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Comparison of analysis results to regulatory benchmarks The California EPA Maximum Contaminant Levels (CA MCLs) represent the water quality objectives at this site Groundwater constituent levels did not exceed any CA MCLs (Table 1) (CA Code of Regulations, 2003) Environmental Screening Levels (ESLs) set by the San Francisco Bay California Regional Water Quality Control Board (CRWQCB) were compared with groundwater constituent concentrations when a CA MCL was not available (CRWQCB, 2004) The ESLs serve as screening values and represent a conservative substitute when no CA MCL exists For constituents that did not have a CA MCL, groundwater levels at the site did not exceed ESLs (Table 1) In particular, no chromium was found in the groundwater For a few constituents, the method detection limits exceeded the CA MCL or ESL values (Table 1) Complete analysis results are reported in the laboratory data sheets (Attachment B) Metal levels measured in aquifer sediments were compared with the ESL values appropriate for deep soils (> 3 m bgs) in a current or potential drinking water aquifer with residential sod usage None of the metals detected in the sediments exceeded the corresponding ESLs (Table 2) Chromium total soil concentrations fell below total sal ESL and Cr(Ill) ESL values, but exceeded the ESL value for Cr(VI) a Because chromium mobilization during ozone treatment is of special concern at this site it is useful compare the chromium levels in aquifer sediments to levels reported in the literature for similar soil types Chromium levels well above literature values could signal some unusual geochemistry or an anthropogenic input The chromium content of natural minerals and sods varies widely with the type and nature of the rocks or sediments The lowest natural chromium levels are generally found in granites (1 —26 mg/kg), carbonates (1 — 16 mg/kg) and sandy sediments (16— 36 mg/kg) In general, the highest chromium content tends to be associated with the finest grain size sods (Alloway 1990, Richard 1991) From boring logs, sediments retrieved from MW-913 consist primarily of well graded coarse to medium sand with 20%fine gray sands (55 ft) and well graded silty sand transitioning to silty clay (60 ft) The sample was homogenized before analysis so constituents from both soil textures ' should-be present -The-total chromium level measured in the sediments from MW-9R, 16 mg/kg, is consistent with the lower chromium levels previously reported in sandy sediments The presence of finer sediments in MW-9R at 60 ft bgs did not elevate levels in the combined sample out of the normal range for sandy sediments, such as those found in MW-9R at 55 ft bgs Possible Chromium Mobilization During Ozone Treatment Mobilization of redox sensitive metals into the groundwater has been reported as a general consequence of in-s,tu oxidation treatments (Cnmi, 2003, USEPA, 2002) The California Regional Water Quality Control Board has expressed particular concern with possible chromium mobilization during ozone treatment, and chromium mobilization has been reported as a consequence of ' permanganate sod treatment-(Cnmi,-2003) This-evaluation-will-thus-focus-on-how-ozone treatment - - C\Documents and ' Settings%crlohnson\DesktopVUnccaJ\GeochemEval_D 2 November 2004 cc doc