GROUND-WATER EXPLORATION AND WELL-SITE LOCATIONS IN A NARROW, SAND-FILLED BEDROCK CHANNEL NEAR VAIL, ARIZONA, U.S.A.
William M. Turner, Ph.D.
A private land developer with large land holdings about 20 miles (32 km) east of Tucson, Arizona in the arid American Southwest was forced to truck water to its property because they were unable to produce adequate amounts of water from their only usable well. AGW was contracted to analyze the occurrence of ground water and conduct a Thermonic well location survey for them.The location of Vail, Arizona is shown in Figure 1.
HYDROGEOLOGY
The Vail area is situated in a region of highly deformed sedimentary and igneous rock units ranging in age from Cretaceous to Quaternary. The undifferentiated Carboniferous rocks in the area are divided into several formations. Pennsylvanian rocks include limestone, mudstone, siltstone and sandstone with some gypsum. Permian rock units are quartzose sandstone and massive limestone with chert. The total thickness of Carboniferous rock would be more than 4,000 ft (1,220 m). But, they are highly faulted and deformed and the stratigraphic section is incomplete.
Overlying the Carboniferous basement rocks is the Pantano Formation of Tertiary age. It is a well indurated sandstone, mudstone and conglomerate of continental origin. It is structurally deformed and dips 15 to 50 degrees to the west. It is thought to be overlain by a thrust sheet of Lower Cretaceous rock in the Vail area.
The major aquifer in the Tucson Basin is alluvial material that was derived from the bordering bedrock mountain ranges. This alluvium was deposited on an old erosional surface developed on the Pantano Formation and Carboniferous rocks. This erosional surface is known as the "Rillito" surface. The overlying alluvium ranges in thickness from a wedge edge at the fringe of the basin to more than 700 feet (213 m) in downtown Tucson. Ground water in the alluvium is perched above the Rillito surface.
When the Rillito surface was exposed at the surface, rivers and streams incised deep channels into it. As alluvium from the surrounding mountains began to bury the Rillito surface, surface water, flowing in the channels, sorted the clastic material and removed the fine-grained material. Alluvium that infills old river channels is better sorted and coarser than sediment that deposited outside of the channels. Wells in the buried channels have the highest production rates and generally the best quality water.
In the project area, ground-water is recharged along the eastward flowing Pantano Wash and along many small arroyos which originate in the bordering bedrock highland.
THERMONICS
The depth to moving ground water in the area of the study is more than 300 feet (91 m).
If the temperature of ground-water recharge is less than the mean annual temperature at the land surface, the temperature of ground water will increase away from the zone of recharge because the ground water absorbs heat from the medium through which it moves. Where ground water moves rapidly through an aquifer, the land surface may be expected to be cooler than elsewhere. Areas where ground water moves rapidly under natural conditions are where the transmissivity of the aquifer is highest and are therefore favorable well sites.
To locate the part of the alluvium-filled, bedrock channel in the project area, thermal data was collected from existing wells and from 16 specially-drilled boreholes along two north-south oriented lines of wells. The two lines of Thermonic observation holes were one mile apart. Temperature data was measured in specially constructed measuring tubes inserted into each borehole.
Variations in moisture content, mineralogy and macropores in shallow soils can significantly affect subsurface thermal data. Additionally, shallow thermal data changes rapidly due to the downward propagation of the seasonal heat wave generated at the land surface. We processed and corrected shallow, subsurface, thermal data using proprietary methods to eliminate errors introduced by these factors such that thermal data could be used. Without these corrections, the data can lead to incorrect interpretation.
RESULTS
Prior to conducting our Thermonic well location study, the client had two wells in the area. The first well had been capped because of the high sulphate content in the ground water. The second well is 847 feet (258 m) deep and had a static water level of 675 feet (206 m). It produced only 60 gallons per minute (3.8 l/s).
Our Thermonic analysis defined a narrow, southwest-oriented zone about 100 feet (30 m) wide and a mile (1.6 km) long.
Our client constructed a 400-foot (122 m) deep well in the middle of our Thermonic target zone. We performed an aquifer-performance test at a pumping rate of 609 gpm (38.9 l/s). We analyzed the data and determined an aquifer transmissivity of 26,907 gpd/ft (102 m2/d). A 600 gpm (38 l/s) pumping rate was recommended to the client.
LATER RESULTS
Since the first well was constructed in the zone we located, our client drilled and constructed five more high capacity wells. All wells produce from 600 to 1,000 gpm (38 - 64 l/s).
Flush with success and convinced that water was to be found everywhere in the area, our client moved out of our Thermonic target zone and drilled a 1,000-foot (305 m), 16-inch (41 cm) diameter dry hole using cable tool methods.
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