1. East Hennepin Avenue Site
Presented to MPCA and EPA
December 12, 2011

2. Overview
Site History
Closure Evaluations
Pump Out System Shutdown – One Year of Monitoring Results
Recent Vapor Intrusion Investigation Methods & Results
Next Steps

3. Site History
Located in residential neighborhood just north of U of MN campus
GMI research laboratory 1930s s
Solvents disposed of on site 1940s -1960s
Property sold in 1977 – currently mixed commercial/industrial use at site buildings
I-35W
Dinkytown
U of MN

4. Site History – Characterization
Began site characterization work and disposal pit excavated in early 1980s
Solvents impacted shallow groundwater (glacial drift) and uppermost bedrock aquifers with VOCs, primarily TCE
Monitoring well network in four hydrogeologic units
Shallow groundwater plume extends off site to the south-southwest
1984 Response Order by Consent between GMI and MPCA

5. Site History – Generalized Geologic Section

6. Site History – Groundwater Remediation
Operation of pump out systems in glacial drift and uppermost bedrock began in 1985 – operated continuously since then
Objectives were to contain the plume and reduce concentrations
GMI began pursuing site closure and delisting in 2010 – MPCA concurred and requested several closure evaluations
Pump out systems shut down September 2010 with one year of subsequent groundwater monitoring
Onsite air stripper tower for groundwater treatment

7. Closure Evaluations Groundwater Soil Vapor Intrusion
Monitor groundwater for one year after pump out system shutdown
1997 well survey indicated no potable use nearby – to be updated before closure
Recent MPCA surface water assessment indicated no concern for adverse impact to Mississippi River
Soil
2001 onsite soil investigation indicated no concern
Vapor Intrusion
1997 vapor survey at onsite building basements/tunnels and nearby buried utilities using field instruments (i.e., PID) indicated no concern
Additional vapor intrusion work completed in November 2011 based on recent MPCA guidance
Institutional Controls
BBD Holdings, Inc. (current Property owner) placed a Restrictive Covenant in 2004 – to be updated following closure

8. Groundwater Monitoring after Shutdown
Pump out systems in glacial drift
Two wells (109, 110) treated with onsite air stripper and discharged to storm sewer
Three downgradient wells (111, 112, 113) discharge directly to storm sewer
Pump out systems in Magnolia Member of the Platteville Formation
Two on-site pump out wells (MG-1 and MG-2) discharge directly to storm sewer

9. Groundwater Monitoring after Shutdown
Four rounds of quarterly monitoring in all pump out wells and selected “sentinel” glacial drift and Magnolia Member wells to monitor any plume migration
September 2010 through June 2011
Continued annual (one round) of monitoring at other wells including Carimona Member and St. Peter Sandstone

10. Groundwater Monitoring after Shutdown – Groundwater Elevations
Flow direction in glacial drift remained to the southwest as was the case both before and during pump out system operation
Flow direction back to pre-pump out system operation directions in the Magnolia Member (northwest)
Water levels similar to previous years with exception of pump out wells (water levels increased)

11. Groundwater Monitoring after Shutdown – TCE Concentrations
Groundwater Flow Direction

12. Groundwater Monitoring Before & After Shutdown – TCE Concentrations
Sept. 2010

13. Groundwater Monitoring Before & After Shutdown – TCE Concentrations
Sept. 2010

14. Groundwater Monitoring after Shutdown – TCE Concentrations
Groundwater Flow Direction

15. Groundwater Monitoring Before & After Shutdown – TCE Concentrations
Sept. 2010

16. Groundwater Monitoring after Shutdown
Generally, data is favorable
TCE concentrations stable or decreasing in most glacial drift and Magnolia Member wells
Immediate increase in concentration at glacial drift well V following shutdown – concentrations stable since then
One round of monitoring in 2012 would be useful to continue to evaluate effect of pump out system shutdown

17. Simplified Vapor Intrusion Model
MPCA has developed screening values for volatile contaminants in groundwater, soil gas, and indoor air
TCE in groundwater: 20 ug/L
TCE in soil gas: 30 ug/m3
TCE in indoor air: 3 ug/m3
Describe general concept.
From MPCA August 2010 Vapor Intrusion Technical Support Document

18. VI Evaluation Steps in September 2011 Work Plan
Step 1 – Groundwater Screening and Receptor Survey
determine if vapor intrusion should be further evaluated based on site conditions and existing groundwater data
Step 2 – Soil Gas Investigations
2A – Soil gas investigation work using existing monitoring well network (if needed based on results of Step 1)
2B – Soil gas investigation on site (if needed based on results of Step 2A)
2C – Soil gas investigation in City R-O-W (if needed based on results of Steps 2A and/or 2B)
Step 3 – Building Specific Investigations (if needed based on results from Steps 2B and 2C)
Step 4 – Response Actions at Buildings (if needed based on results from Step 3)

19. Groundwater Screening and Receptor Survey
Assumed Property Use
# of Properties (Estimated)
Residential
304
School/day care 2
Recreational (Van Cleve Park)
1 (4 parcels)
Commercial 6
Industrial (the Site) 1
Vacant/undeveloped (no structures) 19
TOTAL
336
Describe potential magnitude of problem – area shown in yellow is approximately 78-acres.

20. Surrounding Neighborhood

21. Surrounding Neighborhood

22. Evaluation Steps in September 2011 Work Plan
Step 1 – Groundwater Screening and Receptor Survey -> done as part of Work Plan
Step 2 – Soil Gas Investigations
2A – Soil gas investigation work using existing monitoring well network -> needed based on results of Step 1 – work completed November 2011
2B – Soil gas investigation on site (if needed based on results of Step 2A)
2C – Soil gas investigation in City R-O-W (if needed based on results of Steps 2A and/or 2B)
Step 3 – Building Specific Investigations (if needed based on results from Steps 2B and 2C)
Step 4 – Response Actions at Buildings (if needed based on results from Step 3)

23. Vertical Profiling Investigation at Existing Groundwater Well
Recent Investigation Methods & Results – Part 1: Vertical Groundwater Profiling
Investigation to determine if TCE concentrations vary with depth in the shallow groundwater
If shallowest water is “clean” VI might not be a concern
Used passive bags in existing groundwater wells and analyzed water in bags for TCE
Well cap
Wire hanging assembly for sampler bags
Ground surface
Soil column
Groundwater table
Sampler bags deployed for two weeks at varying depths
Weight
Vertical Profiling Investigation at Existing Groundwater Well

24. Soil Gas Sampling at Existing Monitoring Well
Recent Investigation Methods & Results – Part 2: Soil Gas Sampling at Existing Wells
Collect soil gas samples from existing monitoring wells
Results better evidence of potential VI risk than groundwater sampling but are worst-case since collected at the water table
Groundwater table
Seal
Ground surface
Vacuum canister to collect soil gas sample after purging with vacuum pump
Soil column
Soil gas drawn into well screen under vacuum
Note that MPCA suggested this method/step
Soil Gas Sampling at Existing Monitoring Well

25. Soil gas sampling at monitoring well S

26. Recent Investigation Methods – Sample Locations
Wells selected based on construction (screen length, length of exposed screen) and location (inside of plume, outside of plume). Sampling at existing wells to minimize intrusion/disturbance of nearby neighborhood.

27. Results – Summary Table
CAN PROBABLY DELETE THIS SLIDE – REDUNDANT WITH FOLLOWING FIGURE. Groundwater ISV is 20 ug/L. Some variation with TCE concentration with depth.
10x ISV is 30 ug/m3 (more investigation per our workplan) – shown in pink. Building specific investigations required at 100x ISV (300 ug/m3). Acute ISV is 2,000 ug/m3 (potential short term exposure health risks in indoor air). Remind that our results are not “true” soil gas results but are “worst-case” screening results.

29. Next Steps
Work Plan calls for onsite soil gas investigation in soil column if Step 2A results exceed screening values
One round of groundwater monitoring in 2012 to continue to evaluate effect of pump out system shutdown

30. Questions/Discussion