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Groundwater Monitoring Well Construction Workplan January 2024 Geologic Setting Freshwater-bearing formations and sediments in the Subbasin that may be encountered at the drill site, from youngest to oldest, include Alluvial Fan Deposits, Older Alluvium,Modesto Formation, Los Banos Alluvium, and the Tulare Formation. These formations,except for the Tulare Formation (which is buried by the other sediments), are exposed at ground surface (see Figure 2). Alluvial Fan Deposits Along the southern margin of the Subbasin, are Alluvial Fan Deposits (Qf) from the Coast Ranges. These deposits consist of loosely to moderately compacted sand, silt, and gravel deposited in alluvial fans during the Pliocene and Pleistocene ages. The thickness of these fans is up to 150 feet(DWR 2006). Modesto Formation The Modesto Formation (Qm) is located along the east side of the San Joaquin River and is slightly older that the Alluvial Fan Deposits.The formation consists of granitic sands over stratified silts and sands.Near the southern margin of the Subbasin,there are small occurrences of Los Banos Alluvium (Qlb) and Older Alluvium (Qo) that are of similar age as the Modesto Formation. Tulare Formation The Tulare Formation is Pleistocene in age and consists of semi consolidated,poorly sorted, discontinuous deposits of clay, silt, sand, and gravel. Unlike the other freshwater-bearing formations and sediments that may be encountered at the drill site, the Tulare Formation is not exposed at ground surface in the Subbasin. The formation's sand and gravel deposits are moderately permeable, and most of the larger agricultural, municipal, and industrial operations extract from this formation. Wells completed in this formation can produce up to 3,000 gallons per minute. The thickness of the Tulare Formation is about 1,300 feet at the drill site. At MW- 203 the Corcoran Clay was present between 280 and 360 feet below ground surface (bgs). The lower portion of the Tulare Formation is typically coarser than the upper portion of the formation. The sediments consist of sand and gravel beds that are interbedded with clays and silt. Within the Tulare Formation is the Corcoran Clay, one of the largest lakebed deposits in the San Joaquin Valley.The clay was about 80 feet thick at MW-203. Figure 3 shows the extent and structure of the clay based on geologic profiles and geophysical logs as well as United States Geological Survey(USGS)datasets in the Subbasin. The clay should be present at MW-204. Figure 4 shows how the clay separates the Upper and Lower aquifers in the region. GEI Consultants, Inc. 6