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Administrative Draft Environmental Impact Report Gill Medical Center Project While there are clay and silt zones that form aquitards throughout the geologic formations listed above, the extent of the aquitards is limited and the entire thickness of the principal aquifer is hydraulically connected, meaning that groundwater can move relatively easily from one depth or one zone to another. Based on groundwater contour maps provided in the GSP (Figures 2-37 and 2-38 in ESJGA 2019), groundwater generally flows radially inward from the perimeter of the Subbasin toward a large pumping depression in the center of the Subbasin (see Figure 4.12-3). The pumping depression is located to the east of the City of Stockton. In the Project vicinity, the groundwater surface elevation is approximately 30 feet below sea level. The hydraulic gradient, or slope of the groundwater surface, averages approximately five to 10 feet per mile, which is equivalent to a gradient of about 0.001 to 0.002 ft/ft. In general, groundwater levels within the East San Joaquin Valley Subbasin exhibit minor seasonal fluctuations of a few feet due to increased pumping demand in the summer and increased recharge during the winter and spring. The more significant trend has been a persistent decline in groundwater levels ranging from 20 feet to more than 60 feet in most areas of the Subbasin since the 196Os, especially in the area of the pumping depression shown on Figure 4.12-3. However, groundwater levels have remained relatively stable within the City of Stockton, potentially because municipal water demands tend to be appreciably lower than agricultural water use on a per-acre basis. Figure 4.12-4 shows hydrographs of groundwater levels throughout the Subbasin from 1960 to 2017. The current volume of fresh (i.e., non-saline) groundwater in storage within the principal aquifer in the Eastern San Joaquin Subbasin is estimated to be 53 million acre-feet (ESJGA 2019). The amount of groundwater in storage has decreased by approximately 0.01 percent per year, or about 5,300 acre-feet per year, between 1995 and 2015. According to the GSP, a reduction in beneficial uses, which is an undesirable result under the Sustainable Groundwater Management Act of 2014 (SGMA), would not occur until the volume of water in storage is reduced by 23 million acre-feet, to a total of 30 million acre-feet (ESJGA 2019). Under the current rate of decrease in water storage, it would take several thousand years to reduce the volume in storage to the level of concern identified in the GSP. The GSP established measurable objectives in wells related to chronic lowering of groundwater levels in representative monitoring wells throughout the Subbasin. The two closest representative wells to the Project site are referred to as the Swenson-3 well, located in the western part of the City of Stockton, approximately four miles southwest of the Project site, and State Well Number O2NO7E29BOO1 (referred to as well 29B herein), located approximately five miles southeast of the Project site, near the westernmost edge of the pumping depression identified in Figure 4.12-3, above. The current groundwater level at the Swenson-3 well is -19.3 ft msl and the groundwater elevation is expected to remain at that level through at least 2035. At well 29B, the current groundwater elevation is at an elevation of-49.8 ft msl and the groundwater level is anticipated to decline to -65 ft msl by 2035. The measurable objective for chronic lowering of groundwater in the GSP (ESJGA 2019) is -19.3 ft msl for the Swenson-3 well and -80.4 ft msl for well 29B. Hydrology and Water Quality 4.12-5 October 2021