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Description
The Lompoc subarea is a structural basin that occupies the western most region of the Santa Ynez River basin on the west coast of Santa Barbara county, California. The study area includes the Lompoc plain, upland, and terrace. The nearly flat alluvial deposits of the Lompoc plain and temperate climate make conditions favorable for large scale irrigation farming. Historically, ground water has been the main source of agricultural, municipal, and military water supply in the Lompoc area of the Santa Ynez River basin. At present, ground water from the most important water bearing units in the Lompoc area is only marginally acceptable for most uses. Previous studies have concluded that the quality of ground water in the basin has been degraded to levels that do not meet water-quality standards recommended by the U.S. Environmental Protection Agency. These studies suggest that recharge of irrigation water may be the principal cause of the deterioration. The objective of this study was to describe the ground-water quality of the basin. Water-quality and water-level data were used to determine historical and areal trends within the basin. Chemical analyses of major cations and anions were augmented by stable isotope geochemical analyses (oxygen-18 and deuterium) to determine, if possible, vertical variations in water quality and recharge sources to the alluvial aquifers. The principal water-bearing units in the Lompoc area are divided into the upper aquifer (which includes the shallow, middle and main zones), and the lower aquifer. The main zone of the upper aquifer is the primary source of water to the Lompoc plain. The lower aquifer also is present beneath the Lompoc plain and extends to the Lompoc upland and terrace. In areas adjacent to the Santa Ynez River where there has been little history of agricultural activity, all water-bearing zones have dissolved-solids concentrations less than 1,000 mg/L. These concentrations increase rapidly as the water moves away from the river beneath irrigated fields. Stable-isotope chemistry indicates that the westward accumulation of dissolved solids in the shallow zone is not due to the effects of evaporation, but instead to the accumulation of salts from upgradient fields and (or) the dissolution of soluble salts in the shallow zone. Dissolved-solids concentrations and oxygen-18 isotope values also show that the middle zone of the upper aquifer has been affected by leakage from the shallow zone in the eastern part of the Lompoc plain. Silt and clay layers of the shallow zone are less extensive here, allowing water to move freely between zones. The deterioration of the water-quality in the main zone of the upper aquifer is attributed to increases in agricultural irrigation and municipal pumpage in the eastern part of the Lompoc plain, which intercepts a large percentage of the recharge from the Santa Ynez River. Decreases in recharge to the Lompoc plain have been compounded by the construction of Bradbury Dam 30 miles inland to form Lake Cachuma in 1953. This thesis was prepared in cooperation with the U.S. Geological Survey as part of an ongoing study in the Santa Ynez River Basin.
