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7-Eleven/Stockton 13 <br /> November 1989 <br /> established a constant cone of depression, the recovery of <br /> dissolved hydrocarbons declined sharply to less than 140 ppb TPH- <br /> as-gasoline for a period of approximately two months. In June, <br /> 1988, dissolved-hydrocarbon concentrations in the effluent from <br /> RW-1 were below Method Detection Limit (MDL) . This reduction in <br /> dissolved hydrocarbon concentrations coincides with a sharp <br /> decline in groundwater elevation in MW-1 to below the permeable <br /> gravelly coarse sand unit. It also coincides with the marked <br /> attenuation of the interpreted dissolved-hydrocarbon plume. <br /> Since June, 1988, RW-1 has failed to capture dissolved <br /> hydrocarbons that exist in MW-1. This can be explained by the <br /> discontinuity of permeable sand bodies within the lower sequence. <br /> The geologic cross-section (Figure 3) shows that RW-1 penetrates <br /> permeable sands within the lower sequence; whereas, MW--1 does <br /> not. Therefore, since June, 1988, groundwater pumped from RW-1 <br /> has been derived from discontinuous permeable sands in the lower <br /> sequence that are not impacted by dissolved hydrocarbons. <br /> This subsurface geologic model also suggests that soils and <br /> groundwater impacted by the highest concentrations of dissolved <br /> hydrocarbons may be mostly restricted to the upper stratigraphic <br /> sequence, particularly the basal gravelly coarse sand unit. This <br /> is supported by the sharp permeability contrast which exists at <br /> the base of the gravelly coarse sand and the marked attenuation <br /> of the lateral extent of the dissolved hydrocarbon plume <br /> coincident with the decline of the groundwater elevation below <br /> the base of the gravelly coarse sand. <br /> The apparent existence of a highly-permeable gravelly sand <br /> containing concentrations of adsorbed hydrocarbons within the <br /> vadose zone directly overlying a sharp permeability barrier <br /> presents additional justification for the proposed soil-vent <br /> system expansion. The currently operating vapor-extraction wells <br /> C� liHrrl�hltlll.k ' <br /> 1i,i H wp.r h:1 I�i <br />