A.76 Twelve-Mile Creek/Lake Hartwell, Pickens County, SC
Sangamo Weston, Inc., Twelve-Mile Creek, Lake Hartwell, Operable Unit 2
A.76.1 Contacts
Craig Zeller
USEPA
A.76.2 Summary
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Environment: |
Freshwater lake |
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Scale: |
7 mile stretch of Twelve-Mile Creek and 730 acres of the Twelve-Mile Creek arm of Lake Hartwell |
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Contaminants of Concern: |
PCBs |
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Source Control Achieved Prior to Remedy Selection? |
Yes |
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Final Remedy: |
MNR for Twelve-Mile Creek arm of Lake Hartwell |
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Expected Recovery Time: |
12 years |
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MNR viewed as a success? |
Yes |
A.76.3 Site Description
Primary Pathway: Human health via fish consumption, benthic
Primary source(s): The primary source of the contaminants is from a company named Sangamo Weston who manufactured electrolytic mica and power factor capacitors from 1955 to 1978. PCB use at this plant was terminated in 1977.
Location: The Sangamo Weston, Inc. /Twelve-Mile Creek/Lake Hartwell PCB Contamination Site is located in Pickens County, South Carolina. It is made up of the Sangamo property, portions of Twelve-Mile Creek, and the Twelve-Mile arm of Lake Hartwell.
The affected areas of sediment are a 7-mile stretch of Twelve-Mile Creek and 56,000-acre Lake Hartwell (artificial reservoir created by the construction of Hartwell Dam across the Savannah River) (USEPA 2004b). Twelve-Mile Creek is the primary tributary into the headwaters of the lake and contains three masonry impoundments (private dams) along its length. Sediment in both Twelve-Mile Creek and Hartwell Lake contains PCBs that originated from a Sangamo Weston capacitor plant that discharged PCB-containing wastewater into Town Creek, a tributary to Twelve-Mile Creek. From 1955 to 1977, a yearly average amount of PCBs used at the Sangamo Plant in Pickens County, SC ranged from 700,000 to 2,000,000 pounds. It is estimated that 3%, approximately 400,000 pounds, of the quantity of PCBs used at the plant ended up being discharged into Town Creek.
Sediment PCB concentrations in the lower 7-mile stretch of Twelve-Mile Creek, interchangeably known as the Twelve-Mile Creek Arm and Seneca Creek Arm, and a depositional area, were originally measured in the 1–3 ppm range at the surface and higher in deeper sediments. Portions of the Twelve-Mile Creek Arm were found to contain up to 61 ppm PCBs. In 1991/92, maximum PCB concentrations measured in sediment core samples from the upper section of Lake Hartwell (where Twelve-Mile Creek enters) exhibited concentrations of 5–11 ppm; PCB concentrations in sediment in the lower part of the lake were typically < 1 ppm.
A.76.4 Remedial Objectives
In June 1994, a ROD was issued for the site that specified MNR supplemented by institutional controlsNon-engineered instruments, such as administrative and legal controls, that help minimize the potential for human exposure to contamination and/or protect the integrity of the remedy. as the selected remedy. The selected target cleanup standard for sediment was 1 ppm PCBs based on technical feasibility; the affected area covers approximately 730 acres with a total estimated volume of 4.7 million yd3 of PCB-contaminated sediment. For fish, the FDA action level of 2 ppm PCBs was selected, also based on technical feasibility. A carcinogenic risk-based approach was evaluated by determining the concentration levels in largemouth bass that would result in acceptable risk to anglers through ingestion of fish. Using USEPA risk assessment methods, a fish tissue concentration of 0.036 ppm was associated with a 10–4 risk. The risk-based fish cleanup goal of 0.036 ppm was determined to be technically impracticable. Natural recovery of largemouth bass within Hartwell Lake to below the FDA action level of 2 ppm PCBs was predicted by modeling to occur within 12 years (by 2004).
A.76.5 Remedial Approaches
Sediment cores were collected in Lake Hartwell and provided data used to determine the vertical profile of PCBs in the sediment column. These data indicated that higher PCBs were being buried beneath sediment with lower PCB concentrations. Sediment Transport Modeling predicted net sediment accumulation in the lake to be 5 to 15 cm/yr. Two long-term fate and bioaccumulationThe accumulation of substances, such as pesticides, or other organic chemicals in an organism. Bioaccumulation occurs when an organism absorbs a toxic substance at a rate greater than that at which the substance is lost. Thus, the longer the biological half-life of the substance the greater the risk of chronic poisoning, even if environmental levels of the toxin are not very high. models were constructed to enable predictions of PCB concentrations in sediment and fish in Lake Hartwell over time under various potential remedial approaches. A water-quality model was developed to determine the fate of PCBs in the system over time, and results of this model indicated that PCB concentrations in the water column1) The basic habitat and the medium through which all other fish habitats are connected; 2) a conceptual column of water from surface to bottom sediments. This concept is used chiefly for environmental studies evaluating the stratification or mixing (such as by wind induced currents) of the thermal or chemically stratified layers in a lake, stream or ocean. Some of the common parameters analyzed in the water column are: pH, turbidity, temperature, salinity, total dissolved solids, various pesticides, pathogens and a wide variety of chemicals and biota. Understanding water columns is important, because many aquatic phenomena are explained by the incomplete vertical mixing of chemical, physical or biological parameters. For example, when studying the metabolism of benthic organisms, it is the specific bottom layer concentration of available chemicals in the water column that is meaningful, rather than the average value of those chemicals throughout the water column. and sediment of Lake Hartwell would generally decrease over time, even in the absence of any intrusive remediationThe act or process of abating, cleaning up, containing, or removing a substance (usually hazardous or infectious) from an environment.. The primary mechanisms for PCB reductions over time were boundary transport and burial. A bioaccumulation model was also constructed to complement the water-quality model and to estimate PCB concentrations in fish tissue over time. The results from this model indicated that largemouth bass PCB levels would decrease to < 2 ppm (in fish weighing greater than 3.4 kg) in 12 years under an MNR scenario. Results from these models were used in establishing the ROD for the site.
A.76.6 Monitoring
Annual biota and sediment monitoring has been implemented in the spring of each year since 1994. This effort has included (1) surface sediment sampling at 21 locations in Twelve-Mile Creek and Lake Hartwell; (2) fish tissue analyses at six stations in Lake Hartwell for largemouth bass, catfish, and hybrid bass; (3) fish tissue analyses on forage fish species at three locations in Lake Hartwell; and (4) 28-day caged corbicula analyses at seven stations in Twelve-Mile Creek. The 2004 USEPA Five-Year Review reported, sediment data indicate that surficial sediment PCB concentrations in Twelve-Mile Creek have decreased steadily since 1990 due to ongoing physical processes such as burial, mixing/dispersion1) Pollutant or concentration mixing due to turbulent physical processes; 2) A distribution of finely divided particles in a medium., and PCB dechlorination. However, the same USEPA Five-Year Review concluded that, although sediment concentrations continue to measurably decrease, PCB concentrations in largemouth bass, channel catfish, and hybrid bass have not responded as measurably to the decreased surface sediment trends.
A 2006 technical agreement between the Natural Resource Trustees and the principal responsible party, Schlumberger Technology Corporation, requires, among other things, the removal of two of three dams (Woodside 1 and 2) on the Twelve-Mile Creek Arm of Lake Hartwell. An Explanation of Significant Differences was issued in 2009 to support this aspect of the project as it is expected to enhance the ongoing natural transport of clean sediment downstream to speed burial of the PCB-contaminated sediment in Lake Hartwell. The removal of these two dams was completed as of 2011.
Publication Date: August 2014