Sandra J. Owen-Joyce and C. K. Bell

Population growth in the upper Verde River area is occurring mostly in the Verde Valley where development of additional water supplies will eventually depend on ground water from the regional aquifer. The availability of surface water for use is limited owing to downstream water rights. Ground water occurs in a thick sequence of flat-lying limestone and sandstone that underlies most of the 2,600-square-mile project area. The regional aquifer includes the alluvium along the Verde River, Verde Formation, Coconino Sandstone, Supai Formation, Naco Formation, Redwall Limestone, Martin Formation, and Tapeats Sandstone. Ground water flows downgradient toward the Verde River. Depth to water generally is less than 800 feet below the land surface. Wells that tap these rock units produce from less than 10 to 1,600 gallons per minute. Where present, faulting, fracturing, and solution cavities increase well yields. Other aquifers provide local perched sources of water from volcanic rocks, granitic rocks, alluvium, Kaibab Limestone, and Toroweap Formation in parts of the Black Hills and Plateau uplands.

The ground-water budget comprises 111,000 acre-feet of base flow discharged from the area as surface water, 35,000 acre-feet lost to evapotranspiration, 31,000 acre-feet of surface water consumed by irrigated crops, and 8,000 acre-feet of ground water withdrawn primarily for public and domestic use. Continued ground-water development will eventually decrease the base flow in the streams because of the close association between ground water and surface water. Because no storage facilities for floodflows exist, this decrease will affect the surface-water uses in the Verde Valley as well as the amount of water available to downstream water users.

Ground water and surface water are of acceptable chemical quality for most uses except for the water obtained near Camp Verde. Dissolved-solids, sulfate, arsenic, and fluoride concentrations in the ground water from the Verde Formation and alluvium exceed the maximum contaminant levels for drinking water as recommended by the U.S. Environmental Protection Agency and the State of Arizona. Dissolved solids exceed 500 milligrams per liter throughout most of the Verde Formation. Concentrations are as much as 97,700 milligrams per liter but generally are less than 5,000 milligrams per liter. Sulfate concentrations exceed the maximum contaminant level of 250 milligrams per liter near Middle Verde and Camp Verde owing to the presence of evaporate minerals in the Verde Formation. Sulfate concentrations are as much as 64,700 milligrams per liter but generally are less than 2,900 milligrams per liter. Large concentrations of arsenic occur in some wells and springs that tap the Verde Formation from Cornville and Rimrock to Camp Verde. Arsenic concentrations in this part of the area ranged from 1 to 240 micrograms per liter, and about 30 percent exceeded the maximum contaminant level of 50 micrograms per liter. Rock samples of the Verde Formation from this same area contained from 7 to 88 micrograms per gram of arsenic; the largest concentrations are associated with clay. Arsenic is disseminated throughout the formation rather than confined to a particular bed. Near Camp Verde and Middle Verde, water from nine wells exceeds the maximum contaminant level for fluoride and is as much as 3.4 milligrams per liter. In all the streams except Bitter Creek and the Verde River downstream from Camp Verde, the surface-water quality is well suited for irrigation, its primary use. Mine drainage contaminates Bitter Creek. The salinity hazard is medium to high in the Verde River downstream from Camp Verde. The increase in dissolved solids, mostly sodium and sulfate, is from ground water discharged from the Verde Formation and alluvium.