SU0004094 - Laserfiche WebLink

Home Browse Search

4.Environmental Analysis — Alluvial fan sediments deposited by erosion of the sandstone and siltstone rocks in the Coast — Ranges underlie the area west of the San Joaquin River, near the City of Tracy. The unit of semi- consolidated alluvium materials referred to as the Tulare Formation underlies the younger alluvial 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-hearing 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 aquitard2 reaching a thickness of 100 feet.The lower zone of the Tulare Formation contains fresh water to a depth of approximately 600 feet below msl. Groundwater pumping in the Tulare Formation began in the early 1900s 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 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 Triana/Bames 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 are the deposits to be mined and they 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 specific 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. 2 An aquitard is a relatively impermeable boundary layer that impedes the exchange of water between the upper and lower zones. RMC Pacific Vemalis Quany Mining and Reclamation Project 4.7-2 ESA/203015 Draft Environmental Impact Report May 2006