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4 Environmental Analysis 4.10 Hydrology/Water Quality 4.10.1.3 Groundwater The Project site is just inside the western boundary of the Tracy Subbasin (05-022.15), which is within the larger San Joaquin Valley Groundwater Basin (05-022). The Tracy Subbasin covers an area of 539 square miles and spans parts of Alameda, Contra Costa, and San Joaquin Counties. Four active domestic water wells are located on parcels owned by the current Project landowner (DWR 2006). Existing water supply infrastructure at the 106-acre Project site includes one on-site groundwater well and the other three wells on the landowner's property are outside of the Project site. No wells occur with the 32-acre area that would be developed as part of the Project. Southwest of the Project site there are three existing wells, including a domestic water well and a cattle well, both of which are actively used, and one additional cattle well that is inactive (Ilkhanipour and McManus 2022). A total of 238,429 acres of the Tracy Subbasin fall under a groundwater sustainability plan that covers approximately 1,958 domestic production wells, 373 agricultural production wells, and 74 municipal production wells. The groundwater storage volume in the Tracy Subbasin was estimated to be less than 100,000 acre-feet (af) in 2015 (GEI Consultants 2021). However, groundwater provides only about 3 percent of the water supply used in the general area. 4.10.1.4 Flooding FEMA is responsible for mapping areas subject to flooding during a 100-year flood event (i.e., 1 percent chance of occurring in a given year). The Flood Insurance Rate Maps produced by FEMA for San Joaquin County, and for the adjacent portion of Alameda County, show that the Project site does not lie within a 100-year flood zone or any other special flood hazard zone. It is located within Zone X, an area of minimal flood hazard risk (FEMA 2009, 2022). 4.10.1.5 Dam Inundation Zones The watershed containing the Project site is very small, and there are no dams of any size upstream from the Project site. There are a couple of small ponds identified in the upper reaches of the main channel, located in Alameda County (USGS 1980). However, these ponds are very small and do not contain the volume of water that would pose a threat downstream. As a result, the Project site is not in a dam-breach inundation zone. 4.10.2 Regulatory Setting 4.10.2.1 Federal Federal Clean Water Act Basin Plan — Beneficial Use and Water Quality Objectives (Clean Water Act Section 303) The Project site is located within the jurisdiction of the Central Valley Regional Water Quality Control Board (Region 5). Region 5 is tasked with implementing the adopted Water Quality Control Plan (Basin Plan) for the Sacramento and San Joaquin River Basins through planning, permitting, and enforcement of established water quality objectives. In accordance with state policy for water quality control, Region 5 employs a range of beneficial use definitions for surface waters, groundwater basins, marshes, and mudflats that serve as the basis for establishing water quality objectives and discharge conditions and prohibitions. The Basin Plan identifies beneficial uses for the key surface water drainages throughout its jurisdiction. The Project site is within the Griffith Energy Storage Project 4.10-5 Tetra Tech/SCH 2022120675 Draft Environmental Impact Report August 2023