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.
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
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
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.
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
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
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.