ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec- 4.7 IMPLICATIONS The findings and interpretations presented in this Section have the following implications: • Historical discharge of untreated industrial wastewater containing COCs at the WWTF lagoons explains the widespread distribution of COCs in Upper Aquifer (A-zone) and Intermediate Aquifer groundwater beneath the Study Area and Site. • Because of the highly variable release history and historical variability in groundwater flow directions, concentration versus time trends are difficult to interpret. However, at a general level, COC concentrations have been decreasing over time. This decrease is expected to continue as CDCs are naturally degraded within the aquifer system. • A likely explanation for COC impacts south of the Stanislaus River is southerly migration of COCs from the WWTF, enhanced by the hydraulic mounds formed due to WWTF and Neenah Paper recharge, and by irrigation pumping. Deeper impacts to the Lower Aquifer are likely related to the former Fiscalini irrigation well, which was recently properly decommissioned on behalf of Nestle. Overall, groundwater quality in the Lower Aquifer is expected to improve as a result of decommissioning this well. Continued monitoring will assess the fate of CDCs south of the Stanislaus River. • Because COC discharge likely occurred sometime in the 1950's and 1960's, response actions that focus on the WWTF area are unlikely to have any substantial beneficial effect on groundwater quality— in essence, disposal practices since the 1950's have caused the current groundwater contamination. CDCs have been migrating northward beneath the Study Area for several decades. • The aquifer system may be best managed by ensuring that no further COC discharges occur, including discharges to the WWTF, discharges from the Site (by advection of Upper Aquifer Zone groundwater, currently contained by IRAP pumping), discharges from abandoned dry cleaner sites, and vertical migration via conduit wells. Additional information on past COC use within the Study Area is needed, and consideration should be given to the identification and assessment of other potential COC source areas, as appropriate. • Given that multiple parties have likely discharged CDCs to groundwater and given that multiple parties have impacted groundwater flow, the management of COC- affected groundwater will likely need to be a multi-party cooperative effort. A concerted effort to manage groundwater withdrawals such that the impacts to receptors (i.e. supply wells) are minimized may be considered. • Groundwater extraction near the WWTF is not feasible for many practical reasons: 1) since the COC release likely occurred decades ago, and has been followed by several decades of recharge by presumably COC-free water, groundwater extraction will likely capture water with very low COC concentrations, AMEC Geomatrix, Inc. hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 52