Pump and Treat of Contaminated Groundwater at the Mystery Bridge at Hwy 20 Superfund Site, Dow/DSI Facility, Evansville, Wyoming

Site Name:

Mystery Bridge at Hwy 20 Superfund Site


Evansville, Wyoming

Period of

- Status: Completed (Pump and treat temporarily shut down; MNA studies ongoing)
- Period of Operation: June 1994 through March 2001
- Data Collected: November 1989 through October 1997


Full-scale cleanup (interim results)


Thomas J. Mueller, P.E.
Technology vendor
Western Water Consultants, Inc.
611 Skyline Road
P.O. Box 4128
Laramie, WY 82071
(307) 742-0031

Pump and Treat and Soil Vapor Extraction

- Groundwater was extracted using 3 wells, located on site, at an average total pumping rate of 103 gpm
- Extracted groundwater was treated with air stripping and reinjected using an infiltration trench with 600 ft of surface area
- SVE was used as a source control activity
- MNA currently in progress

Cleanup Authority:
CERCLA Remedial
- ROD Date: 9/24/90

Rebecca Thomas
U.S. EPA Region 8
999 18th Street, Suite 500
Denver, CO 80202-2466
(303) 312-6552

Don Fisher
Solid and Hazardous Waste Division
Wyoming Dept. of Environmental Quality
1222 W. 25th Street
Cheyenne, WY 82002
(307) 672-6457

Chlorinated solvents
- Maximum concentrations detected in September 1989 were trans-1,2-DCE (500 µg/L), TCE (430 µg/L), PCE (540 µg/L), and 1,1,1-TCA (500 µg/L)

Waste Source:
Various contaminant releases, spills, and leaks

Type/Quantity of Media Treated:
- 192.8 million gallons treated as of December 1997
- Groundwater is found at 14-42 ft bgs
- Extraction wells were located in 1 aquifer at the site
- Hydraulic conductivity was reported as 340 ft/day

Purpose/Significance of Application:
Remedial strategy included use of pump and treat for the on-site plume and natural attenuation for the off-site plume.

Regulatory Requirements/Cleanup Goals:
The remedial goal is to reduce the levels of contaminants in the on-site, up-gradient portion of the groundwater plume to below MCLs such that the remainder of the plume off site meets MCLs through natural attenuation within a reasonable time limit. - Remedial goals were established for TCE (5 µg/L), PCE (5 µg/L), trans-1,2-DCE (100 µg/L), cis-1,2-DCE (70 µg/L), 1,1-DCE (7 µg/L), and 1,1,1-TCA (200 µg/L).

- Contaminant concentrations in all wells have declined significantly, yet remain above MCLs. Concentrations of contaminants in three out of four source area wells fell below their respective MCLs in the last two sampling events in 1996; in the fourth well, the total contaminant concentration was 9.4 µg/L.
- Wells in the down-gradient portion of the plume declined from March 1993 to December 1996, but in at least one well (225 ft down-gradient of the site boundary) individual contaminant concentrations remain significantly above their respective MCLs.
- Approximately 21 pounds of contaminants have been removed from the groundwater at this site.

Cost Factors:
Actual costs for groundwater remediation were $918,000 ($305,000 in capital and $613,000 in O&M), which correspond to $5.65 per 1,000 gallons of groundwater extracted and $44,000 per pound of contaminant removed.

Since 1958, the Dow/DSI facility was used as a base for oil field service operations. Dow/DSI used mobile pumps, tanks, and other equipment to perform services for the oil and gas industry. It is believed that wash water from equipment cleaning operations contained chlorinated solvents. In addition, a tank at the site was used to store large volumes of toluene, which was used for cleaning purposes and oil well servicing activities. In 1986, residents complained of poor water and air quality. In response, EPA conducted an Expanded Site Investigation, which led to the discovery of contaminants in the groundwater. The site was placed on the NPL in August 1990 and a ROD was issued in September 1990.

The remedial strategy at this site was to actively treat the on-site groundwater plume using pump and treat with air stripping, and to allow natural attenuation to reduce contaminant levels in the off-site portion of the plume to levels below the MCLs. The pump and treat system operated continuously from June 1994 to March 2001. In March 2001, the system was discontinued due to the discovery of a layer of Light Non-Aqueous Phase Liquid (LNAPL) in one of the monitoring wells. The water surface in the re-injection tank in the pump and treat building had developed an apparent oily film. Results of analyses indicate that the LNAPL is most likely weathered lean oil and is most likely drainage of residual product from the pore spaces in the deep unsaturated zone during a period of low groundwater levels at the site. The pump and treat system has not been restarted. Quarterly groundwater monitoring continues at the site and has indicated that contaminant concentrations have decreased but remain above cleanup goals.

A five-year review was conducted in September 2004 and indicated that PCE concentrations remain above cleanup goals. However, the review found that, "The remedy as implemented is currently protective of human health and the environment." The review recommends that the site implements institutional controls on groundwater use and continues the groundwater monitoring program.

A five-year review is scheduled to take place in August 2009.