Chemical quality of the water in the Tucson basin, Arizona

The Tucson basin is a broad mountain-rimmed area of about 1,000 square miles in the Basin and Range physiographic province in southeastern Arizona. The altitude ranges from 2,000 feet in the basin to as much as 8,000 feat in the mountains. The major streams in the area are the Santa Cruz River and its principal tributaries--Cafiada del Oro, Rillito Creek, and Pantano Wash. The climate is semiarid, and the distribution and amount of precipitation vary greatly. The potential evapotranspiration is about four times the average annual precipitation.

The streamflow is of excellent chemical quality, although most of the flow occurs during floods and generally has large concentrations of suspended sediment. Because of the erratic occurrence and quantity of streamflow and because of the lack of surface-water storage reservoirs, all the water for municipal, industrial, and agricultural uses is obtained from the many wells that tap the permeable sedimentary deposits, which constitute the principal aquifer in the Tucson basin.

The aquifer consists of three sedimentary formations that range in age from middle Tertiary to Quaternary. The aquifer is as much as 2,000 feet thick and is composed mainly of sand, gravel, sandstone, and conglomerate. The upper part of the aquifer is more permeable than the lower part, and most wells obtain water at depths of less than 700 feet below the land surface.

Most ground water contains less than 500 mg/l (milligrams per liter) of dissolved solids and is of suitable chemical quality for most uses. The water to depths of as much as 700 feet is a calcium sodium bicarbonate type, is hard to moderately hard, and contains less than 1.0 mg/l fluoride. Water at greater depth is a sodium bicarbonate type, is soft, and is of excellent chemical quality; however, water below about 1,0.00 feet may contain fluoride in excess of the maximum allowable limit of 1.4 mg/l for public supply.

The ground water of poorest quality for public supply is at shallow depths along the major streams, in the Pantano Formation along the northeast margin of the basin, at depth in gypsiferous mudstone, and along a narrow zone that trends northwestward across the basin. Water from these hydrologic environ- may contain as much as 500 mg/1 dissolved solids an4 in places may contain more than 1,000 mg/1 dissolved solids.

The anomalously large concentrations of calcium, bicarbonate, nitrate and sulfate in the ground water along the major streams, where the water table is from 25 to 150 feet below the land surface, are the result of near-surface phenomena. The large concentrations of these ions are derived from solution of relict salts, which were deposited in marshes along the streams prior to about 1900 by infiltrating surface water. In the narrow zone the trends northwestward across the basin, the large concentrations of calcium and sulfate are the result of the solution of limestone and gypsiferous mudstone in the sedimentary rocks in the headwaters area of Pantano Wash. The largest nitrate concentrations occur in the ground water along the Santa Cruz River; the nitrate probably is derived from irrigation return water, decayed vegetation from the marshes that occupied parts of the channel prior to 1900, and sewage effluent.

Anomalously large concentrations of sodium, sulfate, chloride, and fluoride occur in ground water along the Santa Cruz River near the major faults that displace the older formations. These anomalously large concentrations probably are derived from the upward leakage of deep water that has reacted with the gypsiferous mudstone in the center of the basin and moved along the faults into the near-surface deposits.

In the Tucson basin the water is divided into seven chemical types based on the relative amount of four major ions--calcium, sodium, bicarbonate, and sulfate---and the absolute amount of chloride. Most of the water is either a calcium sodium bicarbonate or a sodium bicarbonate type.

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