Well Water in Western Slope, Colorado

Garfield / Mesa / Delta County · Population Varies by community · Aquifer: Various — Alluvial, Mancos Shale-influenced

Hardness: Hard to Very Hard (high mineral content)

Colorado's Western Slope faces a set of water quality challenges distinct from the Front Range. The Mancos Shale — a Cretaceous marine formation covering roughly 1,250 square miles in the Four Corners region — leaches selenium into groundwater wherever irrigation or natural processes mobilize it. In Garfield County, a three-year study linked methane in water wells directly to gas drilling operations.

Selenium from Mancos Shale

The Mancos Shale is the defining geological feature for Western Slope water quality. Selenium occurs naturally in this formation, locked in pyrite minerals deep in unweathered shale. When irrigation water percolates through Mancos Shale soils, it dissolves highly soluble selenium-bearing salts and carries them to groundwater and streams.

The Lower Gunnison River Basin and the Uncompahgre Valley are the most affected areas. In Mesa County alone, 11 washes have been targeted for action by CDPHE due to impaired water quality from dissolved selenium and iron. The predominant source is groundwater inflow from canal seepage and deep percolation from irrigated lands.

For private well owners, selenium exposure is a concern if your well is in or near areas underlain by Mancos Shale, particularly in irrigated agricultural zones. Selenium is essential in trace amounts but toxic at elevated levels — it bioaccumulates and can cause reproductive failure, neurological effects, and "selenosis."

Methane in Garfield County

A three-year Garfield County study examined over 700 methane samples from 292 locations and found a clear pattern: as the number of gas wells in an area increased from 200 to 1,300, methane levels in nearby water wells increased too.

The study used sophisticated isotopic analysis to match methane in water wells to the same rock layer where gas companies were drilling. One key finding: "in the area where there are large vertical faults you see a much higher instance of water wells being affected" — faults act as migration pathways for gas from deep formations to shallow aquifers.

This is among the first studies to broadly demonstrate methane migration in drilling areas, and it has implications for private well owners throughout Garfield County's gas-producing regions.

Other Contaminants

The Mancos Shale contains more than just selenium. The U.S. Department of Energy documented naturally occurring arsenic, uranium, vanadium, boron, and elevated dissolved organic carbon in Mancos Shale groundwater. In West Rifle Creek, high concentrations of arsenic were found — naturally occurring in copper, iron, and lead ore in the area.

Water from Mancos Shale-influenced formations tends to have high total dissolved solids — the water has been in contact with highly soluble marine sediments for long periods. Expect hard water with elevated mineral content.

What Western Slope Well Owners Should Test For

Testing priorities depend on your specific location:

Colorado Analytical Laboratory in Brighton maintains CDPHE certification for inorganic drinking water analysis. The Weld County lab also serves nearby areas. See our testing guide for the full list.

Every well is different. Two wells on the same street can produce completely different water. The data on this page reflects documented conditions in the Western Slope area, but the only way to know what's in your water is to test it.

Sources

  • USGS — Mobilization of Selenium from the Mancos Shale, Lower Uncompahgre River Basin
  • USGS — Lower Gunnison River Basin Groundwater Selenium
  • Garfield County — Methane in Water Wells Study
  • U.S. Department of Energy — Natural Contamination from the Mancos Shale (2011)
  • CDPHE Water Quality Control Division — Selenium-Impaired Washes, Mesa County