Hydrogeology of the regional aquifer near Flagstaff, Arizona, 1994-97

Sandstones, siltstones, and limestones that are Pennsylvanian to Permian in age underlie the southern
part of the Colorado Plateau near Flagstaff, Arizona, and contain a complex regional aquifer that has
become increasingly important as a source of water for domestic, municipal, and recreational uses.
Ground-water flow in the regional aquifer is poorly understood in this area because (1) depth of the
aquifer limits exploratory drilling and testing and (2) the geologic structure increases the complexity of
the aquifer characteristics and the ground-water flow system.

Four methods were used to improve the understanding of the hydrogeology of the regional aquifer
near Flagstaff.

• Remote-sensing techniques and geologic mapping provided data to identify many structural
features that indicate a more complex structural environment and history than previously
realized.

• Data from surface-geophysical techniques that included ground-penetrating radar, seismic
reflection and seismic refraction, and square-array resistivity, verified that some of the geologic
structures expressed at land surface propagate deep into the subsurface and through the principal
water-bearing zones of the regional aquifer at near-vertical angles.

• A well and spring inventory, borehole-geophysical methods, and well and aquifer tests provided
additional information relating aquifer and ground-water flow characteristics to geologic
structure.

• Water-chemistry data, which included major ion, nutrient, trace-element, and radioactive and
stable-isotope analyses, provided an independent means of verifying the hydrogeologic
characteristics of the aquifer and were used to determine recharge and discharge areas, groundwater
movement, and ground-water age.

Ground-water recharge occurs throughout the area but is greatest at higher altitudes where
precipitation is greater and in areas where heavily fractured rock units of the aquifer are exposed.
The estimated annual average recharge to the regional aquifer in the study area is about 290,000 acre-feet.
Ground water flows laterally and vertically through pore spaces in the rock and along faults and other
fractures from high-altitude areas in the southern part of the study area to regional drains north of the
study area along the Little Colorado and Colorado Rivers, and to drains south of the study area along
Oak Creek and the Verde Valley. Ground-water discharge in these areas—about 400,000 acre-feet per
year—exceeds the annual recharge to the aquifer in the Flagstaff area, but ground water from areas
outside the study area contributes to this discharge as well. The saturated thickness of the regional aquifer averages about 1,200 feet, and the amount of water
in storage could be as much as 4,800,000 acrefeet,
or about 10 percent of the total volume of the
aquifer.

The quality of water in the regional aquifer in
terms of dissolved-solids concentrations is good
for most uses throughout the area. Dissolvedsolids
concentrations generally are less than
500 milligrams per liter. Water in the regional
aquifer is primarily a calcium magnesium
bicarbonate type. In some areas near the
Rio de Flag, the water has significant nitrate and
chloride components, which indicate direct
recharge in these areas from the Rio de Flag.
Oxygen and deuterium data indicate a common
recharge source for water in the aquifer and that
some sites receive recharge from surface waters
where evaporation has occurred. Estimated
carbon-14 ages and tritium activities indicate
ground-water ages from less than 200 years in the
Lake Mary area to more than 5,000 years in the
Wupatki area.

The regional aquifer is heterogeneous and
anisotrophic and has a complex ground-water flow
system. The most productive water-bearing
material tends to be fine- to medium-grained
sandstones, and ground-water flow and potential
well yields are related to geologic structure.
Fracturing associated with structural deformation
increases recharge locally and also increases the
potential for high well yields. Surface-geophysical
techniques provided information on the orientation
of high-angle, deep-seated structure in the
saturated zone. Borehole-geophysical data
identified horizontal to near-horizontal fractures as
significant components of the fracture-flow system
not apparent in the surface-geophysical data.
Structural features that strike northwest appear to
be areas that have the greatest potential for high
well yields. A north-northeastward-striking
structure may be just as promising, but additional
data are needed to verify this relation.