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Mi Sergio Morescalchi - 3 - 8 April 2009 Atlantic Richfield Company 27, section 20415(e)(10) contains statistical procedures recommended for establishing baseline concentrations, which, by reference, requires the collection of data for four consecutive quarters, and obtained during the times of highest and lowest groundwater elevations. Subsections (A) and (B) of Section 20415(e)(10) allow for the use of either historical or contemporaneous data to determine baseline concentrations in accordance with the methods described in Section 20415(e)(7). We cannot accept the baseline concentrations proposed in the Response. Arco must propose baseline concentrations derived in accordance with CCR, Title 27 Section 20415(e)(10). ARCO explains that the rationale behind choosing the highest historical concentrations as baseline concentrations is to avoid triggering contingency measures prematurely. ARCO states that natural fluctuations in the concentrations of these constituents could trigger the contingency plan even if the injected materials do not reach MW-1A. However, there are very few data points: and ARCO has not provided data showing these fluctuations. In addition, the contingency measures start with a 6-month monitoring period to ensure that a 120% exceedence is sustained before extraction is initiated. Data in Table 7 in the February 2005 2004 Annual and Fourth Quarter 2004 Site Status, Groundwater Monitoring, and Remedial Summary Report (2004 Annual Report), taken during and after implementation of the 2004- 2005 nitrate/sulfate bioremediation pilot study, demonstrate that there is no evidence to believe that a 6-month sustained 120% exceedence will occur. This method for calculating baseline concentrations is not sufficiently protective of groundwater quality. Therefore, Regional Water Board staff cannot concur with ARCO's proposed method and baseline concentrations. ARCO is still required to clean up the petroleum hydrocarbon pollution in the groundwater beneath the Site, whether it is via an in-situ or alternative method. If ARCO chooses to proceed with an in-situ remedy, ARCO must (1) demonstrate there are no cost-effective non salt- containing injectants that can effectively clean up the pollution, (2) propose baseline concentrations for dissolved calcium, sulfate. sulfide, and TDS calculated in accordance with Title 27 section 20415(e)(10) with data incorporated frorn Table 7 of the 2004 Annual Report as well as Table 2 in the Response, and (3) propose an alternative contingency plan to fructose infection. By 8 May 2009, please inform Regional Water Board staff of ARCO's chosen path forward. If ARCO wishes to proceed with an in-situ project, the 8 May submittal must include a time schedule to submit a revised work plan that addresses the comments above. If ARCO chooses not to proceed with an in-situ project, the 8 May submittal must include a time schedule to submit a work plan for an alternate cleanup method. If you have any questions or comments regarding this letter, you may contact me at (916) 464- 4811 or.,betaylcr �terboards.ca.gov. BRIAN TAYLOR, P.G. Engineering Geologist cc list on next page