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IMPACT Geology,Soils,and Seismicity—Construction-Related Erosion Hazards.Based on soil types and <br /> 4.7.3 topography, excavation, movement, and grading of soil could result in erosion during project construction, <br /> particularly during periods of strong winds. This impact is considered potentially significant. <br /> Soils in the project area consist of a variety of soil types, including Bisgani loamy course sand, Delhi fine sand, <br /> Delhi loamy sand,Delhi-Urban land complex, Timor loamy sand, and Tinnin loamy course sand. Water erosion <br /> hazards associated with these soil types are slight. In addition,the relatively flat topography of the project area <br /> would minimize the potential for water erosion during a storm event. However, according to the NRCS (see Table <br /> 4.7-3),the wind erosion hazard associated with these soil types in the project area is severe, and could result in a <br /> loss of topsoil during the spring and summer months. <br /> Project construction activities would involve trenching and the excavation and grading of soil. These activities <br /> could result in localized erosion during the construction buildout period. Construction activities would remove <br /> any vegetative cover and could expose disturbed areas to wind and storm events. Because the wind erosion hazard <br /> associated with project area soil types is severe, and construction activities could expose disturbed areas to wind <br /> and storm events,this would be a potentially significant impact. <br /> IMPACT Geology,Soils,and Seismicity—Risks to People and Structures Resulting from Unstable Soil <br /> 4.7-4 Conditions. Soils in the southwest portion of the project area have a very high clay content and are rated by <br /> the NRCS as highly plastic with a high shrink-swell potential. Therefore, it is possible that expansive soils <br /> identified in the southwest portion of the project area could damage wastewater collection system, effluent <br /> outfall, and recycled water distribution system pipelines and related structures. Because expansive soils are <br /> located in the southwest portion of the project area and groundwater levels in the project area are high, this <br /> would be a potentially significant impact. <br /> Expansive soils shrink and swell as a result of moisture change. These volume changes can result in damage over <br /> time to building foundations,underground utilities, and other subsurface facilities if they are not designed and <br /> constructed appropriately to resist the changing soil conditions. Volume changes of expansive soils also can result <br /> in the consolidation of soft clays following the lowering of the water table or the placement of fill. <br /> Shrink-swell potential is the potential for volume change in a soil with a loss or gain in moisture; soils swell when <br /> wet and shrink when dry. Soils with high clay content tend to be most affected by shrink and swell.The potential for <br /> soil to undergo shrink and swell is greatly enhanced by the presence of a fluctuating,shallow groundwater table. <br /> The Galt clay soil type south of the WQCF site (Exhibit 4.7-1)is considered expansive because of its high clay <br /> content,high plasticity index, and high shrink-swell potential(MRCS 1992). In addition,the Arents soil type <br /> south of the WQCF, and the Guard the Egbert soil types along the effluent outfall pipeline alignment are also <br /> considered expansive soils (San Joaquin County 1999), and groundwater levels in the project area were found to <br /> be high(City of Manteca 2003a). <br /> Therefore,it is possible that expansive soils identified in the southwest portion of the project area could damage <br /> wastewater collection system, effluent outfall, and recycled water distribution system pipelines and related <br /> structures. Because expansive soils are located in the southwest portion of the project area and groundwater levels <br /> in the project area are high,this would be a potentially significant impact. <br /> IMPACT Geology,Soils,and Seismicity—Risk of Structural Damage Caused by Corrosive Soils. The <br /> 4.7-5 corrosiveness of project area soils was not tested to determine whether the soils could cause damage to buried <br /> concrete slabs and foundations and buried metal pipes during the operation of the proposed project. However, <br /> because the City's engineering design process for the project would ensure the identification of corrosive soils <br /> and the implementation of project designs that would minimize damage to buried concrete slabs, foundations, <br /> or buried metal pipes, this impact would be less than significant. <br /> Manteca WQCF and Collection System Master Plans EIR EDAW <br /> City of Manteca 4-7.19 Geology,Soils,and Seismicity <br />