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4.7 Geology,Soils,and Seismicity V 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 INN 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 sand or silt can be easily eroded while clayey soils are less susceptible. In general, the soils which comprise 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 r. 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 at Los Banos to about 28 feet east of Mendota(Hotchkiss and Balding, 1971). Land subsidence caused by groundwater withdrawal was not identified in the vicinity of Tracy(Hotchkiss and Balding, 1971). Subsidence is not considered a potential adverse geologic affect and therefore is not discussed further in this chapter. Settlement ` Settlement is the depression of the bearing soil when a load, such as that of a building or new fill material, is placed upon it. Soils tend to settle at different rates and by varying amounts depending on the load weight, which is referred to as differential settlement. Differential settlement can be a greater hazard than total settlement if there are variations in the thickness of previous and new fills or natural variations in the thickness and compressibility of soils across an area. Settlement commonly occurs as a result of building construction or other large projects, such as the proposed RMC Pacific Vernalis Quarry,that require soil stockpiles. Although some settlement may occur within the proposed project site area due to soil stockpiling and other ., operations,the affect to the site would be very minimal considering that the site does not support development that could be adversely impacted by ground settlement. Therefore,the impact of ground settlement is not discussed further in this document. RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.7-9 ESA 1203015 Draft Environmental Impact Report May 2006 r