Well Water in Douglas County Outskirts, Colorado
Douglas County · Population ~370,000 (county); rural areas vary · Aquifer: Denver Basin (Dawson / Arapahoe) and Fractured Granite
Hardness: Moderate to Hard
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.
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