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Groundwater Well Installation Work Plan Sharpe Army Depot Lathrop, California Sharpe was placed on the National Priorities List in 1987 and entered into a four-party agreement in 1989. As described in the FFA,the authority for environmental restoration decision-making rests with a team of remedial project managers from the following four parties: • California Department of Toxic Substances Control • Central Valley Regional Water Quality Control Board • U.S. Environmental Protection Agency, Region 9 • Sharpe Army Depot The DLA transitioned Sharpe back to the U.S. Department of the Army (Army) effective October 1, 2014. The Army is the lead agency responsible for funding and implementing remedial actions at Sharpe. The FFA established two OUs to facilitate environmental restoration at Sharpe. OU 1 addresses groundwater in four saturated monitoring zones (designated as A, B, C, and D) contaminated primarily with volatile organic compounds (VOCs). The OU 1 Record of Decision (ROD; ESE, 1993) documents groundwater extraction, treatment, and discharge of treated effluent as the selected remedy for contaminated groundwater. The Explanation of Significant Differences for OU 1 (URS, 2014) was signed and approved by the regulatory agencies and the Army in September 2014 and adds in situ treatment of high levels of subsurface contamination and land use controls to the remedy. OU 2 addresses soil and soil gas above the water table. The OU 2 Record of Decision (ROD; ESE, 1996) documents the extraction and treatment of VOC-contaminated soil vapor"no further action" decision at multiple sites. In June 2011,the Amendment to the OU 2 ROD (URS, 2011) modified the No Further Action decision to ensure protectiveness by adding land use controls at ten sites and biological monitoring at two sites for the burrowing owl (Athene cunicularia), a California Species of Special Concern. 1.2 Geology and Hydrogeology The Sharpe area contains one complex, heterogeneous aquifer system, in which four hydrostratigraphic zones are located between 14 and 270 feet (ft) below ground surface (bgs). These hydrostratigraphic zones are considered distinct zones to monitor hydraulic gradients and contaminant migration.The zones include several saturated transmissive layers (sands) 5 to 12 ft thick in the following depth intervals: • A Zonewater table to 40 ft bgs • B Zone40 to 90 ft bgs • C Zone90 to 170 ft bgs • D Zone170 to approximately 280 ft bgs In the A Zone, groundwater is generally unconfined; however, the fine-grained deposits in the Lower A Zone and Upper B Zone (also known as the A/B aquitard) locally create confining conditions. Although there is good continuity of the confining layer across Sharpe, groundwater and dissolved contaminants can move from the A Zone through the B Zone and into the C Zone.This movement can occur because the confining layer in some areas is thinner and contains more sand. Pumping onsite and offsite water supply wells has caused strong vertical hydraulic gradients. The B, C, and D Zones have hydraulic characteristics of leaky confined aquifers. Subsurface soils vary from clay and silt to coarse sand. Within each zone, deposits containing each soil type occur; however,the deposits vary in thickness vertically and horizontally across Sharpe. Silt and clay Ahtna Environmental,Inc. 3