ARCHIVED REPORTS_XR0012190 - Laserfiche WebLink

Home Browse Search

The near surface geology of the region is comprised of unconsolidated alluvial deposits of Pleistocene to Holocene age These deposits, sometimes referred to as the "older alluvium", consist of intercalated beds of gravel, sand, silt and clay Underlying the older alluvium are Pilo/Pleistocene continental deposits of similar derivation and lithology The older alluvium functions as the most important aquifer in the site area (USGS Professional Paper 1401-C, 1986) Soils developed on the alluvium are generally well drained, differing from the parent material only in the increased volume of organic matter (DWR Bulletin No 146, 1967) ' The important body of surface water in proximity to the site is Mormon Slough which is approximately 2/3 mile south of the site Existing and potential beneficial uses of this surface water include municipal and domestic water supply, irrigation, stock watering, ' industrial supply, groundwater recharge, freshwater replenishment, recreation, freshwater habitat, wildlife habitat, fish migration, and fish spawning (CRWQCB Water Quality Control Plan, 1991) ' The existing and potential uses of groundwater in the area include irrigation, private and municipal water supply, stock watering, process water, and service supply (CRWQCB Water Quality Control Plan, 1991) The "older alluvium" is the most extensively developed geologic unit in the area and hosts both unconfined and confined zones The unconsolidated, mostly coarse grained nature of the aquifer material results in high well yields Specific capacities of wells completed in the older alluvium average 42 GPM/ft (USGS Professional Paper 1401-D, 1989) Drinking wells in the region are generally completed at depths greater than 100 feet below ground surface due to the relatively poor quality of near surface groundwater ' Regional groundwater flow in the site area is generally in a southwesterly direction toward the San Joaquin River Delta, or easterly toward a regional groundwater depression centered about 4 miles east of Stockton (San Joaquin County Flood Control and Water Conservation District, Spring 1991) Published data from the San Joaquin Flood Control and Water Conservation District indicate that the average long term depth to groundwater at the site has varied from approximately 65 feet below grade (Fall 1991) to 35 feet below grade (Spring 1986) Current depth to groundwater at the site is approximately 561/2 to 571/2 feet below grade 2.2 Soil Lithology ' Sediments encountered during g drillin of onsite and offsite borings during the most recent subsurface investigation consisted of clay, silt, sandy silt, silty sand, sandy silty gravel, and ' sand Descriptions of the materials encountered are shown on the boring logs included in Appendix 4 Lower permeability sediments were generally encountered to a depth of approximately 45 to 50 feet bgs Higher permeability sediments were generally encountered from 50 feet bgs to a maximum explored depth of approximately 80 feet bgs Cross groundxelroeklcip 7