ARCHIVED REPORTS XR0001827 - Laserfiche WebLink

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' wells for both individual properties and municipal water supply source within a distance • of 2,000 feet 4 Chronology of Events five Underground Storage Tanks (USTs)were probably in service from about 1965-1983 USTs were not used after 1983 Principal identified contaminant is gasoline as petroleum hydrocarbon Benzene, 1,2-DCA, and MTBE occur at concentrations of concern MTBE concentrations on site exceed primary MCL of 13 µg/l 1,2-DCA concentrations on site exceed primary MCL of 5 µg/1 Benzene concentrations on site exceed the primary MCL of 1 µg/1 ' 5 Site Stratieraphy and Hydrogeol0 Stratigraphy is at least upper 60 foot thickness of mterbedded medium to fine grained sands and silt or clayey silt lenses that are several feet thick These sediments contain the petroleum hydrocarbon compounds that slowly release ' the compounds into local groundwater Hydrogeology is summarized by flow direction that is southwesterly to southerly at a low-gradient(<001) Local flow reversal and gradient changes due to seasonal variations in the Mokelumne River groundwater has occurred in the past fifty years(DWR, 2043) Current groundwater levels(1988-2004) Ihave varied from 1 to 14 feet msl The most recent groundwater surface elevation has maintained an elevation of about 6 feet ms] 6 Well and Conduit Study A sensitive receptor study is underway to Wmttfy all wells identified to a distance of 2,000 foot radius Two wells within 2,000 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 affected is estimated to be 9,400 cubic yards of petroleum hydrocarbon impacted soil Density of soil is estimated to be 1 9 tons/cubic yard Estimated mass of affected soil 17,860 tons 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) ' The average constituent concentration used to calculate the SCM for the site is benzene 5 mg/kg, TPH-gasoline 400 mg/kg, all other volatile organic compounds ' (toluene, ethylbenzene, xylene) 15 mg/kg, Soil release mass estimate for the individual compounds Is as follow benzene 178 pounds, TPH-gasolme 14,288 pounds, all other volatile organic compounds 534 pounds 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 (1991) Main\D\EnvuonmentalTamwTexaco\R eporls1i23104QMR wpd I