Risk-Based Management Strategies And Innovative Remedies For Surface Water Protection: A Case Study

S. Grant Watkins, P.G., Kristen Wandland, and Brian Ray, ENSR Corporation, 7041 Old Wake Forest Road, Suite 103, Raleigh, NC  27616, Tel: 919-872-6600, Fax: 919-872-7996

Releases of chlorinated aliphatic hydrocarbons including trichloroethene have impacted soils and groundwater at a facility in the Piedmont of South Carolina. Migration of groundwater plumes from on-site sources has contaminated tributaries in drainage sub-basins surrounding the facility, where up to 4,600 mg/L trichloroethene were detected in surface waters. Studies and modeling of the stream flow and aquifer were performed to define interactions of groundwater to surface water and to facilitate selection of surface water protection remedies.

Part of the groundwater plume migrates beneath a watershed divide and discharges into a storm water control system comprised of underground culvert pipes and a retention pond. Surface water studies on this system, combined with flow modeling, demonstrated that >99 percent of chlorinated compounds in the pond and downstream tributary originate from infiltration of contaminated groundwater into piping upstream of the pond. A surface water protection plan was developed using a risk-based approach. This approach, which was approved by the state regulatory agency, proposed plume containment by the subsurface pipe, and in-situ treatment and mixing of water in the pipe and pond. Risk-based components of this remedy included fencing the pond to restrict access, developing wildlife-based water standards for the pond, and using health-based surface water standards downstream of the pond. A successful short-term pilot test was conducted using air diffusion and ozone injections for direct treatment of water inside the culvert pipe.

For tributaries west and south of the site, the current remedy involves passive biological barriers around the tributaries using enhanced reductive dechlorination techniques. This treatment process is accomplished by injections of Hydrogen Release Compound® as a source of lactic acid for a substrate electron donor. Trichloroethene concentrations have been reduced in surface water and in the surrounding aquifer by 43-99 percent. Methods of optimizing the in-situ barriers are being evaluated.

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