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CAM 31ZIA Tables 7 and 8 present the mass estimate calculations for TPHg and benzene in groundwater, respectively A depth interval of 10 feet (for the deep zone) and 20 feet (for the shallow zone), based on the respective screened intervals of monitoring wells in each zone, was assumed to be the affected thickness of groundwater for these calculations The porosity was assumed to be 45 percent in the shallow zone and 37 percent in the deep zone, based on typical porosity values for soil types found at in the screened interval The average TPHg (and benzene)concentration was assumed to be half the value of the higher contour interval For the center contour interval, we assumed that the average concentration was half the highest detected value Based on the approach and assumptions presented above, the estimated TPHg mass in groundwater is approximately 10 5 pounds, and the estimated benzene mass is 0 0127 pounds These contaminant mass estimates are based on available site data, and an assumed static environment versus the true dynamic environment The error associated with these estimates should be considered with any decision-making process Mass estimates for hydrocarbons in soil were not prepared primarily because the available data is five to six years old Using this data would result in a gross overestimate of the current hydrocarbon mass in the soil at this site As shown on Figure 3 the and on the cross sections, the extent of impacted soil is limited to the northwestern dispenser area Other Historical Data Analytical data tables and figures showing the sample locations from the 1996 tank removal are included in Appendix C for reference CONCLUSIONS The data presented in this SCM suggest the release at this site occurred in the vicinity of the northwest fuel dispenser Based on the TPHg and benzene found at depth it appears that the release occurred during a period when groundwater was lower than it is now The relative absence of MTBE at this site suggests the release occurred prior to the introduction of MTBE into gasoline The extent of TPHg and benzene in both soil and groundwater appear to be defined and primarily limited to the area surrounding the northwestern dispenser The TPHg and benzene concentrations appear to be decreasing indicating a stable plume and that biodegradation is occurring • 0797 4