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N <br /> may affect chromium aquifer geochemistry A general review of chromium geochemistry in aquifers is <br /> j attached (Attachment C) <br /> Chromium was not detected in the groundwater from MW-3, indicating that it is not mobile under <br /> aquifer conditions at the time the sample was taken In most reported cases of metal mobilization b <br /> i in-situ oxidation, metal concentrations decreased as the aquifer returned to its pre--treatment <br /> oxidation-reduction (redox) state (Crime, 2003, USEPA, 2002) It is thus likely that, if mobilized by <br /> ozone treatment, chromium levels would decline as the aquifer returned to its pre-treatment redox <br /> condition <br /> In the past, the aquifer was probably anaerobic due to <br /> the presence of gasoline and gasoline <br /> components The effect of the air-sparging system, which has been in operation at the site, on <br /> groundwater redox values is not certain as analyses for redox sensitive parameters have not been fully <br /> completed In anaerobic aquifers, chromium is reduced to Cr(III) Cr(III) forms insoluble chromium <br /> hydroxide and insoluble chromiumfiron hydroxide minerals (Cr(OH)3 and (Cr, Fe)(OH)3) These <br /> minerals are slow to oxidize even in the presence of oxygen without catalysis from manganese oxide <br /> surfaces Thus, if reduced chromium species existed on aquifer solids before sparging began, it is <br /> likely that some still remain bound to the sediments <br /> If Cr(III) was oxidized to Cr(VI) during ozone treatment, the resulting chromate anion could be more <br /> mobile in the aquifer than Cr(Ill) Chromate levels in aquifers are usually controlled by adsorption to <br /> iron oxyhydroxide minerals The extent of chromate adsorption depends on several environmental <br /> factors Chromate adsorption decreases with increasing pH with significant a drop in adsorption <br /> observed by pH 8 Adsorption will also decrease with increasing levels of anions that compete for <br /> adsorption sites (e g silicate, arsenate, phosphate, sulfate) Adsorption will increase with increasing <br /> iron oxyhydroxide surface area(Rai 1989, Richard 1991, Palmer 1991) <br /> Generally, aquifer H values Y q p es measured at the Former Unocal Station No 5098 are near neu#ral, with _ <br /> pH values of 8 or-near 8-reported for some wells It is-not certain how ozone treatment will affect <br /> aquifer pH Oxidation reactions generally produce acidity, however, which would lower pH and <br /> increase chromate adsorption If sulfide minerals are present in the sediments, oxidation of these <br /> sulfides by ozone could increase sulfate concentrations High sulfate levels might lower chromate <br /> adsorption through competition If the aquifer were iron-reducing before ozone treatment, ozone <br /> oxidation of dissolved reduced iron (Fe(11)) to insoluble oxidized iron (Fe(III)) could increase iron <br /> hydroxide surface areas, increasing chromate adsorption Also, it is not known if sulfide minerals are <br /> found on aquifer solids At present, analyses that will help elucidate the aquifer redox state have not <br /> been completed When completed, these analytical results will be closely examined and used to help <br /> design a bench scale study to estimate the extent of chromium mobilization during aquifer ozone <br /> treatment All bench scale tests recommended in this evaluation should use aquifer materials from <br /> gasoline impacted aquifer zones <br /> C\Oocurrtems and <br /> Settings\cgohnsonOesktcp%UnocallGeochemEval_D 3 November 2004 <br /> oc doc <br />