternative 3 most directly addresses the indoor air ISV as a TBC by providing confidence that the VI pathway is disconnected Alternative 3 Balancing Criteria Long-Term Effectiveness and Permanence Alternative 3 provides a systematic plan for O&M of the SSD systems to maintain long-term performance. Property owners can easily assess the operation of the SSD systems through simple visual and auditory methods, as readily as they assess other home appliances. In addition, an SSD systems are a simple technology that is regarded by MPCA and EPA as one of most effective vapor intrusion mitigation strategies for existing or new buildings (MPCA, 2010; EPA, 2015). They effectively disconnect the potential vapor pathway into a building, providing protection when uncertainty exists regarding the level of reduction in future groundwater TCE concentrations needed to eliminate the VI pathway. Additionally, over the long term, natural attenuation processes will eventually reduce TCE concentrations in groundwater. 32

38 Reduction of Toxicity, Mobility, or Volume through Treatment The existing SSD systems significantly reduce the mobility of sub-slab soil gas to migrate into buildings. A systematic approach for O&M of the SSD systems provides a method to maintain the reduction in mobility of sub-slab soil gas into buildings through verification and maintenance of the SSD systems. Alternative 3 does not provide additional treatment of soil gas or groundwater beyond naturally occurring degradation, which over time will reduce groundwater concentrations. Short-Term Effectiveness There would be negligible short-term exposure to TCE because there is no handling of impacted groundwater or soil gas as part of Alternative 3. Because Alternative 3 directly addresses the vapor intrusion pathway and can be implemented quickly, the risk of exposure while implementing the remedy is significantly less than alternatives with a significant implementation stage (e.g., Alternatives 4 and 5). There would be disturbance to property owners and an increased presence in the neighborhood for construction of up to 10 new SSD systems and maintenance of SSD systems. Maintenance events would entail a contractor coordinating with a property owner for installing or repairing SSD system components (e.g., fan, riser pipe, suction pit, manometer). Repair activities would be comparable to other appliances or building maintenance activities performed by property owners. The short-term construction and maintenance impacts for Alternative 3 are similar to Alternatives 1 and 2 and significantly less than Alternatives 4 and 5. Implementability Alternative 3 is readily implementable using standard techniques for SSD installation and O&M assuming cooperation from property owners for access during installation and routine O&M. The services, materials, and technologies are available and proven, and implementation is technically and administratively feasible. Cost The present worth cost to complete Alternative 3 is estimated to be $2 million with a range of costs estimated between $1.4 million and $3.0 million. The cost estimate for Alternative 3 is summarized in Table Alternative 3 Additional Considerations Incremental Reduction in Vapor Intrusion Risk Alternative 3 provides the greatest incremental reduction in current and future risk based on the reliability and effectiveness of the SSD systems in disconnecting the potential VI pathway. Disconnecting the pathway helps assure the VI risk reduction since there is uncertainty of the TCE groundwater concentration necessary to eliminate VI risk and long-term O&M helps assure that the SSD systems continue to operate. Secondary Impacts There are no secondary impacts resulting from implementation of Alternative 3. Sustainability The greenhouse gas emissions, energy footprint, and worker injury risk for Alternative 3 are similar to Alternatives 1 and 2. 33

39 Construction Time Alternative 3 could be implemented in less than a year assuming cooperation of property owners Alternative 3 Summary Alternative 3 builds on the progress made by operating the legacy groundwater extraction system and installing the existing SSD systems by installing additional systems and providing long-term O&M of the SSD systems. SSD systems have been shown to be effective and reliable over the long term for mitigating VI risk and directly disconnect the potential VI pathway. Long-term O&M with Alternative 3 is easy to implement and will be only minimally intrusive for property owners in the Central Area. Alternative 3 has minimal short-term risks due to construction activities and no secondary impacts from remedial activities like Alternatives 4 and Alternative 4 Enhanced Bioremediation via Injection Events Alternative 4 involves installing injection wells within the rights-of-way and performing periodic injection of carbon substrate and other amendments to support bioremediation of approximately 100 million gallons of impacted groundwater over 40 acres in the Central Area. Within this area there are approximately 177 individual property owners with buildings covering approximately eight acres. Road and alleyway rights-of-way cover approximately five acres with significant infrastructure consisting of water, stormwater, wastewater, electrical, and other underground utilities present beneath the rights-ofway and throughout the Central Area. Due to the urban residential development, it is assumed for this feasibility study that the injection wells would be installed primarily along two transects through the Central Area. The carbon substrate and amendments would be injected using mobile tanker trucks. The SSD systems previously installed in the Central Area will continue to provide protection, although Alternative 4 does not provide a systematic plan for O&M of the SSD systems. Additionally, natural attenuation of TCE in groundwater will occur as part of all alternatives, including Alternative 4. Monitoring of natural attenuation processes is not included in Alternative 4, however, performance monitoring of the remedy will be conducted Alternative 4 Threshold Criteria Overall Protection of Human Health and the Environment Although Alternative 4 would reduce TCE concentrations in groundwater within the Central Area, the overall protection of human health and the environment would not necessarily be improved through a reduction in TCE concentrations in groundwater beyond the protection already offered by the existing SSD systems. It is unlikely that Alternative 4 could achieve an effective and consistent reduction in groundwater concentrations throughout the Central Area to a level that would eliminate the need for SSD systems. Additionally, Alternative 4 has the potential to generate methane and vinyl chloride which may pose a greater human health risk than the existing risk of TCE vapor intrusion as described in Section

40 Compliance with ARARs Alternative 4 would comply with chemical-, action-, and location-specific ARARs with appropriate design and planning. It is unlikely that Alternative 4 can reduce the groundwater concentrations sufficiently to meet the RAO Alternative 4 Balancing Criteria Long-Term Effectiveness and Permanence The effectiveness of Alternative 4 in reducing the groundwater concentrations to a level that that would eliminate the need for SSD systems is highly uncertain due to the size of the Central Area, limitations of advective flow distribution within a heterogeneous aquifer with many preferential flowpaths, and access limitations due to the high density residential development. If groundwater remediation is successfully implemented, the long-term effectiveness of Alternative 4 may be somewhat better than Alternatives 1 and 2, similar to Alternative 5, but less effective than Alternative 3. Similar to Alternatives 1 and 2, Alternative 4 does not provide a systematic plan for long-term O&M of the SSD systems. Reduction of Toxicity, Mobility, or Volume through Treatment Alternative 4 would likely reduce the volume of impacted-groundwater through treatment. There is a potential risk that complete degradation would not be achieved by implementing Alternative 4 and that daughter-compounds with higher toxicity (e.g., vinyl chloride) could be created. There is also a risk that complete degradation in an uncontrolled manner would lead to generation of methane. Alternative 4 would provide greater reduction of volume than Alternatives 1, 2, and 3, but less reduction in volume than Alternative 5. Alternative 4 does not provide a reduction in mobility beyond the existing SSD systems that reduce the mobility of sub-slab soil gas to migrate into buildings, and could potentially increase toxicity of the contaminants in groundwater. Short-Term Effectiveness Alternative 4 would expose workers to chemicals used as amendments in the injection solution. It would also expose the neighborhood to increased construction traffic during injection well installation and tanker traffic during injection delivery events. Implementability Alternative 4 is implementable assuming rights-of-way work permits, railroad access, and underground injection control program approvals can be obtained. Alternative 4 is implementable using standard well drilling techniques and fluid injection using tanker trucks and temporary piping connections. Installing the series of injection wells in close proximity to one another will be challenged by the urban setting and the presence of underground utilities and mature trees. The services, materials, and technologies are available and implementation is technically and administratively feasible, but potentially difficult to implement given the numerous stakeholders and approvals required. Alternative 4 is more difficult to implement than Alternatives 1, 2, and 3, but easier to implement than Alternative 5. Cost The present worth cost to complete Alternative 4 is estimated to be $26 million with a range of costs estimated between $6 million to $90 million. There is significant uncertainty regarding the cost estimate for Alternative 4 due to the influence of amendment quantity on total cost. Given the approximate treatment volume of 100 million gallons, the estimated cost per volume of groundwater targeted for treatment is approximately $0.26 per gallon. A review of TCE groundwater treatment projects from the Remediation Technology Cost Compendium (EPA, 2001) and EPA s database of Superfund decision documents (EPA, 2016) indicates that groundwater treatment costs are typically between the 35

41 estimated cost and the high end of the estimated range calculated for Alternative 4. Groundwater treatment completed at those sites was also not completed to a level that would eliminate the need for SSD systems in the Central Area. The cost estimates are prepared to an accuracy of plus 50 to minus 30 percent, with the exception of items involving amendment quantity which have a larger range. The cost estimate for Alternative 4 is summarized in Table Alternative 4 Additional Considerations Incremental Reduction in Vapor Intrusion Risk It is uncertain whether Alternative 4 will be effective at reducing TCE concentrations in groundwater throughout the Central Area sufficiently to eliminate the need to operate the SSD systems. Secondary Impacts There are numerous potential secondary impacts associated with implementing enhanced bioremediation. Providing a carbon substrate to the subsurface creates reducing conditions that can lead to metals liberation in groundwater. The breakdown of TCE via the reductive dechlorination process can generate daughter-compounds that may pose a higher risk to residents than TCE. Two examples of this are the generation of vinyl chloride as a result of incomplete reductive dechlorination and the generation of methane as a result of complete reductive dechlorination. While additional controls of these compounds could be designed, such as an area-wide vapor extraction system, the access limitations present in the Central Area and the limited radius of influence of vapor extraction systems would not provide coverage for the entire Central Area. Vapor extraction systems were screened out as a technology due to the lack of coverage as summarized in Table 4. The process of injecting carbon and amendments can also lead to changes in color, turbidity, and odor of the groundwater. The risk of secondary impacts for Alternative 4 may be greater than the existing risk of TCE vapor intrusion. Sustainability The greenhouse gas impacts are estimated at 3,800 metric tons CO 2-equivalent and the energy footprint is estimated at 66,000 million British Thermal Units (MMBTU) for implementation of Alternative 4. The greenhouse gas impacts for Alternative 4 are equivalent to the impact of driving 800 passenger vehicles for one year or consuming 428,000 gallons of gasoline. The estimated worker injury risk associated with implementing Alternative 4 is 1.0 accidents. Additional details regarding the sustainability impact evaluation are provided in Appendix B. Construction Time Alternative 4 could be implemented in +/- five years assuming necessary permits can be obtained. The timeframe for construction would be dependent on the results of final design, performance monitoring, and the need for installation of additional wells Alternative 4 Summary It is unlikely that Alternative 4 would be effective at reducing groundwater concentrations to a level that meets the RAO given the technical and logistical constraints in the Central Area. Alternative 4 is more intrusive during implementation than Alternatives 1, 2, and 3 and it would have short-term risks due to construction activities and secondary impacts from completing remedial activities. The potential benefit of reducing groundwater concentrations provided by Alternative 4 may be less than the potential risks of implementing this alternative. 36

42 6.2.5 Alternative 5 Enhanced Bioremediation via Recirculation Alternative 5 includes installing extraction and injection wells and conveyance piping within the rights-ofway, and constructing two treatment control buildings for operating a groundwater recirculation system. The recirculation system would inject carbon substrate and other amendments, extract groundwater, mix amendments with the groundwater to maintain conditions suitable for bioremediation, and reinject the amended groundwater. The treatment control buildings would be constructed to allow for adjustments to the substrate and amendment solutions and to switch from anaerobic to aerobic conditions, as necessary to promote reductive dechlorination or aerobic co-metabolic processes. The SSD systems previously installed in the Central Area will continue to operate, although Alternative 5 does not provide a systematic plan for O&M of the SSD systems. Additionally, natural attenuation of TCE in groundwater will occur as part of all alternatives, including Alternative 5. Monitoring of natural attenuation processes is not included in Alternative 5, however, performance monitoring of the remedy will be conducted Alternative 5 Threshold Criteria Overall Protection of Human Health and the Environment Although Alternative 5 would reduce TCE concentrations in groundwater within the Central Area, the overall protection of human health and the environment would not necessarily be improved through a reduction in TCE concentrations in groundwater beyond the protection already offered by the existing SSD systems. There is also significant uncertainty whether Alternative 5 could achieve an effective and consistent reduction in groundwater concentrations throughout the Central Area to a level that would eliminate the need for SSD systems due to the size of the Central Area, access limitations, and the heterogeneity of the glacial aquifer. Additionally, Alternative 5 has the potential to generate methane and vinyl chloride which may pose a greater human health risk than the existing risk of TCE vapor intrusion as described in Section Compliance with ARARs Alternative 5 would comply with chemical-, action-, and location-specific ARARs with appropriate design and planning Alternative 5 Balancing Criteria Long-Term Effectiveness and Permanence The effectiveness of Alternative 5 in reducing the groundwater concentrations to a level that that would eliminate the need for SSD systems is highly uncertain due to the size of the Central Area, limitations of advective flow distribution within a heterogeneous aquifer with many preferential flowpaths, and access limitations due to the high-density residential development. If groundwater remediation is successfully implemented, the long-term effectiveness of Alternative 5 may be somewhat better than Alternatives 1 and 2, similar to Alternative 4, but less effective than Alternative 3. Similar to Alternatives 1, 2, and 4, Alternative 5 does not provide a systematic plan for long-term O&M of the SSD systems. Reduction of Toxicity, Mobility, or Volume through Treatment Alternative 5 provides for the greatest potential for reduction of volume of groundwater impacts through treatment. There is a potential risk 37

43 that complete degradation would not be completed as part of implementing Alternative 5 and daughtercompounds with higher toxicity (e.g. vinyl chloride) could be created. Alternative 5 does not provide a reduction in mobility beyond the existing SSD systems that reduce the mobility of sub-slab soil gas to migrate into buildings, and could potentially increase toxicity of contaminants in groundwater. Short-Term Effectiveness Alternative 5 would expose workers to chemicals used as amendments in the treatment control buildings. Alternative 5 would also expose the neighborhood to heavy equipment traffic during extraction and injection well installation, conveyance piping and treatment control building installation, and maintenance of the control buildings, piping, and wells. Construction of the treatment control buildings would require demolition of several buildings within the Central Area to provide centralized locations for treatment process control. Construction of the conveyance piping would disturb roads during installation and would require temporary road closures. Implementability - Alternative 5 is implementable assuming access for the treatment control buildings, rights-of-way work permits, railroad access, and underground injection control program approval can be obtained. Alternative 5 is implementable using standard well drilling techniques, standard trenching and conveyance piping installation techniques, water treatment and control plant installation processes, and water treatment systems operations. Constructing the treatment control buildings will be challenged by the need to acquire property or obtain access within the Central Area for the buildings. Installing the series of injection wells, extraction wells, and conveyance piping will be challenged by the urban setting and the presence of underground utilities and mature trees. The services, materials, and technologies are available and implementation is technically and administratively feasible, but potentially difficult to implement given the numerous stakeholders and approvals required. Alternative 5 is more difficult to implement than Alternatives 1, 2, 3, and 4. Cost The present worth cost to complete Alternative 5 is estimated to be $42 million with a range of costs estimated at $19 million to $100 million. There is significant uncertainty regarding the cost estimate for Alternative 5 due to the influence of amendment quantity on total cost. Given the approximate treatment volume of 100 million gallons, the estimated cost per volume of groundwater targeted for treatment is approximately $0.42 per gallon. A review of TCE groundwater treatment projects from the Remediation Technology Cost Compendium (EPA, 2001) and EPA s database of Superfund decision documents (EPA, 2016) indicates that groundwater treatment costs per volume are typically between the estimated cost and the high end of the estimated range calculated for Alternative 5. Groundwater treatment completed at those sites was also not completed to a level that would be protective of VI risk. The cost estimates are prepared to an accuracy of plus 50 to minus 30 percent with the exception of items involving amendment quantity which have a larger range. The cost estimate for Alternative 5 is summarized in Table Alternative 5 Additional Considerations Incremental Reduction in Vapor Intrusion Risk It is uncertain whether Alternative 5 will be effective at reducing TCE concentrations in groundwater throughout the Central Area sufficiently to eliminate the need to operate the SSD systems. 38

44 Secondary Impacts There are numerous secondary impacts associated with implementing enhanced bioremediation. Providing a carbon substrate to the subsurface creates reducing conditions that can lead to metals liberation in groundwater. The breakdown of TCE via the reductive dechlorination process can generate daughter-compounds that pose a higher risk to residents than TCE. Two examples of this include the generation of vinyl chloride as a result of incomplete reductive dechlorination and generation of methane as a result of complete reductive dechlorination. The process of injecting carbon and amendments can also lead to a change in color, turbidity, and odor of the groundwater. Alternative 5 provides greater control over subsurface conditions than Alternative 4 by modifying the system operations as appropriate to minimize secondary impacts. Alternative 5 provides relatively greater control to promote complete dechlorination to reduce vinyl chloride generation, to promote aerobic conditions for breakdown of methane, and/or to flush groundwater to promote stabilizing color, turbidity, and odor alterations. The secondary impacts associated with Alternative 5 are less than the secondary impacts associated with Alternative 4, but greater than the secondary impacts of Alternatives 1, 2, and 3. Sustainability The greenhouse gas emissions are estimated at 4,400 metric tons CO 2-equivalent and the energy footprint is estimated at 70,000 MMBTU for implementation of Alternative 5. The greenhouse gas impacts for Alternative 5 are equivalent to the impact of 925 passenger vehicles driven for one year or 495,000 gallons of gasoline consumed. The greenhouse gas impacts and energy footprint estimated for Alternative 5 are slightly higher than Alternative 4. The estimated worker injury risk associated with implementing Alternative 5 is 4.0 accidents, approximately four times higher than Alternative 4. Additional details regarding the sustainability impact evaluation are provided in Appendix B. Construction Time Alternative 5 could be implemented in +/- 10 years assuming treatment control building locations can be identified and purchased, and the necessary approvals can be obtained. The construction timeframe would be dependent on the results of final design and obtaining access for the treatment control buildings Alternative 5 Summary There is significant uncertainty whether Alternative 5 would be effective at reducing groundwater concentrations to a level that meets the RAO given the technical and logistical constraints in the Central Area. Alternative 5 is the most intrusive of the remedial alternatives during implementation and would have short-term risks due to construction activities and secondary impacts from completing remedial activities. The potential benefit of potential VI risk reduction provided by Alternative 5 may be less than the potential risks of implementing this alternative. 6.3 Recommended Alternative The results of the detailed analysis were compiled to evaluate the performance of each alternative relative to the specific criterion and to compare the advantages and disadvantages of each alternative. A summary of the detailed analysis and the comparative evaluation is provided in Table 6. Alternative 3 is the recommended alternative. This alternative provides the best balance of tradeoffs of the five balancing criteria including cost effectiveness as required by CERCLA and the NCP. This 39

45 alternative provides the greatest protection to human health and the environment and complies with ARARs. 40

46 7.0 Summary and Conclusions This feasibility study was performed to evaluate alternatives to address the VI pathway in the Central Area and included alternatives that directly mitigate vapor intrusion and alternatives that decrease the potential for VI through a decrease in soil gas and groundwater concentrations. This feasibility study was completed in general accordance with EPA guidance (EPA, 1988) and RAP Modification #1. This feasibility study was performed for the Central Area, which has been impacted by up-gradient off-site sources in the Northeast Area, and only a portion of which is down-gradient of the Site. The HHRA concluded that exposure pathways in the Central Area, including the VI pathway, are currently either incomplete or insignificant and not expected to be significant in the future. Moreover, because multiple sources of TCE exist in the Northeast Area that have yet to be characterized, remedial actions to address groundwater will not be effective at reducing TCE concentrations in groundwater due to continuing recontamination from these up-gradient sources. Nevertheless, General Mills completed this feasibility study at the request of MPCA. The remedial action objective for this feasibility study focuses on maintaining insignificant potential VI risk. A thorough screening of technology and process options was completed to evaluate technologies for inclusion in remedial alternatives that could be implemented throughout the Central Area. The technologies were screened based on demonstrated effectiveness at similar sites and on implementability given the technical, logistical, and physical access constraints of the Central Area. The technologies that were retained following the technology screening included: Installation of additional SSD systems (at residential properties within the Central Area that currently do not have active SSD systems), O&M of SSD systems, Institutional controls, Monitored natural attenuation, Expanded/modified groundwater extraction, and In-situ bioremediation (considering both reductive dechlorination and aerobic co-metabolic treatment and multiple process delivery options). The five remedial alternatives assessed during detailed analysis were: Alternative 1 No Further Action Beyond Previous Response Actions, Alternative 2 Monitored Natural Attenuation, Alternative 3 Long-Term O&M of SSD Systems, Alternative 4 Enhanced Bioremediation via Injection Events, and 41

47 Alternative 5 Enhanced Bioremediation via Recirculation. Alternative 3 Long-Term O&M of SSD Systems was selected as the recommended alternative. This alternative provides the best balance of tradeoffs of the five balancing criteria. This alternative provides protection to human health and the environment, complies with ARARs, and is cost effective as required by CERCLA and the NCP. 42

48 References Barr Engineering Co. (Barr), 2014a. Final Sub-Slab Sampling and Building Mitigation Work Plan, East Hennepin Avenue Site. Prepared for General Mills, Inc. February Barr Engineering Co. (Barr), 2014b. Summary of Phase 2G Vapor Intrusion Evaluation Results, East Hennepin Avenue Site. Prepared for General Mills, Inc. May 11, Barr Engineering Co. (Barr), 2014c. Disposal Area Investigation Results, 2010 East Hennepin Avenue Site. Prepared for General Mills, Inc. May 23, Barr Engineering Co. (Barr), 2015a. Indoor Air Sampling Report, 2010 E Hennepin Avenue, East Hennepin Avenue Site. Prepared for General Mills, Inc. May 13, Barr Engineering Co. (Barr), 2015b. Sub-Slab Sampling and Building Mitigation Implementation Report, East Hennepin Avenue Site. Prepared for General Mills, Inc. June Barr Engineering Co. (Barr), 2015c. Vapor Intrusion Pathway Investigation Report, East Hennepin Avenue Site. Prepared for General Mills, Inc. July Barr Engineering Co. (Barr), 2015d Sentinel Monitoring Network Report, East Hennepin Avenue Site. Prepared for General Mills, Inc. December Barr Engineering Co. (Barr), Glacial Drift Network Groundwater Monitoring Plan, East Hennepin Avenue Site. Prepared for General Mills, Inc. April 6, General Mills, Inc. (GMI), Letter to Timothy Grape (MPCA) from Larry Deeney (General Mills) regarding Vapor Intrusion Pathway Investigation Response to MPCA Comments; East Hennepin Avenue Site. April 6, Haley & Aldrich, Inc. (H&A), Human Health Risk Assessment Report, East Hennepin Avenue Site. Prepared for General Mills, Inc. July 31, Haley & Aldrich, Inc. (H&A), Letter to Timothy J. Grape (MPCA) from Jay Peters (H&A) regarding Response to the Human Health Risk Assessment Report for the General Mills/Henkel Corporation Site; Minnesota Pollution Control Agency Site ID#: SR3. April 6, Minnesota Department of Health (MDH), 2016a. Trichloroethylene (TCE) in Air, Site Assessment and Consultation Unit, available at January 26, Minnesota Department of Health (MDH), 2016b. Vapor Intrusion, Site Assessment and Consultation Unit, available at February 2, Minnesota Pollution Control Agency (MPCA), Response Order by Consent (Consent Order) between General Mills and the Minnesota Pollution Control Agency (MPCA). October Minnesota Pollution Control Agency (MPCA), Risk Based Guidance for the Vapor Intrusion Pathway, Superfund, RCRA, and Voluntary Cleanup Section, MPCA Document Number c-s4-06. September

49 Minnesota Pollution Control Agency (MPCA), Vapor Intrusion Technical Support Document, Remediation Division, MPCA Document Number c-rem3-01. August Minnesota Pollution Control Agency (MPCA), 2014a. Remedial Action Plan Modification #1 (Exhibit B) to the Response Order By Consent between General Mills and the Minnesota Pollution Control Agency (MPCA), October March 11, Minnesota Pollution Control Agency (MPCA), 2014b. Five-Year Review Report General Mills/Henkel Corp Superfund Site, Minneapolis, Minnesota. Prepared by Minnesota Pollution Control Agency, December Minnesota Pollution Control Agency (MPCA), 2015a. Letter to Larry Deeney (General Mills) from Timothy J. Grape (MPCA) regarding Minnesota Pollution Control Agency Response to the Vapor Intrusion Pathway Investigation Report for the General Mills/Henkel Corporation Site; Site ID#: SR3. November 3, Minnesota Pollution Control Agency (MPCA), 2015b. Letter to Larry Deeney (General Mills) from Timothy J. Grape (MPCA) regarding Response to the Human Health Risk Assessment Report for the General Mills/Henkel Corporation Site; Minnesota Pollution Control Agency Site ID#: SR3. November 12, Minnesota Pollution Control Agency (MPCA), Letter to Larry Deeney (General Mills) from Timothy Grape (MPCA) regarding MPCA Response to the 2015 Sentinel Monitoring Network Report for the General Mills/Henkel Corporation Site ; Site ID#: SR3. January 28, Parsons Infrastructure & Technology Group, Inc. (Parsons), 2010a. Addendum to the Principles and Practices Manual Loading Rates and Impacts of Substrate Delivery for Enhanced Anaerobic Bioremediation. Prepared for Environmental Security Technology Certification Program (ESTCP) Project ER January Parsons Infrastructure & Technology Group, Inc. (Parsons), 2010b. Substrate Estimating Tool for Enhanced Anaerobic Bioremediation of Chlorinated Solvents, Version 2.1. Prepared for Environmental Security Technology Certification Program (ESTCP). November United States Environmental Protection Agency (EPA), Guidance for Conducting Remedial Investigations and Feasibility Studies under CERCLA. Office of Emergency and Remedial Response. October United States Environmental Protection Agency (EPA), Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. Office of Research and Develop. September United States Environmental Protection Agency (EPA), A Guide to Developing and Documenting Cost Estimates During the Feasibility Study. Office of Emergency and Remedial Response. July United States Environmental Protection Agency (EPA), Remediation Technology Cost Compendium. Office of Solid Waste and Emergency Response, Technology Innovation Office. September United States Environmental Protection Agency (EPA), Institutional Controls: A Guide to Planning, Implementing, Maintaining, and Enforcing Institutional Controls at Contaminated Sites. Office of Solid Waste and Emergency Response, Office of Enforcement and Compliance Assurance. December

50 United States Environmental Protection Agency (EPA), OSWER Technical Guidance for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor to Sources to Indoor Air, Office of Solid Waste and Emergency Response. June United States Environmental Protection Agency (EPA), Search for Superfund Decision Documents, available at Accessed March

51 Tables

52 Table 1 Potential Action-Specific ARARs and TBCs East Hennepin Avenue Site Minneapolis, MN Federal Standard Requirement Citation Potentially Applicable/ Relevant and Appropriate TBC 1 Comments RCRA Identification of Hazardous Waste Waste generator shall determine if the waste is hazardous waste. 40 CFR 261 No/No Applicable for any remedial action where waste would be generated. Generation of a hazardous waste is not anticipated. RCRA Generators of Hazardous Waste Generation of contaminated media that is characterized as a hazardous waste. 40 CFR 262 No/No Applicable where hazardous waste would be generated. Generation of a hazardouse waste is not anticipated. RCRA Transporters of Hazardous Waste Transportation of hazardous waste to off-site facilities. 40 CFR 263 No/No ARARs pertain only to activities conducted on-site. RCRA Owners and Operators of Hazardous Waste Treatment, Storage, and Disposal Facilities Management of hazardous waste. 40 CFR 264 No/No Applicable for remedial action where hazardous waste would be treated, stored or disposed of. Generation of a hazardouse waste is not anticipated. RCRA Management of Specific Hazardous Waste and Specific Types of Facilities Management of specific hazardous wastes. 40 CFR 266 No/No These standards do not apply to remedial actions in the Central Area. RCRA Land Disposal Restrictions Restricts certain hazardous wastes from land disposal. 40 CFR 268 No/No Applicable where hazardous wastes with land disposal restrictions are generated. Generation of a hazardous waste is not anticipated. Disposal of Solid Waste that is not a Hazardous Waste Generator of RCRA Subtitle D regulated waste. 40 CFR 257 Yes/-- Applicable if RCRA Subtitle D waste is generated (e.g., drill cuttings). USDOT General Information, Regulations and Definitions Requirements for packaging, labeling, marking, placarding, and motor vehicles used for transportation of hazardous materials. 49 CFR 171 No/No ARARs pertain only to activities conducted on-site. USDOT Hazardous Materials Table, special provisions, communications, emergency response, training and security plans Each person who offers hazardous material for transportation or each carrier that transports it shall mark each package, container, and vehicle in the manner required. 49 CFR 172 No/No ARARs pertain only to activities conducted on-site. USDOT Requirements for Shipments and Packagings Definitions of hazardous materials for transportation purposes; requirements for preparing hazardous materials for shipment. 49 CFR 173 No/No ARARs pertain only to activities conducted on-site. OSHA Work on Contaminated Sites Requirements for workers on uncontrolled hazardous waste sites such as training, personal protective equipment, recording and reporting workrelated fatalities/injuries/illnesses. 29 CFR Recording and Reporting Occupational Injuries and Illnesses 29 CFR Occupational Safety and Health 29 CFR 1926 Safety and Health Regulations for Construction Yes/-- Appropriate health and safey procedures would be implemented during remedial actions. Table 1 Page 1 of 5

53 Table 1 Potential Action-Specific ARARs and TBCs East Hennepin Avenue Site Minneapolis, MN Standard Requirement Citation Potentially Applicable/ Relevant and Appropriate TBC 1 Comments National Pollutant Discharge Elimination System (NPDES) Program Discharge limits, monitoring requirements, and best management practices for surface water discharges. 40 CFR No/No Substantive requirements may apply to extent that remedial action includes discharge to surface waters. Surface water discharges are not anticipated. Clean Water Act Implements requirements for discharge of pollutants to waters of the United States. 33 USC 1251, et seq. Yes/-- Substantive requirements may apply to extent that remedial action includes discharge to waters of the United States. Clean Air Act Regulates air emissions from stationary and mobile sources. 42 USC 7401 et seq. No/No Substantive requirements apply if identified pollutants are emitted in excess of threshold amounts. This is not anticipated to occur. EPA Effluent Guidelines Treatment process must not allow waste to pass through untreated or result in contaminated sewage sludge. 40 CFR No/No Applicable to remedial actions that include discharge to a POTW. Discharge to a POTW is not anticipated. Injection Wells Injection well must be designed for its intended use and injection activity cannot allow fluids to migrate into underground drinking water sources. 40 CFR 144 Yes/-- Substantive requirements applicable to remedial actions that include injection wells. State and Local Hazardous Waste Regulations Onsite waste generation Waste generator shall determine if the waste is hazardous waste. Minnesota Rules Ch through No/No Applicable where waste would be generated. Generation of a hazardous waste is not anticipated. Generators of Hazardous Waste Generation of contaminated soils that are characterized as hazardous wastes. Minnesota Rules Ch through No/No Applicable where hazardous waste would be generated. Generation of a hazardouse waste is not anticipated. Transporters of Hazardous Waste Transportation of hazardous waste to off-site facilities. Minnesota Rules Ch. No/No ARARs pertain to on-site activities only through Owners and Operators of Hazardous Waste Treatment, Storage, and Disposal Facilities Management of hazardous waste. Minnesota Rules Ch through No/No Applicable where hazardous waste would be treated, stored or disposed of. Generation of a hazardouse waste is not anticipated. Management of Specific Hazardous Waste and Specific Types of Facilities Management of specific hazardous wastes. Minnesota Rules Ch through No/No These standards do not apply to potential wastes generated by remedial actions. Table 1 Page 2 of 5

54 Table 1 Potential Action-Specific ARARs and TBCs East Hennepin Avenue Site Minneapolis, MN Standard Requirement Citation Potentially Applicable/ Relevant and Appropriate TBC 1 Comments County Regulation of Hazardous Waste Management Procedures for the MPCA s overview of county hazardous waste programs. Hennepin County has an MPCA-approved county ordinance detailing Minnesota Rules Ch through No/No their hazardous waste programs. Generation of hazardouse waste is not anticipated. Land Disposal Restrictions Restricts certain hazardous wastes from land disposal. Minnesota Rules Ch No/No Applicable where hazardous wastes with land disposal restrictions are generated. Generation of a hazardous waste is not anticipated. Solid Waste General requirements for management of solid waste Requirements and standards for solid waste. Remedial action is not anticipated to include on-site disposal of solid Minnesota Rules Ch through No/No waste. Off-site disposal, if any, will be conducted in accordance with RCRA Subtitle D requirements. Individual Properties Responsibility for management of solid waste. Solid waste requirements would be applicable for storage, transport Minnesota Rules Ch through Yes/-- and disposal of contaminated media generated during remedial activities (e.g. drill cuttings). Industrial Solid Waste Land Disposal Facilities Requirements for industrial solid waste land disposal facilities. Minnesota Rules Ch through No/No Remedial actions do not involve an industrial solid waste land disposal facility. Solid Waste Management Facilities Financial Requirements Requirements for cost estimates and financial assurances documentation. Minnesota Rules Ch through No/No Remedial actions would not involve construction of an industrial solid waste storage facility. Solid Waste Management Facility Specific Technical Requirements Requirements for facilities that dispose of mixed municipal solid waste in or on the land. Minnesota Rules Ch through No/No Remedial actions would not involve management of a mixed municipal waste landfill. Abandonment of motor vehicles and scrap metal Requirement for disposal and reuse of abandoned motor vehicles and other scrap metal. Minnesota Rules Ch through No/No Remedial activities would not involve disposal or reuse of abandoned motor vehicles or scrap metal. Solid Waste Programs and Projects Requirements for application procedure for grantsin-aid, state requirements, approval of applications, and payments for programs or projects which will Minnesota Rules Ch. encourage both the reduction of the amount of through No/No Remedial activities would not involve construction or management of a solid waste management facility. material entering the solid waste stream and the reuse and recycling of solid waste. Infectious Waste Requirements for owners and operators of facilities, commercial transporters, and all infectious waste. Minnesota Rules Ch. No/No Remedial activities would not involve infectious waste through Table 1 Page 3 of 5

55 Table 1 Potential Action-Specific ARARs and TBCs East Hennepin Avenue Site Minneapolis, MN Standard Requirement Citation Water Supply Regulations Potentially Applicable/ Relevant and Appropriate TBC 1 Comments Connection to public sewer State Plumbing Code (MDH). Minnesota Rules Ch No/No Remedial actions do not involve connecting to public sewer or water systems. Public Water Resource Water appropriation permitting, standards and criteria for alterations to structure of public water (DNR). Minnesota Rules Ch Yes/-- Applicable to remedial actions that involve appropriation of water (e.g., groundwater extraction systems). New well construction in contaminated area Allows for designation of special Well Construction Area (MDH). Minnesota Rules Ch Yes/-- The Central Area is located within a Special Well Construction Area. Substantive requirements may apply. Monitoring well installation or abandonment Well and boring construction, use, maintenance, and sealing information (MDH). Minnesota Rules Ch Yes/-- Wells may be installed or abandoned as part of remedial activities. Substantive requirements may apply. Certification of Environmental Laboratories Laboratory accreditation requirements for the State of Minnesota (MDH). Minnesota Statute through Minnesota Rules Ch Yes/-- Laboratories that provide services for this project would be accredited for the appropriate testing methods. Water Pollution Control Act Regulates point source discharges to waters of the state. Minnesota Statute 115 No/No Substantive requirements apply to remedial actions that involve discharge to storm sewers or surface water. Discharges to storm sewers or surface water is not anticipated. Water of the State Classifies waters of the state and establishes standards. Minnesota Rules Ch No/No Substantive requirements apply to remedial actions that involve discharge to storm sewers or surface water. Discharges to storm sewers or surface water is not anticipated. Groundwater Quality Discharge to groundwater Nondegradation goal, prohibition of discharge to saturated zone, limitation on discharge to unsaturated zone, remediation requirements. Minnesota Rules Ch Yes/-- Applicable to remedial actions that involve discharge to groundwater or unsaturated zone. Groundwater use or contact Establishes human health based groundwater standards (MDH). Minnesota Rules Ch and to No/No The geologic unit targeted for remedial actions is not used as a drinking water source. Air Quality Air emissions Duty to notify and abate excessive or abnormal unpermitted air emissions. Minnesota Statute No/No Applicable to remedial actions that involve excessive or abnormal air emissions. No such remedial actions are anticipated. Air emissions Air quality rules. Minnesota Rules Chs. 7005, 7007, 7017 No/No Applicable to remedial actions that involve air emissions. No such remedial actions are anticipated. Air emissions Standards of performance and emissions inventory. Minnesota Rules Chs Air emissions Air emissions and waste management permits. Minnesota Statute No/No No/No These regulations would be applicable to remedial actions that include emissions from stationary sources. No such remedial actions are anticipated. Applicable to remedial actions that involve air treatment and emission. No such remedial actions are anticipated. Table 1 Page 4 of 5

56 Table 1 Potential Action-Specific ARARs and TBCs East Hennepin Avenue Site Minneapolis, MN Standard Requirement Citation Noise Pollution Control Potentially Applicable/ Relevant and Appropriate TBC 1 Comments Sound generation Standards for noise generated during operations. Minnesota Rules Ch Yes/-- May need a waiver of this requirement if operation of construction equipment exceeds noise standards. Health and Safety Worker protection Standards for worker health, safety and training. Minnesota Rules Ch Yes/-- Requirements would be met for health and safety of workers. 1 "Yes/--": If a requirement is potentially applicable, determination of relevant and appropriate or TBC status is not made. Table 1 Page 5 of 5

57 Table 2 Potential Location-Specific ARARs and TBC East Hennepin Avenue Site Minneapolis, MN Standard Requirement Citation Federal Potentially Applicable/ Relevant and Appropriate TBC 1 Comments National Archaeological and Historical Preservation Act Construction on previously undisturbed land would require an archaeological survey of the area. 16 USC c Substantive requirements of 36 CFR 65, National Historic Landmarks Program. No/No There are no known archaeological or historical sites located in the Central Area. Federal National Historic Preservation Act, Section 106 Action to preserve historic properties; planning of action to minimize harm to properties listed on or eligible for listing on the National Register of Historic Places (NRHP). Substantive Requirements of 36 CFR 800, Protection of Historic Properties 16 USC 470, et seq. No/No There are five historic structures currently inventoried by the Minnesota State Historic Preservation Office located in the Central Area; none have been listed on the NRHP or are eligible for listing on the NRHP. Endangered Species Act of 1973 Action to conserve endangered species or threatened species, including consultation with the Department of the Interior. Reasonable mitigation and enhancement measures must be taken, including live propagation, transplantation and/or habitat acquisition and improvement. 16 USC 1531, et seq. No/No There are no known records of endangered plant or animal species located in the Central Area. Migratory Bird Treaty Act of 1972 Protects almost all species of native birds in the U.S. from unregulated take which can include poisoning at contaminated sites. 16 USC 703, et seq. No/No There are no known records of the presence of migratory birds in the Central Area. Wilderness Act Area must be administered in such a manner as will leave it unimpaired as wilderness and preserve its wilderness character. 16 USC 1131 et seq. No/No There are no Federally-owned wilderness areas located in the Central Area. National Wildlife Refuge System Only actions allowed under the provisions of 16 USC Section 688 dd(c) may be undertaken in areas that are part of the National Wildlife Refuge System. 50 CFR 35.1 et seq. No/No There are no National Wildlife Refuge areas located in the Central Area. Fish and Wildlife Coordination Act, Fish and Wildlife Improvement Act of 1978, Fish and Wildlife Conservation Act of 1980 Provides protection for actions that would affect streams, wetlands, other water bodies or protected habitats. Any action taken should protect fish or wildlife. 16 USC e 16 USC 742a-l 16 USC 2901, et seq. No/No There are no streams, wetlands, or other water bodies that could potentially be impacted by activities in the Central Area. Clean Water Act, Section 404 Permitting requirements for dredge and fill materials to waters of the United States. 33 CFR No/No No wetlands will be affected by remedial action. Wild and Scenic Rivers Act Avoid taking or assisting in an action that will have direct adverse effect on a national, wild, or scenic recreational river. 16 USC 1271 et seq. 36 CFR CFR 6.302(e) No/No There are no designated wild, scenic, or recreational areas in the Central Area. Coastal Zone Management Act Regulates activities affecting the coastal zone including lands thereunder and adjacent shoreline. Must conduct activities in a manner consistent with the approved State management programs. 16 USC 1451, et seq. 15 CFR CFR 923 No/No The Central Area is not located within a designated coastal zone. Coastal Barrier Resources Act, Section 3504 Prohibits any new federal expenditure within the Coastal Barrier Resource System. 16 USC 3501, et seq. No/No The Central Area is not located within a designated coastal zone. Clean Water Act Establishes regulations pertaining to activities that affect the navigation of the waters of the United States. 33 CFR USC 1341, et seq. No/No There are no navigable waters in the Central Area. No discharge to navigable waters is anticipated. Table 2 Page 1 of 2

58 Table 2 Potential Location-Specific ARARs and TBC East Hennepin Avenue Site Minneapolis, MN Standard Requirement Citation Potentially Applicable/ Relevant and Appropriate TBC 1 Comments Magnuson Fishery Conservation and Management Act Provides for conservation and management of specified fisheries within specified fishery conservation zones (in federal waters). 16 USC 1801, et seq. No/No There are no fisheries located in the Central Area. RCRA Standards for Identification of Hazardous Waste and Hazardous Waste Generator Requirements Regulates generators of hazardous waste. 40 CFR 261, 262 No/No Applicable for identification of hazardous wastes. Substantive requirements may be applicable if hazardous waste is generated as part of remedial action. Generation of hazardous waste is not anticipated. RCRA Hazardous Waste Treatment, Storage, and Disposal Requirements Regulates treatment, storage, and disposal of hazardous waste. 40 CFR 264 No/No Remedial action unlikely to include treatment, storage, or disposal of hazardous waste. EO 11988, Protection of Floodplains Actions taken should avoid adverse effects, minimize potential harm, restore and preserve natural and beneficial values. 40 CFR 6, Appendix A; excluding Sections 6(a)(2), 6(a)(4), 6(a)(6); 40 CFR No/No The Central Area is not located within a flood plain. Rivers and Harbors Act of 1972 State and Local Permits are required for structures or work affecting navigable waters. 33 USC 403 No/No There are no navigable waters in the Central Area. Endangered Species Protection of endangered species (DNR). Minnesota Rules Ch No/No There are no known records of endangered plant or animal species in the Central Area. Public Water Resources Classifies lakes and wetlands, appropriation permitting (DNR). Minnesota Rules Ch No/No There are no streams, wetlands, or other water bodies impacted by activities in the Central Area. Shoreland and Floodplain Management Shoreland alterations or structures (DNR). Minnesota Rules Ch No/No There is no shoreland in the Central Area. Wetlands Conservation Act Protection of wetlands. Minnesota Statute 103G No/No There are no known wetlands in the Central Area. Wetlands conservation Protection of wetlands, wetland functions for determining public values. Minnesota Rules 8420 No/No There are no known wetlands in the Central Area. Ordinance for work in street rightof-way Regulates obstructions and excavations in the city rightof-way through issuance of permits. City of Minneapolis Code of Ordinances, Chapter 430 Yes/-- Substantive requirements applicable to on-site activities pertaining to obstruction, excavation, and restoration. 1 "Yes/--": If a requirement is potentially applicable, determination of relevant and appropriate or TBC status is not made. Table 2 Page 2 of 2

59 Table 3 Potential Chemical-Specific ARARs and TBCs East Hennepin Avenue Site Minneapolis, MN Standard Requirement Citation Federal Potentially Applicable/ Relevant and Appropriate TBC 1 Comments Safe Drinking Water Act National Primary Drinking Water Standards setting maximum contaminant levels (MCLs). 40 CFR CFR CFR 143 No/No Groundwater is not utilized as a drinking water source. Institutional controls at the Site and Central Area, including a Special Well and Boring Construction Area, require approval from MDH prior to new well construction. Underground Injection Control Requirements Regulates subsurface injection of fluids. 40 CFR 144 Yes/-- Substantive UIC requirements may apply if remedial action includes injection into groundwater. Clean Water Act Federal Water Quality Standards (WQS) Ambient Water Quality Criteria established to protect aquatic life and human consumers 40 CFR 131 No/No Federal WQS are not ARARs. of water or aquatic life. Clean Water Act Wetlands Protection Section 104 Protection of Wetlands 33 CFR 238 No/No No wetlands are anticipated to be affected by remedial action. General Pre-Treatment Regulations for Existing and New Sources of POTWs Effluent limitations and pre-treatment standards and guidelines for existing sources, new sources. 40 CFR 403 No/No Remedial action unlikely to involve discharge of wastewater. National Pollution Discharge Elimination System (NPDES) Regulates discharge of pollutant from point source into waters of the United States. 40 CFR 122 No/No Substantive requirements may be applicable if discharge to surface water is part of a remedial action. Discharge to surface water is not anticipated. National Ambient Air Quality Standards (NAAQS) Establishes acceptable ambient air concentrations. 40 CFR 50 No/No NAAQS are not ARARs. Clean Air Act NESHAP/NSPS/PSD Establishes emission standards for hazardous air pollutants for which no ambient air quality standards exist. 40 CFR Part 53,60,61 No/No Substantive requirements applicable only if identified pollutants will be emitted in excess of threshold amounts. This is not anticipated to occur. Vapor intrusion pathway For evaluating the potential risks to human health caused by vapor intrusion (the migration of volatile compounds from contaminated soil or groundwater to the indoor air of occupied buildings). OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air, OSWER Publication , June 2015 TBC EPA guidance document. The guidance contains vapor intrusion screening levels (VISLs) for indoor air, soil gas, and groundwater concentrations for use when evaluating the vapor intrusion pathway at contaminated sites. The guidance states that vapor intrusion screening levels for groundwater are not intended to be used as cleanup levels. Table 3 Page 1 of 2

60 Table 3 Potential Chemical-Specific ARARs and TBCs East Hennepin Avenue Site Minneapolis, MN Standard Requirement Citation State and Local Potentially Applicable/ Relevant and Appropriate TBC 1 Comments Health Risk Limits for groundwater Establishes human health based groundwater standards (MDH) known as Health Risk Limits (HRLs). Minnesota Rules Ch and to No/No Groundwater is not utilized as a drinking water source. Institutional controls at the Site and Central Area, including a Special Well and Boring Construction Area, require approval from MDH prior to new well construction. Groundwater Guidance Document Framework for evaluating groundwater contamination and managing remediation decisions. MPCA Groundwater Guidance Document, September 1998 TBC Groundwater is not utilized as a drinking water source. Institutional controls at the Site and Central Area, including a Special Well and Boring Construction Area, require approval from MDH prior to new well construction. Ambient Air Quality Standards Establishes acceptable air concentrations. Minnesota Rules Ch No/No Remedial actions do not have emissions that will affect air quality. Standards for Stationary Air Sources Compliance with applicable state air pollution control rules for new and existing emission facilities. Minnesota Rules Ch No/No Substantive requirements applicable only if identified pollutants will be emitted in excess of threshold amounts. Vapor intrusion pathway For evaluating the potential risks to human health caused by vapor intrusion (the migration of volatile compounds from contaminated soil or groundwater to the indoor air of occupied buildings). MPCA Risk-Based Guidance for the Vapor Intrusion Pathway, Superfund RCRA and Voluntary Cleanup Section, September MPCA TBC Vapor Intrusion Technical Support Document, Remediation Division, August MPCA guidance documents. The guidance contains intrusion screening values (ISVs) for indoor air, soil gas, and groundwater concentrations for use when evaluating the vapor intrusion pathway at contaminated sites. ISVs for trichloroethylene (TCE) were used to guide the sub-slab soil gas sampling and building mitigation project. The guidance states that groundwater ISVs should not be used for remedial action levels. 1 "Yes/--": If a requirement is potentially applicable, determination of relevant and appropriate or TBC status is not made. Table 3 Page 2 of 2

61 Table 4 Remedial Technology and Process Option Screening East Hennepin Avenue Site Minneapolis, MN Remedial Technology 1,2,3 Process Option Description Effectiveness 3,4 Implementability 3,5 Relative Cost 6 Screening Comment No Action No Further Action Vapor Mitigation - Additional Installations None No further action beyond completed operation of legacy groundwater extraction system, installation of vapor mitigation systems, and institutional controls already implemented. Effective at addressing vapor risk to buildings with mitigation systems. Legacy groundwater extraction system, vapor mitigation systems, and institutional controls already implemented. Zero Retained for consideration. Install vapor mitigation systems at residences within the Central Area that do not currently have mitigation systems. There are 10 residences that do not currently have mitigation systems in the Central Area (5 owners did not provide access; 3 owners declined mitigation systems; and 2 properties with indoor air sampling TCE concentrations below applicable MPCA intrusion screening values or indoor air criteria). Sub-slab depressurization (SSD) systems and floor sealing HVAC system to maintain positive pressure in building Mitigate remaining buildings within the Central Area as Very effective. Addresses risk to remaining residences appropriate based on sub-slab and/or indoor air sampling within the Central Area. and where access is provided. Modify HVAC systems that maintain a positive pressure in the building basement to prevent soil gas migration to indoor air. Effective. Addresses vapor intrusion risk where it can be implemented. Readily implementable as long as access is provided. Not implementable due to the type, age, and variety of HVAC systems. Moderate Moderate Retained for consideration. Not retained for consideration due to lack of implementability. Vapor Mitigation - O&M Operation and Maintenance of Mitigation Systems Institutional Controls (ICs) ICs pertaining to mitigation systems Containment Operation and Maintenance (O&M) Plan City ordinance or other areawide control Develop and implement an O&M plan for vapor mitigation systems. Ordinance or other area-wide control pertaining to mitigation systems. Very effective. Addresses long-term risk to buildings within the Central Area. Very effective for preventing potential exposure at locations with mitigation systems. Readily implementable as long as access is provided. Requires city (or other regulatory agency) cooperation. Low Low Retained for consideration. Retained for consideration. Physical Barrier Grout cap Place buried horizontal grout cap over groundwater Uncertain effectiveness as this technology has not been impacts via grout injection (horizontal or vertical injection) applied to mitigate vapor intrusion over a large area with Not implementable due to access limitations. to cut off the groundwater to indoor air exposure pathway. access limitations. High Not retained for consideration due to lack of implementability and uncertain effectiveness. Hydraulic Barrier Extraction wells with infiltration galleries Promote a strong downward vertical gradient and clean shallow groundwater by extracting water from the bottom of the surficial aquifer and replenishing it at the top with treated water and/or stormwater via infiltration galleries. Uncertain effectiveness in disconnecting the groundwater to soil gas pathway (unproven technology). Risk that residual soil gas may still be present for many years after implementation. Readily implementable at select locations (within rights-of-way). Moderate Not retained for consideration due to uncertain effectiveness. Physical Removal Soil Excavation at Site Excavators, trucks Excavate source material at the Site. Recent investigations have not identified source material present at the Site. Disposal area drums removed in Implementation on a small scale constrained by Low buildings, streets, and railroads. Not retained since no source material has been identified at the Site. Soil Excavation of Central Area Excavators, trucks Excavate soil impacted by groundwater beneath and throughout the Central Area. This would require a very large volume of excavation to remove a small amount of TCE mass. Not effective as Not implementable due to access limitations. the residual groundwater impacts would not be removed This alternative would require demolition and by soil excavation. No source material from the Site has excavation of the entire Central Area. been found in the Central Area. Very high Not retained due to lack of implementability and effectiveness. Restart Legacy Groundwater Extraction System Extraction wells with submersible pumps and treatment Remove and/or contain contaminated groundwater for treatment and/or disposal. Legacy groundwater system was effective at reducing groundwater concentrations 1-3 orders of magnitude. The groundwater extraction system was turned off after asymptotic recovery was measured. Not effective as a sole remedial technology because restarting the legacy groundwater extraction system would not significantly reduce groundwater concentrations in a reasonable timeframe. Legacy Groundwater Extraction System removed approximately 7,000 lbs of TCE. Implementable if existing NPDES permit can be modified appropriately. Low Not retained for consideration due to limited effectiveness. Table 4 Page 1 of 5

62 Table 4 Remedial Technology and Process Option Screening East Hennepin Avenue Site Minneapolis, MN Remedial Technology 1,2,3 Process Option Description Effectiveness 3,4 Implementability 3,5 Relative Cost 6 Screening Comment Extraction wells with submersible pumps, treatment, and discharge to storm sewer Remove and/or contain contaminated groundwater for treatment and/or disposal. Not effective as sole remedial technology because it does not remove risk of vapor intrusion until a distant timeframe as demonstrated by operation of the legacy groundwater extraction system. Groundwater extraction is effective at altering groundwater flow and providing containment/capture when used in combination with other remedial alternatives. Readily implementable at select locations (within right-of-way) assuming existing NPDES permit can be modified appropriately. Moderate Retained for consideration as part of a combined remedial approach. Expanded/Modified Groundwater Extraction Extraction with submersible wells and reinjection via injection wells Remove and/or contain contaminated groundwater for treatment and/or injection. Not effective as sole remedial technology because it does not remove risk of vapor intrusion until a distant timeframe as demonstrated by operation of the legacy groundwater extraction system. Groundwater extraction is effective at altering groundwater flow and providing containment/capture when used in combination with other remedial alternatives. Readily implementable at select locations (within rights-of-way) assuming underground injection control program approval can be obtained. Moderate Retained for consideration as part of a combined remedial approach. Air Sparging with Soil Vapor Extraction (AS/SVE) Vertical injection and extraction wells Use vertical wells to inject air into the groundwater and extract soil vapor from the vadose zone. Moderate effectiveness within the sparge curtain where volatilization within air-flow channels is occurring. Likely not effective area-wide as right-of-way spacing ( Readily implementable at select locations feet) is greater than radius of influence (10-20 feet) of (within rights-of-way). sparge wells. Sparging may increase TCE vapor concentrations if SVE system does not adequately capture vapors. High Not retained for consideration due to limited effectiveness. Horizontal injection and extraction wells installed by directional drilling Use horizontal wells to inject air into the groundwater and extract soil vapor from the vadose zone. Moderate effectiveness within the sparge curtain where volatilization within air-flow channels is occurring. Likely Not implementable throughout the Central Area not effective area-wide due to access limitations and due to presence of utilities and the offset width of influence (10-20 feet) of sparge wells. Sparging distance required for directional drilling to reach may increase TCE vapor concentrations if SVE system target depth. does not adequately capture vapors. High Not retained for consideration due to lack of implementability and limited effectiveness. Table 4 Page 2 of 5

63 Table 4 Remedial Technology and Process Option Screening East Hennepin Avenue Site Minneapolis, MN Remedial Technology 1,2,3 Process Option Description Effectiveness 3,4 Implementability 3,5 Relative Cost 6 Screening Comment Steam injection and vapor extraction via vertical wells Steam injection via vertical wells to promote volatilization and destruction. Vapor extraction to collect generated vapors. Very effective within the radius of influence of steam injection locations. Likely not effective as right-of-way spacing ( feet) is greater than radius of influence (25-50 feet). Access limitations provide for similar total effectiveness as AS/SVE. Steam injection may increase TCE vapor concentrations if SVE system does not adequately capture vapors. Readily implementable at select locations (within right-of-way). High Not retained for consideration due to limited effectiveness. Steam injection and vapor extraction via horizontal wells Steam injection via horizontal wells to promote volatilization and destruction. Vapor extraction to collect generated vapors. Very effective within the radius of influence of steam injection locations. Likely not effective area-wide as right-of-way spacing ( feet) is greater than width of influence (25-50 feet). Access limitations provide for similar total effectiveness as AS/SVE. Steam injection may increase TCE vapor concentrations if SVE system does not adequately capture vapors. Not implementable throughout the Central Area due to presence of utilities and the offset distance required for directional drilling to reach target depth. High Not retained for consideration due to lack of implementability and limited effectiveness. In Situ Thermal Treatment Electrical resistance heating and soil vapor extraction (SVE) Installing electrode array to heat groundwater through electrical resistance. Vapor extraction to collect generated vapors. Uncertain effectiveness for treatment of TCE in sandy aquifers (unproven technology for this application). Potential risk of increased TCE vapor concentrations in vadose zone if SVE system does not adequately capture vapors. Not implementable as the necessary array spacing (10-20 feet) is less than what could be achieved within the right-of-way. Technology cannot be implemented at singular locations (array required). High Not retained for consideration due to the uncertain effectiveness and lack of implementability. Potential risk of increased TCE vapor concentrations in vadose zone if SVE system does not adequately capture vapors. Not retained for consideration due to uncertain effectiveness and lack of implementability. Potential risk of increased TCE vapor concentrations in vadose zone if SVE system does not adequately capture vapors. Radio frequency heating and soil vapor extraction (SVE) Installing electrode array to heat groundwater through radio frequency. Vapor extraction to collect generated vapors. Uncertain effectiveness for treatment of TCE in sandy aquifers (unproven technology for this application). Potential risk of increased TCE vapor concentrations in vadose zone if SVE system does not adequately capture vapors. Not implementable as the necessary array spacing (10-20 feet) is less than what could be achieved within the right-of-way. High Thermal Conduction and Vapor Extraction Heat aquifer matrix with thermal conductive wells and remove volatilized vapors with vapor extraction wells. Very effective within the radius of influence of thermal conductive wells. Not effective area-wide as right-ofway spacing ( feet) is greater than radius of influence (10-15 feet). Potential risk of increased TCE vapor concentrations in vadose zone if SVE system does not adequately capture vapors. Not implementable due to the well spacing required (approximately feet) within the right-of-way and the concern for adjacent utilities. High Not retained for consideration due to lack of implementability and limited effectiveness. Potential risk of increased TCE vapor concentrations in vadose zone if SVE system does not adequately capture vapors. Ex Situ Thermal Treatment & Injection Biological Monitored Natural Attenuation (MNA) Contained recovery of oily wastes (CROW ) Process Allow native microorganisms to degrade contaminant and monitor progress Heat extracted groundwater and reinject into the aquifer. Establish a monitoring well network and sampling plan to monitor and evaluate groundwater quality over time. Uncertain effectiveness for treatment of dissolved phase TCE in sandy aquifers (unproven technology for this application). Implementable at select locations (within rightof-way). High Not retained for consideration due to uncertain effectiveness. Likely not effective to meet the remedial objectives within a reasonable timeframe. Groundwater monitoring Readily Implementable data indicates that limited natural attenuation is Low Retained for consideration. occurring. Table 4 Page 3 of 5

64 Table 4 Remedial Technology and Process Option Screening East Hennepin Avenue Site Minneapolis, MN Remedial Technology 1,2,3 Process Option Description Effectiveness 3,4 Implementability 3,5 Relative Cost 6 Screening Comment Aerobic co-metabolism via injection wells Use injection wells to deliver carbon substrate and oxygen to the sub-surface to promote aerobic cometabolism. Moderate effectiveness demonstrated at sites with lowlevel impacts over large areas. Pilot testing necessary to confirm site-specific effectiveness. Potential risk of secondary groundwater impacts (e.g. metals liberation). Readily implementable at select locations (within right-of-way). High Retained for consideration as part of a combined remedial approach. Potential risk of secondary groundwater impacts. Aerobic co-metabolism via permeable reactive barrier Install trenches with reactive media to support aerobic cometabolism. Moderate effectiveness demonstrated at sites with lowlevel impacts over large areas. Pilot testing necessary to confirm site-specific effectiveness. Potential risk of secondary groundwater impacts (e.g. metals liberation). Not implementable as there are not accessible locations for reactive barriers within the Central Area due to utilities and other interferences. High Not retained for consideration due to lack of implementability. In Situ Bioremediation Enhanced reductive dechlorination via injection wells Use injection wells to deliver carbon substrate to the subsurface to promote biodegradation of contaminants. Moderate effectiveness demonstrated at sites with lowlevel impacts over large areas. Additional testing likely necessary to evaluate site-specific effectiveness. Potential risk of methane or low-chlorinated compound (e.g. vinyl chloride) accumulation. Potential risk of secondary groundwater impacts (e.g. metals liberation). Readily implementable at select locations (within right-of-way). High Retained for consideration. Potential risk of methane or low-chlorinated compound (e.g. vinyl chloride) accumulation. Potential risk of secondary groundwater impacts. Enhanced reductive dechlorination via permeable reactive barrier or biowall Install trench with reactive media to support enhanced reductive dechlorination. Moderate effectiveness demonstrated at sites with lowlevel impacts over large areas. Additional testing likely necessary to evaluate site-specific effectiveness. Potential risk of methane or low-chlorinated compound (e.g. vinyl chloride) accumulation. Potential risk of secondary groundwater impacts (e.g. metals liberation). Not implementable as there are not accessible locations for reactive barriers within the Central Area due to utilities and other interferences. High Not retained for consideration due to lack of implementability. Bio-geochemical via injection wells Stimulate subsurface micro-organisms to convert naturally occurring minerals or added materials into reducing minerals that abiotically degrade contaminants. Effectiveness is limited by aquifer mineralology. Effectiveness has not been demonstrated with injection wells at sites with low-level impacts over large areas. Readily implementable at select locations (within right-of-way). High Not retained for consideration due to uncertain effectiveness. Bio-geochemical via permeable reactive barrier Install trench with reactive media to support abiotic biogeochemical degradation of contaminants. Moderate effectiveness demonstrated at sites with lowlevel impacts over large areas. Additional testing likely necessary to evaluate site-specific effectiveness. Not implementable as there are not accessible locations for reactive barriers within the Central Area due to utilities and other interferences. High Not retained for consideration due to lack of implementability. Phyto-remediation Chemical Plant trees across the Central Area Selected species of trees are planted across the Central Area to uptake, contain, degrade, or eliminate contaminants Moderate effectiveness demonstrated at similar sites with no access limitations. Not implementable (most of the treatment area is developed) Moderate Not retained for consideration due to lack of implementability. In Situ Chemical Oxidation Inject chemical oxidants via temporary probes Inject chemical oxidants with temporary injection points across the treatment area Uncertain effectiveness as this technology has not been applied effectively to remediate low-level impacts over large areas. Likely not effective due to access limitations, radius of influence (5-10 feet), and one time application of oxidants. Potential risk of secondary groundwater impacts (e.g., metals liberation). Readily implementable at select locations (within right-of-way). High Not retained for consideration due to lack of effectiveness and risk of secondary groundwater quality impacts. Inject chemical oxidants via injection wells Inject chemical oxidants with permanent wells placed across the treatment area Uncertain effectiveness as this technology has not been applied effectively with low-level impacts over large Readily implementable at select locations areas. Likely not effective due to access limitations (within right-of-way). and radius of influence (10-20 feet). Potential risk of secondary groundwater impacts (e.g., metals liberation). High Not retained for consideration due to uncertain effectiveness and risk of secondary groundwater quality impacts. Table 4 Page 4 of 5

65 Table 4 Remedial Technology and Process Option Screening East Hennepin Avenue Site Minneapolis, MN Remedial Technology 1,2,3 Process Option Description Effectiveness 3,4 Implementability 3,5 Relative Cost 6 Screening Comment Add chemical reductants to the subsurface via temporary probes Add chemical reductants to the subsurface via temporary injection points across the treatment area Uncertain effectiveness as this technology has not been applied effectively with low-level impacts over large areas. Likely not effective due to access limitations, radius of influence (5-10 feet), and one time application of reductants. Potential risk of secondary groundwater impacts (e.g., metals liberation). Readily implementable at select locations (within right-of-way). High Not retained for consideration due to lack of effectiveness and risk of secondary groundwater quality impacts. In Situ Chemical Reduction Add chemical reductants to the subsurface via injection wells Add chemical reductants to the subsurface via injection wells across the treatment area. Uncertain effectiveness as this technology has not been applied effectively with low-level impacts over large areas. Likely not effective due to access limitations and radius of influence. Potential risk of secondary groundwater impacts (e.g., metals liberation). Readily implementable at select locations (within right-of-way). High Not retained for consideration due to uncertain effectiveness and risk of secondary groundwater quality impacts. Add chemical reductants to Add chemical reductants to the subsurface by the subsurface via permeable constructing a permeable reactive barrier. reactive barrier Moderate effectiveness demonstrated at sites with lowlevel impacts over large areas. Pilot testing necessary to confirm site-specific effectiveness. Effectiveness is uncertain based on right-of-way spacing ( feet), advective flow rate (800 feet per year) and ability to treat groundwater down-gradient of the barrier. Potential risk of secondary groundwater impacts (e.g., metals liberation). Not implementable as there are not accessible locations for reactive barriers within the Central Area due to utilities and other interferences. High Not retained for consideration due to lack of implementability. Electrical Electrolytic Reactive Barrier Voltage is applied to electrodes within a permeable reactive barrier (PRB) Electrical potential is applied to spaced electrodes within the PRB. Dissolved contaminants are subject to oxidationreduction (or reduction-oxidation) dependent on the sequence of charges applied. Uncertain effectiveness as this technology has not been demonstrated. The technology is effective at reducing flux, but the effectiveness for a larger area is dependent on advective transport to the barrier and diffusion from the aquifer matrix. Implementable at select locations (within rightof-way). High Not retained for consideration due to uncertain effectiveness. Notes: 1 The remedial action objective is defined as the following: Maintain insignificant potential risk to human health from inhalation exposure to TCE in indoor air resulting from TCE concentrations in soil gas and groundwater. 2 The feasibility study area is defined as the Central Area. 3 TCE from one or more potential releases up-gradient of the Central Area are the predominant cause of TCE concentrations in groundwater. Groundwater impacts up-gradient of the Central Area are outside of the scope of this feasibility study. Until the extent and magnitude of the impacts associated with the off-site sources are defined and addressed, remedial action to address groundwater will not be effective at reducing TCE concentrations in groundwater in the Central Area due to continuing re-contamination from these up-gradient sources. 4 Effectiveness is defined as the ability to perform as part of an overall alternative that can meet the objectives within 30 years under conditions and limitations that exist onsite. 5 Implementability is defined as the likelihood that the process could be implemented as part of the remedial action plan under the physical (work completed from public rights-of-way), regulatory, technical, and schedule constraints. 6 Relative cost is for comparative purposes only and is judged relative to the other processes and technologies that involve similar functions. Table 4 Page 5 of 5

66 Table 5 Amendment Sensitivity Analysis East Hennepin Avenue Site Minneapolis, MN Design Period (Years) 1 Design Factor 2 Quantity of Substrate Required (lb) , ,830, ,930, ,210, ,490, ,780, ,680, ,150, ,630, ,150, ,820, ,480,000 Minimum Quantity of Substrate (lb) 4 740,000 Average Quantity of Substrate (lb) 4 7,320,000 Maximum Quantity of Substrate (lb) 4 20,480,000 Notes 1 The design period is the number of years that active treatment is occuring. The amount of substrate required increases as the design period increases because the design tool accounts for flux of competing electron acceptors into the treatment zone. Design periods of 1, 3, and 5 years were evaluated. Active treatment and system operation may occur for 5-10 years for a recirculation approach, however a shorter design period (e.g.would be reasonable because recirculation would limit flux into the treatment zone from up-gradient). 2 The design factor is a safety factor to account for excess substrate demand. A design factor of 3-7 is recommended for slow release substrates like emulsified vegetable oil (Parsons, 2010a), however substrate delivery mechanisms should be considered when selecting a design factor as well. Lower design factors would be reasonable where it is relatively easy to add more substrate (e.g. recirculation systems), conversely higher design factors would be warranted for approaches that rely on one or two injection events. In addition, lower design factors should be used with more soluable substrate (e.g. lactate) to avoid spikes in substrate concentrations which can result in ph depression and excess methane production. 3 Substrate is emulsified vegetable oil, 60% by weight 4 The sensitivity analysis was performed by varying the design period and design factor in the ESTCP Substrate Estimating Tool (Parsons, 2010b). Table 5 Page 1 of 1

67 Table 6 Comparative Analysis Summary by Alternatives East Hennepin Avenue Site Minneapolis, MN Threshold Criteria 2 Balancing Criteria 3 Modifying Criteria 4 Additional Considerations 5 Remedial Alternatives 1 Overall Protection of Human Health and the Environment 6 Compliance with ARARs 7 Long-Term Effectiveness and Permanence 8 Reduction of Toxicity, Mobility, or Volume through Treatment 9 Short Term Estimated Effectiveness 10 Implementability 11 Cost 12 Cost Range 13 State Acceptance 14 Local Acceptance 15 Incremental Reduction in Vapor Intrusion Risk 16 Adverse Secondary Impacts 17 Adverse Sustainability Impacts 18 Construction Time 19 Alternative 1 - No Further Action Beyond Previous Response Actions $0 $0 Baseline None Baseline None Alternative 2 - Monitored Natural Attenuation (MNA) $1.3M $0.9M-$1.9M Minimal None Similar to baseline None Alternative 3 - Long Term O&M of SSD Systems $2.0M $1.4M-$3.0M Added longterm benefit with O&M None Similar to baseline < 1 year Alternative 4 - Enhanced Bioremediation via Injection Event(s) $26M $6M-$90M Significant Uncertainty 20 Risk of methane generation, vinyl chloride generation, and metals liberation. GHG impacts: 3,800 metric tons CO 2 -eq Energy footprint: 66,000 MMBTU Worker injury risk: 1.0 accidents +/- 5 years Alternative 5 - Enhanced Bioremediation via Recirculating System $42M $19M-$100M Significant Uncertainty 20 Risk of methane generation, vinyl chloride generation, and metals liberation. GHG impacts: 4,400 metric tons CO 2 -eq Energy footprint: 70,000 MMBTU Worker injury risk: 4.0 accidents +/- 10 years Scale Does not meet criteria Potentially meets criteria Meets criteria Remedial action objective: Comments: Maintain insignificant potential risk to human health from inhalation exposure to TCE in indoor air resulting from TCE concentrations in soil gas and groundwater. As stated in the RAP Modification #1, General Mills is responsible only for implementing response actions to address impacts that are due to General Mills operations at the Site. The feasibility study area is defined as the Central Area. TCE from one or more potential releases up-gradient of the Central Area are the predominant cause of TCE concentrations in groundwater. Groundwater impacts up-gradient of the Central Area are outside of the scope of this feasibility study. Until the extent and magnitude of the impacts associated with the off-site sources are defined and addressed, remedial action to address groundwater will not be effective at reducing TCE concentrations in groundwater in the Central Area due to continuing re-contamination from these up-gradient sources. Table 6 Page 1 of 2

68 Table 6 Comparative Analysis Summary by Alternatives East Hennepin Avenue Site Minneapolis, MN Notes 1 General Mills has spent $14.5 million on extensive investigation and response action work in the project area since the early 1980s including operating the legacy groundwater extraction system for 25 years and installing SSD systems at 166 properties in the Central Area. 2 Statutory requirements that each alternative must satisfy to be eligible for selection. 3 Technical criteria upon which the detailed analysis is primarily based. 4 Evaluation of state and community acceptance to implemented remedial actions. 5 Additional considerations provide further details on the impact of remedial actions on the community. Additional considerations could be balancing or modifying criteria. 6 The assessment against this criterion describes how the alternative, as a whole, achieves and maintains protection of human health and the environment. 7 8 The assessment against this criterion evaluates the long-term effectiveness of alternatives in maintaining protection of human health and the environment after response objectives have been met. 9 The assessment against this criterion evaluates the anticipated performance of the specific treatment technologies an alternative may employ. 10 The assessment against this criterion examines the effectiveness of alternatives in protecting human health and the environment during the construction and implementation of a remedy until response objectives have been met. 11 This assessment evaluates the technical and administrative feasibility of alternatives and the availability of required goods and services. 12 This assessment evaluates the capital and operation and maintenance (O&M) costs of each alternative This assessment reflects the state s (or support agency s) apparent preferences among or concerns about alternatives. This assessment will be completed after the public comment period. 15 This assessment reflects the community s apparent preferences among or concerns about alternatives. This assessment will be completed after the public comment period. 16 This assessment provides a comparison of the reduction of vapor intrusion risk associated with implementation of each remedial alternative relative to Alternative 1 (No Further Action Beyond Previous Response Actions). 17 This assessment evaluates potential adverse secondary impacts of implementing the remedial alternative. The risk of secondary impacts may be greater than the TCE vapor intrusion risk. 18 The assessment against this criterion describes how the alternative complies with ARARs, or if a waiver is required and how it is justified. The assessment also addresses other information from advisories, criteria, and guidance that the lead and support agencies have agreed is "to be considered". Cost range for alternatives represents the plus 50 minus 30 percent accuracy, except amendment related costs for Alternatives 4 and 5 which vary based on the range of amendment required as detailed in the sensitivity analysis (Table 5).The accuracy range is associated with the most likely cost of the project based on the level of design that has been completed and the uncertainties in the project as scoped. This assessment provides an estimate of the adverse sustainability impacts (e.g., greenhouse gas impacts, energy footprint, worker injury-accident risk) associated with implementing the remedial alternative relative to Alternative 1 (No Further Action Beyond Previous Response Actions). Sustainability impacts were estimated using the SiteWise green and sustainable remediation tool. Additional details regarding the sustainability impact evaluation are in Appendix B. 19 This assessment provides an estimate of the construction time required to implement the remedial alternative. 20 There is significant uncertainty whether Alternatives 4 and 5 could achieve an effective and consistent reduction in TCE concentrations in groundwater throughout the Central Area to be protective of vapor intrusion risk. Table 6 Page 2 of 2

69 Alternative 2 - Monitored Natural Attenuation Table 7 Alternative 2 Cost Estimate East Hennepin Avenue Site Minneapolis, MN Unit Cost Estimated Cost Low Estimate Cost High Estimate Cost Quantity Unit Capital Costs 1.0 Contractor Preparation Subtotal $ 30,000 $ 21,000 $ 45, Monitored Natural Attenuation Work Plan 1 LS $ 30,000 $ 30,000 $ 21,000 $ 45,000 Total Capital Cost $ 30,000 $ 20,000 $ 50,000 Sampling Costs (Annual) 2.0 Annual Groundwater Sampling Costs Subtotal $ 80,000 $ 50,000 $ 120, Well Sampling - VOCs 25 Well/Year $ 1,000 $ 25,000 $ 17,500 $ 37, Well Sampling - MNA Parameters and VOCs 10 Well/Year $ 2,200 $ 22,000 $ 15,400 $ 33, Project Management 1 Year $ 10,000 $ 10,000 $ 7,000 $ 15, Annual Groundwater Report 1 Year $ 20,000 $ 20,000 $ 14,000 $ 30,000 Annual O&M Cost $ 77,000 $ 53,900 $ 115,500 O&M Yrs 1-30 NPV $ 960,000 $ 670,000 $ 1,430,000 Contingency 3.0 Contingency on Total Cost, 25% Subtotal $ 250,000 $ 170,000 $ 370,000 Total Cost (NPV, 30 years, 7% discount rate) $1,300,000 $900,000 $1,900,000 Assumptions 1.1 Costs include preparation of a MNA work plan that details the methods, procedures, locations, and frequency of monitoring. 2.1 Costs include labor, expenses, and laboratory costs for sampling and analyzing groundwater samples for VOCs Costs include labor, expenses, and laboratory costs for sampling and analyzing groundwater samples for VOCs and MNA parameters. MNA parameter list includes 13 parameters recommended in Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater(EPA, 1998). Project management is assumed to be 15 percent of total cost. Assumes preparation of an annual groundwater monitoring report that summarizes groundwater quality trends and natural degradation rates. Contingency based on range of scope and bid contingency recommended in A Guide to Developing and Documenting Cost Estimates During the Feasibility Study (EPA, 2000). Discount rate taken from A Guide to Developing and Documenting Cost Estimates During the Feasibility Study (EPA, 2000). Table 7 Page1 of 1

70 Alternative 3 - Long-Term O&M of SSD Systems Table 8 Alternative 3 Cost Estimate East Hennepin Avenue Site Minneapolis, MN Quantity Unit Capital Costs 1.0 Preparation Subtotal $ 25,000 $ 17,500 $ 37, Mitigation System O&M Plan 1 LS $ 50,000 $ 50,000 $ 35,000 $ 75, Access and Mitigation System Design Meetings 10 Properties $ 2,500 $ 25,000 $ 17,500 $ 37, Engineering and Administration Subtotal $ 25,000 $ 17,500 $ 37, Mitigation System Design Follow-up 5 Properties $ 5,000 $ 25,000 $ 17,500 $ 37, Install Residential SSD Systems Subtotal $ 200,000 $ 140,000 $ 300, Contractor and Support for Installations 10 Properties $ 20,000 $ 200,000 $ 140,000 $ 300, Indoor Air Sampling at 2010 E Hennepin Subtotal $ 150,000 $ 105,000 $ 225, Paired Indoor Air Sampling and Sub-Slab Soil Gas Sampling 1 LS $ 150,000 $ 150,000 $ 105,000 $ 225, Project Management Subtotal $ 60,000 $ 42,000 $ 90, Project Management, Capital Phase 1 Year $ 60,000 $ 60,000 $ 42,000 $ 90,000 Total Capital Cost $ 460,000 $ 320,000 $ 690,000 O&M Costs (Annual) 6.0 Mitigation Systems Subtotal $ 91,800 $ 64,000 $ 138, Electrical Bill for Properties 176 Building/Year $ 60 $ 10,600 $ 7,000 $ 16, Replacement Fan Cost 18 Fan/Year $ 2,000 $ 36,000 $ 25,000 $ 54, O&M Management and Access Coordination 176 Building/Year $ 200 $ 35,200 $ 25,000 $ 53, Project Management, O&M Phase 1 Year $ 10,000 $ 10,000 $ 7,000 $ 15,000 Annual O&M Cost $ 91,800 $ 64,000 $ 138,000 O&M Yrs 1-30 NPV $ 1,140,000 $ 790,000 $ 1,710,000 Contingency 7.0 Contingency on Total Cost, 25% Subtotal $ 400,000 $ 277,500 $ 600,000 Total Cost (NPV, 30 years, 7% discount rate) $2,000,000 $1,400,000 $3,000,000 Assumptions 1.1 Assumes preparation of a mitigation system O&M plan that details the methods, procedures, locations, and frequency of routine mitigation system monitoring. 1.2 Assumes coordination of access and design meetings for engineer and mitigation contractor. Does not include multiple design meetings to satisfy extraordinatry property owner requests. 2.1 Assumes follow-up meetings at 50 percent of the houses without mitigation systems Unit Cost Estimated Cost Low Estimate Cost Assumes installation of a mitigation system including diagnostic testing, contractor management, finish carpentry, and preparation of a property summary report. Not included are sub-slab soil-gas testing, indoor air testing, basement floor replacement, or extensive wall or floor patching. High Estimate Cost Assumes three sampling events, 13 sub-slab soil gas samples per event, 13 indoor air samples per event, data quality and management costs are equal to laboratory costs (based on costs associated with previous project work), sub-slab sampling is a two week effort per event for one person, indoor air sampling is a two week effort per event for two people (includes chemical inventory), and preparation of a Property Summary Report. Project management is assumed to be 15 percent of the total capital costs (Sections 1.0 through 4.0). Does not include meetings with MPCA regarding specific properties Assumes fans are operated 24 hours a day, 365 days a year, half of the fans are RadonAway Model RP145 and half are RadonAway model RP265, fans operated in accordance with manufacturers standards, energy rates based on November 1, 2015 Xcel Energy ratecard for Minnesota, and 176 houses with mitigation systems installed within the Central Area. Assumes replacement of fans every 10 years, replacement of an average of 1/10 of the fans during a given year, half of the fans are RadonAway Model RP145 and half are RadonAway model RP265, half a day of contractor time to install a replacement fan, cost includes property access efforts, and no engineer oversight time is required. Costs include contacting property owners annually regarding systems operating status. Project management is assumed to be 15 percent of O&M costs. Contingency based on range of scope and bid contingency recommended in A Guide to Developing and Documenting Cost Estimates During the Feasibility Study (EPA, 2000). Discount rate taken from A Guide to Developing and Documenting Cost Estimates During the Feasibility Study (EPA, 2000). Table 8 Page 1 of 1

71 Alternative 4 - Enhanced Bioremediation via Injection Table 9 Alternative 4 Cost Estimate East Hennepin Avenue Site Minneapolis, MN Unit Cost Estimated Cost Low Estimate Cost High Estimate Cost Quantity Unit Capital Costs 1.0 Engineering and Administration Subtotal $ 160,000 $ - $ Project Management, Capital Phase 1 Year $ 60,000 $ 60,000 $ 50,000 $ 90, Engineering and Design 1 LS $ 60,000 $ 60,000 $ 50,000 $ 90, Construction Quality Assurance, Monitoring, & Reporting 1 LS $ 40,000 $ 40,000 $ 30,000 $ 60, Planning and Preparation Subtotal $ 740,000 $ 518,000 $ 1,110, Access for Injection Well Installation 1 LS $ 100,000 $ 100,000 $ 70,000 $ 150, Permitting 1 LS $ 100,000 $ 100,000 $ 70,000 $ 150, Injection Design Phase Bench and Pilot Testing 1 LS $ 500,000 $ 500,000 $ 350,000 $ 750, Monitoring Wells 10 Well $ 4,000 $ 40,000 $ 28,000 $ 60, Injection Array Construction and Initial Amendment Delivery Subtotal $ 9,200,000 $ 1,100,000 $ 36,300, Injection Wells 40 Well $ 10,000 $ 400,000 $ 280,000 $ 600, Initial Amendment Delivery 40 Well $ 220,000 $ 8,800,000 $ 756,000 $ 35,700,000 Total Capital Cost $ 10,100,000 $ 1,700,000 $ 37,500,000 O&M Costs (Annual Years 1-3) 4.0 Supplemental Amendment Delivery Subtotal $ 900,000 $ 100,000 $ 3,600, Supplemental Amendment Delivery 1 Year $ 900,000 $ 900,000 $ 76,000 $ 3,570, Monitoring Subtotal $ 1,290,000 $ 904,000 $ 1,930, Performance Monitoring Yrs Samples/Year $ 6,000 $ 1,080,000 $ 756,000 $ 1,620, Injection Well Maintenance 40 EA $ 500 $ 20,000 $ 14,000 $ 30, Annual Groundwater Report 1 Year $ 20,000 $ 20,000 $ 14,000 $ 30, Project Management, O&M Phase 1 Year $ 170,000 $ 170,000 $ 120,000 $ 250,000 Annual O&M Cost Yrs 1-3 $ 2,190,000 $ 1,010,000 $ 5,530,000 O&M Yrs 1-3 NPV $ 3,600,000 $ 1,100,000 $ 11,300,000 O&M Costs (Annual Years 4-6) 6.0 Amendment Delivery Subtotal $ 4,600,000 $ 518,000 $ 18,150, Supplemental Injection Wells 20 Well $ 10,000 $ 200,000 $ 140,000 $ 300, Initial Amendment Delivery at Supplemental Wells 20 Well $ 220,000 $ 4,400,000 $ 378,000 $ 17,850,000 Annual O&M Cost Yr 4 $ 4,600,000 $ 520,000 $ 18,150, Monitoring Subtotal $ 1,740,000 $ 949,000 $ 3,735, Supplemental Amendment Delivery at Supplemental Wells 1 Year $ 440,000 $ 440,000 $ 38,000 $ 1,790, Performance Monitoring Yrs Samples/Year $ 6,000 $ 1,080,000 $ 756,000 $ 1,620, Injection Well Maintenance 60 EA $ 500 $ 30,000 $ 21,000 $ 45, Annual Groundwater Report 1 Year $ 20,000 $ 20,000 $ 14,000 $ 30, Project Management, O&M Phase 1 Year $ 170,000 $ 170,000 $ 120,000 $ 250,000 Annual O&M Cost Yrs 5-6 $ 1,740,000 $ 950,000 $ 3,740,000 O&M Yrs 4-6 NPV $ 6,000,000 $ 1,800,000 $ 19,100,000 O&M Costs (Annual Years 7-30) 8.0 Monitoring Subtotal $ 124,000 $ 82,800 $ 176, Performance Monitoring Yrs Samples/Year $ 2,400 $ 84,000 $ 58,800 $ 126, Annual Groundwater Report 1 Year $ 20,000 $ 20,000 $ 14,000 $ 30, Project Management, O&M Phase 1 Year $ 20,000 $ 20,000 $ 10,000 $ 20,000 Annual O&M Cost Yrs 7-30 $ 124,000 $ 83,000 $ 176,000 O&M Yrs 7-30 NPV $ 1,000,000 $ 700,000 $ 1,400,000 Contingency 9.0 Contingency on Total Cost, 25% Subtotal $ 6,325,000 $ 1,455,000 $ 21,862,500 Total Cost (NPV, 30 years, 7% discount rate) $26,100,000 $6,100,000 $89,800,000 Assumptions Project management is assumed to be 15 percent of capital cost (excluding amendment expense). Engineering and design is assumed to be 15 percent of capital cost (excluding amendment expense). Construction quality assurance, monitoring, and reporting is assumed to be 10 percent of capital cost (excluding amendment expense). Access includes costs associated with coordinating access to the public and railroad right-of-ways for injection well installation and amendment delivery operations. Permitting includes costs associated with applying for and securing permits for the work as required by local, State, and Federal regulations. Injection pilot testing includes installation of one injection well and five monitoring wells for observation of in-situ conditions following injection. Assume 1 percent of amendment and delivery operations costs for full scale. Includes estimated labor and expenses for pilot and bench testing. It is assumed that methane mitigation measures will not be required during pilot testing. Assumes installation of 2-inch stainless steel well screen with carbon steel riser. Assumes installation of 2-inch stainless steel well screen with carbon steel riser, pitless adaptors, and traffic rated vaults Costs include amendment delivery to injection wells via tanker trucks assuming delivery of emulsified vegetable oil. Emulsified vegetable oil was selected based on in-situ longevity, advective flow rates, and delivery method. Given the uncertainty surrounding the quantity of amendments, a sensitivity analysis was completed using the Substrate Estimating Tool for Enhanced Anaerobic Bioremediation of Chlorinated Solvents, Version 1.2, November 2010, Parsons. The estimate cost assumes the average quantity developed during the sensitivity analysis. Costs were estimated using the range of amendment quantity developed during the sensitivity analysis. It is assumed that supplemental amendment will be necessary to maintain conditions suitable for bioremediation. Supplemental amendments are assumed to be completed at the injection wells installed during initial construction. Supplemental amendment and delivery costs are assumed to be 10 percent of initial amendment and delivery costs per year. There is significant uncertainty regarding the quantity of amendments needed. Costs include labor, expenses, and laboratory costs for quarterly sampling at 45 groundwater monitoring wells. Analysis inlcudes 25 parameters for monitoring anaerobic bioremediation processes and secondary water quality. Assumes well redevelopment by a licensed well driller using surging/pumping and biofouling treatment techniques. Assumes preparation of an annual groundwater monitoring report that summarizes groundwater quality trends and natural degradation rates. Project management is assumed to be 15 percent of O&M costs. It is assumed that 20 supplemental injection wells will be required to target areas of insufficient amendment delivery. This is assumed as a one time cost in year 4. Assumed as the same as initial amendment delivery costs on a per well basis. This is a one time cost in year 4. Costs assumed to be 10 percent of initial amendment delivery costs at the supplemental injection wells per year. Costs include quarterly sampling at 45 groundwater monitoring wells. Analysis inlcudes 25 parameters for monitoring anaerobic bioremediation processes and secondary water quality. Assumes well redevelopment by a licensed well driller using surging/pumping and biofouling treatment techniques. Assumes preparation of an annual groundwater monitoring report that summarizes groundwater quality trends and natural degradation rates. Project management is assumed to be 15 percent of O&M costs. Costs include annual sampling at 35 groundwater monitoring wells. Analysis inlcudes 12 parameters for monitoring anaerobic bioremediation processes and secondary water quality. Assumes preparation of an annual groundwater monitoring report that summarizes groundwater quality trends and natural degradation rates. Project management is assumed to be 15 percent of O&M costs. Contingency based on range of scope and bid contingency recommended in A Guide to Developing and Documenting Cost Estimates During the Feasibility Study (EPA, 2000). Discount rate taken from A Guide to Developing and Documenting Cost Estimates During the Feasibility Study (EPA, 2000). Table 9 Page 1 of 1

72 Table 10 Alternative 5 Cost Estimate East Hennepin Avenue Site Minneapolis, MN Table 10 - Alternative 5 Cost Estimate East Hennepin Avenue Site Minneapolis, MN Alternative 5 - Enhance Bioremediation via Recirculation System Quantity Unit Capital Costs 1.0 Engineering and Administration Subtotal $ 1,650,000 $ - $ Project Management, Capital Phase 1 Year $ 620,000 $ 620,000 $ 430,000 $ 920, Engineering and Design 1 LS $ 620,000 $ 620,000 $ 430,000 $ 920, Construction Quality Assurance, Monitoring, & Reporting 1 LS $ 410,000 $ 410,000 $ 290,000 $ 620, Planning and Preparation Subtotal $ 2,200,000 $ 1,600,000 $ 3,300, Access for Wells, Operations Building, and Conveyance Piping 1 LS $ 1,400,000 $ 1,400,000 $ 980,000 $ 2,100, Permitting 1 LS $ 100,000 $ 100,000 $ 70,000 $ 150, Recirculation Design Phase Bench and Pilot Testing 1 LS $ 700,000 $ 700,000 $ 490,000 $ 1,050, Monitoring Wells 10 Well $ 4,000 $ 40,000 $ 28,000 $ 60, Full Scale Recirculation System Construction and Initial Amendment Delivery Subtotal $ 12,700,000 $ 3,700,000 $ 41,900, Injection Wells 40 Well $ 10,000 $ 400,000 $ 280,000 $ 600, Extraction Wells 2 EA $ 15,000 $ 30,000 $ 21,000 $ 45, Conveyance Piping 5100 LF $ 70 $ 360,000 $ 252,000 $ 540, Operations Building Construction 2 EA $ 700,000 $ 1,400,000 $ 980,000 $ 2,100, Operations Equipment Procurement and Installation (Tanks, pumps, valves, etc.) 2 EA $ 820,000 $ 1,640,000 $ 1,148,000 $ 2,460, System Startup 1 LS $ 250,000 $ 250,000 $ 175,000 $ 375, Initial Amendment Delivery via Recirculation System 1 LS $ 8,600,000 $ 8,600,000 $ 770,000 $ 35,700,000 Total Capital Cost $ 16,600,000 $ 5,300,000 $ 45,200,000 O&M Costs (Annual Years 1-5) 4.0 System Operation Subtotal $ 1,400,000 $ 500,000 $ 4,300, Supplemental Amendment Delivery 1 Year $ 860,000 $ 860,000 $ 77,000 $ 3,570, Operations and Management Yrs Year $ 400,000 $ 400,000 $ 280,000 $ 600, Electricity 1 Year $ 30,000 $ 30,000 $ 21,000 $ 45, Treatment Control Building Maintenance 1 Year $ 20,000 $ 20,000 $ 14,000 $ 30, Injection Well Maintenance 40 EA $ 500 $ 20,000 $ 14,000 $ 30, Monitoring Subtotal $ 1,270,000 $ 889,000 $ 1,905, Performance Monitoring Yrs Samples/Year $ 6,000 $ 1,080,000 $ 756,000 $ 1,620, Annual Groundwater Report 1 Year $ 20,000 $ 20,000 $ 14,000 $ 30, Project Management, O&M Phase 1 Year $ 170,000 $ 170,000 $ 119,000 $ 255,000 Annual O&M Cost Yrs 1-5 $ 2,700,000 $ 1,400,000 $ 6,300,000 O&M Yrs 1-5 NPV $ 11,100,000 $ 5,800,000 $ 25,900,000 O&M Costs (Annual Years 6-10) 6.0 System Operation Subtotal $ 170,000 $ 120,000 $ 260, Operations and Management Yrs Year $ 100,000 $ 100,000 $ 70,000 $ 150, Electricity 1 Year $ 30,000 $ 30,000 $ 21,000 $ 45, Treatment Control Building Maintenance 1 Year $ 20,000 $ 20,000 $ 14,000 $ 30, Injection Well Maintenance 40 EA $ 500 $ 20,000 $ 14,000 $ 30, Monitoring Subtotal $ 1,270,000 $ 889,000 $ 1,905, Performance Monitoring Yrs Samples/Year $ 6,000 $ 1,080,000 $ 756,000 $ 1,620, Annual Groundwater Report 1 Year $ 20,000 $ 20,000 $ 14,000 $ 30, Project Management, O&M Phase 1 Year $ 170,000 $ 170,000 $ 119,000 $ 255,000 Annual O&M Cost Yrs 5-10 $ 1,440,000 $ 1,010,000 $ 2,170,000 O&M Yrs 5-10 NPV $ 4,300,000 $ 3,000,000 $ 6,400,000 O&M Costs (Annual Years 11-30) 8.0 Monitoring Subtotal $ 260,000 $ 182,000 $ 390, Performance Monitoring Yrs Samples/Year $ 6,000 $ 210,000 $ 147,000 $ 315, Annual Groundwater Report 1 Year $ 20,000 $ 20,000 $ 14,000 $ 30, Project Management, O&M Phase 1 Year $ 30,000 $ 30,000 $ 21,000 $ 45,000 Annual O&M Cost Yrs $ 260,000 $ 182,000 $ 390,000 O&M Yrs NPV $ 1,410,000 $ 990,000 $ 2,110,000 Contingency 9.0 Contingency on Total Cost, 25% Subtotal $ 8,352,500 $ 3,772,500 $ 19,902,500 Total Cost (NPV, 30 years, 7% discount rate) $41,800,000 $18,900,000 $99,600,000 Assumptions Unit Cost Estimated Cost Low Estimate Cost High Estimate Cost Project management is assumed to be 15 percent of capital cost (excluding amendment expense). Engineering and design is assumed to be 15 percent of capital cost (excluding amendment expense). Construction quality assurance, monitoring, and reporting is assumed to be 10 percent of capital cost (excluding amendment expense). Access includes costs associated with coordinating access to property for installation or construction of infrastructure. This cost is assumed to include purchasing four properties, demolishing existing structures, and negotiating access to public and railroad rights-of-way. Permitting includes costs associated with applying for and securing permits for the work as required by local, State, and Federal regulations. Assumes labor and analytical expense to conduct laboratory bench testing, extraction well pumping test, and injection pilot testing. Bench testing assumes column testing using site groundwater and a variety of amendments. Pump testing includes completion of a pumping test at an extraction well with multiple observation monitoring wells to better develop the radius of influence of extraction and reinjection wells. Pilot testing includes injection of amendments and monitoring at multiple wells. It is assumed that methane mitigation measures will not be necessary during pilot testing. Assumes installation of 2-inch stainless steel well screen with carbon steel riser. Assumes installation of 2-inch stainless steel well screen with carbon steel riser, pitless adaptors, and traffic rated vaults. Assumes installation of 4-inch stainless steel well screen with carbon steel riser, pitless adapters, and traffic rated vaults. Assumes construction of 5,100 feet conveyance piping between the operations building and the extraction and injection wells. Costs include installing 4-inch HDPE pipe at a depth of 8 feet. Assumes trenching will occur in unpaved right-of-way space with the exception of pavement replacement at street crossings where existing pavements and curbs will be demolished and replaced at eight cross-streets. Assumes construction of 3,500-square foot climate controlled building for each treatment control building. Assumes purchase and installation of equipment to collect, amend, and reinject groundwater. Equipment includes 58,000 gallons tankage, redundant injection pumps, mixing vessel, amendment metering and transfer pumps, electrical, instrumentation and controls, safety equipment, valves, piping, and appurtenances. Costs include oversight, troubleshooting, and system modification associated with system startup. Costs include amendment delivery to injection wells utilizing the treatment control building. Assumes emulsified vegetable oil based on in-situ longevity and delivery method. Given the uncertainty surrounding the quantity of amendments, a sensitivity analysis was completed using the Substrate Estimating Tool for Enhanced Anaerobic Bioremediation of Chlorinated Solvents, Version 1.2, November 2010, Parsons. The estimate cost assumes the average quantity developed during the sensitivity analysis. Costs were estimated using the range of amendment quantity developed during the sensitivity analysis. It is assumed that supplemental amendment will be necessary to maintain conditions suitable for bioremediation. Supplemental amendments are assumed to be completed at the injection wells installed during initial construction. Supplemental amendment and delivery costs are assumed to be 10 percent of initial amendment and delivery costs per year. There is significant uncertainty regarding the quantity of amendments needed. Assumes full-time operations engineer onsite for 8 hours a day for 365 days a year and technical support for treatment evaluation and system modifications. Assumes utility expense to operate the equipment and heat the treatment control buildings. Capital maintenance includes expenses for maintaining and replacing equipment. Assumes well redevelopment by a licensed well driller using surging/pumping and biofouling treatment techniques. Costs include labor, expenses, and laboratory costs for quarterly sampling at 45 groundwater monitoring wells. Analysis inlcudes 25 parameters for monitoring anaerobic bioremediation processes and secondary water quality. Assumes preparation of an annual groundwater monitoring report that summarizes groundwater quality trends and natural degradation rates. Project management is assumed to be 15 percent of O&M costs. Operations and management for years 6-10 is assumed to be 25 percent of operations and management costs for years 1-5. Assumes utility expense to operate the equipment and heat the treatment control buildings. Capital maintenance includes expenses for maintaining and replacing equipment. Assumes well redevelopment by a licensed well driller using surging/pumping and biofouling treatment techniques. Costs include quarterly sampling at 45 groundwater monitoring wells. Analysis inlcudes 25 parameters for monitoring anaerobic bioremediation processes and secondary water quality. Assumes preparation of an annual groundwater monitoring report that summarizes groundwater quality trends and natural degradation rates. Project management is assumed to be 15 percent of O&M costs. Costs include labor, expenses, and laboratory costs for quarterly sampling at 45 groundwater monitoring wells. Analysis inlcudes 25 parameters for monitoring anaerobic bioremediation processes and secondary water quality. Assumes preparation of an annual groundwater monitoring report that summarizes groundwater quality trends and natural degradation rates. Project management is assumed to be 15 percent of O&M costs. Contingency based on range of scope and bid contingency recommended in A Guide to Developing and Documenting Cost Estimates During the Feasibility Study (EPA, 2000) Discount rate taken from A Guide to Developing and Documenting Cost Estimates During the Feasibility Study (EPA, 2000) Table 10 Page 1 of 1

73 Figures

74 Service Layer Credits: Copyright: 2013 National Geographic Society, i-cubed USGS 7.5 Minute Quadrangle - Hennepin County 2010 East Hennepin Ave!;N 0 2,000 4,000 LOCATION MAP East Hennepin Avenue Site Minneapolis, Minnesota Note: Pink shaded areas in USGS map indicate residential areas. Feet FIGURE 1

75 12th Ave SE 13th Ave SE 15th Ave SE Mckinley St NE 20th Ave SE Taft St NE Wilson St NE 23rd Ave SE Broadway St NE 2010 East Hennepin Ave Off-Site Property with Documented TCE in Groundwater Based on Review of MPCA Files Arthur St NE Summer St NE Godward St NE Off-Site Property with Potential Solvent Use, Approximate Location of Current and/or Historic Business Cleveland St NE Kennedy St NE Hoover St NE Spring St NE Industrial Blvd Central Area Sub-slab Soil Gas TCE Concentrations < 20 ug/m 3 Winter St NE Roosevelt St NE Harding St NE Delano St NE ug/m ug/m ug/m 3 Johnson St NE 35W 10th Ave SE Garfield St NE Traffic St NE E Hennepin Ave SITE NORTHEAST 29th Ave SE ug/m 3 > 2000 ug/m 3 Maximum sub-slab soil gas TCE concentration at each building shown. 11th Ave SE Como Ave SE 16th Ave SE Brook Ave SE 17th Ave SE CENTRAL 19th Ave SE 21st Ave SE 22nd Ave SE Cole Ave SE 25th Ave SE 26th Ave SE 27th Ave SE Talmage Ave SE 33rd Ave SE Co!;N ,200 Feet SOUTHWEST 18th Ave SE Weeks Ave SE 6th St SE 7th St SE 14th Ave SE 8th St SE Diagonal Rdwy Elm St SE 24th Ave SE Kasota Ave SE 30th Ave SE STUDY AREA East Hennepin Avenue Site Minneapolis, Minnesota Imagery: USGS NAIP (2015) FIGURE 2

76 12th Ave SE 13th Ave SE Mckinley St NE 20th Ave SE Taft St NE Johnson St NE 35W 10th Ave SE 11th Ave SE 6th St SE 7th St SE Winter St NE Kozebar Co./ Cozy Baby Carriage Co Postcard Builder VP30090 MN Tank Co. Gorshe Auto Repair Shop & Storage 15th Avenue Housing VP30330 Garfield St NE 14th Ave SE Arthur St NE Cleveland St NE Como Ave SE Stahl Mfg/ M&M Wire Clamp Co. Jewel Coal Company 15th Ave SE Summer St NE Kennedy St NE 16th Ave SE Brook Ave SE Repair Shop 8th St SE 17th Ave SE Bowen Products Joe Baker's Auto Cargill/ International Sugar Warner MFG Co Joe's Market Diagonal Rdwy East Hennepin Auto Service Inc LUST2477 Shallow GW TCE Conc.: 24 µg/l 18th Ave SE Glidden Paint Traffic St NE E Hennepin Ave Roosevelt St NE 19th Ave SE Office/Warehouse - Traffic St. VP27480 Shallow GW TCE Conc.: 2.7 µg/l Minneapolis Casket Company st Ave SE Anne Gendein Trust Property/ former Scott-Atwater Manufacturing VP13270 Lend Lease Trucking LUST6600 Shallow GW TCE Conc.: 3,600 µg/l Amar's Auto Service 22nd Ave SE CNW East Minneapolis Yard Broadway St NE Harding St NE Twin City Plating Excel Metal Finishing Inc 23rd Ave SE Former Gas Station & Auto Repair Pitcher MFG Co. / United Chemical Elm St SE Cole Ave SE 24th Ave SE Hoover St NE Delano St NE Franks Auto Repair LUST17726 Shallow GW TCE Conc.: 1,620 µg/l 25th Ave SE Como Student Community VP24930 Shallow GW TCE Conc.: 10.7 µg/l 26th Ave SE Kasota Ave SE 27th Ave SE Godward St NE Northwestern Warehouse VP13100 VP13101 Shallow GW TCE Conc.: 610 µg/l Sears/former Scott-Atwater Manufacturing LUST7905 LUST7043 Shallow GW TCE Conc.: 290 µg/l 29th Ave SE Industrial Blvd AmeriPride Services Inc VP24750 LUST16906 Shallow GW TCE Conc.: 7.2 µg/l 2400 Traffic Street VP22300 VP23301 VP23302 Shallow GW TCE Conc.: 41 µg/l Weeks Ave SE Talmage Ave SE 30th Ave SE 33rd Ave SE 4567 Co 2010 East Hennepin Ave Off-Site Property with Documented TCE in Groundwater Based on Review of MPCA Files Off-Site Property with Potential Solvent Use, Approximate Location of Current and/or Historic Business Central Area Sub-slab Soil Gas TCE Concentrations < 20 ug/m ug/m ug/m ug/m ug/m 3 > 2000 ug/m 3 Maximum sub-slab soil gas TCE concentration at each building shown.!;n ,200 Feet POTENTIAL OFF-SITE TCE SOURCES East Hennepin Avenue Site Minneapolis, Minnesota Imagery: USGS NAIP (2015) FIGURE 3

77 12th Ave S E 20th Ave S E 15th Ave S E T a fts tne Mc kinley S tne Wilson S tne Johnson S tne 35W 10th Ave S E 11th Ave S E WinterS tne Ga rfield S tne 13th Ave S E ArthurS tne Clevela nd S tne 16th Ave S E S um m ers tne Brook Ave S E 17th Ave S E S P ug/l(15-20') 274ug/l(38-42') S P ug/l(15-20') 285ug/l(36-40') S tinson Blvd S tinson Blvd NE th Ave S E ") RooseveltS tne T ra ffic S tne Ta lm a d ge Ave S E VP ug/m 3 S P ug/l(18-23') 2.7ug/l(50-54') S P ug/l(15-20') 316ug/l(37-41') 21stAve S E Ha rd ing S tne S P-02 <1.0ug/l(15-20') 1100ug/l( ') VP-01 <0.74ug/m 3 22nd Ave S E K enned y S tne ") S P ug/l(15-20') 13.6ug/l(49-53') 23rd Ave S E VP ug/m 3 S P-08 <1.0ug/l(20-25') 473ug/l(35-40') 584ug/l(43-47') VP ug/m 3 ") S P ug/l(30-35') 90.5ug/l(45-50') VP ug/m 3 24th Ave S E HooverS tne ") ") S P ug/l(15-20') 75.4ug/l(35-40') 1260ug/l(44-48') S P ug/l(40-45') 40.3ug/l(53-57') VP ug/m 3 S P ug/l( ') 685ug/l(34-38') 25th Ave S E Com o Ave S E Dela no S tne ") S P ug/l(25-30') 1810ug/l( ') S P-09 <1.0ug/l(20-25') 1300ug/l(41-45') S P ug/l(20-25') 828ug/l(39-43') 26th Ave S E S P ug/l(18-23') 1080ug/l(29-33') 27th Ave S E Weeks Ave S E S P ug/l(37-41') 78.9ug/l(66-70') EHennep in Ave 29th Ave S E Ind ustria lblvd S a m p le ID T CEin Ground wa ter(t em p WellS c reen Dep th) S a m p le ID T CEin S oilga s 2010 East Hennepin Ave MPCAInvestiga tion Loc a tion S oilprob e Loc a tion ") S oil-ga s S a m p le Loc a tion Centra larea Off-S ite Prop erty with Doc um ented T CEin Ground wa ter Ba sed on Reviewof MPCAFiles Off-S ite Prop erty with Potentia l S olventu se,ap p roxim a te Loc a tion of Cu renta nd /or Historic Business Prop erty Inc lud ed in MPCA s S A249S tud y a s of Ma y 28,2015 Note:MPCAS A249investiga tion d a ta from Novem b era nd Dec em b er2015.s oilga s sa m p ling d ep th of 8ftb gs.!;n ,000 Feet 19th Ave S E Fa irm ountave S E Cole Ave S E Rollins Ave S E 6th S ts E 7th S ts E 8th S ts E 14th Ave S E Dia gona l Rd wy S EElm S t Elm S ts E K a sota Ave S E Imagery: USGS NAIP (2015) MPCAS A249GROU NDWAT ER ANDS OILGAS S AMPLING- T CERES U LT S Ea sthennep in Avenue S ite Minnea p olis,minnesota FIGU RE4

78 12th Ave SE 13th Ave SE 18th Ave SE 20th Ave SE Taft St NE Garfield St NE E Hennepin Ave SITE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Harding St NE NORTHEAST 2010 E Hennepin Ave Central Area Building Mitigation System Installed Building Mitigation System Not Installed in Central Area Central Area Detail Mitigation Declined by Property Owner - Sub-Slab Soil Gas TCE <20 ug/m 3 Talmage Ave SE Mitigation Declined by Property Owner - Sub-Slab Soil Gas TCE >20 ug/m 3 Como Ave SE 16th Ave SE!!!!!!!!!!!!!!!!!!!!! CENTRAL 23rd Ave SE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Indoor Air TCE < Applicable MPCA Intrusion Screening Value (ISV) Did Not Participate in Sub-Slab Soil Gas Sampling Study Property with Post-Mitigation Indoor Air Sampling Residential Properties without Mitigation Systems 15th Ave SE Brook Ave SE 19th Ave SE 21st Ave SE 22nd Ave SE Cole Ave SE!!!!!!!!!!!!!!!!!!!!!!!!!!! Fairmount Ave SE!;N Rollins Ave SE Feet SOUTHWEST 17th Ave SE 8th St SE 7th St SE 14th Ave SE Elm St SE BUILDING MITIGATION STATUS - APRIL 2016 East Hennepin Avenue Site Minneapolis, Minnesota Imagery: USGS NAIP (2015) FIGURE 5

79 14th Ave SE Taft St NE Wilson St NE 2010 East Hennepin Ave Winter St NE Central Area Monitoring Well Locations Garfield St NE!A 302GS 303GS!A Traffic St NE!A!A 301GS 301GD!A Glacial Drift Monitoring Well!A Glacial Drift Nested Monitoring Well 305GS 305GD!A!A * Glacial Drift Pump-Out Well E Hennepin Ave !A SMW1!A!A 307GS 307GD!A!A 306GS 306GD 304GS!A 308GS 308GD!A!A!A B!A!A 309GS 309GD Talmage Ave SE SMW3!A *!A!A!A 311GS 311GD!A! A!A 13th Ave SE Como Ave SE 16th Ave SE 17th Ave SE!A!A *!A!A!A 21st Ave SE 312GS 312GD 310GS Q SMW6 313GS 313GD 110 SMW22!A SMW25 23rd Ave SE 25th Ave SE 26th Ave SE 27th Ave SE 15th Ave SE SMW10 * 111!A 314GS 314GD!A!A!A!A 315GS 315GD!A Fairmount Ave SE 19th Ave SE S 20th Ave SE!A SMW19 T!A 22nd Ave SE Cole Ave SE Weeks Ave SE!;N !A V!A 316GS 316GD * *!A SMW16 Feet!A 318GS!A X 7th St SE 8th St SE W!A!A 317GS Elm St SE 24th Ave SE Kasota Ave SE GLACIAL DRIFT GROUNDWATER MONITORING NETWORK East Hennepin Avenue Site Minneapolis, Minnesota Imagery: USGS NAIP (2015) FIGURE 6

80 14th Ave SE 15th Ave SE 20th Ave SE E Hennepin Ave Stinson Blvd NE Taft St NE Harding St NE Hoover St NE 2010 East Hennepin Ave Central Area Proposed Injection Well Railroad Property Parcels 16th Ave SE 17th Ave SE 18th Ave SE 21st Ave SE Talmadge Ave SE 22nd Ave SE 23rd Ave SE 24th Ave SE 25th Ave SE Brook Ave SE 19th Ave SE Como Ave SE Fairmount Ave SE Cole Ave SE Weeks Ave SE!;N Feet Rollins Ave SE 8th St SE Diagonal Rdwy Elm St SE SE Elm St Imagery: USGS NAIP (2015) ALTERNATIVE 4 - ENHANCED BIOREMEDIATION VIA INJECTION EVENT(S) East Hennepin Avenue Site Minneapolis, Minnesota FIGURE 7

81 14th Ave SE 15th Ave SE 20th Ave SE E Hennepin Ave Stinson Blvd NE Taft St NE Harding St NE Hoover St NE!? 2010 East Hennepin Ave Central Area Proposed Extraction Well (Existing Location) Proposed Extraction Well Proposed Injection Well 16th Ave SE 17th Ave SE 18th Ave SE!? 21st Ave SE Talmadge Ave SE 22nd Ave SE 23rd Ave SE 24th Ave SE 25th Ave SE Forcemain Treatment System Control Building (Typical / Location Not Specified) Railroad Property Parcels Brook Ave SE!? 19th Ave SE Como Ave SE Fairmount Ave SE Cole Ave SE Weeks Ave SE!;N Feet Rollins Ave SE!?!? 8th St SE Diagonal Rdwy Elm St SE SE Elm St Imagery: USGS NAIP (2015) ALTERNATIVE 5 - ENHANCED BIOREMEDIATION VIA RECIRCULATION East Hennepin Avenue Site Minneapolis, Minnesota FIGURE 8