Ground-water Quality in the Lompoc Plain, Santa Barbara County, California, 1983

This report summarizes the results of a comprehensive study of ground-water quality in the Lompoc plain in western Santa Barbara County. Water-quality characteristics in 1983 are described, and changes in water quality since the last comprehensive study in the early 1970’s are documented. Ground water is the main source of water in the Lompoc plain. The younger alluvium is the major aquifer and consists predominantly of unconsolidated deposits of sand and gravel. The younger alluvium is as much as 180 feet thick and is divided into an upper member and a lower member throughout most of the plain. The lower member is the main water-bearing zone in the Lompoc plain. Long-term ground-water levels in most of the plain have not changed significantly since the 1940’s. A seasonal water-level decline of about 15 feet in 1982 in the central part of the plain may be attributed to nearby military and agricultural pumping.

Ground-water quality in the plain in 1983 tended to deteriorate from east to west. Dissolved-solids concentrations throughout the plain exceeded the U.S. Environmental Protection Agency secondary maximum contaminant level of 500 milligrams per liter for drinking water; in most of the plain, concentrations exceeded 1,000 milligrams per liter. In samples from some wells, concentrations of one or more of the following constituents sodium, chloride, nitrate, and iron exceeded primary or secondary maximum contaminant levels for drinking water. Concentrations of constituents in some samples also exceeded recommended levels for irrigation water. The predominant ions generally were calcium, magnesium, sulfate, and bicarbonate – except in the western part, where sodium and chloride were the predominant dissolved ions.

From 1972 to 1983, dissolved-solids concentrations in the main water-bearing zone generally decreased in the central part of the plain but increased throughout most of the study area. The largest increases, greater than 1,000 milligrams per liter, were in the extreme western part of the plain.