ARCHIVED REPORTS XR0001827 - Laserfiche WebLink

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' 10 Areal extent of petroleum hydrocarbon groundwater plume Identified(62,500 square feet) without having southern boundary delimited ' Average concentration of constituents in the contaminant groundwater plume are estimated from existing site assessment work to be as follow benzene(20 ug/1), toluene(30 ug/l), ' ethylbenzene(10 ug/l), xylene(30 ug/l), TPH-gas(1,000 ug/1) The average constituent concentration used to calculate the SCM for the site is I benzene 20 ug/l, TPH-gasoline 1,000 ug/l, all other volatile organic compounds(toluene, ethylbenzene, xylene) 70 ug/l, I The contaminant mass in the groundwater plume is estimated to be approximately five feet thick Based on a porosity of 22% in the host reservoir an estimated 19 million liters of impacted groundwater is present beneath the site IGroundwater release mass estimate for the individual compounds is as follow benzene 38 pounds, TPH-gasolme 1946 pounds,all other volatile organic compounds 13,6 pounds 11 Concentration of Chemicals vs Distance To the identified zero line- 200 feet on cross or downgradient(Figure 6, Muir 2003) I12 Concentration of Chemicals vs Time TPH-gas and benzene in water have been reduced by natural degredation Since 1994 natural degredation has resulted m about 80-percent reduction of original estimated mplace concentrations Benzene, 1,2-DCA, and MTBE remain over primary MCL's on site 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 in Igroundwater Rernammg Threat_ i 1,2-DCA, benzene, and MTBE continues to threaten local water supply wells within a 1,000 foot distance from site I Remaining Data Gap 1 Horizontal and vertical extent of soil and groundwater petroleum hydrocarbon contamination not identified Figures 13, 14 and 15 of Muir(2003)depict the Site Conceptual Model in the form of neap and cross-sectional data Figure 16 of Muir(2003) depicts the Site Conceptual Model Exposure Pathways I The horizontal extent of petroleum hydrocarbon contamination groundwater plume for this reporting period is estimated to be at least 14,560 square feet This minimum documented groundwater plume extent is about the same size of the previous monitoring event m the first quarter of 2003 MalnxDWnvironnia tallPamurTemcolRep m\I23104QMR v pd 7 I I