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v Mr. Robert Mihalovich - 2 - 18 July 2000 3. The historical data show that no samples have been tested for polynuclear aromatic hydrocarbons (PAHs). If the pipeline has been used to transport TPHd, which could contain PAHs, Chevron should test the groundwater for PAHs. 1999 Annual Progress Report The report states that MW-5 and MW-8 define the lateral extent of BTEX; MW-10 and MW-14 define the downgradient extent; and ASW-1 defines the vertical extent. I concur with this statement. In fact, these wells also define the downgradient and vertical extent of TPHd and TPHg. MW-11 defines their lateral extent. However, there is no well to define the upgradient extent. As stated in Item 2 above, Chevron should install a monitoring well to the south to accomplish this. In addition, Table 1 (historical data) and Figure 2 do not show the presence of ASW-1 and MW-11. The report should include this information. A review of the files reveals that staff approved the decommissioning of MW-11. However, since ASW-1 still exists, Chevron should include it in the monitoring program as it defines the vertical extent of BTEX and TPHd/g. Low-Risk Closure Request Chevron followed the Appendix B of the Tri-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of Underground Tank Sites and provided all the required information. Chevron also outlined the four criteria for low risk groundwater and provided justification as to why the site meets each criterion. Chevron further states that petroleum hydrocarbons (PHCs) may be left in place as long as compliance with water quality objectives (WQOs) is achieved within a reasonable time period, which is predicated upon the probable timing of groundwater use. Chevron also states that the City of Tracy has prohibited the use of groundwater beneath the site for reasons unrelated to PHCs. This allows for a longer period in which to restore the beneficial uses of groundwater because the likelihood of future use of groundwater is extremely low. Based on BTEX trends, Chevron estimates that the WQOs would be met in 20-30 years. Chevron estimates a longer remediation time (unspecified) for TPHd and TPHg. The latest monitoring report shows the highest concentrations of BTEX and TPHg as 810, 300, 3,500, 430, and 27,600 ug/1, respectively. The WQOs for these constituents are 0.35, 29, 42, 17, and 5 µg/l, respectively. Therefore, all constituents exceed their WQOs. Chevron is proposing to rely on natural attenuation(NA)to remediate the site to meet WQOs. State Water Resources Control Board Resolution 92-49 states that the Regional Board shall determine whether water quality objectives can reasonably be achieved within a reasonable period by considering what is technologically and economically feasible. Furthermore, the National Contingency Plan (NCP)preamble suggests that a"reasonable"time frame for a remedy relying on NA is a"time frame comparable to that which could be achieved through active restoration."The NCP further states, "the most appropriate time frame, must, however,be determined through an analysis of alternatives." Therefore, Chevron should perform an evaluation of remedial alternatives, including NA,with estimates of the time it would take for each alternative to remediate the site. NA would be an acceptable alternative if its time to achieve cleanup is comparable to that of active remediation.