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2012 Annual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California February 2013 <br /> 8.0 RECOMMENDATIONS <br /> The CVRWQCB issued a new MRP, number R5-2012-0814, dated 5 September 2012 <br /> (CVRWQCB, 2012) which requires groundwater sampling during full-scale in situ treatment, <br /> and specifies the locations to be sampled, frequency of sampling, and analyses to be performed. <br /> SPX implemented the MRP No R5-2012-0814 as part of the fourth quarter 2012 monitoring <br /> program and will continue to implement the MRP going forward. <br /> SPX has performed the required sampling plus additional sampling or analyses for more detailed <br /> plume evaluation, for more than four years after completing the full-scale in situ treatment <br /> program. Data collected during this follow-up period have demonstrated that Cr and Cr+6 <br /> concentrations across large portions of the former groundwater plume have been reduced to <br /> below the WQO (50 µg/L) without a lasting accumulation of undesirable byproducts (i.e., SO4, <br /> As, or other metals). Therefore, consistent with the MNA approach identified in the RAIWP and <br /> the decision logic diagram (Figure 5), MWH has proposed reductions to the sampling frequency <br /> (Table 9) for select wells in the North Yard. <br /> Cr and Cr+6 concentrations at MW-213, EW-3, and TW-13 are fluctuating (Graphs 23, 48, 53). <br /> As presented in Section 4.2.1.2 the water table elevation in the Shallow Zone experienced 20+ <br /> foot decline and 15 foot increase over the past five years. The recent rise allows ground water to <br /> leach soluble hexavalent chromium from the high concentrations immediately under the retort pit <br /> area and could thus mobilize hexavalent chromium in the deeper portion of the vadose zone. <br /> Continued quarterly monitoring is recommend for these locations to assess the impact that <br /> fluctuating groundwater elevations are likely having on the long term contaminant <br /> concentrations. MW-213, EW-3, and TW-13 are located in the north yard and the groundwater <br /> extraction and treatment system provides hydraulic control of contaminants at these wells. <br /> Concentration increases at perimeter wells and off-site wells have not been observed, indicating <br /> that migration of contaminants is being controlled and is limited to confines of the North and <br /> South yards. <br /> (D MWH, <br /> 8-1 <br />