By John A. Izbicki
U.S. Geological Survey Fact Sheet 125-96
Seawater Intrusion in the Upper Aquifer System
Prior to 1989, most of the wells used to monitor chloride concentrations in the upper aquifer system were abandoned (unused) agricultural supply wells, and many of these wells were screened in more than one aquifer. As part of this study, 32 monitoring wells were installed in the upper aquifer system at 14 multiple-well sites within the area believed to be intruded by seawater (Densmore, 1996). These wells are screened in individual aquifers (the Oxnard, Mugu, or perched aquifer) to enable collection of aquifer-specific water-level and water-quality data.
Chloride concentrations in water from most wells completed as part of this study were less than 250 mg/L (fig. 3), and in water from many wells, chloride concentrations were not much higher than that of native (fresh) ground water. However, chloride concentrations in water from some wells near the Hueneme and Mugu submarine canyons were as high as 17,000 mg/L in the Oxnard aquifer, and as high as 6,300 mg/L in the Mugu aquifer (Izbicki and others, 1995). Chloride concentrations in the perched aquifer near the coast were as high as 23,000 mg/L. For comparisons, seawater has a chloride concentration of 19,000 mg/L; the EPA Maximum Contaminant Level (MCL) for chloride in a public water supply is 250 mg/L; salt-sensitive crops common in the study area, such as strawberries, are adversely affected by chloride concentrations as low as 100 mg/L.
Results of sample analyses showed that water from abandoned agricultural wells was not representative of ground water in the Oxnard or Mugu aquifers. This is because these wells have (1) large diameter casings and are difficult to purge of stagnant water prior to sample collection, (2) screened intervals that may be open to more than one aquifer (as a result, chloride concentrations in individual aquifers cannot be defined), and (3) steel casings that are subject to corrosion and failure-especially in areas where water in the perched aquifer is saline.
Corrosion and failure of abandoned wells may result in leakage of water from the perched aquifer into underlying aquifers. For example, chloride concentrations in water from well 27R1, located in what was believed to be a highly intruded part of the upper aquifer system, increased from less than 70 mg/L in the early 1960's to 1,900 mg/L in 1990 (fig. 4). The increase in chloride concentrations was used as evidence of increasing seawater intrusion in the Oxnard aquifer. However, less than 150 feet away, water from well 27R4 (screened in the Oxnard aquifer) had a chloride concentration in 1990 of 180 mg/L. At the same time, water in well 27R5, screened in the perched aquifer, had a chloride concentration of 23,000 mg/L. The water level in well 27R5 was higher than the water level in wells 27R1 or 27R4 (fig. 4), suggesting that water could move from the overlying perched aquifer into the Oxnard aquifer through the failed casing of well 27R1. Leakage of water at a rate of 9 gal/min was confirmed on the basis of a heat-pulse velocity log (R.T. Hanson, U.S. Geological Survey, written commun., 1994). Similar results were obtained at two additional locations where multiple-well sites were located near abandoned wells. Predmore (1993) identified more than 160 abandoned, and potentially failed, wells in the area believed to be intruded by seawater.
Results of a direct-current surface-resistivity survey in the area believed to be intruded by seawater (Zohdy and others, 1993) agreed with water-quality data from multiple-well sites and showed that the areal extent of seawater intrusion in the upper aquifer system was smaller than previously believed. Interpretation by Stamos and others, (1992) of geologic, water-quality, and surface-resistivity data suggested that poor-quality water from fine-grained deposits may be an additional source of high-chloride water to wells. The presence of poor-quality water in the fine-grained, lagoonal deposits near Point Mugu was confirmed using test-drilling data and electromagnetic logs (R.T. Hanson, U.S. Geological Survey, written commun., 1994).
Major-ion data (Izbicki, 1991) and delta oxygen-18 and delta deuterium isotopic data (Izbicki, 1996) were used to identify the source of high-chloride water to wells and to redefine the areal extent of seawater intrusion in the Oxnard and Mugu aquifers. Using this approach, seawater cannot be positively identified from other sources of high-chloride water at chloride concentrations less than 500 mg/L. The redefined areal extent of seawater intrusion in 1993 was 5.5-square miles in the Oxnard aquifer and 3.3-square miles in the Mugu aquifer, (fig. 3)-considerably less than the 23-square miles (of the upper aquifer system) mapped in previous studies. Much of the apparent reduction in the areal extent of seawater intrusion results from the identification of additional sources of high-chloride water to wells in the Oxnard and Mugu aquifers. For example, near Mugu submarine canyon the seawater front is preceded by water from fine-grained lagoonal deposits that has chloride concentrations greater than 500 mg/L in both the Oxnard (4.5-square miles of high-chloride water) and Mugu (3.1-square miles) aquifers.
On the basis of the redefined areal extent of seawater intrusion, seawater in the Oxnard and Mugu aquifers advanced only 2.7 and 1.9 miles, respectively, from aquifer outcrops in the submarine canyons between 1955 and 1992 (fig. 3). As a consequence, the seawater must be moving slower than previously believed. This is consistent with estimates of the rate of movement of the seawater front made on the basis of tritium and chlorofluorocarbon data (Michel and others, in press).