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wponARa &CURRAN The unconfined aquifer in the region of the City of Tracy is primarily composed of older and younger alluvium. The older alluvium tends to be loosely to moderately compacted silt, sand, and gravel deposits with a thickness of approximately 150 feet. Younger alluvium is generally unconsolidated silt, sand, and gravel with a thickness of less than 100 feet. Groundwater recharge to the unconfined aquifer is from the Coast Ranges, and groundwater may be discharging to the San Joaquin River. Groundwater elevations in the unconfined aquifer have been relatively steady over time, and groundwater flows from south to north towards the Sacramento-San Joaquin Delta. The confined aquifer is primarily composed of the Tulare Formation and is separated from the unconfined aquifer by the Corcoran Clay. The Tulare Formation is moderately permeable, and most of the larger production wells extract from this formation. The thickness of the Tulare Formation is approximately 1,400 feet. Groundwater elevations in the confined aquifer are monitored by the City of Tracy, who operates several production wells for municipal use. Historically, groundwater production by the City of Tracy has resulted in a pumping depression. In its most recent UWMP, however, the City of Tracy reported that groundwater elevations have risen steadily in the past decade and the pumping depression has decreased in size as the City of Tracy has reduced its reliance on groundwater (EKI, 2016). The base of fresh water, defined as water with a TDS concentration of greater than 2,000 milligrams per liter(mg/L), ranges from 800 to 2,000 feet below ground surface(ft bgs),and is generally higher in the vicinity of the City of Lathrop. Since the portion of the Tracy Subbasin where the City pumps its groundwater is located east of the San Joaquin River, conditions are similar to the Eastern San Joaquin Subbasin near the San Joaquin River. Groundwater levels in some portions of the Eastern San Joaquin Subbasin have been declining for many years, while groundwater levels in other areas have remained stable or increased in recent years. The western and southern portions of the Eastern San Joaquin Subbasin,near the City of Lathrop,have experienced less change in groundwater levels in part due to minimal groundwater pumping in the Sacramento-San Joaquin River Delta area and the import of surface water for agricultural and urban uses. Groundwater storage capacity in the Tracy Subbasin has not been estimated. However, based on values reported for the Tracy-Patterson Storage Unit(Hotchkiss and Balding, 1989), it has been inferred that the storage capacity of the southern portion of the Tracy Subbasin—extending from the southern boundary of the basin to one-mile north of the City of Tracy — is 1.3 million AF. A discussion of the hydrogeology of the Tracy Subbasin is provided in the Tracy Regional Groundwater Management Plan (GWMP) (GEI, 2007) and is summarized in Section 2.1.2.2. Although groundwater quality constraints in the vicinity of the City of Lathrop represent potential uncertainties, groundwater from the Tracy Subbasin is generally sufficient, in terms of both volume and water quality, to meet the projected water demand associated with the proposed Project. 2.1.2.2 Historical Groundwater Management In the Tracy Subbasin, Byron-Bethany Irrigation District, Banta-Carbona Irrigation District, the City of Tracy, and San Joaquin County formed a Groundwater Advisory Committee to facilitate the development of a regional groundwater management plan for the Tracy Subbasin. The City of Tracy received a grant from DWR to develop the plan, and in 2007, 7 years prior to the passage of SGMA,the entities adopted the Tracy GWMP. The Tracy GWMP concluded that the Tracy Subbasin experiences groundwater quality issues in portions of the basin associated with nitrate, boron,sulfate,chloride,and TDS.As such, many of the groundwater management options that were recommended focused on creating available storage and managing pumping in order to increase water quality within the basin. 2.1.3 Wastewater and Recycled Water Wastewater from the City is treated at two facilities:the regional Manteca Wastewater Quality Control Facility(MWQCF) and the City-owned Lathrop Consolidated Treatment Facility(CTF). Tertiary effluent from the Lathrop CTF is currently Califia(0011533.00) 19 Woodard&Curran, Inc. River Islands Phase 2 Development Water Supply Assessment September 2020