ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec— gradient of 0.001, the horizontal groundwater flow velocity is likely on the order of 0.6 ft/d in the Lower Aquifer. 4.5 HISTORICAL CONTROLS ON GROUNDWATER FLOW Groundwater flow in the Upper and Intermediate Aquifers is strongly controlled by water use (land disposal of industrial and municipal wastewaters and pumping). The objective of this section of the report is to estimate historical groundwater flow regimes beneath the Study Area prior to installation of the monitoring well network in the late 1980's. This is a critical component of the SCM because known TCE use dates back to 1957, and therefore TCE could conceivably have been released as early as 1957 onsite, and perhaps earlier at other locations by offsite unidentified industries. 4.5.1 Groundwater Pumping (1957— 1986) The major known water users in 1957 in Ripon were the Nestle plant, pumping from supply well N-1 at an annual average rate of approximately 340 gpm, and the City of Ripon, utilizing three wells for municipal supply: MW-1, MW-2 and MW-3 at an estimated combined pumping rate of approximately 650 gpm (RUST, 1994a). At this time most of the pumping was concentrated within an area less than a mile to the north of the lagoons. All of these wells pumped from the Intermediate and Upper Aquifers. As the population of the city grew, additional supply wells were drilled to meet increasing municipal demand. In 1966, Simpson Paper (Simpson) began its operations. Simpson pumped groundwater from three production wells installed north of the Stanislaus River (S-1 through S-3) at a combined annual average rate of approximately 1,100 gpm (RUST, 1994a), thereby exerting additional stress on the aquifer system. Simpson and Nestle likely maintained relatively consistent groundwater production rates to support operations; it is unlikely that the operations in these industries were seasonal. However, municipal pumping was likely historically seasonally variable, as it currently is (Figure 14); therefore, groundwater pumping by Simpson and Nestle would likely exert a greater control on groundwater flow during the winter months, when municipal water demand was lower. Historical groundwater pumping rates are summarized in Table 5. It is important to note that accurate records of all pumping and recharge activities within the Study Area dating back to 1957 are not available. The annual average pumping rates shown in Table 5 are based on those reported in ECM (2008c) and modified based on Table 1; all of the records before 1994 are derived from best estimates of water demand based on population and industrial operations, rather than documented quantities of groundwater withdrawals. Estimates of municipal demand based on water use can be overestimated in areas where many single family residences operate domestic wells; this may have been the case decades ago in Ripon. The influence of agricultural pumping has not been considered in this evaluation because no AMEC Geomatrix, Inc. hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 38