Laserfiche WebLink
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 /> • SO4 (may be formed as calcium polysulfide [CaSX] reacts with Cr+6 or other reducible <br /> species) <br /> • General parameters (indicator of redox conditions in the treatment zone) <br /> 4.2.1 Hexavalent and Total Chromium <br /> Analytical results for total dissolved Cr and Cr+6 collected for the injection program are <br /> summarized in Table 5 and presented on the graphs. Cr concentration contours for the first- <br /> quarter (January 2013) monitoring and sampling event are shown in Appendix A (Figure 1 <br /> [Shallow Zone] and Figures 2 and 3 [Intermediate Zone]). The second-quarter (April 2013) <br /> monitoring and sampling event total dissolved Cr concentrations are shown in Appendix A <br /> (Figure 4 [Shallow Zone] and Figures 5 and 6 [Intermediate Zone]). <br /> 4.2.1.1 Background and Compliance Wells. Initial background sampling for the full- <br /> scale in situ Cr+6 reduction treatment implementation was completed in January 2008. During <br /> the first and second quarters of 2013, Cr concentration detections in background and compliance <br /> wells ranged from non-detect (MW-201, MW-302, and MW-401 in January 2013) to 14 µg/L <br /> (MW-201 in April 2013). Cr was not detected in the background and compliance wells at <br /> concentrations above the WQO (50 gg/L) during the current reporting period. Table 6 presents <br /> the proposed upper background concentration limits for background wells in comparison to <br /> historical results and WQOs. Cr concentrations were below the proposed upper background <br /> concentration limits for the background and compliance wells. <br /> First and second quarter 2013 sampling for the background and compliance wells did not include <br /> Cr+6, consistent with MRP No. R5-2012-0814. <br /> 4.2.1.2 Injection Area Wells. Total dissolved Cr and/or Cr+6 were detected at <br /> concentrations above the WQOs in 17 of the 44 wells sampled during the first quarter 2013 and <br /> 13 of 27 wells sampled during second quarter of 2013 (not including the background and <br /> compliance wells). The highest reported concentration of total Cr and Cr+6 was 27,000 and <br /> 26,200 µg/L, respectively, at well EW-3 in April 2013. EW-3 is located in the North Yard at the <br /> former retort pit (source area). A summary of the concentration trends for wells where Cr <br /> © M W H 4-2 <br />