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The direction of regional groundwater flow is generally to the north, but the shallow water <br /> ' table aquifer can be locally influenced by the pumping of large diameter municipal wells <br /> owned and operated by the City of Lathrop's Department of Public Works Since the <br /> hydraulic conductivity of the silt to fine sand aquifer beneath the site is only in the range <br /> Of 0 1 to 10 feet per day, and the groundwaterradient across the site e is a very gentle <br /> 0 001 feet per foot, the calculated groundwater flow velocity (v=Kdh/ndl) is only 0 004 to <br /> ' 0 04 feet per day As a result, groundwater movement is nearly static, allowing Y onI minor <br /> migration of the contaminant plume, which at this time is still restricted to the Langston <br /> ' property The size of the contaminant plume however, is much larger now, than when first <br /> measured in the April, 1992 investigation The difference in groundwater plume size and <br /> configuration during the past year can be primarily attributed to seasonally heavy rains over <br /> the past 6 months The above-average rainfall has contributed to a 3-foot rise in the <br /> elevation of the water table in the Lathrop area over the past year This rainfall has <br /> subsequently led to an increase in the amount of gasoline contamination reaching the water <br /> table in the vicinity of the former tanks Since the near surface soil in this area still <br /> contains a high level of gasoline, up to 0 5 gal/ft3, it is conceivable that hundreds of gallons <br /> of gasoline may still be entrained in the soil Thus, surface water percolating through this <br /> zone of contaminated soil has provided and will continue to provide a source of <br /> ' groundwater contamination until the soil is removed <br /> ' The increase in contamination reaching the water table, has virtually doubled the size of <br /> the contaminant plume since April, 1992. <br /> Specific work performed during this phase of investigation included <br /> 1 Preparation of the Problem Assessment Workplan and application for monitoring <br /> g <br /> well permits. <br /> 2 Drilling of five soil borings and the conversion of four of the borings to groundwater <br />' monitoring wells, MW-1, MW-2, MW-3 and MW-4. <br />' 2 <br />