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Administrative Draft Environmental Impact Report Gill Medical Center Project 4.12 HYDROLOGY AND WATER QUALITY This section of the EIR describes existing surface water and groundwater conditions at the Project site and evaluates the potential impacts on these resources in accordance with impact significance criteria provided in Appendix G of the CEQA Guidelines. Information presented in this section is based on the following technical studies: Test Boring, Well Installation and Sampling, and Aquifer Testing Summary Report, August 11, 2021 (Terracon 2021 a) Percolation Test Results Letter dated August 11, 2021 (Terracon 2021 b) Water Supply Assessment for the Gill Medical Center dated September 9, 2021 (ECORP 2021) These studies are included with this draft EIR as Appendix G — Hydrology and Water Quality. Publicly- available data from the Federal Emergency Management Agency(FEMA), the California Department of Water Resources (DWR), and the California Central Valley Regional Water Quality Control Board (RWQCB) were also reviewed and incorporated into this analysis. 4.12.1 Environmental Setting The environmental setting consists of the existing hydrologic conditions in the region and at the Project site. Existing conditions are described below for both surface water and groundwater, and for water quality. The existing conditions define the baseline for the evaluation of potential environmental impacts. 4.12.1.1 Climate The Project site is in the northern part of the San Joaquin Valley. The San Joaquin Valley has a Mediterranean climate with cool, wet winters and hot, dry summers. Regionally, temperature and precipitation vary with elevation, with the lower temperatures and higher precipitation typically occurring at higher elevations. The nearest meteorological station to the Project site from which long-term precipitation data are available is the Stockton Fire Station 4 station, located 4.5 miles to the southwest. This location is also designated as Station 048560 as part of the National Weather Service Cooperative Network (WRCC 2021). The average annual high temperature is 74.5 degrees Fahrenheit (deg F) but monthly average high temperatures can range from 54 deg F in January to 93 deg F in July. The average annual low temperature is 46 deg F, with monthly average low temperatures ranging from 36 degrees in January and December to 57 degrees in July. Rainfall data are available from March 1906 through December 2017 from the Stockton Fire Station 4 station, but consistent data exist for the period from January 1926 through December 2017. In the discussions in this report, the rainfall data are presented for a water year. A water year in this region of California begins on October 1 and extends through September 30 of the subsequent calendar year. A water year better represents rainfall and hydrologic patterns than a calendar year does. In the discussions Hydrology and Water Quality 4.12-1 October 2021