Groundwater Contractors in Coconino County, AZ

Groundwater Contractors in Coconino County, Arizona

Coconino County is Arizona’s largest county by land area and one of its most geologically distinctive. North of the Mogollon Rim, the landscape transitions dramatically from the hot desert basins of southern Arizona to the Colorado Plateau — a high-elevation landscape of ponderosa pine forests, volcanic fields, canyon country, and the Grand Canyon. Groundwater here behaves very differently from the alluvial basin aquifers of Phoenix or Tucson.

Groundwater Geology of Northern Arizona

Coconino County sits atop a stack of sedimentary formations that dip gently to the north toward the Grand Canyon. The key aquifer units include:

Coconino Sandstone: A cross-bedded aeolian sandstone deposited during the Permian period, this formation is one of northern Arizona’s most important aquifer targets. It typically occurs at depths of 300–700 feet beneath the Colorado Plateau surface near Flagstaff and can be very productive. However, uranium occurs naturally in some areas at elevated concentrations — test all Coconino Sandstone wells before use.

Redwall/Muav Limestone: A carbonate aquifer underlying the Coconino Sandstone across much of the plateau. Yields from the Redwall can be high where dissolution cavities (karst) exist, but water quality can include elevated hardness, calcium, and sometimes sulfate.

Volcanic Basalt: The San Francisco Volcanic Field, centered on the San Francisco Peaks north of Flagstaff, has produced extensive basalt flows over the past million years. Basalt aquifers occur in vesicular zones between flows, and fracture systems in cooled basalt can be locally productive. Depths vary from 100 to 600+ feet depending on the stacking sequence.

Martin/Naco Formation: Older carbonate units that can be productive in specific structural settings.

Flagstaff’s Water Supply Context

Flagstaff is unusual among larger Arizona cities in that it relies primarily on surface water from snowmelt (Upper Lake Mary and other reservoirs) supplemented by groundwater from the Coconino Sandstone. The city has experienced water supply stress during drought years, raising interest in additional groundwater development and water banking.

Rural residents and communities outside the Flagstaff Water Services area depend entirely on private wells into the volcanic or sedimentary formations.

Water Quality in Coconino County

Uranium: The Coconino Sandstone has naturally elevated uranium in parts of the county. ADWR and ADEQ advise testing all Coconino Sandstone wells for uranium.
Hardness: Moderate to hard in carbonate formation areas; lower in basalt-sourced wells.
Radon: Elevated in some wells due to uranium decay chains in the sandstone.
Fluoride: Some wells exceed the secondary MCL for fluoride.
TDS: Generally lower than southern Arizona wells; 200–600 mg/L is typical.