The die-casting facility is located approximately 1,000 feet to the northwest of Town Branch. Creek bottom materials consist of karstic limestone that is covered by a thin layer of silty to gravelly sediments. The Town Branch Creek empties into the Mud River flowing from south to north. PCB DNAPLs from the site enter the creek channel in two locations seeping from the bedrock.
Targeted Environmental Media:
 Dense Non-aqueous Phase Liquids (DNAPLs) - Fractured Bedrock
Not identified in the references cited.
Major Contaminants and Maximum Concentrations:
- Polychlorinated biphenyls (PCBs) (8 µg/L)
No technologies selected.
- Other (Multi-layered cap over gravity fed DNAPL collection system)
Comments:
A multi-layered cap and DNAPL collection system were completed in 2005. The multi-layer cap consisted of three zones: (1) a top zone promotes ecological restoration; (2) a middle zone isolates the creek to the top of the rock both hydraulically and physically; and (3) a third zone collects DNAPL and provides hydraulically control immediately above the rock top. Caps consist of compacted clay, concrete geoweb or pre-cast concrete, HDPE geomembrane, gravel, and geotextile. DNAPL is collected in horizontal drains located within gravel at the top of the bedrock and is collected in fluid-collection standpipes equipped with sealed DNAPL sumps. The drains also convey groundwater through a pumping system to a treatment plant.
Reduce PCB concentrations in fish to meet the state consumption advisory level in fish ý to allow 6 meals/year.
Between 2006 and 2010, more than 35 liters of DNAPL have been recovered. Total PCB concentrations have been reduced from a maximum of 8.3 microgram/liter (ug/L) to less than 0.5 ug/L in surface water. Total PCB concentrations in whole-body sunfish have been reduced substantially from 206 mg/kg to 14 mg/kg. Total PCB concentrations in fish fillet concentrations have reduced from 20 mg/kg in 2003 to 6 mg/kg in 2009 and samples of fish downstream of the remediation meet consumption advisory level.
Remediation system continues to function well, over 4 years after initial installation.
References:
White, Keith; Michael J. Gefell; and Majo Thurman. 2010. Remedy for PCB DNAPL Seeps in a Fractured Bedrock Creekbed. From Seventh International Conference on Remediation of Chlorinated and Recalcitrant Compounds, May 2010, Monterey, CA.
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