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4. ENVntONNIENTAL ANALYSIS 4.7 GEOLOGY,SOILS,AND SEISbnCrrY — GEOLOGIC HAZARDS This following section provides a description of common geologic hazards that currently exist or that could result from quarry development of agricultural areas in the project vicinity. These geologic hazards are the basis for the Significance Criteria(discussed later in this chapter)which determines the level of impact significance. This section discusses only those geologic hazards that pertain to the site and vicinity and are necessary to provide adequate environmental setting _ information. Slope Instability and Landslides — Slope failures,commonly referred to as landslides; include many phenomena that involve the downslope displacement and movement of material,either triggered by static (i.e., gravity)or dynamic (i.e.,earthquake)forces. Exposed rock slopes undergo rockfalls,rockslides,or rock avalanches, while soil slopes experience shallow soil slides,rapid debris flows, and deep-seated rotational slides. Under existing conditions,the proposed project site is flat agricultural area with no hill or slope features susceptible to landsliding,however,change in topography proposed as part of the project would require steep slopes in certain mining areas. Currently,there are existing aggregate mines near the proposed site that maintain excavations into material similar to that underlying the project site. The potential for slope failure is discussed in the Impact and Mitigation Measures section of this chapter. Soil Erosion Soil erosion is the process whereby soil materials are worn away and transported to another area either by wind or water. Rates of erosion can vary depending on the soil material and structure, placement,and human activity. The erosion potential for soils in the project area ranges from slight to moderate. Soil containing high amounts of silt can be easily erodable while sandy soils with clay are less susceptible. In general,the soils which compose the proposed project site have a slight to moderate hazard of water erosion(USDA NRCS, 1992;USDA NRCS 1997). Erosion and sediment control plans are common requirements for quarry reclamation plans. Soil erosion is discussed in the Impacts and Mitigation Measure section of this chapter. Land Subsidence Subsidence is the gradual lowering of the land surface due to loss or compaction of underlying materials. Subsidence can occur as the result of hydro-compaction;groundwater,gas and oil extraction; or the decomposition of highly organic soils. Hydro-compaction is the process of volume decrease and density increase upon saturation of moisture deficient deposits(Ireland,et al., 1984). Subsidence due to hydro-compaction is occurring in the Delta area at a variable rate specific to the site or area. Groundwater overdraft occurs when groundwater pumping from a subsurface water-bearing zone (aquifer)exceeds the rate of aquifer replenishment. Significant subsidence due to excessive groundwater withdrawal occurred in the Los Banos-Dos Palos area south of Tracy. During the period between 1920 and 1966,land subsidence ranged from 1 foot north of Los Banos to about RMC Pacific Vernalis Quarry Minting and Redamation Froject 4.7-8 FSA/203015 Adn inismtive Draft©R Match 2006