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Re-Revised Groundwater Monitoring Well Construction Workplan August 2023 Groundwater Levels The Corcoran Clay is a hydraulic barrier to groundwater movement in the Subbasin. Groundwater aquifers above the clay are generally confined where groundwater levels below the clay are confined. The aquifers were grouped together into an Upper Aquifer(above the clay) and a Lower Aquifer(below the clay). Upper Aquifer The depths to groundwater and trends vary based on location in the Subbasin. Groundwater levels are much deeper in the non-Delta upland portions of the Subbasin where groundwater levels are affected by pumping, discontinuous recharge disconnect from streams and channels, and deep percolation of water from agricultural fields. Groundwater levels are deeper towards the south and shallower near the San Joaquin and Old rivers. In 2020, the groundwater levels in the Upper Aquifer ranged from 80 feet below ground surface (bgs) near the foothills to within 5 feet of ground surface near the San Joaquin River. Groundwater levels typically have greater seasonal fluctuations locally up to 40 feet, due to groundwater pumping and seasonal recharge. Even with these seasonal changes, the depths to groundwater have remained similar, except for those near the southeastern portion of the Subbasin where groundwater levels started to decline around 2010 (to present), due to increased and apparent continued reliance of groundwater since the 2012 to 2016 drought. Lower Aquifer The depths to groundwater in the Lower Aquifer(where the water is confined) are typically deeper than those in the Upper Aquifer. Groundwater levels (piezometric heads')range from about 20 to 270 feet bgs and in some locations, are below sea level. Groundwater elevations in the Lower Aquifer range in the Subbasin from about-60 to 80 feet elevation. The groundwater levels are always above the top of the Corcoran Clay by about 200 to 240 feet. 1 Piezometric head(or hydraulic head)is a specific measurement of liquid pressure above a vertical datum. It is usually measured as a liquid surface elevation,expressed in units of length,at the entrance of a piezometer. GEI Consultants, Inc. 10