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in depth from 35 to 55 feet and in thickness from 10 to 20 feet Diesel concentrations generally exceed <br /> 10,000 mg/kg at the base of the channel along its axis and may reach 20,000 mg/kg The volume of <br /> contaminated soil remaining in place is estimated to exceed 7,000 cubic yards <br /> Groundwater flows southward across the site at a gradient of 0 011 f1/ft, but appears to turn <br /> southwestward near the southern property boundary Over most of the site, groundwater is present <br /> below a depth of 65 feet within silty sand of moderate permeability Locally, however, a shallower zone <br /> of perched groundwater is present along the ams of the contaminated, lugh-permeability channel <br /> deposit This perched zone is probably absent during dry years Groundwater in the perched aquifer <br /> approaches free-product diesel concentrations, groundwater in the underlying aquifer has dissolved <br /> concentrations that locally exceed 3,000 ppb but in most wells are below 1,000 ppb The dissolved <br /> diesel plume occupies an area of approximately 1/2 acre <br /> Monitoring of the groundwater gradient, flow direction, and contamination plume should be continued <br /> to provide data on contaminant transport and migration A risk assessment analysis will likely be <br /> needed to determine whether the contaminants will degrade passively or whether active remediation <br /> methods will be needed to bring the site into compliance with current water quality goals <br />