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.� California RE'onal Water Quality ( ,ntrol Board -: <br /> Central Valley Region <br /> Robert Schneider,Chair gc'°ltdE1�'j Ai. HEAL}11 <br /> Winston H. Hickoz I FSi•t/T�;�' Gray Davis <br /> Secretary for Sacramento Main Office Governor <br /> Environmental Imcme[Address: http://www.swreb.ca.gov/-rwgcb5 <br /> Protection 3443 Routier Road,Suite A,Sacramento,California 958271n3 JUL 30 PM 2' '`e <br /> Phone(916)255-3000-FAX(916)255-3164 <br /> 29 July 2003 <br /> Mr. Mike Lescisin <br /> Diamond Walnut Growers Inc. <br /> 1050 South Diamond Street <br /> Stockton, California 95201 <br /> REVIEW OF CONCEPTUAL REMEDIATION DESIGN, DIAMOND WALNUT <br /> GROWERS, STOCKTON PLANT, SAN JOAQUIN COUNTY <br /> We have reviewed the 25 June 2003 Cleanup Plan/Conceptual Remediation Design (Conceptual <br /> Plan) submitted by ATC Associates Inc. (ATC) on behalf of Diamond Walnut Growers Inc. <br /> (DWG) for its Stockton Plant. The Conceptual Plan contains background information that <br /> details site investigation work performed thus far and proposes a conceptual remedial design. <br /> The Conceptual Plan states, "As a combined remediation effort of soil and groundwater,we <br /> propose to extract shallow groundwater,remediate the groundwater, introduce a ferrous <br /> hydroxide solution, and reinject the treated groundwater near Vaults 7 and 8 to reduce Cr VI in <br /> the subsurface to Cr III." We have the following comments on the Conceptual Plan. <br /> 1. The Conceptual Plan says, "The remediation system consists of shallow groundwater <br /> extraction... [from] one extraction well installed ... near Vaults 7 and 8." Groundwater <br /> extraction should occur from the source area that shows the highest concentration of the <br /> pollutant. Monitoring well MW-3,which is an angle boring terminating beneath Vault 8, <br /> routinely contains the highest pollutant concentration. During the 22 April 2003 sampling <br /> event MW-3 contained hexavalent chromium at 3,800 µg/1. Extraction should occur from <br /> the area of highest pollutant concentration, which reports show is beneath Vault 8. <br /> The depth to groundwater in MW-3 has varied from 80 feet below ground surface (bgs) to <br /> 47 feet bgs. The design of the shallow aquifer extraction well should consider this variation <br /> in depth to groundwater. <br /> 2. DWG proposes to treat the extracted groundwater using an"electrochemical cell with <br /> sacrificial iron anodes." This makes sense, because the electrochemical cell produces <br /> ferrous hydroxide, which reacts quickly and is oxidized to ferric hydroxide while the <br /> hexavalent chromium is reduced to trivalent chromium. Typically, a flocculation tank is <br /> incorporated into the treatment chain to facilitate the removal of the iron, chromium and <br /> California Environmental Protection Agency <br /> Q? <br /> Rerycled Paper <br /> The energy challenge facing California is real. Every Californian needs to take immediate action to reduce energy consumption. For a list of <br /> simple ways you can reduce demand and cut your energy costs,see our Web-site at http://www.swrcb.ca.gov/rwgcb5 <br />