View File - Development Services - City of Oxnard

WATER RESOURCES TECHNICAL REPORTFormation. These deposits reach a maximum thickness of more than 5,000 feet in the Piruarea of the Santa Clara River Valley. The sand and gravel layers range from 10 to 100 feetthick and are separated by silt and clay layers that are generally 10 to 20 feet thick.Late Pleistocene and Holocene Epoch. The Late Pleistocene and Holocene deposits areunnamed and consist of relatively flat-lying marine and continental unconsolidateddeposits. These deposits are derived from local sources from the Santa Clara River andCalleguas Creek, and were deposited unconformably on the older unconsolidated depositsand contain basal conglomerates that are laterally extensive and produce substantialgroundwater supplies.Alluvial and fluvial sand and gravel deposits with interbedded fine-grained deposits of theHolocene epoch unconformably overlie the Late Pleistocene deposits. Basal deposits of theHolocene epoch are relatively more permeable than underlying deposits, and are potentialmajor sources of water to wells completed in the saturated parts of these deposits.Interbedded sand layers occur within the fine-grained deposits. On the Oxnard Plain, thebasal zones are overlain with fine-grained deposits of low permeability, which createshallow, perched groundwater of poor quality that is not considered to be part of theregional upper aquifer and lower aquifer system.2.6 GroundwaterGroundwater is the main source of water in the Santa Clara-Calleguas hydrologic basin. Theonshore part of the alluvial groundwater basins covers approximately 310 square miles inVentura County. The lithologic units of the aquifers comprising the onshore alluvialgroundwater basins extend offshore covering an additional area of approximately193 square miles of continental shelf. The groundwater basins within the Santa Clara-Calleguas Basin are shown in Figure 2-1 from the USGS modeling study and thegroundwater basins within the UWCD service boundary are shown in Figure 2-3.Groundwater basins along the Santa Clara River Valley, from upstream to downstream,include the Piru basin, Fillmore basin, and the Santa Paula basin. Groundwater basins alongthe Coastal areas, from north to south, include the Mound, Oxnard Plain Forebay, andOxnard Plain. Basins further inland from the coastal areas include the Las Posas basin, SantaRosa Valley basin, and Pleasant Valley basin. Figure 2-1 also provides ocean bottombathymetry, which shows the continental shelf that extends several miles offshore and isbisected by the Hueneme submarine canyon near Port Hueneme and by the Mugusubmarine canyon near Pt. Mugu. These submarine canyons bisect the aquifers on theOxnard Plain that extend offshore making them vulnerable to seawater intrusion.Water wells were first drilled in Ventura County using machinery instead of hand laborbeginning in the 1880s. A steady increase in groundwater demand for farming and urbanuses since the late 1800s has resulted in groundwater overdraft conditions. In general terms,groundwater overdraft is the withdrawal of potable water from an aquifer system in excessof replenishment from natural and artificial recharge. This overdraft has resulted in thefollowing conditions within the aquifers of the Oxnard Plain and Pleasant Valley:• Groundwater storage reductions• Declining groundwater levels to below sea levelW112003002SCO LW1458.DOC/ 033390002 15