Seawater Intrusion in the Lower Aquifer System

The lower aquifer system also contains multiple sources of high-chloride water to wells. These sources include seawater intrusion and movement of high-chloride water from partly consolidated marine and volcanic rocks that surround and underlie the basin. Although not discussed specifically in this report, leakage of water through the corroded and failed casings of abandoned wells also is a source of high-chloride water to wells in the lower aquifer system. As part of this study, 25 monitoring wells (some as deep as 1,500 feet) were installed in the lower aquifer system and underlying marine rocks at 10 sites near Hueneme and Mugu submarine canyons (Densmore, 1996).

Chloride concentrations in water from wells installed in the lower aquifer system as part of this study were as high as 2,500 mg/L near Mugu submarine canyon and 7,100 mg/L near Hueneme submarine canyon. Chloride concentrations in water from wells installed in the surrounding and underlying marine rocks were as high as 3,400 mg/L (Izbicki and others, 1995).

High-chloride water from wells in the lower aquifer system near Mugu submarine canyon is enriched in iodide relative to a mixture of native water with seawater (fig. 5). These data suggest that high-chloride water in the lower aquifer system near Mugu submarine canyon is not the result of seawater intrusion. This interpretation was confirmed by delta oxygen-18 and delta deuterium data, which show that high- chloride water near Mugu submarine canyon is meteoric, rather than oceanic, in origin (Izbicki, 1991). The chemical and isotopic composition of high-chloride water from wells near Mugu submarine canyon is similar to that of high-chloride water from wells as far as 15 miles inland. These inland wells have been invaded by high-chloride water from the marine and volcanic rocks that surround and underlie the lower aquifer system. Analyses of water-quality data suggest that the source of most of the high-chloride water to wells in the lower aquifer system near Mugu submarine canyon is the partly consolidated marine rocks, although the volcanic rocks are a locally important source.

Figure 5. Chloride-to-iodide ratio as a
function of chloride in water from wells in
the Oxnard Plain.

The delta oxygen-18 and delta deuterium data confirm that high-chloride water near Hueneme submarine canyon is a mixture of native water and seawater. Seawater intrusion in the lower aquifer system was first identified in November 1989 in water from a well near Hueneme submarine canyon (fig. 3). This well is screened between 720 and 760 feet below land surface. By May 1991, water from a well at the same site, but screened between 500 and 520 feet, also was intruded by seawater. In August 1993, no increase in chloride concentration and no evidence of seawater intrusion was found in water from a well at the site screened at a depth of 830 to 870 feet or in water from other multiple-well monitoring sites inland from Port Hueneme.

Electromagnetic logs show that seawater intrudes the lower aquifer system through relatively thin layers within the aquifer (R.T. Hanson, U.S. Geological Survey, written commun., 1994). Intruded layers are presumably areally extensive and more permeable than the unintruded layers. Clay deposits having low permeability restrict the vertical movement of seawater between more permeable layers. The layered nature of seawater intrusion within the lower aquifer system was simulated using a solute-transport model developed by the U.S. Geological Survey (Tracy Nishikawa, written commun., 1996).