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j;111,11 <br /> 7-Eleven/Stockton 7 <br /> February 1990 <br /> SITE SUBSURFACE CONDITIONS <br /> The application of continuous coring during the drilling of <br /> MW-15 and MW-15 provided for a more-accurate characterization of <br /> the subsurface soil profile at the site. These additional <br /> details have aided the interpretation of the subsurface geology <br /> and hydrogeology. <br /> Subsurface materials encountered during the recent drilling <br /> suggest that the subsurface stratigraphy is composed of two <br /> distinct sequences. A generalized geologic cross-section <br /> (Figure 3) illustrates the site's subsurface geology. The upper <br /> sequence is marked by a basal gravelly coarse sand, overlain by <br /> fine sands and silts, and capped by a silty clay. The lower <br /> sequence consists of laterally discontinuous lenticular sand <br /> bodies encased within silts and silty clays. <br /> This model aids in the interpretation of the history of <br /> �.. groundwater recovery since the installation of the on-site <br /> recovery well (RW-1) in March., 1988. Following the installation <br /> of RW-1, the dissolved-hydrocarbon concentrations recovered <br /> initially were 7,800 parts per billion (ppb) TPH-as-gasoline. <br /> Once RW-1 had established a constant cone of depression, however, <br /> the recovery of dissolved-hydrocarbons declined sharply to less <br /> than 140 ppb TPH-as-gasoline for a period of approximately two <br /> months. And in June, 1988, dissolved-hydrocarbon concentrations <br /> in the effluent from RW-1 were below Method Detection Limit <br /> (MDL) . This reduction in dissolved hydrocarbon concentrations <br /> coincides with a sharp decline in groundwater elevation in MW-1 <br /> to below the permeable gravelly coarse sand unit due to the <br /> regional drought conditions. It also coincides with the marked <br /> -- attenuation of the interpreted dissolved-hydrocarbon plume. <br /> C GROUNDWATER <br /> TECHNOLOGY.I.W. <br />