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2011 Annual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California January 2012 <br /> 8.0 RECOMMENDATIONS <br /> The MRP requires groundwater sampling during full-scale in situ treatment, and specifies the <br /> locations to be sampled, frequency of sampling, and analyses to be performed (CVWRQCB, <br /> 2007). SPX has performed the required sampling, often including additional locations or <br /> analyses for more detailed plume evaluation, for more than 3 years after completing the full- <br /> scale in situ treatment program. Data collected during this follow-up period have demonstrated <br /> Cr and Cr+6 concentrations across large portions of the former groundwater plume have been <br /> reduced to below the WQO (50 µg/L) without a lasting accumulation of undesirable byproducts <br /> (i.e., SO4, As, or other metals). Furthermore, conditions in many areas across the site are <br /> conducive to further attenuation of Cr in groundwater (for example, soluble forms of Fe and Mn <br /> that support Cr reduction, and TOC and SO4 to support Cr removal by SRB). Therefore, <br /> consistent with the NINA approach identified in the RAIWP and the decision logic diagram <br /> (Figure 5), MWH has proposed reductions to the sampling frequency. This reduction is outlined <br /> in the Request for Adjustment to Well Sampling Schedule (MWH, 2011c) which was submitted to <br /> the CVRWQCB and DTSC on 7 October 2011. <br /> As noted in Table 8, an additional round of in situ treatment injections is being considered to <br /> expedite Cr reduction in areas where concentrations are above the WQO (50 µg/L) and <br /> groundwater conditions appear less conducive to natural attenuation. Any such injection <br /> program would be coordinated with the State agencies. <br /> (a MWH <br /> 8-1 <br />