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Former Beacon Station No. 12474 March 18, 2002 Stockton California Problem Assessment Report/Corrective Action Plan samples from monitoring wells MW-2, MW-6, MW-11 and MW-12, vapor extraction. wells • VW-1, VW-4 and VW-7, and air sparge well AS-1. Groundwater generally occurs in silty sand, silt and sandy clay that are assumed to have an overall average porosity of 0.30 (30%). For the purpose of estimating the mass of TPHg in groundwater, the estimated lateral extent of TPHg is assumed to be a circle 160 feet in diameter (5026 ft2) as shown on Figure 7. The thickness of TPHg and benzene-impacted groundwater is assumed to be the distance from the water current level, or approximately 55 feet, to the bottom of the lowermost screened interval of the impacted wells (monitoring well MW-12) or approximately 89 feet. The resulting assumed volume of TPHg and benzene-impacted aquifer is 5236 ft2 x 34 ft = 178,024 ft'. The average dissolved TPHg concentration in the plume is assumed to be the average of the TPHg concentrations reported in groundwater samples or 17,074 parts per billion or ppb (0.000017074 ppb). The estimated volume of dissolved TPHg in groundwater, V9aW, is: Vag„. _(178,024 ft') x (0.30) x (0.000017074 TPHg) x (7.48 gallons/ ft) = 6.8 gallons The average dissolved benzene concentration in the plume is assumed to be the average of the benzene concentrations reported in groundwater samples. Benzene was reported in seven groundwater samples at concentrations of 1.3 to 28,000 parts per billion (ppb) for an average of 8,517 ppb (0.000008517 ppb). The estimated volume of dissolved benzene in groundwater, Vbg,v, is: Vbam= (178,024 ft') x (0.30) x (0.000008517 benzene) x (7.48 gallons/ ft3) 3.4 gallons benzene 474PARCAP3-02.doc 10 Project No. 1474.23 HORIZON ENVIRONMENTAL INC.