Hydrogeology

The Oxnard Plain, 60 miles northwest of Los Angeles, has an area of 120-square miles and is underlain by a complex system of aquifers more than 1,400 feet thick. These aquifers (like many similar coastal aquifers in southern California) can be divided into an upper and a lower aquifer system (fig. 2).

Figure 2. Generalized geologic sections
through Hueneme (A in figure 1) and Mugu
submarine canyons (B in figure 1).

The upper aquifer system consists of relatively flat-lying alluvial deposits about 400 feet thick and contains two aquifers that have been developed for water supply-the Oxnard and Mugu aquifers. The Oxnard aquifer, about 180 feet below land surface, is the primary water-yielding zone. The Oxnard aquifer is underlain by the Mugu aquifer and overlain by a thick, areally extensive clay deposit. This clay deposit separates the Oxnard aquifer from a shallow unconfined aquifer that previous researchers have referred to as the `perched aquifer.' (Use of this name in this report does not imply that perched conditions exist in the Oxnard Plain.) The Oxnard and Mugu aquifers crop out in the Hueneme and Mugu submarine canyons less than one-quarter mile offshore (fig. 2). The perched aquifer crops out immediately offshore all along the coast in the study area.

Native water in the Oxnard and Mugu aquifers is generally fresh and chloride concentrations are about 40 mg/L. However, in some areas (especially near Mugu submarine canyon), interbedded fine-grained deposits in the Oxnard and Mugu aquifers contain saline water. Prior to the onset of seawater intrusion, the Oxnard and Mugu aquifers were extensively pumped for water supply. The perched aquifer contains fresh and saline water, but it is not used as a source of water supply. Saline water in the perched aquifer results from a combination of (1) seawater that recharged the aquifer through offshore outcrops or infiltrated into the aquifer through coastal wetlands or during coastal flooding [in some cases this water was partly evaporated prior to recharge], (2) concentration of dissolved minerals resulting from the evaporative discharge of ground water, (3) infiltration of irrigation return water.

The lower aquifer system consists of alternating layers of alluvial sand and clay about 5 to 50 feet thick. These deposits grade to marine near the coast and overlie fine-grained marine sands that are more than 100 feet thick and are separated by marine silt and clay interbeds as much as 50 feet thick. The deposits of the lower aquifer system have been folded and faulted. Marine seismic-reflection data (Greene and others, 1978) and test-drilling data show that the lower aquifer system crops out in Hueneme submarine canyon, but it does not crop out in Mugu submarine canyon because of offshore faults and uplift of partly consolidated marine and volcanic rocks (fig. 2).

Native water in the lower aquifer system generally is fresh and chloride concentrations range from 40 to 100 mg/L. The lower aquifer system is surrounded and underlain by partly consolidated marine and volcanic rocks that contain saline water. Chloride concentrations are as high as 3,400 mg/L in the partly consolidated marine rocks and can exceed 1,000 mg/L in the volcanic rocks.

Ground-water pumping has caused water levels in parts of the Oxnard and Mugu aquifers to decline below sea level and below the water level in the perched aquifer. Seawater entered aquifers through outcrop areas in Hueneme and Mugu submarine canyons in the mid-1950's and advanced inland in response to changes in the amount and distribution of pumping (fig. 1). By 1989, about 23-square miles of the upper aquifer system was believed to be intruded by seawater. Because of increasing chloride concentrations, pumping was shifted from the upper system to the lower system; subsequently, water levels in the lower aquifer system declined to below sea level. Increasing chloride concentrations were observed in the lower aquifer system near Mugu submarine canyon as early as 1985, and high-chloride water in the lower aquifer system near Hueneme submarine canyon was discovered as part of this study in 1989 (Izbicki, 1991).

After 1993, a combination of ground-water management strategies and increased availability of water from the Santa Clara River for ground-water recharge caused water levels in wells near the coast to rise above sea level and above water levels in the perched aquifer. Water levels in parts of the lower aquifer system near Hueneme submarine canyon also rose above sea level at that time, but water levels in the lower aquifer system near Mugu submarine canyon were still below sea level as late as 1996.