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W OnARa &CURRAN The City plans to utilize its existing groundwater wells (excluding Well 9 and Well 21)to supply water in the future and has no plans to expand its groundwater productions beyond potentially bringing Well 21 and Well 9 online. In previous water supply planning documents (RBF, 2009; West Yost, 2013), the City has included plans to construct a wellfield surrounding Well 21 that includes up to three additional wells (Well 22 through 24). However, due to the conditions of the semi-confined aquifer at Well 21 and the need for wells to be placed farther apart to avoid interfering with each other, the feasibility of such a wellfield is in question. There are also concerns that a wellfield at this location would induce the migration of higher total dissolved solids(TDS)water towards the City's other wells. 2.1.2.1 Groundwater Basin The City is located within the Tracy Subbasin (DWR Bulletin 118 number 5-22.15),3 within the San Joaquin Valley Groundwater Basin (DWR 5-22) (DWR, 2003).4 In 2014, the State Legislature passed the Sustainable Groundwater Management Act(SGMA), requiring the formation of Groundwater Sustainability Agencies (GSAs)and preparation of Groundwater Sustainability Plans (GSPs)to sustainably manage groundwater supplies. The California Department of Water Resources(DWR)has identified the Tracy Subbasin as a Medium priority non-critically overdrafted groundwater basin that is subject to the requirements of SGMA.Accordingly,the Subbasin must submit a GSP by January 31,2022. The GSP will define sustainable management goals, minimum thresholds, and measurable objectives and provide an implementation plan to evaluate, monitor, and manage the use of the Tracy Subbasin sustainably so as to avoid undesirable results (e.g., lowering of groundwater levels, depletion of groundwater storage, surface water depletion, water quality degradation, land subsidence, and seawater intrusion). To the extent that the Tracy Subbasin GSP implements restrictions to groundwater pumping to the Project, the Project's water supply may change; however, the Tracy Subbasin GSP is in early stages of development(it is due to DWR in January 2022)and no decisions relating to such restriction have occurred to date. The City of Lathrop has formed a GSA and participates with other Tracy Subbasin GSAs to support the preparation and implementation of a subbasin GSP. The Stewart Tract GSA has been formed to allow the developer of River Islands Project to participate in the GSP process (as the primary landowner within Reclamation District 2062) for the portion of the City of Lathrop west of the San Joaquin River. Including the Stewart Tract GSA and the City of Lathrop GSA, seven GSAs are working cooperatively to develop a single GSP that covers the Tracy Subbasin. The Tracy Subbasin, shown in Figure 5, is bounded to the north and the east by the San Joaquin River(except for a portion of the Subbasin boundary that extends east of the San Joaquin River to follow the jurisdictional boundary of the City of Lathrop), to the south by a combination of the San Joaquin-Stanislaus County line and the jurisdictional boundaries of water agencies, and to the west by the extent of sedimentary deposits bounded by the Diablo Range. The Tracy Subbasin has a surface area of 345,000 acres (539 square miles) and consists of two primary aquifers, a shallow, unconfined aquifer and a deeper confined aquifer,separated by a regional aquitard called the Corcoran Clay. 3 DWR's Bulletin 118 is an inventory and assessment of available information on the occurrence and nature of California's groundwater to inform decisions affecting the protection,use,and management of the resource.DWR publishes Bulletin 118 to meet requirements of the California Water Code(Section 12924)to identify California's groundwater basins,investigate patterns of groundwater extraction and recharge within those basins,and define basins that are subject to critical conditions of overdraft(DWR,2019). 4 The City submitted a basin boundary modification request in June 2018 to modify the boundaries of the Eastern San Joaquin Subbasin and the Tracy Subbasin to align with the City's City Limit.This modification was approved in 2019 and therefore the entire City of Lathrop lies within the Tracy Subbasin. Califia(0011533.00) 17 Woodard&Curran, Inc. River Islands Phase 2 Development Water Supply Assessment September 2020