ARCHIVED REPORTS_SEMI ANNUAL GROUNDWATER MONITORING OCTOBER 2010

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AXOM Mr. James L. L. Barton Regional Water Quality Control Board —Central Valley October 27, 2010 Page 3 included in Attachment D. Updated with data from the semi-annual event in August 2010, the concentration data show the following trends: MW-2 (Unit B)—TPHg and benzene concentrations have a downward trend at 160 µg/L and <0.5 pg/L, respectively, which is two orders of magnitude reduction in TPHg and three orders of magnitude reduction in benzene from the April 2008 baseline concentrations (15,000 µg/L and 350 µg/L, respectively). • MW-3 (Unit B)—TPHg and benzene concentrations have a downward trend at 500 µg/L and <0.5 pg/L, respectively, which is two orders of magnitude reduction in TPHg and three orders of magnitude reduction in benzene from the April 2008 baseline concentrations (32,000 µg/L and 4,500 µg/L, respectively). • MW-4(Unit B)—The TPHg and benzene concentrations in MW-4 have remained at non- detect concentrations since August 2008. • MW-6 (Unit B)—The concentrations of both TPHg and benzene in groundwater were non- detect, which is a decrease of three orders of magnitude for TPHg from the April 2008 baseline concentration (230 µg/L). Benzene concentrations have been consistently non- detect with the exception of laboratory"estimated"values of 0.6 µg/L, 2 µg/L, and 3 µg/L in August 2008, October 2008, and February 2010, respectively. • MW-8D (Unit D)—The TPHg and benzene concentrations in MW-8D, located in the deepest hydrostratigraphic unit, have remained at non-detect concentrations since January 2009. These non-detect concentrations represent three orders of magnitude reduction over the April 2008 baseline concentrations (7,300 µg/L and 1,800 µg/L, respectively). • MWA1 (Unit B)—TPHg and benzene concentrations were 460 pg/L and 56 µg/L, respectively, which are one and two orders of magnitude lower than the April 2008 baseline concentrations (5,200 pg/L and 1,300 µg/L, respectively). • MW-25D (Unit B)—TPHg and benzene were non-detect, as they were in the first quarter 2010. These non-detect concentrations represent three and one orders of magnitude reduction from the April 2008 baseline concentrations (500 pg/L and 1 pg/L, respectively). Due to previous concerns of cross-contamination, G-R submitted equipment blank rinsate samples collected from pumps used to sample monitoring wells MW-2, MW-3, and MW-8D following sampling and decontamination. These samples were listed as EB-2, EB-3, and EB-8D on the COC and analyzed for the same constituents as the groundwater monitoring well samples. The rinsate blank samples were non-detect for these constituents during the August 27, 2010, sampling event. Site Conceptual Model Update In a letter dated February 8, 2002, the CVWB requested that SCM updates be included in quarterly monitoring reports for the Site and stated that, at a minimum, the SCM must include two