ARCHIVED REPORTS_XR0012633 - Laserfiche WebLink

Home Browse Search

7-Eleven/stockton 8 August 1990 Silty clay. The lower sequence consists of lateral discontinuous lenticular sand bodies encased within silts and silty clays. A generalized geologic cross-section (Figure 3) illustratez the site's subsurface geology. 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 instal- lation 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-hydro- carbons in the recovered groundwater declined sharply to less than 140 ppb TPH-as--gasoline for a period of approximately two MI 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 M11-1 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 continue to be detected in groundwater samples analyzed from monitoring well M11-1. 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) shows that recovery well RW-1 penetrates permeable sands below the water table, Therefore, RW-1 is pumping fresh aquifer water that has not been impacted by _ dissolved hydrocarbons. Monitoring well IfiRl-1 is screened in clay below the water table. Hence, some of the water has seeped down through to impacted soil, picking up dissolved hydrocarbons. Tt:CH\nLrK�C��C