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2013 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site,Stockton,California August 2013 <br /> 7.0 RECOMMENDATIONS <br /> SPX will continue to optimize performance monitoring. MRP No. R5-2012-0814 (CVRWQCB, <br /> 2012) outlines that following four quarters of performance monitoring, following an injection <br /> event, sample frequencies may change. The third quarter 2013 sampling of the South Yard <br /> marks the fourth quarter of performance monitoring and SPX anticipates reducing the number of <br /> samples collected quarterly at the South Yard as part of the fourth quarter 2013 monitoring <br /> event. The reduction determination will be made based on the analytical results. Cr and Cr+6 <br /> concentrations at MW-4, MW-449D, MW-4505, and EW-3 have increased (Graphs 3, 44, 45, <br /> and 48). As presented in Section 4.2.1.2 the water table elevation in the Shallow Zone <br /> experienced 20+ foot decline and 15 foot increase over the past five and a half years. The recent <br /> rise allows ground water to leach soluble hexavalent chromium from the high concentrations <br /> immediately under the retort pit area and could thus mobilize hexavalent chromium in the deeper <br /> portion of the vadose zone. Continued quarterly monitoring is recommended for these locations <br /> to assess the impact that fluctuating groundwater elevations are likely having on the long term <br /> contaminant concentrations. Groundwater extraction and treatment provides hydraulic control of <br /> contaminants at these wells. Concentration increases at perimeter wells and off-site wells have <br /> not been observed, indicating that migration of contaminants is being controlled and is limited to <br /> confines of the North and South yards. <br /> (a mm" <br /> 7-1 <br />