First Environment supported the owners of the Former Potter Company in providing CERCLA remedial investigation/ feasibility study (RI/FS) services at their plant in Wesson, Mississippi. The site was proposed for inclusion on the U.S. Environmental Protection Agency’s (EPA’s) National Priorities List (NPL) and consists of a former industrial plant that manufactured electronic components from 1953 until the mid-1990s. The manufacturing process involved the use of degreasing materials, primarily trichloroethylene and polychlorinated biphenyls (PCBs) as dielectric oil in electronic components. Both the degreasing materials and PCBs were inadvertently discharged into the environment. The site is also located over the primary drinking water aquifer for the City of Wesson, with drinking water wells positioned within 1,000 feet of the plant site.
Site Scoping, Characterization and Preliminary Assessment
To gain a thorough understanding of prior operations which led to the contamination as well as the local and regional geologic framework, First Environment performed detailed initial site characterization activities that began with a comprehensive review of available documents and preliminary assessment of site-specific manufacturing activities; we also developed a Conceptual Site Model (CSM). We completed project scoping activities and developed comprehensive site and remedial investigation work plans to accurately characterize the subsurface in terms of chlorinated organic compounds migration, fate and transport both in soil and groundwater, and to delineate the extent of PCBs in shallow soils, stream sediments, and pond sediments on and off site. In 1988, First Environment conducted the first of four phases of investigation to delineate soil and groundwater contamination. In the first phase of investigation, over 600 soil samples were collected both on- and off-site, many of which were analyzed in the field by a portable gas chromatograph to control investigation costs and provide real-time data to guide the field investigation. This initial investigation identified two chlorinated solvent contamination source areas onsite and an area 7.3 acres in size (on-and off-site), including a private pond and stream sediments containing PCBs above site target cleanup levels. In addition, groundwater onsite was found to contain tetrachloroethene (PCE), trichloroethene (TCE) and their breakdown products above federal maximum contaminant levels (MCLs). The assessment activities led to the subsequent discovery of chlorinated VOCs in the City of Wesson public water supply wells. The public wells were taken out of service and replaced with two new wells located away from the VOC plume as part of an emergency interim response action.
First Environment performed three subsequent phases of investigations to complete the delineation of 14 on-site and off-site soil areas where soil was contaminated by PCBs. These subsequent soil delineation activities consisted of the collection and field screening analysis of hundreds of additional soil samples.
The groundwater investigation ultimately consisted of the installation of 50 monitoring wells on and off the Potter property to delineate a 48-acre chlorinated VOC plume (primarily TCE) contained within three groundwater zones ranging from a shallow surficial aquifer to a 350-foot deep high-yielding drinking water aquifer. First Environment conducted geophysical logging of nearly all the boreholes and numerous aquifer hydraulic conductivity tests to determine the interconnectivity of the groundwater zones. Assessment activities were completed in 1993.
Based on the fact that the site was located in a primarily residential area and both PCB and groundwater contamination was located on a number of private properties, and along a state waterway, First Environment attended frequent public meetings throughout the course of the project to address the status of the site investigation.
Evaluation of Remedial Alternatives
Once groundwater and soils were fully characterized, First Environment conducted a thorough evaluation of remedial alternatives and completed a feasibility study of options for remediation of the groundwater in 1994. For the evaluation, we took into account the remedial action objectives, ability to implement, short and long term effectiveness, protection of human health and the environment, potential migration pathways, potential impacts to receptors, site-specific geologic framework, and costs to implement. We selected groundwater recovery and treatment as the primary remedial option due to the high conductivity of the formation, deep extent of VOCs in the aquifer, and use of the formation as a high-yielding drinking water aquifer. The need to protect four existing and uncontaminated public supply wells located 2,500 feet downgradient of the site also factored in our final decision.
Due to the presence of PCBs in sensitive locations and on residential properties, First Environment worked with the Mississippi Department of Environmental Quality (MDEQ) and EPA Region 4 to develop a fast-tracked initial remedial action plan for PCBs and VOCs in surficial soils, which we implemented and completed in 1989. Upon completion of the soils assessment and interim abatement, First Environment identified remedial alternatives to control direct contact with the PCB-contaminated soil and off-site migration of PCB material, as well as protect public health and the environment from PCBs transported through runoff. The remedial alternative that best met these objectives was the excavation of PCB contaminated soil above 10 ppm and capping any remaining deeper soil that contained PCBs above 1 ppm. The full-scale remedial effort was initiated in 1992, concluded in 1993, and involved the removal of approximately 14,200 tons of PCB contaminated soil from the project area for off-site disposal. First Environment provided full-time oversight of the remedial activities.
Remedial Action Work Plan and Groundwater System Design
First Environment designed and constructed a 1 mgd groundwater recovery system consisting of 14 extraction wells and a treatment system consisting of two parallel air stripping units in 1994 and 1995 at the site. To assist the groundwater recovery and treatment systems, First Environment installed a soil vapor extraction (SVE) system at the site to address the potential vadose zone source area for VOC. The SVE system volatilized and extracted residual phase VOCs from vadose soils near the former plant site over a five-year period (1995-2000) until offgas TCE concentrations were negligible. Based on the system monitoring data, an estimated 300 pounds of VOC were removed from the soil. To date, the groundwater recovery and treatment system has removed approximately 4.7 billion gallons of TCE contaminated water and the downgradient public supply wells remain free of contaminants. Based on the system monitoring data, an estimated 3,400 pounds of TCE have been removed from the groundwater during the period of operation. First Environment has operated the treatment system since its construction and has been responsible for completion and submittal to MDEQ of all monitoring and remediation effectiveness reports since 1995.
Subsequent to the initial remedial responses which were driven by public safety considerations, First Environment conducted additional investigation and full scale treatability studies to address pockets of potential source area DNAPL and recalcitrant VOCs located in an isolated, poorly conductive formation near the former source area. First Environment evaluated vacuum-enhanced groundwater extraction, in situ chemical oxidation, and air sparging technologies. The enhancement approved by MDEQ includes in situ chemical oxidation combined with vacuum-enhanced groundwater extraction in an area of approximately 100 by 200 feet. Early results show the combined treatment has reduced source concentrations by two-thirds to date and the action is expected to significantly shorten the operational period for the groundwater recovery system.