Description and analysis of the geohydrologic system in western Pinal County, Arizona

Abstract

Western Pinal County is between Phoenix and Tucson in the Basin and Range physiographic province of southern Arizona and consists of about 2,000 square miles of valley floor with low relief surrounded by mountains. It is the second largest agricultural area in the State, and about 25 percent of the ground water pumped in the State is from this area. The study area has been divided into four parts. Three of these--the Casa Grande-Florence area, the Eloy area, and the Stanfield-Maricopa area--are in the lower Santa Cruz basin; the fourth--the Gila River area--is a long narrow strip along the Gila River from the Ashurst-Hayden Dam to the confluence of the Gila and Santa Cruz Rivers. The project was undertaken to provide a better understanding of the ground-water supply in relation to the present and potential water use in this area of extensive ground-water development. The arid climate of western Pinal County--combining high temperatures and low humidity--causes most of the precipitation to be returned to the atmosphere by evapotranspiration, which leaves only a very small part for recharge to the ground-water reservoir. The computed potential evapotranspiration--44. 97 inches--is five times greater than the average precipitation. In general, the subsurface materials in western Pinal County are unconsolidated alluvial deposits underlain by consolidated alluvium and crystalline rocks and bounded by mountains consisting of crystalline and minor sedimentary rocks. The crystalline and sedimentary rocks of the mountains are not known to be water bearing in western Pinal County. The impermeable rocks underlying the basin are called the hydrologic bedrock unit in this report. Although the unit may consist of several different rock types, the distinction between them is relatively unimportant in this study because none of them yield appreciable amounts of water. The lower Santa Cruz basin in western Pinal County is divided into two sections by a buried ridge of the hydrologic bedrock unit, referred to in this report as the Casa Grande ridge. The ridge trends in a north-south direction from the Sacaton to the Silver Reef Mountains. The unconsolidated deposits constitute the main storage reservoir for ground water in western Pins/ County. The deposits are divided into four units---the local gravel unit, the lower sand and gravel unit, the silt and clay unit, and the upper sand and gravel unit--all of which are major water-yielding units except the silt and clay unit. The local gravel unit, which is present only in the western section of the lower Santa Cruz basin, ranges in thickness from 0 to nearly 1,000 feet and is generally a productive aquifer. The lower sand and gravel unit, Which is a heterogeneous mixture of sand, gravel, and clay, ranges in thickness from 0 to about 500 feet. Where the lower sand and gravel unit is overlain by the silt and clay unit, it generally contains water under artesian conditions; where it is not overlain by the silt and clay unit, it is indistinguishable from the upper sand and gravel unit, and the water is under water-table conditions. The silt and clay unit is the least permeable deposit of the unconsolidated alluvium, and ranges in thickness from 0 to about 2, 000 feet. Generally it is less productive than the other units of the unconsolidated alluvium, although it yields moderate amounts of water from numerous thin stringers and lenses of highly permeable sand and gravel. The upper sand and gravel unit is at the land surface in most of the area; it ranges in thickness from less than 50 to about 600 feet. The unit has the highest average permeability of all the unconsolidated alluvial units; however, the permeability of the unit varies vertically and laterally, which results in a wide range of well yields. As of 1964, the static water levels in most wells in the basin were still in the upper sand and gravel unit. However, the unit is being dewatered in most of the basin, and water levels in