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

Abstract

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. 1.0 INTRODUCTION 1.1 Purpose and Scope, and Acknowledgments GROUND-WATER QUALITY IN THE LOMPOC PLAIN IS DOCUMENTED AND EVALUATED This report documents ground-water conditions in the Lompoc plain and evaluates changes in ground-water quality. Ground water is the main source of water for municipal, irrigation, and industrial supplies in the Lompoc plain. Salts from the oxidation of minerals in the soil and from streamflows, recycled irrigation waters, and effluents from wastewater-treatment facilities eventually enter the ground-water system. Miller (1976) noted that ground-water quality in the plain had deteriorated during the past several decades as a result of increasing salinity. There is concern about adverse effects of high salinity on crops and on human health. This report, prepared in cooperation with Vandenberg Air Force Base, documents ground-water conditions and evaluates changes in ground-water quality from 1972 to 1983 in the main water-bearing zone of the Lompoc plain, the lower member of the younger alluvium. The scope of the study included: (1) describing ground-water levels and movement in the main water-bearing zone in the study area; (2) collecting and analyzing ground-water samples from wells selected as representative of ground water in the main water-bearing zone; (3) classifying ground water as to predominant ions; (4) detecting areas where specific chemical constituents in the ground water exceed recommended limits for agricultural, municipal, and domestic uses; and (5) comparing groundwater quality in the main water-bearing zone in 1983 to that of 1972. The cooperation and assistance of the city of Lompoc Water Department in supplying data on chemical quality of water for this study are gratefully acknowledged. The author also expresses his gratitude to the many farmers who made possible the collection of water samples and field data, and to Mr. Bill Gillingham of the Johns-Manvil le Company who furnished hydrologic data and assisted in sampling wells in San Miguelito Canyon.