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Mr. Philip Isorena GEOMATRIX California Regional Water Quality Control Board September 14, 2000 Page 3 In this comment, RWQCB has requested that Chevron "refine"the evaluation of alternatives and develop more "accurate" remediation time frames. To address this comment, Geomatrix will develop revised remedial time frames based on a number of underlying assumptions. It is important to recognize that the revised time frames may appear to be more precise than the order-of-magnitude estimates already developed, however, they should not be considered to be more accurate. Additionally, the revised time frames are not likely to change the fundamental conclusion from the analysis already performed: except for complete removal of affected soil, all remediation alternatives will likely require a relatively long period of time before concentrations of biodegradation byproducts in groundwater are permanently below the taste and odor WQOs for diesel. In this comment, RWQCB also requested that Chevron evaluate chemical oxidation and thermal desorption/destruction. To address this comment,these alternatives will be evaluated in the final closure request. C. Total Petroleum Hydrocarbons as Diesel(TPHd) Testing Protocol Chevron states that the WQOs for biogenic byproducts measured as TPH have not been established and assumes the WQOs for those constituents have been exceeded in order to proceed with the evaluation of the site against Regional Board policies, regulations, and guidelines. The use ofsilica gel cleanup prior to TPHd analysis results in removal of byproducts of TPH degradation. It is infeasible to identify all of these byproducts. Using representative constituents in lieu of all the degradation byproducts will result in not detecting some byproducts which could pose a threat to water quality. Therefore, TPHd analysis without silica gel cleanup should be used to determine the threat to water quality. If the TPH at a site falls within the diesel range, staff will compare the TPH concentrations with the TPHd WQO of 100,ug/1. As described in a June 29, 2000 work plan prepared by Geomatrix, we will collect groundwater samples from the existing site monitoring wells and a grab groundwater sample from the location of former soil boring GB-613. The groundwater samples will be analyzed for TPH quantified as diesel both with and without a silica gel preparation procedure. This work was approved in an August 3, 2000 letter from the RWQCB to CEMC. The results of this work will be presented in the final closure request. Future groundwater sampling analyses at other sites will include TPH quantified with and without silica gel preparation procedure. D. Conformance to Tri-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of Underground Tank Sites Chevron evaluates the site against the four criteria for low risk groundwater in Appendix B of the Tri-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of Underground Tank Sites and provides justification as to why the site meets each criterion.