Subsurface geology and the chemical quality of ground water in Buckeye Valley, Arizona

Abstract

Buckeye Valley, one of the largest irrigation districts of the lower Salt River Valley, includes an area of 150 square miles within Maricopa County. It is about 42 miles southwest of Phoenix and the Gila River is the main drainage in the area. The rock units in Buckeye Valley are: (1) Precambrian igneous and metamorphic rocks, (2) volcanic rocks of Quaternary age, and (3) sedimentary rocks of Tertiary and Quaternary age divided into surficial sediments (A), and valley fill (B)0 The valley fill is divided into four subunits: (1) basal conglomerate rocks, (2) Buckeye silty clayey beds, (3) alluvial deposits, and (4) alluvial mantle. The sedimentary rocks of the valley fill are the main aquifers. The ground water occurs under artesian conditions in the basal conglomerate rocks below the Buckeye silty clayey beds, and under nonartesian conditions in the alluvial deposits. The artesian aquifer and the water-table aquifer are referred to as the basal conglomerate aquifer and the Buckeye salty aquifer, respectively. The Buckeye salty aquifer has a saturated thickness ranging from 50 to 200 feet in different parts of the valley and wells yield from 100 to 3, 500 gallons per minute. The amount of water in storage in the Buckeye salty aquifer, assuming 12 percent specific yield, is about 750, 000 acre-feet. The saturated thickness of the basal conglomerate aquifer is not known. The yield of wells producing water is low, mostly less than 50 gallons per minute. The movement of ground water in the Buckeye salty aquifer is toward the south and southwestern parts of the valley. Water is discharged via underfiow into the Gila River channel in the southwestern corner of Buckeye Valley. The main sources of recharge are from irrigation water and from floods in washes during the rainy season. For the period 1952-60 the water-table decline was about 8 feet in most of the valley, but was 23 feet in the northwestern corner for the same period of time. The chemical quality of water from the Buckeye salty aquifer is poor, ranging from 2, 000 to 6, 000 parts per million of total dissolved solids. This water is not adequate for domestic use but is used mostly for irrigation. The water has a high to very high salinity hazard and sodium adsorption ratio. It can be used on salt-tolerant crops only with adequate drainage and the addition of chemical and organic matter. The water of the basal conglomerate aquifer has total dissolved solids of about 600 ppm0 It is generally suitable for domestic use. However, it has a fluoride content of 1. 6 ppm. The water from both the basal conglomerate aquifer and the Buckeye salty aquifer is classified as sodium chloride water.