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IFeasibiluy Study Report • 1140 W Hammer Lane, Stockton, CA laterally, but impacted soils remained in the interval from approximately 35 feet bgs to at least 45 feet bgs tin bonngs SBI, SB 12, and SB 14 and to 50 feet bgs in bonng MW5 The analytical results of soil samples collected during drilling of the deep bonngs in 1998 indicated that very low levels of gasoline constituents persist to a depth of at least 98 feet bgs Soil contamination appeared to attenuate significantly with depth, however, as the highest hydrocarbon concentrations in the soil were detected between 40 and 50 feet bgs Tl-us depth interval corresponds to the zone of historic groundwater fluctuation consists predominantly of fine-grained soils such as clays with some silts and clayey sand ' 5.4 Dissolved Hydrocarbons in Groundwater Subjective analyses of groundwater in site morutonng wells indicated that no wells exhibited any free- floating hydrocarbons during the last monitoring on February 9, 2001 However, groundwater from well MW5 exhibited gasoline odors at that time Free floating petroleum product was observed in groundwater in well MW5 in December 1990 and petroleum product sheen was noted in December 1993 10 Analytical results of groundwater samples collected from site wells in February 2001 indicated that TPHg and benzene is present in two site wells Consistently low levels of dissolved hydrocarbon constituents have been detected in well MW 1, located at the edge of the plume Elevated levels of ' dissolved hydrocarbon constituents persist well MW5, located approximately 30 northeast of the location of the nearest former product dispenser An isoconcentration map showing the extent of ' dissolved benzene in groundwater in February 2001 is presented on Figure 8 Historical groundwater analytical results are presented in Table 3 I Based upon the extent of dissolved petroleum hydrocarbons in groundwater beneath the site and ' assuming a soil porosity of 25% within the saturated zone, the mass of contaminants beneath the site was calculated to be approximately 12 pounds (approximately l 8 gallons) of TPHg and 16 pounds ' (approximately 0 25 gallons) of benzene Contaminant mass calculations are summarized in Table 8 1 5.5 Toxicity, Persistence and Potential for Contaminant Migration The toxicity of residual contaminants at the site can be discussed in terms of occupational exposure G 1DnW1GROUNDZEIKNOWi.CS HAMIREP0RTS1rCU%Lbd,1)Report dna 7