View File - Development Services - City of Oxnard

WATER RESOURCES TECHNICAL REPORT• Water quality degradation, including seawater intrusion• Ground subsidenceThe hydrogeology and groundwater resources are discussed below, with an emphasis onthe Oxnard Plain Forebay, Oxnard Plain, and Pleasant Valley basins, which are those basinsthat will be affected by the GREAT Program. Groundwater levels and quality are activelymonitored by the UWCD and USGS, in addition to other entities, as part of watermanagement activities. These data are critical to understanding the historical and currentoverdraft conditions. The locations of wells from which monitoring data are currentlycollected are summarized on the four maps shown in Figure 2-9, which were providedby UWCD.2.6.1 Aquifer UnitsThe Santa Clara-Calleguas Basin consists of multiple aquifers that are grouped into a UASand an LAS. These aquifers contain gravel and sand materials that were deposited along theancestral Santa Clara River, within alluvial fans along the flanks of mountains, and withincoastal plain/delta complex at the terminus of the ancestral Santa Clara River. The surfacegeology of the outcropping stratigraphic units that comprise the UAS, LAS, and deeperbedrock are shown in Figure 2-7, and the stratigraphy of the materials comprising theseunits is shown in Figure 2-8.The UAS consists of the Shallow, Oxnard, and Mugu aquifers, which include theunconsolidated deposits of the late Pleistocene and Holocene epochs. The Shallow Aquiferis also referred to as the Semi-Perched Aquifer. The UAS is separated by an unconformityfrom the LAS. The LAS consists of the upper and lower Hueneme, Fox Canyon, and GrimesCanyon aquifers, which include the unconsolidated deposits of the Pliocene and Pleistoceneepochs. The sediments of the LAS extend to about 1,600 feet below sea level in the OxnardPlain. All of these aquifers extend offshore within the continental shelf. The onshore part ofthe Oxnard Plain is subdivided into a confined region and an unconfined region. Theconfined region includes the Oxnard Plain Forebay and the northeastern part of the OxnardPlain where the Semi-Perched Aquifer is absent. The sediments of the UAS and LAS areunderlain by older bedrock of upper Cretaceous and Tertiary age that is consolidated andmostly nonwater bearing.These aquifer systems are bounded below and along mountain fronts by consolidatedbedrock that forms a relatively impermeable boundary to groundwater flow. Numerousfaults act as low permeability boundaries to groundwater flow. The aquifer systems extendoffshore where they crop out along the edge of the submarine shelf within the coastalsubmarine canyons. Submarine canyons have dissected these regional aquifers, providing ahydraulic connection to the ocean through the submarine outcrops of the aquifer system.Regionally, groundwater flows from the inland areas toward the ocean, east to west.Locally, however, overdraft has resulted in groundwater elevations below sea level thathave resulted in flow from the ocean inland (landward flow), which has resulted inseawater intrusion in both UAS and LAS.Three generally east-west cross sections from the USGS groundwater modeling report, B-B’,C-C’, and D-D’, are provided in Figure 2-10 to the subsurface geology of these units in theOxnard Plain and Pleasant Valley Areas. The cross sections are aligned with the coastline.W112003002SCO LW1458.DOC/ 033390002 16