SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Mr.Russell Chapin 1766 W.Monte Diablo Avenue Page 4 of 7 Under the same discussion, ATC stated that: The flow and transport model was calibrated by comparing total petroleum hydrocarbons, modified for TPHg isocontours generated from the January 2002 groundwater laboratory data and TPHg isocontours generated by the model and adjusting the horizontal hydraulic conductivity of the soil until the positions of the two sets of isocontours were in approximate agreement. Therefore the model takes the effects of the SVE system into consideration but does not take the effects of the GWE system into consideration. The EHD agrees that this action will not incorporate the effects of the GWE into the model, but does not understand how adjusting the horizontal hydraulic conductivity of the soil will incorporate the effects of the SVE system. An explanation, utilizing available authoritative sources to demonstrate and validate this technique, should be provided. It appears that plume matching has been utilized to solve for two independent variables: the horizontal hydraulic conductivity, and the effects-of SVE on plume migration and concentration distribution. Point 6: ATC states, "Subsequent to the calibration of the flow model, the transport model was calibrated by adjusting the concentration of the recharge flux until the longitudinal extents of the two sets of TPHg contours were in approximate agreement." At this point, the values for three fairly independent variables have been assigned by matching the modeled plume to an actual plume distribution. Having selected a flow direction oblique or counter to the directions predominantly inferred from field data, there is little data left from which to determine the prevailing gradient. ATC used an average" gradient of 0.0125 ft/ft. The data utilized to derive the average gradient should be provided to the EHD. Also, the sensitivity of the model to significant changes of gradient, i.e., 0.025 or 0.007 ft/ft., should be tested, with the results provided to the EHD. Point 7: ATC discussed the dissolved hydrocarbon mass estimate and noted that the MODFLOW model utilized the recharge package to simulate the loading of the hydrocarbon plume due to leaching of hydrocarbons sorbed in the soil to groundwater. In the Closure Report, ATC noted that the recharge concentration (85,000 micrograms per cubic foot) was assigned based on laboratory data (which was not demonstrated or discussed further) and that the recharge flux (3.17E-08 feet per second) corresponded to a typical rainfall of 12 inches per year. If the recharge concentration can be demonstrated to be reasonable, the contaminant loading to the plume from the vadose zone will have been addressed. However, the concern of the EHD with contaminant loading from the submerged impacted soil mass remains unaddressed, except through adjustment of the hydraulic conductivity as a result of plume matching, as described above, which the EHD does not understand. Soil data from boring CB-5 shows that significant concentrations of sorbed hydrocarbons extend at least 30 feet into the subsurface, well below the water table, which has been as shallow as 10 feet bsg. Point 8: ATC discusses Criterion 3 for a 'No Further Action Required' under Section 6.0 of Appendix A of the Tri-Regional Recommendations, which states in part "Contaminant remaining in the vadose zone cannot migrate in soil vapor or leach at concentrations that would cause groundwater to exceed water quality objectives. ATC maintains that calibrating the model to field data (not presented or discussed), while utilizing the MODFLOW recharge package and the MT31D chemical reaction package, does account for contaminant flux from the impacted vadose and submerged soil. The EHD has discussed much of this topic above and does not understand how the model incorporates contaminant flux from submerged impacted