Why Colorado Well Water Is the Way It Is

From the Denver Basin aquifer stack to Pikes Peak granite to Mancos Shale — the geology beneath your well determines what's in your water.

The Denver Basin Aquifer System

The Denver Basin is a structural basin on the eastern flank of the Rocky Mountain Front Range — approximately half a mile thick and 70 miles wide — containing about 3,000 feet of sediments that hold the groundwater supply for hundreds of thousands of Colorado households.

Four Aquifers, Stacked Like an Onion

The Denver Basin contains four major bedrock aquifers, stacked from shallowest to deepest, separated by confining layers of impermeable bedrock:

Aquifer	Area	Max Depth	Thickness	Key Characteristics
Dawson (shallowest)	~1,400 mi²	~600 ft	400-1,200 ft	Most commonly tapped by domestic wells. TDS <100 mg/L in best areas. Thins dramatically near Black Forest.
Denver	~3,500 mi²	~1,300 ft	800-1,000 ft	Lots of claystone and shale. Acts as leaky confining layer between Dawson and Arapahoe. Declining ~3.2 ft/year.
Arapahoe	~4,700 mi²	~1,700 ft	0-400 ft	Conglomerate and sandstone. Yields up to 700 GPM for municipal wells. Declining ~1.9 ft/year.
Laramie-Fox Hills (deepest)	~6,700 mi²	~3,000 ft	0-350 ft	TDS up to 2,000 mg/L. Declining ~9.9 ft/year — the fastest of any layer.

The basin extends from Greeley on the north to Colorado Springs on the south, and from the foothills west to Limon east. The center lies just west of the town of Parker.

Water Quality Varies by Depth

Total dissolved solids in Denver Basin groundwater range from less than 100 mg/L in the Dawson to about 2,000 mg/L in the Laramie-Fox Hills. The deeper you go, the older the water and the more minerals it has dissolved. This gradient is critical: as shallower aquifers deplete and wells are drilled deeper, water quality declines.

Confining Layers: Why the Aquifers Are Isolated

Between each aquifer sits a confining layer that isolates them from each other. Because of these barriers and because of limited connection to surface water, the groundwater in the Denver Basin is not renewable on any human timescale. When it's pumped out, it's gone.

The 100-Year Rule and the Depletion Timeline

In 1974, the Colorado Division of Water Resources established that Denver Basin water should not be extracted at a rate greater than 1% per year — a 100-year aquifer life. We are now 50 years into that timeline.

El Paso County adopted a more conservative 300-year rule, upheld by the Colorado Supreme Court. But the reality on the ground is sobering:

Deep wells have a practical lifespan of 15-20 years before they're no longer economical to pump. Water districts cycle through wells continuously — drilling new ones as old ones fail. New deep wells cost over $1 million for districts. Individual homeowners face $15,000-$50,000+ for replacement wells.

Front Range Fractured Rock

West of the Denver Basin's sedimentary aquifers, the Front Range foothills and mountains expose Precambrian crystalline bedrock — rocks 1 to 1.7 billion years old. These are granites, gneisses, and schists with fundamental differences from the Denver Basin:

• Porosity below 2% — water exists only in fractures, not in the rock matrix
• Permeability in hundredths of a millidarcy — water moves slowly
• Recharged by precipitation — unlike the Denver Basin, this water is renewable, but supply is limited
• Uranium-bearing minerals produce radon — especially the Pikes Peak granite

Well yields in fractured rock are inherently unpredictable. Your neighbor's well can produce completely different water at a completely different rate, because it taps a different set of fractures. There's no way to predict yield before drilling.

The Pikes Peak Granite

The Pikes Peak granite is a 1.08-billion-year-old intrusion with unusually high uranium content. It underlies Teller County, western El Paso County, and parts of Park and Fremont counties. This is the primary source of radon in the Pikes Peak region, and weathered sediments from this granite contribute uranium and radon to the Denver Basin aquifer system.

See our radon guide for the complete story on how this geology affects well water.

Mancos Shale — Western Slope

The Mancos Shale is a Cretaceous marine formation — deposited in an ancient seaway 80-90 million years ago — that crops out over approximately 1,250 square miles in the Four Corners states. On Colorado's Western Slope, it's the dominant geological influence on water quality.

The shale contains selenium locked in pyrite minerals. When irrigation water or natural processes mobilize these minerals, selenium enters groundwater and surface water. In Mesa County, 11 washes have been designated as selenium-impaired. The Lower Gunnison River Basin is a major area of concern.

The Mancos Shale also contains naturally occurring arsenic, uranium, vanadium, and boron. Water in contact with this formation tends to have high total dissolved solids.

The San Luis Valley Closed Basin

The San Luis Valley is Colorado's largest intermontane basin — a high-altitude closed basin at 7,500 feet. No surface water flows out. The valley has both unconfined (shallow) and confined (deep) aquifers.

The closed-basin geology means that minerals accumulate over time rather than being flushed downstream. Combined with decades of drought and overpumping, this creates conditions where arsenic and other contaminants concentrate in the remaining groundwater. See our San Luis Valley community page for the full story.

Colorado's Exempt Well System

Understanding Colorado's well permitting system is important for any well owner:

• Exempt wells are small-capacity wells (typically 15 GPM) exempt from the traditional water court adjudication process
• 35-acre rule: Properties of 35+ acres can get an exempt well for up to three dwellings, one acre of irrigation, and livestock watering
• Under 35 acres: Properties subdivided after May 8, 1972 are limited to indoor use only — no irrigating lawns, gardens, or watering livestock
• Tributary vs. non-tributary: All Colorado groundwater is presumed tributary (connected to surface streams) unless proven non-tributary. This classification affects your water rights and what you can do with your water
• Denver Basin wells are allocated based on overlying land ownership assuming a 100-year aquifer life

The Colorado Division of Water Resources handles well permits. Know your permit type and its restrictions.