WORK PLANS - Laserfiche WebLink

Home Browse Search

school, is located approximately 700 feet west of the Site. There are no known surface water bodies within 1,000 feet of the Site. GEOLOGY AND HYDROGEOLOGY REGIONAL GEOLOGY AND HYDROGEOLOGY The Site is located within the central portion of the Great Valley Geomorphic province, an elongate alluvial plain extending from the Tehachapi Mountains in southern California to the Klamath mountain in northern California that is bound by the Sierra Nevada range to the east and the Coast Ranges to the West. The Great Valley is divided into the Sacramento Valley in the north with the San Joaquin Valley occupying the southern portion (Hackel, 1966). The San Joaquin portion of the Great Valley has been filled with as much as six miles of sediment deposited by rivers and streams draining from the western margins of the Sierra Nevada (USGS, 1986). Locally, the Site is located within the Eastern San Joaquin Subbasin of the San Joaquin Groundwater Basin, covering a surface area of 1,105 square miles. The Site and surrounding area are underlain by the Alluvium and Modesto/Riverbank Formations, formed by coalescing alluvial fans of Recent to Late Pleistocene age. These formations consist of more than 150 feet of sands and gravels within the alluvial fan deposits and clay, silt and sand within the interfan deposits. Groundwater within these deposits is unconfined(DWR, 2006). SITE GEOLOGY AND HYDROGEOLOGY Near surface soils in the vicinity of the Site are described as Holocene aged flood basin deposits consisting of clay silts and some sands (USGS, 1986). Previous subsurface investigations completed at the Site, and in the Site vicinity, have involved drilling of wells and exploratory borings to depths up to 125 feet below ground surface (bgs). Soils encountered during well drilling and advancement of conepenetrometer (CPT) borings have consisted of interbedded layers of clay, clayey silts, sandy silts, clayey sands and poorly graded sands of varying thicknesses. Three primary water bearing zones have been interpreted to occur between the approximate depths of 25 and 55 feet bgs (shallow zone), 70 and 80 feet bgs (intermediate zone) and greater than 95 feet bgs (deep zone). The permeable stratigraphy in these water bearing zones do not appear to be laterally continuous in thickness or extent, based on lithologic logs recorded during drilling. The direction of groundwater flow in the shallow water bearing zone has been interpreted to range from northeasterly to southeasterly. Groundwater flow in the intermediate water bearing zone has been interpreted to be predominately northeasterly to east-northeasterly. Only one well has been installed in the deeper water bearing zone and does not allow for hydrogeologic interpretation of groundwater flow direction. Historically, depth to groundwater in the shallower screened monitoring wells has ranged between approximately 13 and 32 feet bgs. Depth to groundwater in the intermediate and deeper screened monitoring wells has ranged between approximately 14 and 26 feet bgs. FREY Environmental,Inc. 2 Project No. 1095-01