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7-Eleven/Stockton 9 May 1990 This conceptual model aids in the interpretation of the history of groundwater recovery since the installation of the on- site recovery well (RW-1) in March, 1988. Following the installation of RW-1, the dissolved--hydrocarbon concentrations in the recovered groundwater initially were 7,800 parts per billion (ppb) TpH-as-gasoline. Once RW-1 had established a constant cone of depression, however, the concentrations of dissolved- hydrocarbons in the recovered groundwater declined sharply to less than 140 ppb TFH-as-gasoline for a period of approximately two months. In June, 1988, dissolved-hydrocarbon concentrations in the effluent from RW--1 were below Method- Detection Limit (MDL) . This reduction in dissolved hydrocarbon concentrations _.. coincides with a sharp decline in groundwater elevation in MI-1 to below the permeable gravelly coarse sand unit due to the regional drought conditions. It also coincides with the marked attenuation of the interpreted dissolved-hydrocarbon plume. Between June, 1988, .and January 1990, RW-1 has failed to. capture dissolved hydrocarbons that continues to be detected in samples from MW-1. This can be explained by the discontinuity -^ and limited areal extent of permeable sand lenses within the lower sequence. The geologic cross-section (Figure 3) shawl that recovery well RW-1 penetrates permeable sands within the lower sequence, whereas monitoring well MW-1 does not. Therefore, groundwater pumped from RW-1 has been derived from discontinuous permeable sands in the lower sequence that have not been impacted by dissolved hydrocarbons. This subsurface geologic model also suggests that the highest concentrations of hydrocarbons in the adsorbed phase may be primarily restricted to the unsaturated upper stratigraphic sequence, particularly the basal gravelly coarse sand unit. This Ca GROLtNDWATER TECHNOLOGY.INC.