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amec— <br /> COCs. <br /> By the mid-1970's, Simpson Paper began discharging its industrial wastewater to the <br /> ground for direction infiltration, which would have caused localized groundwater mounding. <br /> This mounding would have been enhanced after Simpson stopped pumping from wells S-1, S- <br /> 2 and S-3 in the early 1990s, and began pumping from S-4 and S-5 near the northeast portion <br /> of the property, see Table 5). At this time, if wells such as S-2 were idle, downward flow <br /> through these and other wells at relatively high rates is likely, because of the strong downward <br /> gradients due to surficial recharge. Also, during this time, horizontal flow directions towards the <br /> south, beneath the Stanislaus River, may have been enhanced because of the combined <br /> effects of hydraulic mounding at the WWTF and paper company properties. This phenomenon <br /> has been observed during recent monitoring events (Appendix F). <br /> Therefore, COC impacts to the Upper and Intermediate Aquifer domestic and monitoring wells <br /> south of the Stanislaus River appear to be related to the southwesterly migration of CDCs <br /> from the east WWTF lagoon area, consistent with reported discharge of untreated industrial <br /> wastewater at this area. This transport would have been facilitated by irrigation pumping south <br /> of the Stanislaus River, causing deeper groundwater (A-zone and C-zone) to flow beneath the <br /> Stanislaus River depths. Shallow groundwater is probably prevented from flowing beneath the <br /> Stanislaus River which acts as a hydraulic boundary (either a recharge or discharge boundary, <br /> depending on the time of year). <br /> Directions of the lateral hydraulic gradient in the Lower Aquifer historically have been toward <br /> the west and have not been influenced by surficial recharge processes in the Study Area. This <br /> suggests that the Corcoran Clay which overlies the Lower Aquifer is an effective aquitard, <br /> isolating the Lower Aquifer from the Intermediate and Upper Aquifer flow systems. Therefore, <br /> vertical migration of groundwater through the Corcoran Clay is negligible absent the presence <br /> of vertical conduits (unpumped wells screened across the Corcoran Clay). Vertical gradients <br /> have increased across the Corcoran Clay recently, with greater than 10 feet of head difference <br /> between collocated C-zone and D-zone wells. This increase in downward vertical gradient will <br /> increase the rate of groundwater flow and the rate of COC migration through conduit wells. <br /> Other conduit wells may exist near the WWTF lagoons and at locations throughout the Study <br /> Area. If they exist, conduit wells could give rise to relatively narrow, localized VOC plumes, in <br /> which the distribution of VOCs likely reflects the complex history of temporally variable vertical <br /> gradients, conduit wells, and pumping. A similar mechanism for VOC impacts would explain <br /> the variable concentration trends for M-6D. Although a specific conduit well near M-6D has yet <br /> to be identified, a conduit well is likely located north of the WWTF lagoons, closer to the Site <br /> and M-6D. Nestle is continuing to identify any wells that may serve as conduits to the Lower <br /> Aquifer so that they can be properly decommissioned (ECM, 2008c). <br /> AMEC Geomatrix, Inc. <br /> I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 51 <br />