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City of Pleasanton—Hidden Canyon Residences and Preserve Draft EIR Geology, Soils, and Seismicity FirstCarbon Solutions 3.6-3 https://adecinnovations.sharepoint.com/sites/PublicationsSite/Shared Documents/Publications/Client (PN-JN)/2148/21480016/EIR/3 - Draft EIR/21480016 Sec03-06 Geology.docx lean clay alluvium, and the southeastern portion of the project site along Devany Creek with alluvial terrace deposits comprised of sand and gravel.3 Groundwater Fluctuations in groundwater can vary based on natural factors such as rainfall and soil characteristics, and non-natural processes including irrigation practices. The City of Pleasanton is located on the Livermore Valley Groundwater Basin which is located in the lower elevations of the Tri-Valley and percolates in a southwest direction from the Altamont Pass in the east at 600 feet AMSL to the Arroyo de la Laguna in the west at 280 feet AMSL and flows into the Sunol Groundwater Basin south of Pleasanton.4 The closest groundwater well to the project site within the Livermore Valley Groundwater Basin is approximately 1.2 miles northwest of the project site with the most recent spring and fall measurements listed at 20 feet and 25 feet below the surface respectively.5 The Updated Preliminary Geotechnical and Geologic Report indicated that groundwater was not encountered in test pits excavated to a depth ranging from 3 to 18 feet beneath the surface within the higher elevations on the western and southern portions of the project site. However, shallow groundwater may be present in swale areas and along Devany Creek on the eastern side of the project site, as perched groundwater that is separate from the aquifer.6 Subsidence Subsidence is the gradual and widespread sinking of the surface due to the removal of groundwater or other mineral resources within the subsurface from overexploitation, causing compaction of sediments. Subsidence may also occur from earthquakes, additional loading of structures, or sinkhole formation. Soils highly susceptible to subsidence are clays and silts due to their shrink and swell potential depending on moisture content. As previously mentioned, subsurface conditions on the project site consist of Diablo clay and Millsholm silt loam where both soils consist of a clayey texture and fluctuate in moisture content seasonally from California’s Mediterranean climate. The Updated Preliminary Geotechnical and Geologic Report indicated most soils on the project site having a clayey texture within residual soil and colluvium. Although soils on the project site display characteristics susceptible to subsidence, areas in California that have experienced subsidence were generated from excessive groundwater pumping in the twentieth century. The closest area that experienced subsidence to the project site is the Santa Clara Valley where extensive agriculture overexploited the groundwater supply in the early 1900s with subsidence greatest in San Jose, California experiencing gradual sinking of 14 feet between 1910 and 1995. However, efforts from the Santa Clara Valley Water District and the State Water Project successfully imported water for groundwater recharge and subsidence has ceased.7 3 Cornerstone Earth Group. 2020. Updated Preliminary Geotechnical and Geologic Report. March 31. 4 California Department of Water Resources (DWR). 2006. California Groundwater Bulletin 118, Livermore Valley Groundwater Basin. January 20. 5 ArcGIS. California’s Groundwater Live: Groundwater Levels. n.d. Website: https://storymaps.arcgis.com/stories/b3886b33b49c4fa8adf2ae8bdd8f16c3. Accessed February 25, 2025. 6 Cornerstone Earth Group. 2020. Updated Preliminary Geotechnical and Geologic Report. March 31. 7 United States Geological Survey (USGS). n.d. Website: https://ca.water.usgs.gov/land_subsidence/california-subsidence-areas.html. Accessed February 26, 2025.