Laserfiche WebLink
amec�9 <br /> The results of a pilot test for carbohydrate injection performed at the Site showed that when <br /> geochemical conditions favor biotransformation of TCE to cDCE, concentrations of arsenic in <br /> groundwater are increased to levels above the 10 pg/L (the Maximum Contaminant Level for <br /> drinking water). This result likely occurs because of the reductive dissolution of aquifer solids <br /> that contain arsenic (Geomatrix, 2006). Given the widespread distribution of cDCE beneath <br /> the Study Area, it is likely that the disposal of organic rich wastewaters at the WWTF have <br /> created conditions that favor reductive dissolution of arsenic from aquifer solids. This is an <br /> important concern for groundwater pumping near the WWTF. <br /> 4.6.4 Lower Aquifer— D-Zone Wells <br /> The spatial distribution of the maximum TCE, cDCE, and vinyl chloride concentrations <br /> reported from groundwater samples collected from the Lower Aquifer (D-zone monitoring <br /> wells) is shown in Figure 33. The highest concentrations of VOCs detected in D-zone wells <br /> were reported for Neenah Paper Company well TH-10. This well is constructed with well <br /> screens in both the Intermediate and Lower Aquifer, at depths between 120 and 210 feet bgs. <br /> The TCE, cDCE, and vinyl chloride concentrations reported for the April 2008 sample were 61, <br /> 170, and 250 pg/L, respectively. Downward flow was documented in this well at a rate of <br /> 0.4 gpm (ECM, 2008c). Therefore, samples from this well represent a starting concentration <br /> for D-zone VOC impacts near the lagoons, where VOCs from the Intermediate Aquifer bypass <br /> the Corcoran Clay and enter the Lower Aquifer. This is the first documented VOC source for <br /> the Lower Aquifer; although others likely exist, in particular, non-utilized municipal wells MW-4 <br /> and MW-6. <br /> The concentration versus time plot for Lower Aquifer well M-6D is shown in Appendix C. TCE <br /> and cDCE were not detected in this well from when it was first sampled in March 1990, until <br /> July 1991, when both TCE and cDCE were reported at concentrations of 1.4 and 2.2 pg/L, <br /> respectively, and concentrations subsequently increased to a 13 and 9.8 pg/L, respectively, in <br /> July 1994, before declining to non-detect by October 1996. Both TCE and cDCE were not <br /> detected in groundwater samples from M-6D until May 2003 (when they were reported at <br /> concentrations of 4.9 and 0.7 pg/L, respectively). Concentrations of TCE and cDCE then <br /> increased to maximum values of 76.1 and 41 pg/L, respectively in groundwater samples <br /> collected from M-6D in May 2007, and decreased slightly since then. The concentration versus <br /> time history for TCE and cDCE is complex because of the nature of conduit-type release <br /> mechanisms, which give rise to relatively narrow dissolved plumes, and the temporal <br /> variations of groundwater use within the Study Area. The period of time where TCE and cDCE <br /> were not detected in groundwater samples from M-6D is from 1996 —2002. As shown in Table <br /> 5, many changes to the groundwater flow system in the Intermediate and Lower Aquifers likely <br /> occurred during this time, caused by: <br /> AMEC Geomatrix, Inc. <br /> hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 47 <br />