The Significance of Filling Data Gaps and Developing Good Conceptual Site Models Prior to Remedy Implementation Under Fixed Price Performance Based Remediation Contracts

APTIM Authors/Contributors

Background/Objectives

At a former Air Force base in San Antonio, Texas, site closure (SC) was proposed for one of the sites, under a 10-year, fixed-price, performance-based remediation (PBR) contract, during the 2011-2021 period of performance (PoP). The site is approximately 3 acres and consisted of multiple disposal/evaporation pits. Chrome plating waste from reconditioning of aircraft engine cylinders was discharged into these pits from the 1940s to 1955. The contaminants of concern (COCs) at this site are tetrachloroethene, trichloroethene (TCE), cis-1,2-dichloroethene, vinyl chloride, and chromium. A stream flows along the western boundary of the site. A groundwater extraction and treatment system to remediate the solvent plume and prevent discharge to the nearby stream was installed in 1993 and modified in 2003. Excavation was performed in 2005 to remove the contaminated soil, and the excavation was backfilled with clean soil. Native subsurface geology consists of mostly tight clays, silts, silty sands and silty clays, with a gravel layer near the bottom of the saturated zone. A several hundred feet thick Navarro clay acts as the base of the saturated zone at a depth of 35 to 40 feet below ground surface (bgs). Groundwater occurs at a depth of approximately 25 feet bgs.

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