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2012 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California October 2012 8.0 RECOMMENDATIONS The CVRWQCB issued a new MRP, number R5-2012-0814, dated 5 September 2012 (CVRWQCB, 2012) which requires groundwater sampling during full-scale in situ treatment, and specifies the locations to be sampled, frequency of sampling, and analyses to be performed. SPX will implement the MRP No R5-2012-0814 as part of the fourth quarter 2012 monitoring program. SPX has performed the required sampling, often including additional locations or analyses for more detailed plume evaluation, for more than four years after completing the full-scale in situ treatment program. Data collected during this follow-up period have demonstrated Cr and Cr+6 concentrations across large portions of the former groundwater plume have been reduced to below the WQO (50 µg/L) without a lasting accumulation of undesirable byproducts (i.e., SO4, As, or other metals). Therefore, consistent with the MNA approach identified in the RAIWP and the decision logic diagram (Figure 5), MWH has proposed reductions to the sampling frequency (Table 8) for select wells in the North Yard. Cr and Cr+6 concentrations at EW-3 and MW-213 are fluctuating. As presented in Section 4.2.1.2 the water table elevation in the Shallow Zone experienced 20+ foot decline and 15 foot increase over the past five years. The recent rise allows ground water to leach soluble hexavalent chromium from the high concentrations immediately under the retort pit area and could thus mobilize hexavalent chromium in the deeper portion of the vadose zone. Continued quarterly monitoring is recommend for these locations to assess the impact that fluctuating groundwater elevations are likely having on the long term contaminant concentrations. EW-3 and MW-213 are located in the north yard and the groundwater extraction and treatment system provides hydraulic control of contaminants at these wells. Concentration increases at perimeter wells and off-site wells have not been observed, indicating that migration of contaminants from EW-3 and MW-213 is being controlled and if occurring is limited to confines of the North and South yards. The newly installed wells MW-449S and MW-449D, installed in the North Yard, and MW-450S and MW-450D, installed in the South Yard, will help provide additional information on the distribution of Ct and Cr6+ concentrations in the intermediate zone. Data from these newly installed wells will be documented in the 2012 Annual Report, which is due by February 2013. 9 mm" 8-1