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Groundwater Well Installation Work Plan Sharpe Army Depot <br /> Lathrop, California <br />Ahtna Environmental, Inc. 2 <br />chlorinated solvents). Releases of these chemicals contaminated installation soil and groundwater <br />through a variety of fate and transport processes. <br />Sharpe was placed on the National Priorities List in 1987 and entered into a four-party agreement in 1989. <br />As described in the FFA, the authority for environmental restoration decision-making rests with a team of <br />remedial project managers from the following four parties: <br />• California Department of Toxic Substances Control <br />• Central Valley Regional Water Quality Control Board <br />• U.S. Environmental Protection Agency, Region 9 <br />• Sharpe Army Depot <br />The DLA transitioned Sharpe back to the United States Department of the Army (Army) effective October <br />1, 2014. The Army is the lead agency responsible for funding and implementing remedial actions at <br />Sharpe. <br />The FFA established two OUs to facilitate environmental restoration at Sharpe. OU 1 addresses <br />groundwater in four saturated monitoring zones (designated as A, B, C, and D) contaminated primarily <br />with VOCs. The OU 1 Record of Decision (ROD; ESE, 1993) documents groundwater extraction, treatment, <br />and discharge of treated effluent as the selected remedy for contaminated groundwater. The Explanation <br />of Significant Differences for OU 1 (URS, 2014) was signed and approved by the regulatory agencies and <br />the Army in September 2014 and adds in situ treatment of high levels of subsurface contamination and <br />land use controls to the remedy. OU 2 addresses soil and soil gas above the water table. The OU 2 ROD <br />(ESE, 1996) documents extraction and treatment of VOC-contaminated soil vapor “no further action” <br />decision at multiple sites. In June 2011, the Amendment to the OU 2 ROD (URS, 2011) modified the No <br />Further Action decision to ensure protectiveness by adding land use controls at ten sites and biological <br />monitoring at two sites for the burrowing owl (Athene cunicularia), a California Species of Special Concern. <br />1.2 Geology and Hydrogeology <br />The Sharpe area contains one complex, heterogeneous aquifer system, in which four hydrostratigraphic <br />zones are located between 14 and 270 feet (ft) below ground surface (bgs). These hydrostratigraphic <br />zones are considered distinct zones to monitor hydraulic gradients and contaminant migration. The zones <br />include several saturated transmissive layers (sands) 5 to 12 ft thick in the following depth intervals: <br />• A Zone — water table to 40 ft bgs <br />• B Zone — 40 to 90 ft bgs <br />• C Zone — 90 to 170 ft bgs <br />• D Zone — 170 to approximately 280 ft bgs <br />In the A Zone, groundwater is generally unconfined; however, the fine-grained deposits in the Lower A <br />Zone and Upper B Zone (also known as the A/B aquitard) locally create confining conditions. Although <br />there is good continuity of the confining layer across Sharpe, groundwater and dissolved contaminants <br />can move from the A Zone through the B Zone and into the C Zone. This movement can occur because the <br />confining layer in some areas is thinner and contains more sand, and because strong vertical hydraulic