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September 29, 2014 Monitoring Well Abandonment and Installation Workplan within the City of Lathrop.The MBR WRP-1 is regulated under Waste Discharge Requirements and Master Reclamation Permit(WDRs) Order No.RS-2006-0094.The City of Lathrop is named as the Discharger.As part of Lathrop's MBR WRP-1 operations,recycled water is stored in lined storage ponds and applied to LAAs.The LAAs contain landscaped areas,turf,and agricultural crops,and in the future may include infiltration basins. The WDRs for the WWTF and MBR WRP-1 require quarterly groundwater monitoring. Due to the relatively close proximity of the two treatment plants and associated storage ponds, data from some monitoring wells are utilized to monitor groundwater conditions required by both permits. 1.2. Purpose On March 27,2014,the City of Lathrop submitted a Report of Waste Discharge (RWD) for the Crossroads WWTF that proposes to convert LAA 3 to percolation ponds.Subsequent to the submittal of the RWD,the City determined that planned percolation pond 4 is not needed and therefore will not be constructed. Four piezometers (PZ-1,PZ-2,PZ-3,and PZ-4)located within that part of LAA 3 to be converted to percolation ponds 1, 2,and 3 need to be destroyed prior to the conversion (Figure 1).Additionally,RWQCB staff requested at least three monitoring wells be installed outside of LAA 3.This workplan describes the well destruction and construction plans to meet this request.The City also recommends removing and destroying one WWTF monitoring well from its network due to well construction problems.This workplan provides justification for that recommendation and also describes how that well will be destroyed. 1.3. Local Hydrogeologic Conditions The Lathrop area is located in the basin-trough geologic zone of the San Joaquin Valley, which consists of continental Pleistocene and Holocene stream-laid and still-water deposits3.These deposits are derived from a mixture of Sierra Nevada and Coast Range rocks¢.The near surface deposits are mapped primarily as Quaternary alluvial flood plain deposits5.The local depositional environment is a sedimentary river basin,and the shallow sediments are primarily flood plain and over bank deposits of sand,silt and clay.The near surface deposits are characterized by discontinuous inter-bedded silty sand,sandy silt,silty 3Jennings,C.W.and Strand, R.G.,1958, "Geologic map of California,Santa Cruz Sheet",California Division of Mines and Geology,Sacramento,California. °Miller, R.E.,Green,l.H., Davis,G.H...1971, U.S. Geological Survey Professional Paper 497-E. 5 Atwater, B.F.,1982,Geologic maps of the Sacramento-San Joaquin Delta, California, US Geological Survey Miscellaneous Field Studies Maps,MF-1401. HydroFocus, Inc. 2