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f <br /> Introduction <br />' Thiseo <br /> g chemical evaluation describes how aquifer geochemistry, and particularly chromium <br />' chemistry, might affect and be affected by the proposed in-situ oxidation treatment at the Former <br /> Unocal Station No 5098 at 5606 Pacific Avenue, and Former Montgomery Wards Auto Service <br /> located at 5400 Pacific Avenue, Stockton, California (Figures 1 and 2) When available data is <br /> insufficient to draw a conclusion, a method to obtain the needed information is recommended Topics <br /> discussed include <br /> o Comparison of groundwater and aquifer sediment constituent concentrations to appropriate <br /> regulatory benchmarks, <br /> o Discussion of how the ozone treatment might affect chromium groundwater concentrations, <br /> o Calculation of chromium levels in groundwater under a worst-case mobilization scenario, <br /> o Calculation of the aquifer's capacity to resist a drop in pH, and, <br /> o Discussion of how the aquifer's q er's reduction capacity could reduce the efficacy of the ozone <br /> treatment <br /> Sample Collection and Analysis <br /> Two soil samples were collected from MW-9R, 55 ft and 60 ft below ground surface {b s} in the <br /> saturated zone (Fig 2) The samples from these two depths were combined prior to analysis <br /> Groundwater from MW-3 was analyzed for total and hexavalent chromium Groundwater from MW-1 <br /> was analyzed for metals and other groundwater constituents Samples analyzed for metals were field <br /> filtered _ The aquifer sediment samples and-both groundwater samples were collected from the same <br /> geologic unit Laboratory data sheets, quality control test results, and chain of custody forms are <br /> included in Attachment B EPA Method 3050 was used to digest the soil samples before metals <br /> analyses Aqueous soil extracts are reported as a 1 10 dilution, mass soil mass water <br /> MW-1 is within the proposed ozone injection area MW-3 and MW-9R are outside this area Use of <br /> the sediment samples from MW-9R was approved by the California Regional Water Quality Control <br /> Board, Central Valley Region, since there should not be any significant differences in intrinsic soil and <br /> ground water parameters between these well locations <br />�i <br /> 13'Documents and <br /> SethngslcgohnsonlDesktaplUnocailGeochemEvaJ_D 1 November 2004 <br /> oc doe <br />