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SECTION 5.0 RESULTS AND DISCUSSIONS ' water level The permeability of the sand pack may be one or more orders of magnitude greater than the water-bearing zone, depending on the stratigraphy 5.4 ESTIMATION OF MASS OF PETROLEUM HYDROCARBONS IN SOIL AND GROUNDWATER The masses of TPH-g and benzene in soil and groundwater underlying the Stockton Yard site were estimated based on soil analytical data collected at the time of drilling, and on September 1999 groundwater analytical data The areal extent of TPH-g and benzene in soil at concentrations greater than 10 mg/kg and 10 µg/kg are depicted on Figures 5-4 and 5-5, respectively The mass of petroleum hydrocarbons in the soil was calculated based on the bulk density of the soil and the average concentration of petroleum hydrocarbons over a discrete volume The bulk density used, 1,650 kg/M3, was referenced in a report entitled Human Health Risk Assessment and Groundwater Impact Analysis, Caltrans Shop 10 Maintenance Station, Stockton, California, prepared by URSGWC and dated February S, 1999, for the Shop 10 Maintenance Station (URSGWC, 1999) Soil analytical data from the GEOCON investigation and the Alton I0 investigation were used to estimate the mass of petroleum hydrocarbons (GEOCON, 1996b, Alton, 1992) GEOCON estimated approximately 1,150 m3 of soil were impacted with TPH-g The areal extent of TPH-g and benzene in soil are shown in Figures 5-6a and 5-6b URSGWC has estimated the volume of TPH-g and benzene-impacted soil to be 2,300 m3 (3,000 yd ) and 16,200 m3 (21,200 yd ), respectively The masses of TPH-g and benzene were estimated to be approximately 220 kg (485 pounds) and 43 6 kg (S pounds), respectively The volume and the corresponding mass of petroleum hydrocarbons in groundwater beneath the site were estimated using TPH-g and benzene data from the Third Quarter 1999 groundwater monitoring event, conducted by Ninyo & Moore in September 1999 The areal extent of TPH-g and benzene above 100 gg/L and 1 µg/L, respectively, the height of the water column in the wells, and the saturated soil porosity (approximately 0 35) were used to approximate the volume of impacted groundwater The areal extent of TPH-g and benzene-impacted groundwater are shown in Figure 5-7 Approximately 2 grams (4 4 x 10 -' pounds) of TPH-g, and 0 7 grams (1 5 x 10-1 pounds) of benzene have been estimated in groundwater at the site A summary of the mass estimated for groundwater including assumptions, equations used and sample calculating are presented in Table 5-5 5 5 NATURAL ATTENUATION EVALUATION Based on the results of the water quality parameters and inorganic analyses in Table 5-1, it is • unclear whether the subsurface conditions at the site are aerobic or anaerobic In addition, S 4PROJW1040100044SYYARMMMNTYARIDSAUOC 5-4 URS Greiner Woodward Clyde