Colorado Plateaus

Sandstone country in Utah, Arizona, and the Four Corners — uranium country, Navajo Nation country, Lake Powell country

States

UT, AZ, CO, NM

Type

Confined Mesozoic sandstone aquifers (Navajo, Entrada, Wingate, Coconino) plus shallow alluvial

Status

Generally stable in quantity; pervasive uranium and radium from natural and mining sources; arsenic in pockets

The Colorado Plateau is the high desert region of southern Utah, northern Arizona, southwestern Colorado, and northwestern New Mexico — a structurally stable plateau uplifted to elevations of 5,000-10,000 feet, deeply dissected by the Colorado River and its tributaries. The aquifer system is built from the Mesozoic sedimentary rocks that make the region's iconic landscape: the Navajo Sandstone, the Wingate Sandstone, the Entrada, the Kayenta, the Coconino. Where these sandstones are saturated, they form the regional aquifer; where they're exposed at canyon walls, they reveal the deep time of their deposition.

For private well owners, the Plateau presents a particular challenge: naturally elevated uranium and radium are pervasive, derived from the same uranium-bearing Jurassic-age sandstones that fueled the US uranium mining industry from the 1940s through the 1980s. The Plateau is the geographic heart of US uranium mining, and the legacy of that mining — combined with the natural background — produces some of the most uranium-contaminated private well water in the country, particularly across the Navajo Nation.

What it is, geologically

The aquifer system divides into several named units, generally from oldest/deepest to youngest/shallowest:

• Coconino Sandstone (Permian) — the deepest major aquifer; supplies parts of northern Arizona including Flagstaff.
• Glen Canyon Group (Triassic-Jurassic) — Navajo, Kayenta, and Wingate Sandstones; the most productive aquifer regionally.
• San Rafael Group (Jurassic) — Entrada Sandstone and others; aquifer in some sub-regions.
• Younger Cretaceous and Tertiary units — variable importance.
• Quaternary alluvium in canyon bottoms and along major rivers.

The deeper sandstones are confined across most of the plateau interior and unconfined where they outcrop along the canyon margins. Recharge happens at the outcrops and travels through the formations at slow rates — modern groundwater ages in the deep aquifer are commonly thousands of years.

Population centers

• Flagstaff, AZ — Coconino Sandstone is the primary supply.
• St. George, UT — Navajo Sandstone.
• Page, AZ — combination of groundwater and Lake Powell surface water.
• Cortez and Durango, CO — surface water primarily, with groundwater for rural areas.
• Farmington, NM — San Juan River surface water plus groundwater.
• Navajo Nation — over 175,000 residents across NE AZ, NW NM, SE UT; many areas without piped water rely on private wells, hauled water, or unregulated sources. About 30% of Navajo households still haul their drinking water.

Uranium mining legacy

The Colorado Plateau hosted the bulk of US uranium mining from the 1940s through the 1980s. The Navajo Nation alone has over 500 abandoned uranium mines, most of them small "dog hole" mines from the early Cold War period that were never properly reclaimed. These sites continue to contaminate downgradient surface and groundwater with uranium, radium, arsenic, and other associated metals.

The 2007 EPA Five-Year Plan and subsequent multi-agency efforts have begun to address the Navajo Nation contamination but progress is incremental. Many Navajo households still draw drinking water from sources with uranium concentrations exceeding the EPA MCL, sometimes by orders of magnitude. The Trans-Canyon Drinking Water Pipeline and similar infrastructure projects are slowly extending piped water to historically unserved areas.

Water quality

• Uranium — pervasive, both natural and mining-derived. The single most important test for any Colorado Plateau private well. See uranium.
• Radium — companion to uranium; less universally tested but commonly elevated. See radium.
• Radon — present in some sandstone units; generally lower than crystalline-rock regions but not negligible. See radon.
• Arsenic — pockets, including some Navajo Nation areas. See arsenic.
• Total dissolved solids and salinity — high in some deep formations; the Mancos Shale and Paradox Formation contribute high-TDS, high-sulfate water in some sub-basins.
• Hardness — moderate to hard depending on formation.

Known contaminant concerns

Communities on this aquifer