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ENS& <br /> ENSR International <br /> 10411 Old Placerville Road <br /> Suite 210 <br /> Sacramento,CA 95827-2508 <br /> November 24, 2004 (916)362-7100 <br /> FAX (916)362-8100 <br /> D ensr.com <br /> ' Mr. James Barton <br /> Regional Water Quality Control Board NOV z 9 2004 <br /> ' Central Valley Region ENVIRONMENT <br /> 11020 Sun Center Drive, Suite 200 PERMIT/SERVICES <br /> Rancho Cordova, CA 95670 <br /> RE: Addendum to the Geochemical Evaluation <br /> Regarding Potential Chromium <br /> ' Mobilization During Ozone Treatment <br /> Former Unocal Station No. 5098 at 5606 Pacific Avenue, and Former Montgomery Wards <br /> Auto Service located at 5400 Pacific Avenue, Stockton California <br /> ' Dear Mr. Barton: <br /> On behalf of Union Oil Company (Unocal) of California, and in accordance with the California <br /> Regional Water Quality Control Board, Central Valley Region (RWQCB) letter dated <br /> October 4, 2004, ENSR Corporation (ENSR) has prepared this addendum to ENSRs <br /> Geochemical Evaluation Regarding Potential Chromium Mobilization During Ozone Treatment <br /> dated November 2, 2004. In an RWQCB letter dated November 22, 2004, the Geochemical <br /> Evaluation was approved pending the submittal and approval of this addendum. This addendum <br /> provides a work plan for a Bench Scale test for the former Unocal facility. <br /> Proposed Work Plan <br /> Soil and water samples will be sent to Prima Environmental (Prima) of Sacramento, California <br /> to conduct bench scale testing to evaluate the following: <br /> • Determine whether ozonation of site materials can remove chemicals of concern (CDCs) <br /> • Determine whether removal is due to destruction and or volatilization <br /> • Estimate the ozone demand of soil and water <br /> ' Determine the effect of ozonation on secondary groundwater parameters <br /> • Assess the potential formation and fate of chromium VI (Cr(VI)) <br /> • Measure changes in total suspended solids (TSS) due to ozonation in order to assess <br /> potential of in-situ treatment to reduce permeability of the aquifer <br /> Methods and procedures used by Prima to conduct the testing are included in Attachment A. The <br /> activities outlined will begin no later than 15 days after the attached workplan is approved by the <br /> RWQCB. Once the samples are submitted, the bench scale test is expected to take a minimum <br /> of 45 days. <br /> \\Dcsacrramento\projects\Projects\0694015098-293\FinaR5O98 GeoChem Eval Addendum.doc <br />