Evaluation of VI Data Relative to Separation Distance Screening Criteria A Michigan Case Study

Transcription

1 GROUNDWATER & ENVIRONMENTAL SERVICES,INC. Evaluation of VI Data Relative to Separation Distance Screening Criteria A Michigan Case Study Chris Mulry, GES, Inc. March 19, 2014

2 Presentation Outline Study Objectives and Limitations Michigan DEQ (MDEQ) Background and Regulatory Framework > Applicable Guidance > Default Assumptions > Practice Considerations LNAPL presence Residential vs. non-residential screening criteria application Case Study Locations and Settings Data Summary Comparison to EPA OUST and ITRC draft screening criteria > Applicability > Key Findings Recommendations for further study

3 MDEQ Vapor Intrusion Guidance - Comprehensive Compendium - All sites require a complete CSM - Step-wise, risk-based approach - 4-Step Process: 1. Pathway Screening Assessment i. Media-specific look-up values 2. Soil Gas Investigation i. Consult MDEQ Analyst for soil gas point location 3. Building Specific Vapor Invest. i. If proper soil gas samples cannot be collected 4. Response Actions

4 Guidance Application Use of Guidance Requires: GW depth > 3m > Necessary for standard RBCA applications in MI for pathway elimination J&E model only accepted * Petroleum-sites may use alternate guidance (Appendix B-3) > Lateral inclusion distance reduced to 30 feet > Presence of subsurface O 2 > 5% required to use 30 ft.. distance Therefore, subsurface investigation required - Alternative approaches may be considered * BioVapor acceptable for petroleum compounds only

5 Guidance Application Overview

6 Screening Process Step 1

7 Soil Gas Investigation - Step 2 Soil gas investigations required for UST sites either due to screening value exceedence or NAPL indicators > Presence of LNAPL is condition driving most VI investigations > Inability to refute NAPL potential existence invalidates RBCA NAPL Indicators - Non-codified & variable references cited in practice for over 10 years: > TPH-GRO > 100 mg/kg in sand or 200 mg/kg in clay; or > TPH-DRO > 5 mg/kg in sand or 18 mg/kg in clay > Benzene in GW > 1,000 μg/l > Persistent dissolved GW plume > TPH-GRO > 500 mg/kg > TPH-DRO > 100 mg/kg

8 Soil Gas Investigation Flow Chart

9 Petroleum-only Site Application Appendix B-3 of 2013 Guidance 3 primary status conditions established: 1. Biodegradation Zone i. Aerobic conditions - O 2 > 5% (via vertical profiling) ii. iii. > 30 ft.. lateral and 10 ft.. vertical source/bldg. separation < 2,500 ft 2 bldg. footprint 2. Vapor Intrusion Zone i. Conditions inhibit O 2 ii. Inadequate separation distances iii. iv. Large building footprint Preferential pathway(s) connecting source area to bldg. 3. Transition Zone i. Inadequate or intermediate separation distances ii. iii. Preferential pathways within 30 feet of a vapor source Evidence of O 2 beneath large buildings

10 VI Screening Values

11 VI Guidance Screening Values (Cont d) Appendix D-1 (Residential) and D-2 (Non-Residential) Comprehensive compendium of look-up values by media Example values for Benzene: - Shallow soil gas: 4 ft.. bg (sub-slab) - Residential vs non-res classification is a major screening differentiator - Defaults to MCLs and/or method detection limits for many compounds in the sump sample column

12 Michigan s Quaternary Geology

13 Vapor Intrusion Sites

14 Geologic Settings a closer look

15 VI Investigation Sites 32 Total sites: > 31 are UST (or former UST) locations with gasoline stored/dispensed; one is an adjacent residential property > All buildings are slab-on-grade > No indoor air sampling conducted > No preferential pathways tying source(s) to building identified Soil Gas investigation required due to: > LNAPL indicators identified, or > site failed preliminary screening, or > < 30 foot lateral separation from PHC detections to building, or > Vertical separation distance < 10 feet (Appendix B-3) CVOCs identified in soil/gw at only 1 of 32 sites prior to VI investigation

16 Vapor Point Installation Locations biased towards: > areas of impact (1), > between potential sources and buildings (1+), > Between potential sources and property boundaries (2) > Sub-slab if documented subsurface impact within 30 ft.. of bldg... (minimum 2 for 2,500 ft 2 bldg.) Number and location determined through consultation with MDEQ Analyst Soil vapor points installed: Sub-Slab Points installed: 30 of 32 sites 13 of 32 sites 97 total (avg. = 3.2/site) 43 total (avg. = 3.3/site) Total points per site range from 1 to 9; avg. = 4.7/site SG vapor point screens range from ft.. depth

17 Sampling Program Soil gas/sub-slab samples collected in 1 L summa canister each calendar quarter per guidance Full VOC scan analyzed via EPA TO-15 method Helium shroud placed to demonstrate sample integrity Full QA/QC followed Careful laboratory coordination recommended

18 Comparing Screening Applications Vertical Separation ITRC (draft): 5 foot vertical separation for non-napl sources 15+ feet for NAPL sources USEPA OUST (draft): 5 to 6 foot vertical separation for non-napl sources 15 to 18 feet for NAPL sources MI Case Study Sites: 16 of 32 (50%) have groundwater conditions deeper than 4 feet below grade on average and might be amenable to separation distance and source strength criteria screening Only 3 of 32 sites have depth to GW > 10 feet (MDEQ guidance vertical separation criteria for Biodegradation Zone )

19 Data Findings Field Fixed Gases CO 2 detected in soil gas at 31 of 32 sites, typically in 0.5% to 10% range CH 4 detected at 20 sites, 13 at concentrations > 10% > 10 of these 13 sites failed MDEQ soil gas look-up criteria O 2 ranges in soil gas were significant at all sites > 10 of 11 sites where any O 2 reading was < 1% failed MDEQ soil gas screening criteria > Note the MDEQ uses 5% O 2 as base for biodegradation > No correlation noted between O 2 and GW depth strictly a function of source strength?

20 Data Findings - CVOCs One or more chlorinated compounds found in soil gas at 22 of 32 sites None exceeded MDEQ soil gas look-up values Only 1 of 32 sites had prior detections of CVOCs in soil or GW > These constituents not typically included in service station investigation SOWs Count by compound: PCE 11 Dichlorodiflouromethane 5 Chloroform 5 1,1,1 TCE 4 TCE 4 1,3 dichlorobenzene - 3 Methylene Chloride - 2

21 Screening Applicability Summary Half of the 32 case study sites would be ineligible to screen using ITRC or EPA distance criteria alone due to shallow water table Of the 16 remaining cases, 7 met MDEQ VI screening criteria 9 sites with GW depth > 4 feet failed to meet soil gas screening criteria why? > 5 had max PID readings near or in excess of 500 ppmv > All 9 had at least one soil gas point/value where O 2 < 1% > 6 of 9 had CH 4 in soil gas > 10%; 5 > 50% For the 16 shallow GW sites, 9 met MDEQ VI screening values and 7 did not. For the 7 that did not: > Benzene and Hexane exceeded criteria in 5 of 7 > 4 of 7 sites had elevated PID or CH 4 readings indicative of highstrength source

22 Site Deep GW Sites Failing VI Screening DTW (ft.. below TOC) CH 4 (%) CO 2 (%) O 2 (%) VOC (ppmv) PID Compound(s) Exceeding MDEQ (Appendix D) value Benzene Benzene, Hexane Benzene Benzene Benzene, Ethylbenzene, Hexane Ethylbenzene Ethylbenzene, Hexane ,000 Benzene, Hexane Benzene, Ethylbenzene, Hexane 22

23 Deep GW Screening Summary Site # Vapor Points Exceeding Exceeds Residential Exceeds Non-Residential Notes 1 1/4 1,2 2 1/3 3 1/2 4 1/3 5 1/4 6 1/ /6 8 1/5 9 1/5 23 Notes: 1. Only exceeded shallow soil gas residential 2. Only one of two sampling events exceeded

24 Where would ITRC/EPA Screening Have Led? Approximately 16 sites may have been eligible to screen based upon vertical separation distance criteria alone > 7 met MDEQ VI screening values > 9 failed, but 8 of these show evidence of an LNAPL source Therefore, 8 of 32 sites (25%) would meet separation distance and source strength screening criteria proposed by ITRC and EPA resulting in a 7/8 agreement (88%) with investigative results sub slab investigation planned for single outlier site Of 16 shallow GW sites, 9 passed MDEQ VI screening; 7 failed > 4 of 7 failures show evidence of high-strength source

25 Sites Requiring Additional Work 9 Sites with GW > 4 feet deep Failed MDEQ VI Screening Compounds exceeding soil gas criteria: Benzene = 7 (78% of failures) Hexane = 5 all also failed for at least one other compound Ethylbenzene = 4 TMBs = 2 Naphthalene = 1 CVOCs = 0 All 9 sites showed at least some soil gas O 2 readings < 1% Average CO 2 reading (for highest point) = 12% 6 of 9 yielded max CH 4 > 10% with 5 of 9 > 50% 5 of 9 had PID screening max values in excess of 500 ppmv A Multiple Lines of Evidence approach would have flagged 8 of 9 sites

26 Summary and Follow-Up Actions Application of ITRC and/or EPA screening would provide 89% agreement with MDEQ guidance outcomes Source strength and residential vs. non-residential screening applications are critical CSM and management considerations Re-visit CSM for one outlier site (deep GW with no indications of NAPL-strength source) that failed MDEQ VI screening criteria Incorporate risk arguments for sites where soil gas data failing to meet screening criteria are not located near buildings or property boundaries

27 Follow-Up Actions One site proposed for elimination of VI pathway due to soil gas results < 10% of Appendix D screening values All other sites will complete 4 quarters of soil gas/sub-slab sampling as per guidance to evaluate data trends and seasonality

28 VI Investigation Compliance Costs Soil gas point installation (2-3) $ 3,500 - $4,500 Sub-slab point installation (2) $ 2,000 Sampling event $ 2,500 - $3,000 Annual Cost per site $ 13,000 - $18,000

29 Recommendations for Further Analysis Work with data sets to evaluate O 2 ranges in time and space large variability noted which readings correspond to exceedences? Consider vertical profiling for sites failing to meet VI screening values Evaluate spatial relationships of soil gas data failing compound screening > Does the CSM account for this? > Is risk profile altered by findings? Bring site-specific risk arguments into CSM and decision process > Example: if one of 4 vapor samples fails screening criteria, but is > 30 ft.. from the nearest building and no NAPL-strength source is identified, is further work necessary? Evaluate data seasonality via continued sampling > Recommended in Guidance unless vapor samples return < 10% of look-up values > What changes in data values and required actions result from additional sampling? Give greater consideration to the potential role of CVOCs > Via CSM past practices, neighboring property issues, sanitary or water leaks > Are any of these laboratory issues?

30 GROUNDWATER & ENVIRONMENTAL SERVICES,INC. Questions? Thank you