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DISCUSSION AND CONCLUSIONS ' As discussed above,the objectives of the SSI, reducing concentrations with time Both analytes developed in close coordination with the have reached concentrations that are below the ' PHS/EHD, were to collect the remaining normal reporting limits of 50 and 0 5 ug/L, additional site data necessary to request site respectively Benzene concentrations have been closure These data were specifically identified in below the California MCL of 10 ug/L for the last ' meetings with PHS/EHD and the RWQCB, two sampling events (August 7, 1997 and July 19, PHS/EHD correspondence, and have been 1999) Analytical data from the July 19, 1999 documented in HLA work plan addendums and sampling event of all seven wells, and the .reports(e g , HLA 1997a) Specifically,these data Hydropunch data from HP-2 (depth 4 5 to 7 5 feet and other requests from PHS/EHD included the bgs) demonstrate that detectable hydrocarbons are following no longer present in groundwater at the Site Based on these data there is no measurable free ' • Additional analytical data to complete product and no TPHg mass remaining in delineation of the lateral and vertical extent of groundwater at the Site impacted soil and allow calculation of the ' contaminant mass in the remaining impacted As discussed above in the Results of Insitu soil Bioremediation Feasibility Studies Section above, • Depth-specific groundwater analytical data to the plate count results collected during SSI-Phase ' allow calculation of the remaining mass of One investigation provide strong evidence that the dissolved hydrocarbons proposed remedial method for the Site, insitu 0 Demonstration that free product is not biodegradation, is occurring(Table 4) By present comparing the magnitude of the HCU populations MS Site specific physical and chemical data to within similar soil types in impacted-versus assess the efficacy of remedial measures unimpacted-soils, the HCU population in impacted ® Analytical data on fuel oxygenates and 1,2- soils is shown to be approximately nearly two ' DCA from selected soil and groundwater orders of magnitude higher than in unimpacted samples, and from a complete final soils This indicates that msitu biodegradation is groundwater sampling round occurring at the Site This conclusion is further ' corroborated by two observations 1)the impacted These data have all been collected and presented soil contains relatively low TPHg concentrations in the sections above and are summarized here (1 3 mg/kg), and 2)the magnitude of the total ' microbial populations are actually greater in the The lateral and vertical extent of impacted soil non-impacted sample than the impacted soil have been assessed and are shown on PIates 4 and sample In addition,the strongest evidence of the ' 6 These data and the analytical data on soil efficacy of the proposed remedial measure (insitu samples within the TPHg soil plume shown on biodegradation) is provided by the decreasing Plates 4 and 6 were used to estimate the concentration gradient evidenced in Plate 5 and by contaminant mass remaining in the impacted soil the fact that dissolved hydrocarbons are no longer ' These estimates, and the assumptions used, are present onsite at concentrations above normal presented in the Estimate of Remaining reporting limits Contaminant Mass Section above The mass of ' TPHg in the remaining impacted soils is calculated At the specific request of PHS/EHD, and in to be approximately 380 kilograms, or 130 gallons compliance with RWQCB policy on oxygenate of gasoline analytical requirements for site closure, analyses ' for the five oxygenates and 1,2-DCA were Historical and current TPHg and benzene performed on one representative soil and concentrations in well MW-4(most impacted)are Hydropunch groundwater sample In addition, ' shown on Plate 5 The data show a trend of these analyses were performed on groundwater L 130000087371WPL38737N19 Harding Lawson Associates 21