View File - Development Services - City of Oxnard

WATER RESOURCES TECHNICAL REPORTThe northern cross section, B-B’, runs through the northeastern tip of the Oxnard Plain,through the Oxnard Forebay and northwestern Oxnard Plain, and out to the edge of thecontinental shelf. The middle cross section, C-C’, runs through the central portion of theOxnard Plain and terminates at Hueneme Canyon. The southern cross section, D-D’, runsthrough Pleasant Valley, through the southern Oxnard Plain, and terminates at MuguCanyon.Clay layers, as schematically shown in the DWR 1976 cross section in Figure 2-11, separatethe individual aquifers within the UAS and LAS. As further described below, the UAS isunconfined in the Oxnard Forebay area making this an important recharge area on theOxnard Plain. In addition, the clay layers separating the aquifer units of the UAS from theLAS are also notably less significant in this area, making recharge to the LAS possible. Claylayers separate the UAS from the LAS across the remainder of the Oxnard Plain andremaining Santa Clara-Calleguas Basin, making natural recharge to the LAS slow.The Department of Water Resources (DWR) 1976 cross section also shows seawaterintrusion extending inland from the outer continental shelf into the onshore portion of theOxnard Aquifer of the UAS. This is a useful schematic diagram to illustrate seawaterintrusion; however, two items are noted as follows. First, as further described below,seawater intrusion also exists within the other aquifers of the UAS and LAS. Second,although seawater intrusion has likely occurred into the offshore continental shelf as shown,the inland seawater intrusion affecting coastal groundwater likely resulted from thesubmarine canyons short-circuiting the travel distance from the ocean to inland areas.The individual aquifer systems are discussed below, from shallowest to deepest.Upper Aquifer SystemShallow Aquifer or “Semi-Perched” Aquifer. The shallow aquifer beneath the Oxnard Plainand Pleasant Valley basins consists of fine to medium-grained sand with interbedded claylayers and is referred to as the “semi-perched aquifer.” Clay layers separate the shallowaquifer from the underlying Oxnard aquifer. Water quality is poor throughout most to theOxnard Plain and Pleasant Valley sub-basins in this unit; consequently, few wells areperforated in this interval. The shallow aquifer extends to a depth of approximately 100 feetbelow ground surface (bgs).Oxnard Aquifer. The Oxnard aquifer lies at the base of the Holocene deposits, which consistsof sand and gravel deposited by the ancestral Santa Clara River and Calleguas Creeksystem. The base of the aquifer ranges from about 150 to 250 feet bgs throughout most of theOxnard Plain. The basal deposits are a major source of water to wells in the Forebay areaand on the Oxnard Plain. Alluvial fans derived from the mountains to the north pushed theSanta Clara River south where the clay layers were eroded and sand and gravel weredeposited in their place. As noted above, this is an important groundwater recharge areadue to the absence of these low permeability clay layers. Elsewhere, clay layers throughoutthe Oxnard Plain and Pleasant Valley basin separate the Shallow and Oxnard aquifers.These clay layers confine or partly confine the Oxnard aquifer limiting surface recharge.Mugu Aquifer. The Mugu aquifer is composed of the basal part of the unnamed upperPleistocene deposits in the Oxnard Plain Forebay and Oxnard Plain basins. These depositsconsist of sand and gravel interbedded with silt and clay deposited by the ancestral SantaW112003002SCO LW1458.DOC/ 033390002 17