LOCATION OF OPTIMUM WELL SITES FOR CONGRESS JUNCTION, ARIZONA, U.S.A.
William M. Turner, Ph.D.
Congress Junction, Arizona is a very small community 67 miles northwest of Phoenix on the road to Prescott, Arizona. It is shown in Figure 1.
Historically, the community had a chronic shortage of stable water supply. Their four wells averaged 900 ft (275 m) in depth and would produce a total of only 25 gpm (1.6 l/s) for four hours before sucking air. Supplemental water was hauled to the community by the Santa Fe Railroad in tank cars.
American Ground Water Consultants (AGW) was contracted to conduct a hydrogeological investigation and a Thermonic, ground-water-exploration geophysical survey to locate new well sites.
HYDROGEOLOGICAL SETTING
Congress Junction is at the northern end of an alluvium-filled, intermontane basin of the Basin and Range Physiographic province. The community is bordered to the north, west and east by high mountains of granitic rocks.
The area is situated within a fault block basin which had its major development at the close of the Cretaceous period. At that time, the granite that presently forms the mountains and the underlying basement was subject to significant subareal erosion. This erosion carved a very uneven surface and deep canyons that are now overlain and filled by alluvium derived from the mountains.
The most likely drilling targets are zones, near the mountain front, where deeply incised subcrop canyons have been filled with coarse, well-sorted alluvium. Ground water occurs at a depth of from 850 to 950 ft (244 -290 m). Wells will yield more water if they are situated in water-bearing alluvium along the axes of the deep bedrock canyons. If wells encounter basement pinnacles and ridges above the water table, wells will not produce adequately.
The land surface at Congress Junction slopes at a low angle to the south..
Drainage in the area occurs in ephemeral stream channels that drain surface-water-catchment areas in the surrounding mountains. Martinez Wash, the major ephemeral stream channel, is deeply incised into granitic basement rock east of Congress Junction. Southeast of the community, it has incised a broad valley into the alluvial sediment. Surface drainage is primarily to the southwest and the Hassayampa River.
Ground water moves away from Martinez Wash which suggests it is the major zone of ground-water recharge.
The method of well-site location used in this study is based on the variable flow rate of ground water under the influence of gravity and its effect on the upward flow of geothermal heat from the hot interior of the earth.
THERMONICS
We carried out the Thermonic survey in two phases. We first acquired all available geologic and hydrologic information and analyzed the ground-water-flow system. We carried out a geological reconnaissance of the area, measured water levels in existing water wells and analyzed data from two aquifer-performance tests.
After our analysis of the ground-water-flow system was completed, we collected subsurface heat-flow data from a depth of about 3 ft (1 m) in holes that were augered for installation of electric power poles. We were looking for the axes of bedrock canyons filled with water-bearing alluvium. We analyzed the shallow data using our proprietary methods to eliminate highly variable effects caused by daily temperature fluctuations at the surface and variable thermal properties of the soil. We used our proprietary "valley mapping function" to delineate the deep water-bearing zones of alluvium. We confirmed our interpretation from the very shallow data by measurement of heat-flow data in several deeper Thermonic observation holes augered to a depth of 20 ft (6 m).
RESULTS
We identified 10 well sites. The community's consulting engineer eliminated five sites because of land access problems, their distance from storage and pumping facilities, and their location with respect to active, surface stream channels.
The first site drilled encountered well-sorted, coarse sand and was completed as a well. We tested the well at a sustained yield of 75 gpm (5 l/s). This was very satisfactory for the community and additional wells were not needed.
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