ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec- known irrigation pumping has been described in previous reports or discussions with the City of Ripon Department of Public Works staff. However, the presence of orchards north of the WWTF is visible in aerial photos taken in 1957 until at least 1989 (they were not present in 1992) (Appendix G). It is unknown whether irrigation wells were used to irrigate these orchards in the past; Nestle is investigating this as part of the Lower Aquifer IRAP (ECM, 2008c). 4.5.2 Groundwater Recharge (1957— 1986) Available records indicate that the major historical anthropogenic sources for groundwater recharge in the Study Area are the City of Ripon domestic and industrial wastewater disposal practices and Simpson Paper (after 1975; prior to 1975 Simpson Paper discharged industrial wastewater to the Stanislaus River). The history of domestic and industrial wastewater practices is somewhat complex, and is summarized based on an account by Ed Feightmeir, former mayor of Ripon (his notes were provided to AMEC by Ted Johnston, Director of Public Works for the City of Ripon). Key points from this account are summarized below. Recharge of Domestic and Industrial Water at the City Lagoons Prior to 1968, the sewer system was run as a district and was not under control of the City. The treatment facility was comprised of an Imhoff tank (i.e. a large, two-story septic tank) for treatment of domestic sewage and 10 acres of disposal ponds divided into two sections; one for untreated industrial discharge, and the other for sanitary sewer discharge. The ponds can be seen in the 1957 aerial photo (Appendix G) as a series of water-filled trenches; however, the area shown in the 1957 aerial photo appears to be approximately 20 acres. At some time prior to 1963, the industrial sewer discharge was directly to the Stanislaus River without any pretreatment. The users of the industrial sewer at this time were Nestle, the Meyenberg Milk Plant, and Ripon Cannery; it is unknown if other industries discharged to the industrial sewer at this time. After 1963, some of the domestic wastewater was diverted to the industrial wastewater ponds because of insufficient capacity, and some excess industrial wastewater may have been discharged to the Stanislaus River. During the 1960's, the 10 or 20 acres of disposal ponds were insufficient to hold the combined domestic and industrial flows, and were likely operated at capacity for the entire time. This use focused recharge in this area, very likely creating a groundwater mound. These ponds occupied the site of the current easternmost aeration ponds shown in Figure 13. Domestic discharge flows were likely on the order of 186 gpm (approximately 0.27 MGD) in the 1950's and 1960's assuming domestic sewage inflow was less than the domestic pumping by a factor of 3.5 (i.e. domestic inflow = 650 gpm=3.5; the 2007 ratio was approximately 3.5 gallons of groundwater pumped for domestic use per gallon of domestic sewage inflow). As discussed in Section 4.3.2, an average infiltration capacity of approximately 27 gpm/acre has been calculated for the soils in the vicinity of the WWTF disposal ponds. If the total area of AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 39