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San Joaquin County Community Environmental Setting,Impacts,and Mitigation Measures Development Department Geology,Soils,and Seismicity Other Geologic Conditions Land Subsidence — Historically, land subsidence has been a significant problem in the Sacramento– San Joaquin Delta and the southern half of the San Joaquin Valley. Subsidence occurs in three ways: drying and oxidation; wind erosion of peat soils; and — hydrocompaction from drafting of groundwater from certain types of substrates. In the project area,peat soils are absent, and land subsidence due to hydrocompaction has not been observed. Volcanic Activity Volcanic activity is not a local concern. The nearest volcanoes lie in Mono County, in the Mammoth Lakes/Long Valley area, to the northeast of Tulare — County,and Lassen Peak in Lassen County. Landslides Within the limits of ground disturbance of the project area,there is no risk of naturally occurring large landslides, since it is essentially flat and topographically featureless. Localized landsliding as a result of project activities is discussed in the "Impacts and Mitigation Measures"section. Mineral Resources The state has identified a number of regionally significant sand and gravel aggregate resource areas in San Joaquin County. Their importance is home out in the County's increasingly greater share of the state's production of sand and gravel. The extraction sites are concentrated in three areas: the northeastern end of the county along the Mokelumne River, along the San Joaquin River west of Manteca, and to the south and southwest of Tracy. The most significant of these lies near Tracy(San Joaquin County 1992). The primary extractive resources in the county are sand,gravel, and natural gas. Pursuant to the California Surface Mining and Reclamation Act of 1975 (SMARA), the mineral deposits in San Joaquin County have been identified and classified by the state geologist. Also required by SMARA, the State Mines and Geology Board has designated the sand and gravel deposits that are of regional and statewide significance. The principal deposits where extraction is occurring are in the southwestem portion of the county along the Corral Creek alluvial fan and along the major rivers in the eastern portion of the county. The San Joaquin County General Plan(San Joaquin County 1992)identifies four areas in the county, referred to as sectors,containing regionally significant deposits of high-grade aggregate(sand and gravel). The project area is located in _ Zone MRZ-3, an area containing mineral deposits, the significance of which cannot be evaluated from available data. DeSilva Gales Quarry Project Draft Environmental Impact Report 3.F-6 J85 05105 OS