ARCHIVED REPORTS XR0001845 - Laserfiche WebLink

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reversal and gradient change due to seasonal variation in the Mokelumne River groundwater is strongly possible Current groundwater levels(1988-2003)have varied from 1 to 14 feet msl The most recent groundwater surface elevation has been about 6 feet ms] 6 Well and Conduit Study A sensitive receptor study is underway to identify all wells identified to a distance of 2,000 foot radius Two wells within 500 foot radius of release site have been identified to date No horizontal or vertical conduits have been identified to date 7 Estimation of Release Mass Soil is estimated to have 9,400 cubic yards of petroleum hydrocarbon impacted soil Average concentration of constituents in the contaminant mass are estimated to be as follow benzene (1 to 10 mg/kg), toluene (2 to 50 mg/kg), ethylbenzene (2 to 15 mg/kg), xylene (5 to 220 mg/kg), TPH-gas (10 to 2, 100 mg/kg) Source has been stopped with UST removal However petroleum hydrocarbon contaminated soil has continued to contaminate site groundwater 8 Source Removal Activities UST's removed in 1988 9 Remediation Activities Removal of several thousand gallons of possible free-product(199 1) 10 Areal extent of petroleum hydrocarbon groundwater plume Identified (62,500 square feet) without having southern boundary delimited �r 11 Concentration of Chemicals vs Distance To identified zero line - 200 feet on cross or downgradaent(Figure 14) 12 Concentration of Chemicals vs Time TPH-gas and benzene in water have been reduced by natural degredation Since 1994 natural degredation has resulted In about 80-percent reduction of estimated original concentrations being present (Figure 15) Benzene, 1,2- DCA, and MTBE remain over MCL's on property in core of release area 13 Spread of Constituents in Groundwater The groundwater flow direction is variable but generally southwest to south with a low gradient Transport of the petroleum hydrocarbons in this area may most likely be controlled by chemical diffusion to groundwater Remaining Threat 1 1,2-DCA,benzene,and MTBE continues to threaten local water supply wells within a 1,000 foot distance from site Remaining Data Gap 1 Horizontal and vertical extent of soil and groundwater petroleum hydrocarbon contamination not identified Figures 13, 14,and 15 depict the Site Conceptual Model in the form of map and cross-sectional data Figure 16 depicts the Site Conceptual Model Exposure Pathways Mam1D =nwnmentalParaTexawSReprtslSAR1203 wpd Page 24