ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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interbedded,two to five foot thick strata of medium-dense silty and"clean"sand within the upper 40 feet of subgrade soils may be susceptible to liquefaction(Kleinfelder 1997,2004). SUBSIDENCE, SETTLEMENT AND LATERAL SPREADING Land surface subsidence can be induced by both natural and human phenomena. Natural phenomena include: subsidence resulting from tectonic deformations and seismically induced settlements; soil subsidence from consolidation,hydrocompaction, or rapid sedimentation; subsidence from oxidation or dewatering of organic-rich soils, and subsidence related to subsurface cavities. Subsidence related to human activity includes subsurface fluid or sediment withdrawal. Pumping of water for residential, commercial and agricultural uses from subsurface water tables causes more than 80% of the identified subsidence in the U.S. (Galloway et al. 1999). Lateral spreading is the horizontal movement or spreading of soil toward an open face, such as a streambank,the open side of fill embankments, or the sides of levees. The potential for failure from lateral spreading is highest in areas where there is a high groundwater table,where there are relatively soft and recent alluvial deposits, and where creek banks are relatively high. By 1970, subsidence in excess of one foot had affected one-half of the San Joaquin Valley(more than 5,200 square miles of farmland). The maximum subsidence, over 28 feet,was recorded near Mendota. Land subsidence in the San Joaquin Valley since the 1970s has generally slowed from reductions in groundwater pumping and the subsequent recovery of groundwater levels as a result of a greater emphasis on surface water irrigation. In the late 1980s,pumping of groundwater during a period of extended drought resulted in rapid decline in groundwater levels and renewed subsidence (Galloway et al. 1999). TIDAL WAVES AND SEISMIC SEICHES Earthquakes may affect open bodies of water in two ways: by creating seismic sea waves and by creating seiches. Seismic sea waves (often called"tidal waves")are caused by abrupt ground movements(usually vertical) on the ocean floor in connection with a major earthquake. Because of the distance of the project area from the ocean (i.e., greater than 20 miles), seismic sea waves would not be a factor. A seiche is a sloshing of water in an enclosed or restricted water body such as a basin,river, or lake. It is caused by earthquake motion; the sloshing can occur for a few minutes or several hours. In 1868, for example, an earthquake along the Hayward fault in the San Francisco Bay area is known to have generated a seiche along the Sacramento River. The project area is approximately two miles from the San Joaquin River, and is located in an area of relatively flat topography.A high percentage of Delta levees are subject to overtopping and subsequent failure, and seiches in the Delta could be damaging to the areas protected by those levees.Although the San Joaquin River is lined by levees in the region,the risk of damage to the project area from a levee break is minimal.None of the earthquakes caused by faults listed in Table 4.7-1 are known to have resulted in seismic seiche events along the river. In addition,the potential for damaging seiches is considered very low to negligible because of the absence of a deep, large open body of water adjacent to or in the project area. Therefore,the risk of seiche is considered low. SOIL RESOURCES Soil properties can affect the construction and maintenance of roads,building foundations, and infrastructure. Among these properties are permeability, shrink-swell potential,water retention capacity, and corrosion potential. The project area is located at the northern end of the San Joaquin Valley, approximately two miles east of the San Joaquin River. Surface soils at the project area consist of terrace deposits formed by ancient channels of the Stanislaus River(Atwater 1982) and more recent alluvial deposits of the San Joaquin River. These soils are Manteca WQCF and Collection System Master Plans EIR EDAW City of Manteca 4-7.5 Geology,Soils,and Seismicity