SITE INFORMATION AND CORRESPONDENCE

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Ms. Katie Hower • - 2 - • 8 March 1999 2. The vertical extent of groundwater contamination at MW-2, MW-6, and potentially at MW-7, GP- 3, and GP-10 needs to be defined Geomatrix states, "There is no horizontal plume in the upper water table where groundwater is in contact with petroleum-affected soil (and product at MW-2), therefore, there can be no plume in the vertical direction because the water table acts as a horizontal no-flow boundary." It is true that there is no horizontal plume in the upper groundwater. However, because the contaminated soil -� extends below the water table, leaching of contaminants can occur at any portion of the contaminated soil layer in the saturated zone. Once contaminants leach out in the saturated zone, they follow the groundwater flow. For example, at the Thompson Chevrolet site in Patterson, separate phase product was detected in two separate borings at 20 and 25 feet below ground surface (bgs) which are five and 10 feet below the water table, respectively. Total petroleum hydrocarbons in the diesel range also were detected in a grab groundwater sample collected at 55 feet bgs. The presence of a separate phase product and leachable concentrations in the saturated zone has impacted the groundwater at those locations. To determine the full impact to water quality, full characterization of the extent of contaminant mass is needed at the Thompson Chevrolet site. Similarly, at the former Lyoth Loading Station, Chevron should characterize the lateral and vertical extent of the contaminant mass to evaluate fully the threat to water quality and the remedial alternatives which can be used to clean up the contamination. 3. The threat to water quality is not being adequately addressed because the test methods... Geomatrix states, "It is standard practice to not regulate chemicals to concentrations that may be potentially lower than the lowest practically achievable method detection limit (MDL)." In cases where the water quality objectives (WQOs) for contaminants are lower than their MDLs, the Board applies the bI)Ls as the WQOs. This procedure also will be applied at this site. However, in reporting the data, Geomatrix should show results of trace detections. For example, if a trace of benzo[a]pyrene is detected, then we would assume that the WQO of 0.029 µg/1 has been exceeded since the reporting limit of 0.1 µgll is almost 3.5 times higher than the WQO. Chevron would then be required to consider alternatives to remediate the groundwater. 4. The technical approach should be revised and made consistent with State Water Board Resolution Nos. 88-63 and 92-49 and Appendix B of the Tri-Regional Recommendations. Geomatrix states that its Technical Approach to Site Evaluation and Decision Making is consistent with State Water Board Resolutions Nos. 88-63 and 92-49 since groundwater is being considered a potential source of drinking water and restoration of beneficial use within a reasonable timeframe is a remedial goal. However, the approach does not have any provision for restoration of beneficial uses of groundwater within a reasonable timeframe. This should be included in the technical approach. Based on Figure 2 in the response, crude oil has migrated to at least 22 feet below the invert of the former pipeline. If active remediation will not be proposed at this site, Geomatrix should shoe what the fate of the contaminants will be. As specified in Appendix B of the Tri- Regional Recommendations, Geomatrix should demonstrate that"by the time the groundwater is anticipated to be used, water quality objectives will be achieved and beneficial uses will be protected."