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Appendix A Monitoring Well Abandonment Workplan City of Lathrop Consolidated Treatment Facility 1. General Information The City of Lathrop's Consolidated Treatment Facility(CTF) treats and disposes domestic wastewater generated within the City of Lathrop. On February 5, 2015,the Regional Water Quality Control Board approved new Waste Discharge Requirements (WDRs) (Order No. R5-2015-2006). This Monitoring Well Abandonment Plan describes the proposed abandonment procedures for wells identified in the WDRs or identified as damaged or dry during the September to October 2015 well reconnaissance outlined in the Well Condition Survey Report. 1.1. Purpose The WDRs slated six monitoring wells for abandonment to simplify the monitoring network(CLSP- 2, CLSP-3, CLSP-4, MWM-14, MWM-16,and MWM-26).1 However,field reconnaissance revealed that grading activities around one of these wells (CLSP-3) increased land surface level adjacent to the well, and excavation will be required to expose the buried concrete well pad in order to properly abandon the well. Because the well is accessible to the City,it is secure and functional, provides water level data adjacent to the San Joaquin River,and there is uncertainty in the timing and extent of future land use plans for the site,the City recommends keeping CLSP-3 as an active well within the current network. Additionally,the well inspections during September and October 2015, identified three damaged wells (MWR-6, MWR-13, and CLSP-10) and two low-yielding wells (MWR-15 and MWR-20) recommended for abandonment. The well locations are shown in Figure 1 (two wells are located within CLSP,three are located within Mossdale,and four are located on River Islands).This workplan outlines the general procedure to abandon and seal the monitoring wells consistent with California Well Standards Bulletin 74-90 and San Joaquin County Well Standards. 1.2. 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 depositsz.These 1 Recommended Groundwater Monitoring Well Network and Sampling and Analysis Plan.City of Lathrop. March 19, 2014. Prepared by HydroFocus, Inc. 2 Jennings,C.W.and Strand,R.G., 1958,"Geologic map of California,Santa Cruz Sheet",California Division of Mines and Geology,Sacramento,California.