A geohydrologic study of the regional ground-water system in Avra Valley, Pima and Pinal counties, Arizona

Abstract

Ground water in Avra Valley, southeastern Arizona, is used for irrigation and as part of the City of Tucson municipal supply. The unconfined regional aquifer consists of about 1,300 feet of basin-fill sediments that can be subdivided into two lithostratigraphic units. The upper 300-400 feet of sediments contain permeable sands interbedded with gravel, silt, and clay. The lower 900-1,000 feet are composed of silty clay in the northern part of the valley and poorly to highly consolidated sands and gravels in the central and southern parts. Ground water enters Avra Valley as underflow from the south, flows northerly through the valley, and discharges as underflow at the northwest end of the valley. Most wells yield water at rates of 500 gpm or more. Approximately 3.6 million acre-feet of ground water were withdrawn from the aquifer during 1940-1978, resulting in water-level declines of more than 140 feet in the central part of the valley.