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

By: Donald J. BillsMargot TruiniMarilyn E. FlynnHerbert A. PierceRufus D. Catchings**Michael J. Rymer

                            https://doi.org/10.3133/wri004122

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#### Abstract

Sandstones, siltstones, and limestones that are Pennsylvanian to Permian in age underlie the southernpart of the Colorado Plateau near Flagstaff, Arizona, and contain a complex regional aquifer that hasbecome 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 theaquifer limits exploratory drilling and testing and (2) the geologic structure increases the complexity ofthe aquifer characteristics and the ground-water flow system.

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

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

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

• A well and spring inventory, borehole-geophysical methods, and well and aquifer tests providedadditional information relating aquifer and ground-water flow characteristics to geologicstructure.

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

Ground-water recharge occurs throughout the area but is greatest at higher altitudes whereprecipitation 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 otherfractures from high-altitude areas in the southern part of the study area to regional drains north of thestudy area along the Little Colorado and Colorado Rivers, and to drains south of the study area alongOak Creek and the Verde Valley. Ground-water discharge in these areas—about 400,000 acre-feet peryear—exceeds the annual recharge to the aquifer in the Flagstaff area, but ground water from areasoutside 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 waterin storage could be as much as 4,800,000 acrefeet,or about 10 percent of the total volume of theaquifer.

The quality of water in the regional aquifer interms of dissolved-solids concentrations is goodfor most uses throughout the area. Dissolvedsolidsconcentrations generally are less than500 milligrams per liter. Water in the regionalaquifer is primarily a calcium magnesiumbicarbonate type. In some areas near theRio de Flag, the water has significant nitrate andchloride components, which indicate directrecharge in these areas from the Rio de Flag.Oxygen and deuterium data indicate a commonrecharge source for water in the aquifer and thatsome sites receive recharge from surface waterswhere evaporation has occurred. Estimatedcarbon-14 ages and tritium activities indicateground-water ages from less than 200 years in theLake Mary area to more than 5,000 years in theWupatki area.

The regional aquifer is heterogeneous andanisotrophic and has a complex ground-water flowsystem. The most productive water-bearingmaterial tends to be fine- to medium-grainedsandstones, and ground-water flow and potentialwell yields are related to geologic structure.Fracturing associated with structural deformationincreases recharge locally and also increases thepotential for high well yields. Surface-geophysicaltechniques provided information on the orientationof high-angle, deep-seated structure in thesaturated zone. Borehole-geophysical dataidentified horizontal to near-horizontal fractures assignificant components of the fracture-flow systemnot apparent in the surface-geophysical data.Structural features that strike northwest appear tobe areas that have the greatest potential for highwell yields. A north-northeastward-strikingstructure may be just as promising, but additionaldata are needed to verify this relation.

#### Study Area

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