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Groundwater Monitoring Well Construction Workplan August 2023 Monitoring Well Design Justification Four-level nested monitoring wells will be constructed at this drill site. The levels (depths) of the screen intervals were selected to provide groundwater levels above and below the Corcoran Clay, groundwater use in the area,potential differences in water quality, and to assess whether the groundwater pumping is raising the base of fresh water. These monitoring wells (four-levels) will also better characterize groundwater conditions by establishing groundwater outflow gradients from or to the Northern Delta-Mendonta Subbasin into the TSb. The lithologic information will also help to confirm/support Airborne Elecromagnetic (commonly know as AEM) survey findings. Figure 8 shows the preliminary monitoring well designs. The monitoring wells screen locations were developed based on the depth and thickness of the Corcoran Clay and the base of fresh water. Corcoran Clay is present beneath the drill site. Figure 3 shows the extent and structure of the Corcoran Clay based on geologic profiles and geophysical logs as well as USGS datasets in the Subbasin (Page 1968). The top of the clay is about 230 feet bgs and is about 80 feet thick at the proposed monitoring well location. The base of fresh water is at elevation -1,200 feet amsl or about 1,400 feet bgs (refer to Figure 4). The shallowest monitoring well (named MW-203A) will be constructed in the Upper Aquifer, above the Corcoran Clay. Aquifers above the Corcoran Clay are used by domestic wells that are constructed to depths of 30 to 150 feet bgs. Some irrigation wells also use this aquifer. This monitoring well will be used to monitor groundwater levels in the Upper Aquifer to track changes and inform the GSAs if lowering of groundwaters could affect these domestic well owners. Three monitoring wells (named MW-203B, C, D) will be screened below the Corcoran Clay. The screens for MW-203B will be positioned just below the Corcoran Clay, 390 to 410 feet bgs where low salinity groundwater may be present, based on other occurrences in the Subbasin. MW-203C screens will be positioned about half-way between the bottom of the Corcoran Clay and the base of fresh water, about 710 to 740 feet bgs. Both MW-203B and MW-203C will be positioned to track groundwater level changes and inform the GSAs if lowering of groundwaters could affect agricultural well owners whose wells depths range from 277 to 800 feet bgs. The screens of MW-203D,the deepest monitoring well,will be positioned in the first aquifer above the base of fresh water to detect changes in groundwater levels and salinity and to evaluate whether pumping may be causing upwelling of saline water from the underlying marine sediments. MW-203A through D will be outfitted with transducers and telemetry to measure groundwater levels. The measurements will be reported to DWR's Monitoring Network Module. Water quality samples may be collected from the monitoring wells, but a schedule has yet to be established. GEI Consultants, Inc. 15