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Pilot Study Workplan November 7, 2014 Project#54.62583.0001 Page 7 chromium by EPA Method 7199, volatile organic compounds by EPA Method 82606, and arsenic, total chromium, copper, total iron, total manganese, and zinc by EPA Method 6020. The three soil samples collected from S611 were analyzed for Hexavalent chromium and total chromium. Results of previous site assessment show that the extent of impacted soil is limited to the immediate vicinities of the closed vaults. 4.2 POLLUTANTS IN GROUNDWATER The monitoring well network at the site consists of eight wells screened in the shallow aquifer (MW-1 through MW-8) and one well screened in the deeper aquifer (MW-9). Quarterly or semiannual monitoring has been performed since the wells were installed. The most impacted well at the site is MW-3, located adjacent to Vault 8. Concentrations of hexavalent chromium detected in the well have increased over time, likely due to migration from soil. The highest concentrations of hexavalent chromium in the well have been detected in groundwater samples collected in more recent years, ranging from 0.72 mg/L in August 2006 to 10 mg/L in April 2009. The second most impacted well is MW-6, located immediately downgradient from MW-3. Hexavalent chromium concentrations in groundwater samples collected from MW-6 in more recent years range from 0.025 mg/L in November 2006 to 0.348 mg/L in November 2012. The range of concentrations of hexavalent chromium in the remaining shallow monitoring wells, MWA, MW-2, MW-4, MW-5, MW-7, and MW-8 are generally similar and have been reported from nondetect at <0.001 mg/L to 0.077 mg/L. The concentrations of hexavalent chromium in the deeper screened monitoring well, MW-9, range from 0.0013 mg/L to 0.05 mg/L. Groundwater analytical data is summarized in Table 2. Figure 4 shows the lateral distribution of hexavalent chromium in groundwater at the site. 5.0 BENCH SCALE TREATABILITY TESTING In order to evaluate the feasibility of calcium polysulfide in site hexavalent chromium reduction as a remedial technology for the Diamond Foods site, ATC Associates Inc. (now Cardno ATC) submitted soil and groundwater samples to Prima Environmental in 2005 for bench-scale treatability testing using both Fe (II) and calcium polysulfide. The calcium polysulfide used in the treatability study was a commercially available product known as Cascade®, which is also available under the trade name CalmetO. Cascade® is a 29% aqueous calcium sulfide solution designed as a metal precipitating agent for use in various treatments systems. The bench-scale groundwater treatability testing was performed on the water collected from monitoring well MW-3 at the Diamond Foods site in which hexavalent chromium was detected at a concentration of 4.2 mg/L. The testing was conducted in the presence and absence of soil to assess the potential for in situ remediation. Laboratory testing demonstrated that Cascade® could decrease the concentration of hexavalent chromium in groundwater in the presence and absence of soil. Specifically, in the absence of soil, Cascade® applied at concentrations of both 4.0 and 0.8 ml of Cascade® per L of groundwater (ml/L) decreased the concentration of hexavalent chromium in groundwater from the initial concentration of 7.90 mg/L to below the analytical