Garfield / Mesa / Delta County · Various — Alluvial, Mancos Shale-influenced

Well Water in Western Slope, Colorado

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.

Hardness: Hard to Very Hard (high mineral content)

130 fttypical depth drilled (middle half 73–220 ft)

56 fttypical depth to water when drilled

12 gal/mintypical pump-test yield

7,715household wells on the state's records here

Typical (median) depth of household wells completed in each period, drawn to scale; the dashed outline reaches the depth that the deeper quarter of wells go past. Blue is where water stood when the wells were finished, where enough records give it. The rock pattern is a sketch of this area's ground, not a log of any one well.

What tested wells showed

Uranium limit 30 µg/L3% of 217 tests over the limit

Arsenic limit 10 µg/L3% of 297 tests over the limit

Nitrate limit 10 mg/L1% of 372 tests over the limit

Bacteria limit any coliform22% of 134 tests over the limit

Garfield, Mesa, Delta Counties, CDPHE private-well lab results 2011–2020. People usually test because they have a reason to, so these rates describe tested wells, not every well. Bacteria counts any coliform detected.

What to test for and where →

400 permits here are for wells that replaced an earlier well. Wells get replaced when they fail, run dry or are re-drilled deeper, among other reasons.

Every well on record around Western Slope

Depth drilledunder 100 ft100–200200–400400–700700 ft +

A random sample of up to 2,500 household wells from the state's permit records, placed by their recorded location and coloured by how deep each was drilled. Locations on permits are often approximate, sometimes only to a section, quarter-section or 40-acre parcel, so a dot marks an area, not a house.

How deep wells have been drilled, decade by decade

Each line hangs from the ground to the median depth of household wells completed that decade; the shaded band is the middle half. Wells drilled later are not the same wells, so a deeper decade can mean falling water, new homes on higher or drier ground, or deeper aquifers being permitted. It is a record of what was drilled, not a single cause.

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:

Near irrigated Mancos Shale areas (Mesa, Delta, Montrose counties): selenium, TDS, sulfate, iron, basic mineral panel
In Garfield County gas-producing areas: dissolved methane, VOCs, basic mineral panel, bacteria
General Western Slope: arsenic, uranium, hardness, TDS, bacteria, nitrates

Mesa County Public Health's Regional Lab in Grand Junction tests private well water for bacteria, and Mesa and Delta county health departments accept samples for courier to the state lab. For chemistry (arsenic, selenium, uranium), use a CDPHE-certified lab that takes homeowner samples. See our testing guide for labs and current prices.

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