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Workplan for Additional Pilot Study May 25, 2018 <br />Injection Testing Page 1 <br />1.0 PURPOSE <br />On behalf of Diamond Foods, Inc., ATC Group Services LLC (ATC) has prepared this workplan to <br />perform additional pilot study testing to evaluate the effectiveness of in-situ chemical reduction for site <br />remediation at the facility located at 1050 South Diamond Street in Stockton, California. <br />In April 2014, Cardno ATC (now ATC) prepared a Feasibility Study/Remedial Action Plan (FS/RAP) that <br />analyzed remediation alternatives for reducing hexavalent chromium in soil and groundwater. Calcium <br />polysulfide (Cascade®) was identified as a potential remedial technology for mitigation of hexavalent <br />chromium impacts at the site, and a 2005 treatability study found that the technology could successfully <br />be applied at the site (Prima, 2005). The FS/RAP recommended performing a pilot test in order to <br />determine the effectiveness of in-situ injection of ferrous iron solution and calcium polysulfide. A pilot <br />study workplan was requested by the Central Valley Regional Water Quality Control Board in <br />correspondence dated September 18, 2014. <br />In June 2015, ATC performed the first pilot test consisting of the injection calcium polysulfide into the <br />water bearing formation below the site via existing well MW-3 to remediate hexavalent chromium. <br />Historically, MW-3 has been the most impacted well at the site. The results indicate that more than two <br />years after the injection of calcium polysulfide, the reported hexavalent chromium concentration in this <br />well remains less than the laboratory detection limit and has not rebounded. <br />In February 2017, ATC performed additional remediation testing to treat hexavalent chromium in <br />groundwater and determine a radius of influence in the vicinities of monitoring wells MW-5 and MW-6 <br />and the shallow and deep observation wells. ATC supervised the injection of 4,700 gallons of CPS into <br />the water bearing formation below the site through eight temporary direct push borings placed <br />immediately adjacent to the wells. The reported Cr+6 concentrations detected in the groundwater <br />samples collected from these wells have all exhibited a downward trend subsequent to injection <br />activities. <br />Based on the results of the pilot testing, ATC is proposing additional injection of CPS in the source area <br />to further reduce the highest concentrations of hexavalent chromium in groundwater at the site. <br />2.0 BACKGROUND <br /> <br />2.1 DESCRIPTION OF FACILITY <br />Diamond Foods, Inc. operates a food production facility located northeast of the intersection of Charter <br />Way and South Diamond Street in Stockton, California (Figure 1). The facility is approximately <br />755,000 square feet in size. Walnuts and other types of nuts are processed and packaged at the <br />facility. The site boundaries along with pertinent features are shown on Figures 2 and 3. <br /> <br />2.2 PREVIOUS INVESTIGATIONS AND REMEDIAL ACTIVITIES <br />The Diamond Foods facility has been primarily operating as a walnut processing plant since the 1950s. <br />Five underground concrete vaults were located north and east of the main processing building and <br />formerly contained refrigerants used in the plant cooling systems. Several of these vaults are believed <br />to be the source of the hexavalent chromium impacts at the site.