Description
Historical activity that resulted
in contamination.
| This 37-acre site has been occupied by retail shopping mall since 1960. The site is zoned for commercial land use and is located in a commercial district. A PCE release occurred from a former drycleaning business that operated in the center. The drycleaner had two large in-ground cement tanks for storing PCE, and over a period of many years these tanks leaked. The sanitary sewer line that served the dry cleaner also served as a source of PCE contamination to the underlying soil and groundwater. |
Contaminants
Contaminants present and the highest amount
detected in both soil and groundwater.
| Contaminant |
Media |
Concentration (ppb) |
Nondetect |
| Benzene |
groundwater |
22 ppb |
|
| cis-1,2-Dichloroethene |
groundwater |
100 ppb |
|
| ethylbenzene |
groundwater |
120 ppb |
|
| methylene chloride |
groundwater |
34 ppb |
|
| methylene chloride |
soil |
1,300 ppb |
|
| Tetrachloroethene (PCE) |
groundwater |
41,000 ppb |
|
| Tetrachloroethene (PCE) |
soil |
11,000,000 ppb |
|
| Trichloroethene (TCE) |
groundwater |
840 ppb |
|
Site Hydrology
| Deepest Significant
Groundwater Contamination: |
|
|
| Plume Size: |
|
Plume Length: 250ft
|
| Average Depth
to Groundwater: |
|
29.25ft |
Lithology and Subsurface Geology
|
|
Fine to medium, red-brown sand
Depth: 0-11ft bgs
11ft thick
Conductivity: 6.5ft/day
Gradient: 0.01ft/ft
|
|
|
Silty to clayey moderately stiff, red-brown till
Depth: 11-22ft bgs
11ft thick
|
|
|
Silty, fine to medium, tan-brown sand
Depth: 22-26ft bgs
4ft thick
|
|
|
Dry, stiff to very dense, clayey, fine to medium, gray sand
Depth: 26-28ft bgs
2ft thick
|
|
|
well sorted fine to medium gray sand, which is laterally discontinuous and averages less than 3 ft in thickness
Depth: 28-31ft bgs
3ft thick
|
Decorah shale
|
|
shale bedrock
Depth: 31ft bgs
|
Pathways and DNAPL Presence
 Groundwater
 Sediments
Soil
DNAPL Present
|
Remediation Scenario
Cleanup
Goals: |
|
Groundwater: Treatment required until a soil cleanup goal of 5 mg/L PCE is achieved and a steady state level of PCE in groundwater is achieved.
Soil: 19,900 µg/kg
|
Technologies
In Situ Air Sparging |
|
Why the technology was selected:
Because these alternatives were found to meet the remedial objective for the project while being less costly, more readily implemented and achieve the remedial objectives sooner than the other alternatives.
Date implemented:
February 3, 1999
Final remediation design:
Multi-phase extraction (MPE) wells were placed in Area 1 (near the source area) of the site, while air sparging (AS)/soil vapor extraction (SVE) was implemented in Area 2 (100-200 ft downgradient of the source area). There were a total of 10 MPE wells, 7 SVE wells, and 2 AS wells.
The Area 2 system operated for one year before being dismantled. The Area 1 system operated for 3-4 years before shut down. The remedial plan specified that the Area 1 system should be operated until PCE levels in soil reached the MPCA's risk-based cleanup criteria for PCE in soil, and until the PCE levels in groundwater reached a point of diminishing returns, i.e., an asymptotic contaminant mass recovery rate was reached for groundwater.
Results to date:
Mass removal, for both MPE and SVE from Feb 2, 1999 through June 8, 2001, was 2,313 lbs. Average daily contaminant recovery declined from a high of 22 lb/day to 0.2 lb/day (by Jan 2001). Exponential decay analysis indicated that the average daily recovery reached an asymptotic limit, which satisfied the first stage of the clean up goals. The MPE system was authorized to be shut down on December 11, 2001.
The AS/SVE system was shut down after 1 year of operation, based upon an agreement between the MPCA and the owners of the Midway Plaza Shopping Center to facilitate continuing construction at the site.
As specified in the remedial plan, follow-up groundwater monitoring was done to check for rebound of PCE levels in groundwater, and soil was sampled to identify PCE concentrations. An acceptable level of PCE was found in soil, and the degree of rebound was minimal.
The MPCA staff approved a permanent remedial system shutdown in September 2003, along with approval to abandon monitoring and extraction wells.
|
In Situ Multi Phase Extraction |
|
Why the technology was selected:
Because these alternatives were found to meet the remedial objective for the project while being less costly, more readily implemented and achieve the remedial objectives sooner than the other alternatives.
Date implemented:
February 3, 1999
Final remediation design:
Multi-phase extraction (MPE) wells were placed in Area 1 (near the source area) of the site, while air sparging (AS)/soil vapor extraction (SVE) was implemented in Area 2 (100-200 ft downgradient of the source area). There were a total of 10 MPE wells, 7 SVE wells, and 2 AS wells.
The Area 2 system operated for one year before being dismantled. The Area 1 system operated for 3-4 years before shut down. The remedial plan specified that the Area 1 system should be operated until PCE levels in soil reached the MPCA's risk-based cleanup criteria for PCE in soil, and until the PCE levels in groundwater reached a point of diminishing returns, i.e., an asymptotic contaminant mass recovery rate was reached for groundwater.
Results to date:
Mass removal, for both MPE and SVE from Feb 2, 1999 through June 8, 2001, was 2,313 lbs. Average daily contaminant recovery declined from a high of 22 lb/day to 0.2 lb/day (by Jan 2001). Exponential decay analysis indicated that the average daily recovery reached an asymptotic limit, which satisfied the first stage of the clean up goals. The MPE system was authorized to be shut down on December 11, 2001.
The AS/SVE system was shut down after 1 year of operation, based upon an agreement between the MPCA and the owners of the Midway Plaza Shopping Center to facilitate continuing construction at the site.
As specified in the remedial plan, follow-up groundwater monitoring was done to check for rebound of PCE levels in groundwater, and soil was sampled to identify PCE concentrations. An acceptable level of PCE was found in soil, and the degree of rebound was minimal.
The MPCA staff approved a permanent remedial system shutdown in September 2003, along with approval to abandon monitoring and extraction wells.
|
In Situ Soil Vapor Extraction |
|
Why the technology was selected:
Because these alternatives were found to meet the remedial objective for the project while being less costly, more readily implemented and achieve the remedial objectives sooner than the other alternatives.
Date implemented:
February 3, 1999
Final remediation design:
Multi-phase extraction (MPE) wells were placed in Area 1 (near the source area) of the site, while air sparging (AS)/soil vapor extraction (SVE) was implemented in Area 2 (100-200 ft downgradient of the source area). There were a total of 10 MPE wells, 7 SVE wells, and 2 AS wells.
The Area 2 system operated for one year before being dismantled. The Area 1 system operated for 3-4 years before shut down. The remedial plan specified that the Area 1 system should be operated until PCE levels in soil reached the MPCA's risk-based cleanup criteria for PCE in soil, and until the PCE levels in groundwater reached a point of diminishing returns, i.e., an asymptotic contaminant mass recovery rate was reached for groundwater.
Results to date:
Mass removal, for both MPE and SVE from Feb 2, 1999 through June 8, 2001, was 2,313 lbs. Average daily contaminant recovery declined from a high of 22 lb/day to 0.2 lb/day (by Jan 2001). Exponential decay analysis indicated that the average daily recovery reached an asymptotic limit, which satisfied the first stage of the clean up goals. The MPE system was authorized to be shut down on December 11, 2001.
The AS/SVE system was shut down after 1 year of operation, based upon an agreement between the MPCA and the owners of the Midway Plaza Shopping Center to facilitate continuing construction at the site.
As specified in the remedial plan, follow-up groundwater monitoring was done to check for rebound of PCE levels in groundwater, and soil was sampled to identify PCE concentrations. An acceptable level of PCE was found in soil, and the degree of rebound was minimal.
The MPCA staff approved a permanent remedial system shutdown in September 2003, along with approval to abandon monitoring and extraction wells.
|
Costs
Cost
for Assessment:
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Cost
for Operation and Maintenance:
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Total
Costs for Cleanup:
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Lessons Learned
1. The multi-phase extraction process was able to adequately remove a significant amount of contaminant mass from soil and groundwater.
2. The site geology complicated the system design, because the soil is primarily a fine-grained till material, and the aquifer is deep and thin (approximately 35-40 feet bgs).
3. The site cleanup decisions were made during a time period when the MPCA was taking a more cautious approach to groundwater cleanup than typically occurs at present. The lower perceived risk to potential groundwater receptors posed by this site might have resulted in a different outcome if site decisions were being made today. Perhaps the remedial option chosen would have been source area cleanup and long-term monitoring of the plume to demonstrate natural attenuation, rather than active groundwater cleanup.
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Contacts
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