i-admin /15/14 2 of 3 Doc Type: Board Memo/Issue Statement

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Leslie Benson
5 years ago
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2 On September 13, 2010, with MPCA approval GMI discontinued the groundwater extraction and treatment because TCE concentrations had decreased below the cleanup concentrations established in the Consent Order. After the pump and treat system was shut down, MPCA required General Mills to begin quarterly groundwater monitoring to determine if the groundwater contamination would increase in concentration without the system operating. Vapor Intrusion VI) Investigation Across South East Como Neighborhood ) More recently, professional experience, both of MPCA staff and others around the country working on contaminated properties, shows that even relatively low VOC concentrations in groundwater can contaminate the air contained in the soil. The air in the void spaces in the soil is called soil gas. The soil gas has the potential to make its way into homes and buildings through utility openings and cracks in the basement floor or walls and create an indoor air contamination exposure risk. The risk of soil gas migrating into homes is referred to as the vapor intrusion potential or VI potential. The VI potential from the contaminated groundwater plume prompted MPCA to request that GMI conduct a soil gas survey in the vicinity of the General Mills Site and adjoining VOC groundwater plume. Beginning in April 2012, soil gas sampling was conducted in three phases in the public rights of way in the South East Como Neighborhood. Soil gas samples were collected from borings at a depth of about eight feet below grade the approximate depths of area basements). In early October 2013, 14 of 40 soil gas samples confirmed the presence of TCE in soil gas above risk criteria established by the MPCA. Soil gas data were compared with the known groundwater plume footprint. Data analysis of the groundwater plume and the newer soil gas plume, led to the public notice of the VI potential. See Figure 1: The Vapor Intrusion Pathway) Public Involvement November 2013 to present) The soil gas survey data mapped an area containing 195 parcels with a VI potential risk the Vapor Study Area, shown by the orange outline on the attached map). On November 6, 2013, the MPCA mailed 950 letters to tenants, residents and property owners. The joint Minnesota Department of Health MDH) MPCA letter communicated new information that the groundwater contaminant plume in the area poses vapor intrusion risks, requested assistance and cooperation of residents and property owners, and announced two public meetings scheduled for November 12, Following the public meetings, MPCA staff along with MDH and GMI) attended more than five community group meetings as well as office hours sessions where tenants, residents and property owners could meet with MPCA, MDH, GMI and its primary environmental consultant, Barr Engineering. Since early November 2013, the groundwater contamination and VI story remains widely covered by the local media and is frequently discussed in public meetings. Sub slab Soil Gas Sampling Throughout the Vapor Study Area November 18, 2013, to present) MPCA requested that property owners within the Vapor Study Area, and properties adjacent to properties which have been tested to show a VI potential risk, enter into an access agreement with GMI to allow sub slab soil gas sampling of their homes/buildings. To determine if contamination in soil gas is entering a building creating an indoor air exposure, a sub slab sample an air sample collected from beneath the basement floor) is collected. A GMI environmental consultant installs a sampling ports) through the foundation floor to obtain a representative soil gas samples). The concentration of TCE in the sample is evaluated against a MPCA established criteria for soil gas beneath buildings. The risk based criterion for sub slab soil gas is 10 times the concentration established by MDH as safe for the air inside the building i.e., residential or commercial/industrial value). For example, the MDH indoor air standard for TCE in residential use application is 2 µg/m³. Residential buildings with sub slab TCE concentrations greater than 20 µg/m³ receive a vapor mitigation system after the building owner signs a Mitigation Installation Agreement. If initial TCE concentrations in the sub slab air of residential buildings are between 2 and 20 µg/mᶟ, a second sampling event is scheduled to confirm TCE concentrations are below 20 µg/m³, the cut off for mitigation. GMI s work is being conducted under the oversight of the MPCA. This oversight includes MPCA conducting field duplicate sampling and laboratory analyses, as well as field audits by MPCA staff and an MPCA consultant. i-admin /15/14 2 of 3 Doc Type: Board Memo/Issue Statement

3 Sub slab Vapor Mitigation Installations November 28, 2013, to present) The sub slab vapor mitigation system is the same as a typical radon mitigation system. Such systems consist of a hole in the building floor, with a sealed pipe attached that leads to a low wattage, in line turbine or fan placed outside of the living space i.e., in the attic or outdoors). The fan pulls soil gas from beneath the foundation and discharges it to ambient air through a stack above the roofline and away from any building intakes. Full foundation sub slab depressurization is assured though field diagnostic testing and the mitigation system design. Installations are tailored to each building. The end result is an operational depressurization system that eliminates the vapor intrusion pathway. See Figure 2: The In Home Vapor Mitigation System) Additional Remedial Actions for Contaminant Cleanup ) After properties that need mitigation are identified and the mitigation systems are installed, General Mills, under MPCA oversight, will do additional investigation and cleanup of the contamination. This work is needed to address the contamination in the soil gas that extends from the contaminated groundwater. Attachments Attachment 1: General Mills/Henkel Corp. Superfund Site Study Area Sampling Status Map Attachment 2: Figure 1: The Vapor Intrusion Pathway Attachment 3: Figure 2: The In Home Vapor Mitigation System i-admin /15/14 3 of 3 Doc Type: Board Memo/Issue Statement

event Mitigation needed Results less than 2 ug/m3 in first event or less than 20 ug/m3 in second event* Commercial/Industrial results less than 60 ug/m3 Results 2-20 ug/m3* in first event sample

4 ATTACHMENT 1 General Mills/Henkel Corp. Superfund Site Study Area Sampling Status E Hennepin Ave k j General Mills/Henkel Corp. Superfund Site historic disposal area Vapor Study Area Approximate Groundwater TCE Plume Sub-slab Vapor Sampling Status No mitigation required No mitigation required Will Receive a second sampling event Mitigation needed Results less than 2 ug/m3 in first event or less than 20 ug/m3 in second event* Commercial/Industrial results less than 60 ug/m3 Results 2-20 ug/m3* in first event sample Results greater than 20 ug/m3* Ventilation system installed* First sample was 2-20 ug/m3 Second sample completed *Sampling status current as of Feb. 5, 2014, 4PM. Wright Anoka Washington k j denied 23rd Ave SE Sampling access agreement denied* Sampling access agreement signed signed but not scheduled* scheduled Sampling event scheduled* complete Results not yet available* Hennepin Ramsey 7th St SE Meters 0 Feet :6, Map Vicinity Carver Scott Dakota

5 Figure 1: The Vapor Intrusion Pathway ATTACHMENT 2

6 ATTACHMENT 3 FIGURE 2: The In Home Vapor Mitigation System In-home vapor mitigation systems

Best management practices for vapor investigation and building mitigation decisions

www.pca.state.mn.us Best management practices for vapor investigation and building mitigation decisions Purpose This best management practices (BMPs) document describes the processes used to conduct vapor