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ekienvironment & water 1 INTRODUCTION Pursuant to requirements by Lathrop's Monitoring and Reporting Program (MRP) (R5-2016-0028- 03 REV3) for the Consolidated Treatment Facility(CTF),this 2023 Semiannual Report summarizes the Spring (i.e. First Semiannual) groundwater monitoring event for the period January 2023 through June 2023. The report summarizes data from wells monitoring groundwater beneath recycled water storage ponds and Land Application Areas (LAAs) located in Mossdale, Central Lathrop Specific Plan (CLSP), River Islands, the CTF Facility, and Pond S6 areas (Figure 1). Sample collection and analysis was performed in accordance with standard Unites States (US) Environmental Protection Agency (EPA) methods. Consistent with the requirements of the MRP (italicized below), this groundwater monitoring report includes the following: • A narrative description of all preparatory, groundwater monitoring, sampling, and analytical testing activities for the first semiannual groundwater monitoring event in compliance with the standard EPA methods. Field logs for each well sampled are provided in Appendix A. The field logs document depth to groundwater; parameters measured before, during, and after purging; method of purging; calculation of casing volume; and total volume of water purged. • Calculation of groundwater elevations and contours, a groundwater elevation map referenced to mean sea level (provided in Figure 3), and an assessment of groundwater flow direction, gradient, and seasonality, including a comparison with previous conditions (provided in Section 3.1). • Cumulative data tables containing depth to groundwater (provided in Appendix B) and water quality analytical results (provided in Appendix C). Current water level data and analytical results are provided in Table 3 and Table 5, respectively. • Scaled maps showing relevant structures,features, and monitoring well locations (shown on Figure 1 and Figure 2). 2 GROUNDWATER MONITORING ACTIVITIES The 2023 Spring (i.e. First Semiannual) monitoring event occurred from 24 March through 5 April 2023. Current compliance wells' identified in the MRP2 for semiannual water level and water quality were monitored during this period. Monitoring well locations are shown on Figure 2. Additional wells that are not part of the water quality compliance monitoring network (i.e., MWM-5, MWM-8, MWM-9, and MWM-13) were also sampled in March 2023 to monitor 1 CLSP-1, CLSP-3,CLSP-8, CLSP-9,CLSP-11, CLSP-12 and MWM-19 are considered current compliance wells as of the second semiannual 2018 monitoring event. 2 Revised Monitoring and Reporting Program R5-2016-0028-03 REV3 City of Lathrop 1 13 July 2023 EKI B90088.03