ARCHIVED REPORTS (3) - Laserfiche WebLink

Home Browse Search

2011 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California January 2012 October 2011. Table 6 presents the proposed upper background concentration limits for background wells in comparison to historical results and WQOs. 4.2.4.2 Injection Area Wells. During the third and fourth quarters of 2011, the As concentrations in injection area wells ranged from below the RL of 5 µg/L in 4 wells to 17 µg/L (MW-6 in July 2011 and TW-13 in October 2011). As was detected at concentrations above the WQO in wells MW-104, TW-12, and TW-13 during the third and fourth quarters of 2011; As was also detected above the WQO in wells MW-6 and TW-14 during the third quarter of 2011 but not the fourth quarter. As concentrations in well TW-13 indicate a decreasing trend. The As concentrations in MW-104 have increased over the first and second quarter 2011 results but are comparable to the third and fourth quarter 2010 results. As concentrations in TW-12 have an increasing trend since October 2010. This well will continue to be monitored and verify if the concentration is going to continue rising. As the groundwater becomes re-oxidized, the As III is anticipated to convert to less-soluble As V. No additional action is recommended at this time regarding the observed As concentrations. 4.2.5 Sulfate SO4 is a common byproduct of the chemical reactions caused by the use of CaSx (reductant solution) for groundwater remediation. The reductant solution used included adding ethanol as a carbon source, to aid in the growth of sulfate-reducing bacteria (SRB), which have the ability to reduce SO4 back to the sulfide form, thus enhancing the reduction and the ultimate precipitation as elemental sulfur. During Phase II, no significant long-term increase in the SO4 concentration was noted. SO4 concentrations in groundwater were monitored before and after injection. Analytical data for SO4 sampled during the injection program are summarized in Table 5 and presented on Graphs 1 through 53. 4.2.5.1 Background and Compliance Wells. During the third and fourth quarters of 2011, the SO4 concentrations in background and compliance wells remained low. SO4 concentrations in background and compliance wells ranged from 5.6 milligrams per liter (mg/L) (MW-325 in July 2011) to 54.1 mg/L (MW-201 in July 2011). SO4 was not detected at concentrations above the WQO (250 mg/L) during the current reporting period. Table 6 presents (a MWH 4-6