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- Former Beacon Station No 12419 March 18,2002 <br /> Stockton, California Problem Assessment Report/Corrective Action Plan <br /> . For the purpose of estimating the mass of TPHg in groundwater, the estimated lateral extent <br /> of TPHg is assumed to be a rectangle 140 feet long by 70 feet wide as shown on Figure 7 <br /> The thickness of TPHg and benzene-impacted groundwater is assumed to be the distance <br /> from the current water level, approximately 55 feet bsg, to the estimated low groundwater <br /> level, approximately 95 feet bsg, or approximately 40 feet The resulting assumed volume of <br /> TPHg-impacted aquifer is 140 feet x 70 feet x 40 feet= 392,000 ft3 <br /> The average dissolved TPHg concentration in the plume is assumed to be the average of the <br /> TPHg concentrations (ranging from 64 to 150,000 ppb) reported in the groundwater samples <br /> The average concentration of TPHg in nine groundwater samples is 17,949 ppb <br /> (0 000017949 ppb TPHg) in the assumed lateral extent area The estimated volume of <br /> dissolved TPHg in groundwater, Vggw, is <br /> Vg., _ (392,000 ft3) x (0 30) x (0 000017949 TPHg) x (7 48 gallons/ft) <br /> = 16 gallons <br /> For the purpose of estimating the mass of benzene in groundwater, the estimated lateral <br /> extent of benzene is assumed to be a rectangle 120 feet long by 80 feet wide, as shown on <br /> Figure 8 The thickness of benzene-impacted groundwater is assumed as above for TPHg, or <br /> approximately 40 feet The resulting assumed volume of benzene-impacted aquifer is 120 <br /> feet x 80 feet x 40 feet= 384,000 ft3 <br /> The average dissolved benzene concentration in the plume is assumed to be the average of <br /> the benzene concentrations (ranging from 1 3 to 12,000 ppb) reported in groundwater <br /> samples The average benzene concentration in six groundwater samples is 2,117 ppb <br /> (0 000002117 ppb benzene) The estimated volume of dissolved benzene in groundwater, <br /> VbJ' is <br /> Vbgw = (384,000 ft) x (0 30) x (0 000002117 benzene) x (7 48 gallons/ ft) <br /> = 1 8 gallons benzene <br /> 419PARCAP3-02 doe 16 <br /> Project No 1419 23 HORIZON ENVIRONMENTAL INC. <br />