ARCHIVED REPORTS_XR0012730 - Laserfiche WebLink

Home Browse Search

PRELIMINARY HYDROGEOLOGIST ASSESSMEN7 REPORT 14ti O West Froalage Road Manteca,California Delta Project No.40439 479 F: Page 7 3.2 Site Soil and Georg ly The surface of the site consists of a 2-inch-thick layer of asphalt concrete overlying a 4-inch-thick 1-r aggregate base. Subsurface geology at the site has been determined from soil boring logs prepared by ETS and borings completed by Delta. The soils encountered during these drilling activities were predominantly f silty and sandy clays and sand to silty sand. G� The uppermost silty sand layer contains an occasional silt and clay lense. Beneath the silty sand,a sandy clay grades into a clayey sand with depth. A sand layer 3 to 5 feet thick lies beneath the clayey sand. This sand unit was found between 34 and 39 feet below the surface in every boring. Below the sand unit Iies a silty clay stratum. The silty clay stratum is the deepest stratum penetrated by soil borings at the r site. Sediments are mostly fine-grained but-ange from very fine to coarse. Mica is a common component in each unit. The subsurface relationships of the units across the site are illustrated in the geologic cross- sections presented in Figure 4. The location of the cross-sections is shown in Figure 5. Soil boring logs are presented in Appendix A. .— 3.3 Regional Mdro2eola k, Ground water in the Sacramento Valley occurs in confined and unconfined conditions. Confined ground j water is located In older deeper formations. Unconfined water can be found in alluvial fan,flood plain, `M and stream channel deposits. Ground water is found at shallower depths in the central part of the f.,. Sacramento Valley Basin(5 feet)and deeper toward the basin margins,where it may be greater than 100 feet in depth. Regional ground water flow under natural conditions is from the Sacramento Valley margins toward the Sacramento River Delta. Development and use of ground water has created a major depression in the ground water surface in the Stockton area. This depressed portion of the ground water masques the regional gradient and is currently acting as a sink toward which ground water flows (DWR Bulletin 146, 1967). Recharge to the water table occurs through percolation of water from streams,rain,snowfall,and irrigation. i '- Because of its quality,ground water throughout the valley is suitable for most uses. Ground water in the eastern half of the valley is of better qualitywith respect to mineral content than water fromthe western side of the valley. On the west side,rocks of the Coast Range may contain many soluble minerals and saline rester. High concerarations of sodium, chloride, and sulfate in the ground water of the western 6, 1978}. valley can render it unsuitable for irrigation and drinking (DWR Bulletin 118 - i i