Douglas County · Denver Basin (Dawson / Arapahoe) and Fractured Granite

Well Water in Douglas County Outskirts, Colorado

Douglas County is one of the fastest-growing counties in Colorado, and groundwater from the Denver Basin is its primary water source. While the suburban core relies on water districts, outlying communities like Larkspur, Perry Park, and the Pike/Rampart area depend on private wells. Water levels are declining across all four Denver Basin aquifers, and Perry Park made headlines when radium levels tested at three to four times the EPA limit.

Hardness: Moderate to Hard

460 fttypical depth drilled (middle half 310–606 ft)

280 fttypical depth to water when drilled

12 gal/mintypical pump-test yield

2,880household 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/L2% of 64 tests over the limit

Arsenic limit 10 µg/L2% of 501 tests over the limit

Nitrate limit 10 mg/L2% of 430 tests over the limit

Bacteria limit any coliform8% of 307 tests over the limit

Douglas County, 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 →

187 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 Douglas County Outskirts

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.

Declining Water Levels

Rapid population growth has driven increased pumping from the Denver Basin aquifer system. A USGS study of Douglas County wells from 2011-2019 found 12 of 13 monitored wells showed declining water levels:

Aquifer	Median Annual Decline
Upper Dawson	-0.23 feet/year
Lower Dawson	-0.31 feet/year
Denver	-0.92 feet/year
Arapahoe	-2.26 feet/year

The Arapahoe aquifer — widely used by municipal water providers — is declining fastest. Because the confining layers between aquifers prevent significant vertical recharge, the groundwater in the Denver Basin is effectively being "mined" — used up without replacement.

Perry Park: Radium in the Water

Perry Park, an unincorporated community west of Larkspur, sits among natural red rock formations — geology rich in minerals including radioactive elements. In 2022, Perry Park Water and Sanitation District notified residents that radium levels were three to four times higher than normal. A sample tested at more than twice the EPA limit.

Radium in drinking water increases the risk of bone cancer over long-term exposure. It occurs naturally in formations with elevated uranium and thorium — exactly the geology of the Perry Park area.

If you're on a private well near Perry Park, test for radium, uranium, and radon. The community water system found it; private wells in the same geology face the same risk.

The Pike/Rampart Area

Nearly a third of Douglas County is the Pike/Rampart area — western and southwestern portions including Pike National Forest. This area's geology is fundamentally different from the rest of the county: instead of Denver Basin sedimentary aquifers, wells here tap fractures in granite rock.

These fractured granite aquifers are renewable (recharged by precipitation), unlike the Denver Basin's fossil water. But water availability is limited by the low permeability of the fractures. Well yields tend to be low, and water quality varies dramatically from well to well depending on which fracture network you tap.

See the Evergreen / Conifer page for more on fractured rock well issues common to this geology.

What Douglas County Well Owners Should Do

Test your water for radium, uranium, radon, hardness, iron, bacteria, nitrates, pH, and TDS. Given the Perry Park findings, radioactive contaminants should not be overlooked in the Douglas County area.

Track your well's yield and water level if possible. If you notice the pump running longer for the same amount of water, your aquifer may be depleting.

Douglas County provides Denver Basin Aquifer information and the Rural Water Authority of Douglas County tracks regional groundwater conditions. See our testing guide for labs and costs.

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 Douglas County Outskirts area, but the only way to know what's in your water is to test it.

Sources

USGS — Groundwater Levels in Denver Basin Bedrock Aquifers of Douglas County, 2011-19
CBS Colorado — Perry Park Radium Contamination Report (2022)
Douglas County — Denver Basin Aquifer Information
Rural Water Authority of Douglas County — Denver Basin Geology and Background