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SECTION 5.0 RESULTS AND DISCUSSIONS surrounding sand pack on the rate of rise and fall of the 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 underlying the Shop 10 site were estimated based on laboratory analyses conducted at the time of drilling, and on September 1999 analytical data The areal extent of TPH-g and benzene in soil at concentrations greater than 10 mg/kg and 10 µg/kg, respectively, are depicted on Figures 5-5 through 5-6 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 8, 1999, for the Shop 10 Maintenance Station Soil analytical data from EA and GEOCON investigations were used to estimate the mass of petroleum hydrocarbons (GEOCON, 1996, Environmental Assessors, 1994) GEOCON estimated approximately 2,000 to 3,000 cubic yards of soil were impacted with TPH-g In this report, the impacted volume with respect to TPH-g is estimated to be approximately 1,500 cubic 1 meters (20,000 yd 3) It is estimated that approximately 1,500 kg (3,300 pounds) of TPH-g, and 10 kg (22 pounds) of benzene are present in soil beneath the site A summary of the mass estimated for soil including assumptions, equations used and sample calculations are presented in Table 5-4 The volume and 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 collected by Ninyo & Moore in September 1999 The areal extent of TPH-g ' and benzene above 100 µg/L and I 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 110 kg (242 pounds) of TPH-g, and 20 kg (44 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 Also, without S 1PRO114104010004MHOP10lSHOPiOSADOC 5-4 URS Greiner Woodward Clyde