ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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ameO potentiometric surface north of the WWTF could not be assessed. Water levels were generally about five to ten feet lower in July 1990 compared to July 2006. January 1990 (Figure 26): Six Intermediate Aquifer monitoring wells were monitored during this event; the general flow direction appears to be to the north towards Site production well N-1 and municipal supply well MW-4. Estimated Groundwater Flow Direction in 1974-1985 (Figure 27): As shown in Table 5, MW-7 was heavily utilized in 1985, with annual average production of greater than 550 gpm. Industrial wells N-1/N-3 and Paper Company wells were also highly utilized. This pumping scenario, with recharge at the WWTF and Paper Company Lagoons, would likely cause a northerly groundwater flow direction from the WWTF area towards N-1/N-3 and MW-7 pumping, and an easterly component of groundwater flow from the eastern WWTF area towards S-1 and S-3. Estimated Groundwater Flow Direction in 1966-1974 (Figure 28): As shown in Table 1, and Figure 28, N-1/N-3 were heavily utilized in 1974, with annual average production of greater than 600 gpm. Simpson Paper wells S-1, S-2, and S-3 and municipal wells MW-1 through MW-4 were also pumped, but at lower rates than wells N-1/N-3. This pumping scenario, with recharge at the WWTF and Simpson Paper property, would likely cause a northerly groundwater flow direction from the WWTF area towards wells N-1/N-3, MW-1 and MW-4, which together would have created a large cone of depression in the vicinity of the Site. A strong easterly component of groundwater flow from the eastern WWTF area towards S-2 also probably occurred, given the close proximity of S-2 to discharge at the unlined WWTF ponds. The influence of S-2 would be stronger at this time, because Simpson Paper was discharging its wastewater to the Stanislaus River, rather than direct infiltration to groundwater. No groundwater mounding conditions on the Simpson Paper area would have been occurring. The expansion of the WWTF ponds in 1972 would have allowed for greater quantities of groundwater recharge by domestic and industrial wastewater disposal at the WWTF, which may have enhanced groundwater flow velocities towards the pumping wells. Estimated Groundwater Flow Direction in 1957-1966 (Figure 29): As shown in Table 5, and Figure 29, wells N-1 and MW-1, MW-2 and MW-3 were the only known municipal and industrial wells operating in 1957. The 10 to 20 acres of WWTF disposal land was probably near capacity even in 1957 (based on the aerial photo which shows the ponds as being filled), such that wastewater was recharging groundwater at a rate of several hundred gpm, and overflow industrial discharge was likely being sent to the Stanislaus River. In the 1960s, pumping at the Site production wells and municipal pumping increased, maintaining a AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 43