ARCHIVED REPORTS (2) - Laserfiche WebLink

Home Browse Search

2013 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site,Stockton,California August 2013 concentrations exceeded the WQO during the first and second quarters of 2013 is presented below. Wells where Cr concentrations have decreased to below the WOO. Quarterly sampling in the first half of 2013 showed that concentrations of total dissolved Cr and Cr+6 in well TW-13, which were below the WQO in third quarter 2012, increased above the WQO in fourth quarter 2012 (262 µg/L and 254 µg/L) and then decreased to below the WQO in the first quarter 2013. The increased concentrations of Cr and Cr+6 in well TW-13 in the fourth quarter of 2013 are not typical of the historic trends for this well (see Graph 53). Concentrations of Cr and Cr+6 decreased at monitoring well MW-321 (I-9), located in the South Yard area. The first and second quarter 2013 Cr concentration (<10 and 10 µg/L, respectively) and Cr+6 concentration (<0.5 and 7.16 µg/L, respectively) were both all-time lows (Graph 32). Wells where Cr concentrations are above the WOO. Quarterly sampling in the first half of 2013 showed that concentrations of total dissolved Cr and Cr+6 is above the WQO in wells MW- 4, MW-8, MW-102, MW-106, MW-204 (S-8), MW-205 (S-7), MW-213, MW-4475, MW-447D, MW-4485, MW-448D, MW-449S, MW-449D, MW-450S, MW-450D, EW-3, and I-11. Cr and Cr 6+ concentrations decreased during the reporting period at wells MW-8, MW-204 (S-8), MW- 213 (I-7), MW-447S, and MW-447D, . Notable trends in Cr and Cr6+ concentration changes during the reporting period are summarized below on an individual well basis. MW4: Concentrations of Cr and Cr 6+ increased relative to when the well was last sampled in third and fourth quarter 2012. Both the first and second quarter 2013 Cr and Cr+6 concentrations were all-time highs at the time of collection. Well MW-4 is located in the intermediate zone at the western edge of the North Yard area. MW-8: Concentrations of Cr and Cr 6+ decreased during the reporting period. As shown on Graph 6, concentrations at this well fluctuate but had been gradually increasing since mid-2008 through second quarter 2012. Concentrations declined in third and fourth quarter, and the first quarter results were the lowest since January 2011. MW-8 is located in the shallow zone adjacent to MW-4 at the western edge of the North Yard. © M W H 4-3