WORK PLANS - Laserfiche WebLink

Home Browse Search

• LFR Inc. 1.3 Site Geology and Hydrogeology Soils encountered at the Site during previous investigations consist of silty gravel with interbedded clayey silt lenses to a depth of approximately 25 feet below the average gravel pit elevation. Silt and sand with varying amounts of gravel and clay occur at a depth of 25 to 300 feet below ground surface (bgs). The Corcoran Clay is encountered at depths ranging from 300 to 380 feet bgs (Ecology and Environment, Inc. 2002). The soil encountered during drilling activities for monitoring wells MW-6 and MW-7 and soil borings SB-01 and SB-02 from the surface to approximately 25 feet bgs consisted of silty gravel with minor layers of fines (resembling alluvial fan deposits). From approximately 25 feet to 125 feet bgs, the soils generally consisted of silt and sands with varying amounts of clay and gravels assumed to be part of the Tulare Formation. A 10-foot-thick clay layer was noted in the cliff face from 85 to 95 feet bgs but was not detected in the intermittent soil samples collected from borings. Groundwater occurs in two water-bearing zones at the Site: an upper water-bearing zone and a lower water-bearing zone. The upper water-bearing zone (i.e., the water table) is encountered at an approximate depth of 75 feet bgs. The lower water-bearing unit is encountered at an approximate depth of 380 feet bgs, and is separated from the upper zone by the Corcoran Clay (Ecology and Environment, Inc. 2002). Regionally, recharge of the upper water-bearing unit is primarily from irrigation and seepage of surface water through the alluvium with little contribution from rainwater (U.S. Geological Survey 1971). The project site receives an approximate average of 10.5 inches of precipitation per year and has a total estimated evapotranspiration rate of 61 inches per year as measured 3 miles south of the Site. Based on data collected from six on-site groundwater monitoring wells during a recent groundwater sampling event, groundwater flow within the upper water-bearing unit is toward the northwest, with a calculated gradient of approximately 0.001 (Figure 3). The nearest surface-water features to the project site are Corral Hollow Creek to the north, an intermittent natural drainage course; the Delta-Mendota Canal, an irrigation canal; and the California Aqueduct. The topographic nature of the Site precludes it as a potential source of runoff to any of these water features. Because the Site is a former gravel mine, it is essentially a closed basin with no outfall. All rainwater that falls onto the Site infiltrates into the soil substrate and serves as local groundwater recharge. "The Water Quality Control Plan, Sacramento River and San Joaquin River Basins" ("Basin Plan"; Regional Water Quality Control Board 1998) designates beneficial uses, establishes water-quality objectives, and contains implementation plans and policies for all waters of the Basin. The designated beneficial uses of the groundwater, as specified in the Basin Plan, are domestic and municipal, agricultural, and industrial supply. As stated previously, there is no surface-water flow into or out of the project site because it is located in an excavated pit. Page 2 wp-MW-08_Insult-Feb06-final-0902506.dm:LFR