Unconditional Closure of a Chlorinated Solvent Disposal Area at the Pinellas County, Florida, Site - 21229

The 4.5 Acre Site was originally a part of the US Department of Energy (DOE)-owned Pinellas Plant (now the Pinellas County, Florida, Site), but was sold in 1972 and has remained privately owned since then. Chlorinated solvents were discovered in site groundwater in 1984, leading to the execution of a remediation agreement between DOE and the US Environmental Protection Agency in 2000 under Comprehensive Environmental Response, Compensation, and Liability Act authority. The Florida Department of Environmental Protection (FDEP) was granted Resource Conservation and Recovery Act authority in 2000 and assumed enforcement authority for the site at that time. A source removal effort in 1985 consisting of excavating previously disposed drums and contaminated soil was initially thought to be successful, but left much contaminant mass in place. Subsequent voluntary corrective actions for contaminant mass reduction included soil and drum removal, groundwater pumping with ex situ treatment, dual-phase extraction, bio-sparging, and a second pump-and-treat system. These actions were also partially successful, but persistent concentrations of trichloroethene (TCE) in the groundwater indicated that the source was still in place, potentially including dense nonaqueous phase liquids. A high-resolution characterization effort in 2007 delineated the location of the remaining mass, and an extensive soil excavation event in 2009 successfully removed the contaminant source using large diameter augers. However, residual dissolved-phase contamination remained around the perimeter of the former source areas in the downgradient portions of the surficial aquifer, at low concentrations above the cleanup standards. DOE identified enhanced in situ bioremediation as the preferred technology to cost effectively address the remaining low-level contaminant mass by accelerating the naturally occurring biodegradation process. Specifically, DOE chose to inject a mixture of emulsified soybean oil and the bacteria Dehalococcoides mccartyi, a microbe proven to degrade TCE to benign byproducts. Concurrent with the bio-injection corrective actions, DOE supported the property owner in selling the property to a third party in 2018, which entailed educating the buyer regarding the remaining contamination, the remediation process, and associated regulatory liabilities. The initial goal was to reduce contaminant mass and concentrations in the targeted hot spots enough for a conditional closure under FDEP Risk-Based Corrective Action rules. Based on the success of the bio-injection technology, DOE elected to continue the biotreatment and ultimately reduced groundwater concentrations to levels meeting the standards for an unconditional closure, retiring the remediation agreement in 2020. DOE has no other remedial or legal obligations at this property. (authors)