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Mr. John MacLeod -2 - Chevron Corral Hollow and Grant 6%.w Road, Tracy 17 September 2009 and depth to water was about nine feet below ground surface (bgs)_ Maximum groundwater concentrations of TPHg, TPHd, and benzene were 740 pg/L, 1,300 Ng/L, and 2.0 pg/L, respectively. Naphthalene was detected at a maximum concentration of 40 pg/L. The Closure Report presents two data analytical procedures to evaluate concentration trends, the Mann-Kendall and Sen's Slope analyses. Elevated concentrations of TPHg, TPHd, and benzene are historically observed in MW-6, MW-7 and MW-12. The results of Chevron's Mann-Kendall analysis showed significant downward trends for these constituents of concern (COCs) in all three wells. The Sen's Slope analysis indicated that COCs were decreasing at rates that varied from -0.0286 to -2.12 pg/L. Chevron has conducted groundwater monitoring for natural attenuation parameters since 1995, and concluded that natural attenuation is occurring. Chevron assumed that groundwater at the Site is not a potential source of drinking water and used San Francisco Bay Region Environmental Screening Levels (ESLs) to propose compliance levels of 210 pg/L for TPH, 1 pg/L for benzene, and 24 Ng/L for naphthalene. Using a median Sen's Slope value, Chevron estimates that the proposed compliance levels for benzene and TPH will be attained within three to four years. Chevron did not provide an estimated time to attain the compliance level for naphthalene, and the Closure Report states that it is reasonable to assume that the compliance level would be attained during the same time period as those for TPH. Our comments are included below: 1. Central Valley Water Board staff does not concur with the use of ESLs as cleanup goals for groundwater in the Central Valley. The 15 September 1998 Water Quality Control Plan for the Sacramento and San Joaquin River Basins (Basin Plan) considers all groundwater to be a potential drinking source and requires cleanup to background concentrations, if technically and economically feasible. Since petroleum hydrocarbons do not naturally occur at this Site, the cleanup level for all petroleum hydrocarbons is nondetectable at normally-achievable laboratory method detection limits. If background levels cannot be achieved, the Basin Plan provides Water Quality Objectives (WQOs) for a select list of commonly occurring contaminants. The WQOs for Corral .Hollow COCs include 100 pg/L for TPHd, 5 pg/L for TPHg, and 17 pg/L for naphthalene. 2. The U.S. EPA has produced a guidance document that provides a methodology for consistently estimating the time required to achieve cleanup. The November 2002 Calculation and Use of First Order Rate Constants For Monitored Natural Attenuation Rate Studies is available on the internet at http://www.epa.gov/ada/download/issue/540S02500.pdf. Using this methodology, Central Valley Water Board staff calculated first order rate constants for Corral Hollow COCs based on their respective WQOs. The first order rate constants for TPHg, TPHd, benzene, and naphthalene were -0.2427 pg/L/year, -0.2014 Ng/L/year, -0.432 pg/L/year, and -0.1678 pg/L/year, respectively. Using these constants only benzene is expected to attain its WQO within Chevron's estimated four-year period. Central Valley Water Board staff estimates that TPHg, TPHd, and naphthalene will attain their respective WQOs by 2023, 2035, and 2023, respectively. These time periods are considerably longer that those estimated by Chevron.