SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Mr. Russell Chapin 1766 W. Monte Diablo Avenue Page 4 of 7 Model of Groundwater Plume — ATC utilized MODFLOW to model the hydrogeology and MT3D to model contaminant fate and transport in the MODFLOW model. The MODFLOW model was a simplified single-layer (25 feet thick) model, the hydraulic gradient was assumed to be westward at 0.0125. As noted above, EHD believes the soil profile for the site indicates a 2- or 3-layer model may be more appropriate for the site. The MT3D model was simulated through two runs. Both runs included modeling for dispersion, advection and biodegradation. Run 1 was simulated for a 10-year period at 1-year intervals with no recharge reflux (percolating rainwater carrying additional contaminants to ground water); this would be the more optimistic model with processes operating to diminish the plume, and none operating to increase it. Run 2 simulated fate and transport for a 30-year period over 3-year intervals and did incorporate recharge reflux. Neither model incorporated reflux from the submerged sorbed hydrocarbon mass (impacted soil in the saturated zone). ATC presented a contaminant mass estimate of 2.59 pounds of dissolved TPH-g currently impacting groundwater. It would appear from Figure 7 and ATC's calculation that analytical data from groundwater samples collected from the extraction wells was not utilized and the dissolved contaminant mass has been underestimated (VW-6 2800 ppb, EW-1 4300 ppb, EW-2 2900 ppb and EW-3 8300 ppb TPH-g). EHD believes the initial dissolved mass (or average concentration) utilized in the models to be too low. Run 1, the more optimistic scenario, predicts TPH in MW-3 will continue to decline, from 1700 ppb in November 2003 to approximately 3 ppb in three years, similar concentration decreases were predicted for TPH in MW-1 and MW-4. ATC states that the predicted contaminant concentration trend is in accordance with the trends observed over the last 5 years. However, the observed contaminant concentration trends incorporated active remediation, the predicted trends are for natural attenuation only. Run 2 predicted a TPH high concentration of 6906 ppb in MW-3 in 1998 decreasing to 270 ppb in 2004, remaining fairly stable at that concentration over a period of years due to equilibrium between recharge reflux and contaminant degradation. The average TPH concentration in 1998 was 7450 ppb; the only reported TPH-g concentration in 2004 was 2100 ppb; clearly the model for natural attenuation for this well was not validated even with active remediation ongoing during the general period. EHD believes that ATC's model is not valid and cannot be utilized to justify site closure or ending ground water monitoring. Groundwater Receptor Survey—Appears to be adequate. -No Further Action Required (NFAR) Reporting Criteria — EHD has no disagreement with the first two criteria. Criterion 3: Contaminants remaining in the vadose zone cannot migrate in soil vapor or leach at concentrations that would cause groundwater to exceed water quality objectives — This may be true, but undemonstrated for vadose zone soil; however the soil model doesn't account for rising or falling ground water elevation or for contaminant flux from submerged impacted soil.