The soils in the area of the Missouri Electric Works (MEW) site property are somewhat permeable and the bedrock is highly fractured. Two distinct groundwater regimes have been identified at the site: (1) fractured bedrock and (2) alluvium below the wetland area. The groundwater is shallow (20 feet below ground surface [bgs]) in some areas. A wetland area is located immediately south of the site. The wetland area is underlain by saturated alluvium. The alluvium receives groundwater from the adjacent bedrock aquifer. Site runoff flows into Cape La Croix Creek, which discharges into the nearby Mississippi River (located about two miles from the Site). These conditions facilitate contaminant transfer into groundwater.
Targeted Environmental Media:
 Fractured Bedrock
Volatile organic compounds (VOCs), chlorinated hydrocarbons, and polychlorinated biphenyls (PCB) have been identified in site groundwater. Non aqueous phase liquid (NAPL) has not been identified at the site.
Major Contaminants and Maximum Concentrations:
- Trichloroethene (0 µg/L)
- Polychlorinated biphenyls (PCBs) (0 µg/L)
- Chlorobenzene (0 µg/L)
No technologies selected.
- Thermal Treatment (In Situ)
- Pump and Treat
- Other (Monitored Natural Attenuation (contingency))
Comments:
The site owner erected barriers to prevent further PCB migration by drainage ditches and conducted sampling of an on-site structure. The EPA determined that the owner did not adequately implement these activities, resampled the structure, and erected new barriers across the drainage ditches. In 1990, the EPA selected a remedy including on-site incineration of the PCB-contaminated soil and pumping and treating of the groundwater via air stripping (followed by carbon adsorption). After the site soils had been incinerated, further groundwater investigations were implemented. In 1994, EPA issued an Explanation of Significant Differences (ESD) changing the remedy to in situ thermal desorption at high temperature with off gas treatment. PCB-contaminated soils were excavated and stock-piled. Performance tests were conducted during October 1999, December 1999 and early Spring 2000 to provide a basis for development of the conceptual design. Performance test results identified a target treatment temperature of 100ýC in the interwell regions was needed to effectively achieve the remedial standards for TCE. Full-scale operation of the thermal desorber began during April 2000. The thermal treatment system consisted of 24 heater borings/wells. The heater-vacuum wells were placed around the perimeter of the treatment zone to ensure that vapors were pulled back towards the treatment zone and not pushed outward. In addition, the system consisted of a low-flow extraction system design system to lower the groundwater table within the treatment zone, thereby creating upward hydraulic gradients across the bottom of the treatment zone. Heaters were extended into the upper 10 to 15 feet of bedrock and the power output was increased in the bottom potion of the heaters to establish a ýhot floorý. This ýhot floorý helped provide a barrier to vertical migration of contamination. After 148 days of operation, the average temperature goal of 100ýC was achieved in the entire treatment volume.
Treatment of over 30,000 tons of PCB-contaminated soils was completed by July 25, 2002. Remedial action took longer than anticipated because additional PCB contamination was identified to depths greater than 20 feet bgs. The original remedial plan consisted of removing all trees located on the eastern perimeter of the site. However, Red-Tailed Hawks were nesting in some of the trees and a modification was made to protect this habitat.
The OU2 groundwater remedy was selected in 2005 and included 4 elements to address the site based on the fractured bedrock at the site. Groundwater in the fractured bedrock would be addressed with (1) a technical impracticability waiver due to the highly variable and fractured nature of the bedrock in the upland area of the site (waiver for specific ARARs); (2) institutional controls (ICs); (3) wellhead treatment systems (activated carbon or air stripper systems installed to remove contaminants of concern (COCs) from the drinking water supply); and (4) long-term monitoring of groundwater from bedrock wells. Groundwater in the alluvium would be addressed with ICs, wellhead treatment systems, monitoring, and enhanced biodegradation. The Record of Decision (ROD) also included monitored natural attenuation of the alluvium groundwater as a contingent remedy.
None provided
Soil contamination at the site has been addressed. Over 30,000 tons of PCB contaminated soil was treated to remove and destroy the PCBs. A vegetative cover was then established. The First Five-Year Review in 2004 concluded the soil remedy was protective of human health and the environment. It recommended continued groundwater monitoring to evaluate migration and whether natural attenuation was occurring. It also recommended further evaluation of the wetland to determine if ecological impacts are occurring that must be addressed. EPA and the PRPs began negotiating a new Consent Decree in 2009 which would address groundwater. Negotiations were still ongoing in 2011.
The 2004 five-year review indicated that ecological risk in the area south of the Missouri Electric Works (MEW) property needs to be evaluated. The second five-year review in 2009 found that the soil remedy was still protective, and the selected groundwater remedy was expected to be protective once it was fully implemented.
References:
EPA. Missouri Electric Works." Missouri Electric Works. Environmental Protection Agency. Web. 2012. March 29. Modified by Ben Washburn, EPA Region 7. http:www.epa.govregion7cleanupnpl_filesmod980965982.pdf
FRTR Web Site: http:webapp1.dlib.indiana.educgi-binvirtcdlibindex.cgi5315320FID1abstracts00000116.html%3B1
EPA. 2004. Five-Year Review for Missouri Electric Works Site.
EPA. 2009. Five-Year Review for Missouri Electric Works Site. http:www.epa.govsuperfundsitesfiveyearf2009070002989.pdf
Heron, Gorm. 2008. Use of Thermal Conduction Heating for the Remediation of
DNAPL in Fractured Bedrock. May 19-22. Presented Remediation of Chlorinated and Recalcitrant Compounds: Proceedings of the Sixth International Conference. http:www.terratherm.compdfwhite%20papersChlor708HeronPaper.pdf
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