Technical MemorandumTo: Gary Krueger ï MPCAFrom: Richard Manser ï Barr EngineeringSubject: Soil Vapor SamplingDate: June 20, 2012Project: East Hennepin Avenue Site, Minneapolis, MN

Summary of Phase 2B Soil Vapor Results and Path Forward

Barr Engineering, on behalf of General Mills (GMI), conducted Phase 2B of the soil vapor evaluation for

the East Hennepin Ave site located at 2010 East Hennepin Avenue, Minneapolis, Minnesota (Site). The

purpose of the Phase 2B investigation was to further evaluate the soil vapor conditions over groundwater

TCE plume at the site. Locations for the Phase 2B investigation borings were identified in the Phase 2B

work plan submitted to the MPCA on February 9, 2012 and modified on April 5.

On January 13, 2012 a groundwater sample was collected from existing glacial drift monitoring well B

located on the Site. Well B was selected for groundwater analysis based on historical TCE concentrations

in groundwater that exceed 100 ug/L and its location appears to be outside of the on-site source area. A

groundwater sample was collected from well B to support the idea that a soil gas sample collected in the

vicinity would overlie TCE impacted groundwater.

On April 25, Matrix Environmental, LLC, contracted to Barr, advanced soil borings using a direct-push

rig for purposes of collecting soil samples for field screening and groundwater and soil gas samples for

laboratory analysis. Soil borings were advanced at two locations on the Site; one location, DP002, is

approximately 50 feet west of well B and one location, DP001, was advanced in the northeast portion of

the Site. One soil boring was advanced off-site, in the City of Minneapolis right of way in front of 907

19th Avenue Southeast, approximately 200 feet north of well S. Soil boring locations are shown on

Figure 1.

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To: Gary Krueger ð MPCAFrom: Richard Manser ð Barr EngineeringSubject: Soil Vapor SamplingDate: June 20, 2012Page: 2Project: East Hennepin Avenue Site

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The following sections describe the methods for the collection of the soil, groundwater, and soil gas

samples.

Soil

From soil borings DP001, DP002, and DP003, soil was collected continuously in acetate liners for

purposes of documenting soil characteristics and to field screen for organic vapors using a

photoionization detector. Soil boring DP001 was advanced to a depth of 25 feet below ground surface

(bgs) and soil borings DP002 and DP003 were advanced to a depth of 20 feet bgs.

Field screening activities did not indicate detectable chlorinated volatile organic compounds (CVOCs) in

soils, except for a PID reading of 0.3 ppm from a soil sample collected above the water table at DP001.

The stratigraphy observed in the borings DP001 and DP002 was consistent with past results. The

Decorah Shale was present at a relatively high elevation in boring DP003 which had not been seen in

previous borings downgradient of the site. As a result, the base of the unconsolidated aquifer is higher and

the aquifer thinner at this location.

Groundwater

Temporary groundwater monitoring wells were installed in soil borings DP001 and DP003 for the

purpose of collecting groundwater samples for laboratory analysis. Temporary monitoring wells were

constructed using 1-inch diameter PVC casing in each borehole. Temporary wells were purged with

disposable polyethylene tubing and silicone tubing that was connected to a peristaltic pump.

Groundwater samples were collected from temporary wells by stopping the peristaltic pump, removing

the tubing from the temporary well, and filling laboratory supplied vials from the bottom of the tubing.

Groundwater samples were submitted to TriMatrix Laboratories located in Grand Rapids, Michigan

(TriMatrix) and analyzed for the presence of TCE using EPA Method 8260B. The groundwater sample

collected from the glacial drift monitoring well B on January 13 was also submitted to TriMatrix and

analyzed for the presence of TCE using EPA Method 8260B.

To: Gary Krueger – MPCAFrom: Richard Manser – Barr EngineeringSubject: Soil Vapor SamplingDate: June 20, 2012Page: 3Project: East Hennepin Avenue Site

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The water table was measured at depths between 14.1 and 18.15 feet bgs. Results from the laboratory

analysis of groundwater samples showed detections of TCE in all samples, except DP001, the most

upgradient location, where TCE was not detected at a reporting limit of 1 ug/L. Groundwater analytical

results are shown on Figure 2.

Soil Gas

Separate temporary soil gas monitoring points were installed at locations DP001, DP002, and DP003 for

the purpose of collecting soil gas samples for laboratory analysis. Temporary soil gas monitoring points

were constructed using direct-push equipment, specifically the Post-Run Tubing System fabricated by

Geoprobe Systems, and sealed at the ground surface with hydrated bentonite. Soil gas samples were

collected from separate nearby borings to minimize interference between soil gas and groundwater

samples.

Soil gas samples were collected through the direct-push equipment using disposable teflon tubing

connected to stainless steel swagelock fittings, t-fittings, valves, and 1.4 liter summa canisters at the

ground surface. At each soil boring location, soil gas monitoring points were constructed to depths of 4

feet bgs, 8 feet bgs, and 3 feet above the water table as measured at the closest temporary monitoring well

or well B. Initial and final vacuum measurements were recorded for each summa canister. Sample valves

and fittings were decontaminated by running ambient air through the assembly for 6 minutes and tested

for vacuum integrity prior to collecting the soil gas samples. Each temporary soil gas monitoring point

was purged by removing three tubing volumes of soil gas with a disposable syringe. Summa canisters

were filled with soil gas at a rate of approximately 300 ml/minute. Soil gas samples were submitted to

Legend Technical Services location in Saint Paul, Minnesota and a split sample from one location

(DP003) was submitted to Pace Analytical Services in Minneapolis, Minnesota. Soil gas samples were

analyzed for TCE using Method TO-15.

Analytical results are summarized in Table 1 and shown on Figure 3. Results from the laboratory analysis

of the soil gas samples showed TCE detections in all samples except for, the soil gas samples collected

from a depth of 4 feet bgs where TCE was not detected at the laboratory reporting limit of 2.7 ug/m3,.

For each soil boring location, the TCE concentration measured in the soil gas sample collected from 8 feet

bgs was approximately one order of magnitude less than the TCE concentration measured in the soil gas

To: Gary Krueger – MPCAFrom: Richard Manser – Barr EngineeringSubject: Soil Vapor SamplingDate: June 20, 2012Page: 4Project: East Hennepin Avenue Site

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sample collected from a depth of 3 feet above the water table. The decrease in TCE concentration

indicates that attenuation appears is occurring between soil gas near the water table and soil gas collected

at 8 feet bgs and 4 feet bgs.

Strategies For Moving Forward

The goal of the next round of sampling will be to collect additional soil gas data to assess subsurface

conditions over a broader area to further assess the strategy for moving the soil gas sampling program

forward. Lessons learned from other soil vapor investigations conducted in MN along with the results

from the analysis of soil gas and groundwater samples collected at the East Hennepin site will be taken

into consideration as we move forward to understand the nature and extent of TCE in the subsurface.

We wish to confirm with MPCA and EPA that field data collection and laboratory analytical procedures

employed are appropriate for addressing the soil vapor conditions that exist at the East Hennepin Ave.

Site. We plan on utilizing the same soil vapor sampling protocol described above for the next phase of

soil vapor sampling. In particular, we will use the 1.4 liter summa canisters and anticipate a minimum

TCE detection level of 2.7 ug/m3. We plan once again to utilize local analytical laboratories to minimize

shipping of soil vapor samples.

The overall proposed scope and rationale of the next sampling phase is to install an additional 10

temporary borings to collect soil gas samples at 3 intervals consistent with the previous round of sampling

(depths of 4, 8, and 3 ft above water table) and collect groundwater samples at the water table from

temporary borings at these locations.

The locations of the proposed 10 monitoring locations that will be used in the next phase of the

investigation (Phase 2C) are shown in Figure 4.Barr’s approach to selecting the locations for the

additional soil gas samples included the following:

- Continue down the assumed spine of the plume; generally in a south/southwest direction throughMonitoring Wells 110 and S; focus on areas that have historical elevated concentrations of TCEin groundwater and assess corresponding concentrations in overlying soil gas

- Limited lateral transects to help identify lateral extent across plume in the vicinity of Well S- Perimeter samples to begin to help identify the outer edge of soil gas area

To: Gary Krueger – MPCAFrom: Richard Manser – Barr EngineeringSubject: Soil Vapor SamplingDate: June 20, 2012Page: 5Project: East Hennepin Avenue Site

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All soil gas and groundwater samples will be analyzed for TCE using the methods used in Phase 2B of

the investigation.

Table 1. Summary of Analytical Results from April 25, 2012 Sample Event