ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec�9 use of PCE by dry cleaners in Ripon has been verified, and dry cleaners typically discharge to the sanitary sewer (Izzo, 1992). Direct Infiltration of Untreated Industrial Wastewater at the WWTF Lagoons: Inflows from the industrial sewer were percolated into shallow soils adjacent to the Stanislaus River (Section 4.5.2). TCE present in this wastewater stream would have been discharged along with other biodegradable organics (domestic sewage and high BOD industrial wastewater), causing the natural breakdown of TCE to less chlorinated compounds, including cDCE and vinyl chloride. Much of the infiltrated wastewater would have rapidly discharged to the Stanislaus River, because the lagoons are at a higher elevation and land surface gently slopes towards the Stanislaus River (Figure 35). Recharge of deeper (A-zone and C-zone) groundwater also appears to have occurred, because of steep downward gradients due to the presence of the hydraulic mound due to surface discharge to groundwater (discussed in Section 4.6.6). Direct Discharge of Untreated Industrial Wastewater to the Stanislaus River: During much of the 1960s the WWTF ponds were at capacity. Industrial wastewater was reportedly directly discharged to the Stanislaus River (likely at a rate of several hundred gpm; Section 4.5.2). Recent information (2000 — present) indicate that Stanislaus River flows are typically 400 - 500 cubic feet per second (cfs)for much of the year, although seasonal highs occur, and the high stage of early 2006 resulted in a peak flow higher than 6,000 cfs (Appendix E). Assuming an average historical flow of 400 cfs (approximately 180,000 gpm) during the 1960s, discharges of industrial flows would likely have been diluted by a factor of 1,000 or more and infiltration to groundwater would only occur during times when the river stage was higher than groundwater, which is unlikely if the WWTF ponds were at capacity. Therefore, it can be assumed that discharges of industrial wastewater to the Stanislaus River were unlikely to have a long-term substantial impact on groundwater quality. 4.6.7 Historical Migration of COCs in Groundwater from Offsite Sources Industrial sewer leaks located farther from the WWTF, away from the mounds (such as near M-813 near Stockton Avenue and 4th Street), would have been subject to relatively narrow flow paths towards pumping wells N-1, N-3, and perhaps to a less extent, well MW-1. This pattern generally fits with the observed distribution of B-zone CDCs (primarily TCE). The discharge of CDCs at the WWTF lagoons would have initially been subject to a flow direction that was primarily northward (Figure 29), until the mid-1960s when Simpson Paper began pumping from wells S-1, S-2, and S-3 in 1966. By this time, COCs may have migrated farther to the north towards wells N-1 and MW-1, but a shift in flow direction near the WWTF towards the east, particularly pumping at S-2, would have promoted an easterly migration of AMEC Geomatrix, Inc. hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 50