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4. ENVIRONMENTAL ANALYSIS 4.7 GEOLOGY,SOILS,AND SEISM1CrrY consolidated alluvium materials referred to as the Tulare Formation underlie the younger alluvial F materials. The Tulare Formation extends to approximately 600 feet below sea level and consists of ancient floodplain, alluvial fan and terrace deposits. Beneath the city of Tracy,the upper and lower unit of the Tulare Formation forms the prominent water-bearing units. These water-bearing units are divided by the Corcoran Clay,one of several clay layers deposited within alluvial materials along the western margin of the San Joaquin Valley. The Corcoran Clay formed in lacustrine(marshy environments)and its presence can influence the occurrence and quality of groundwater. The Corcoran clay is an aquitard 2 reaching a thickness of 100 feet. The lower zone of the Tulare Formation contains fresh water to a depth of approximately 600 feet below mean sea level. Groundwater pumping in the Tulare Formation began in the early 1900's and increased gradually until 1940 when the pumpage in the lower zone increased dramatically (KJC, 1990). As a result, water levels dropped over 100 feet. Refer to Section 4.8,Hydrology and Water Quality for additional discussion of groundwater occurrence underlying the proposed project site. RMC Lonestar advanced 35 exploratory borings from depths ranging from of 79 feet to 143 feet below ground surface (bgs)within areas of the project site to characterize the existing aggregate resource(Geomatrix, 2002). The Applicant referred to these two investigations as the A and L _ Ranch Exploration Program(1989) and the TrianaBarnes Exploration Program(1994). Exploratory logs identify the rocky surface topsoil layer, considered overburden,extending from the surface to depths ranging from about 5 to 12 feet bgs. The overburden topsoil is underlain by silty sand and gravel unit,with interbedded silty clay layers varying in thickness from 1 to 15 feet. These deposits extend to depths of at least 100 feet bgs. In the southeast portion of the project site,groundwater was encountered within these alluvial deposits at a depth of 100 feet bgs. Groundwater production wells on the proposed project site indicate that groundwater elevations range between 85 to 140 feet bgs,which is consistent with the groundwater depth encountered at 135 feet bgs at the adjacent DSS Quarry (Geomatrix,2002). SOILS The Soil Survey for San Joaquin County identifies seven soil types at the proposed project site. These soil units occur on slopes ranging between 0 and 2 percent. Table 4.7-1 lists the spec characteristics of these. The Capay clay,the Zacharias clay loam,and the Zacharias gravelly clay ' loam, occupies most of the north and northwest sections of the site. The Vernalis clay loam and the Cortina gravelly sandy loam overlie the remainder of the site, with interspersed areas of the _ other mentioned soil types. In general,the soils which cover the proposed project site have a slight to moderate hazard of water erosion. Prime Farmland Soils, as defined by the NRCS,are those that have the best combination of physical and chemical characteristics for crop production. Prime Farmland Soils possess soil quality,growing season,and moisture supply needed to produce sustained high yields of crops when treated and managed. Soils of Statewide Importance are soils other than Prime Farmland 2 An aquitard is a relatively impermeable boundary that impedes the exchange of water between the upper and lower zones. RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.7-2 AdESA/203015 mitdwa ivc Draft EIR March 2004