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

Future development in the upper Verde River area probably will be dependent on water obtained from units of the regional aquifer. Additional surface-water development is limited owing to existing downstream water rights. Ground water is presently used as the principal source of domestic and public supplies; annual draft on the system is estimated to be 8,000 acre-ft with less than 10 percent used for irrigation. Ground water occurs mainly in the regional aquifer, which includes the alluvium along the Verde River, Verde Formation, Coconino Sandstone, Supai Formation, Naco Formation, Redwall Limestone, Martin Formation, and Tapeats Sandstone. All the units are hydraulically connected. The particular rock unit that produces water is dependent on areal location because some units have been eroded and some are above the water table. Other aquifers that provide local water supplies are the volcanic rocks, granitic rocks, alluvium, Kaibab Limestone, and Toroweap Formation.

In most of the area, ground water is unconfined; however, in parts of the Verde Formation, Supai Formation, and Redwall Limestone, ground water is confined. Some wells that obtain water from the Verde, Supai, and Redwall flow at the land surface near Rimrock, Cornville, Cottonwood, and Page Springs. Depth to water in wells that tap the regional aquifer ranges from 47 ft above the land surface to 917 ft below the land surface but generally is less than 800 ft below the land surface. Well yields range from less than 10 to 1,600 gal/min. The highest yields, those greater than 500 gal/min, occur in the Verde, Redwall, and Coconino and are probably associated with solution features, faulting, or fracturing. Aquifer-test data indicate that the transmissivity of the regional aquifer ranges from 20 to 16,000 ft2/d. The wide range in values is a result of secondary permeability, which causes the higher values.

Ground water obtained from the regional aquifer throughout most of the area is of acceptable chemical quality for most uses; however, water from the Verde Formation locally exceeds the maximum contaminant levels for drinking water as recommended by the U.S. Environmental Protection Agency and the State of Arizona for dissolved solids, sulfate, arsenic, and fluoride. Water from the Coconino, Supai, Redwall, and Martin generally contains less than 500 mg/L of dissolved solids, mainly calcium, magnesium, and bicarbonate. Ground water changes composition as it flows downgradient through the Verde Formation. North of Cottonwood, the major ions are calcium, magnesium, sodium, and bicarbonate. South of Camp Verde, sodium, magnesium, and sulfate are the major ions. Dissolved-solids concentrations range from 209 to 97,700 mg/L and have a median value of 424 mg/L. The percentage of Verde wells having water with more than 500 mg/L of dissolved solids increases in the downgradient direction and correlates with the presence of evaporate minerals in the Verde Formation. The result is a marked increase in sodium and sulfate from Middle Verde to south of Camp Verde. Ground water in the alluvium along the Verde River south of Camp Verde also contains large concentrations of dissolved solids, 806 to 3,790 mg/L, which are mainly magnesium, sodium, calcium, and sulfate. Arsenic concentrations range from 1 to 240 micrograms/liter, and those samples that exceed the 50 micrograms/liter maximum contaminant level for arsenic occur from Cornville and Rimrock to Campe Verde. Drill cuttings from the Verde Formation contain from 7 to 88 micrograms/gram of arsenic; the highest values were found associated with clay. Fouride concentrations in water from nine wells near Middle Verde and Camp Verde exceeded the maximum contaminant level of 1.4 micrograms/liter.

Locally perched water in the volcanic rocks, granitic rocks, and alluvium is the only developed source of water in some parts of the Black Hills and Plateau uplands. Depth to water in wells and well yields depend on location. Perched water generally is acceptable for its uses.

Base flow in the Verde River and its perennial tributaries is maintained by discharge from the regional aquifer. Base flow has remained virtually unchanged since 1915 north of Clarkdale. Since 1935, winter base flow has remained virtually unchanged between Clarkdale and Chasm Creek; whereas, summer base flow decreased, which probably is associated with increased water use by evapotranspiration and irrigation during dryer summers. This change in the system indicates withdrawal of water from the Verde River and not from the aquifer away from the river. Evapotranspiration and irrigation are highest from Clarkdale to the East Verde River. The principal use for surface water is irrigation, for which the water generally is well suited except in Bitter Creek because of mine drainage and in the Camp Verde area where the salinity hazard is medium to high owing to ground-water discharge that increases the dissolved solids in the surface water.

The ground-water system still represents equilibrium conditions. No change in winter base flow and no lasting water-level declines were detected throughout the study area. No significant change in average inflow or aquifer discharge was detected in flow-duration curves. Ground-water use is 4 percent of the inflow to the area. The total base flow that leaves the area as surface water at the outflow point is 111,000 acre-ft/yr. Underflow is negligible into the study area from the Big Chino Valley area and out of the study area near the outflow point.

Demands on ground water with continuing population growth along the Verde River flood plain and along the lower reaches of the tributary streams will ultimately affect the amount of base flow available in the streams for use locally and downstream. As the area is developed and more wells are drilled, the volume of water in the regional aquifer may decrease, and a corresponding decrease in the amount of base flow may result. Land use is changing as farms and ranches are subdivided, and changes in how water is used are closely related. Ground-water development is not evenly distributed over the entire study area but concentrated in the main population centers. Concentration of pumping in these areas could cause local overdrafts in the regional aquifer. Continued monitoring is needed at both the upstream and downstream ends of the Verde Valley to document changes in the surface-water outflow as development in the valley continues. Operation of the Verde River near Camp Verde gaging station would also provide additional data to help redefine the enveloping base-flow hydrographs. Monitoring is needed in high stress areas for water-level changes in order to understand the effects on the ground-water system. Water-quality monitoring is needed for ground water and surface water to determine effects that may result from further development. The hydraulic interaction between the Verde River, alluvium, and Verde Formation needs to be better understood to determine the effects of continued ground-water development on surface water and whether surface water recharges the ground-water system along the river. Little is known about the water in the alluvium owing to lack of data. Along most of the Verde River, the alluvium is thin and not an important source of ground water. At the south end of the Verde Valley near Camp Verde, the alluvium is an important source of ground water because the water quality is better than water from the Verde Formation. Analysis of streamflow data, the concentration of irrigation occurring on the alluvium particularly south of Camp Verde, and the chemical similarities to water contained in the alluvium and Verde Formation indicate that the alluvium plays an important role in the ground-water system that is not fully understood. Fossil Springs provides about 17 percent of the water leaving the study area but little is known about the origin of this water. Additional sampling and age dating for geochemical modeling might help in understanding this part of the system.