Well Water in Castle Rock, Colorado

Douglas County · Population ~83,000 · Aquifer: Denver Basin bedrock (Dawson / Denver / Arapahoe / Laramie–Fox Hills)

Hardness: Hard to very hard (Denver Basin bedrock groundwater)

Castle Rock Drinks From the Denver Basin

Unlike most Front Range cities that lean on mountain surface reservoirs, Castle Rock's supply is dominated by deep Denver Basin bedrock groundwater — the Town's own site states the community gets most of its water from underground aquifers, and historically relied on them solely. The supply also includes alluvial wells along East Plum Creek (roughly 10–20 percent), renewable surface water from East Plum Creek treated at the Plum Creek Water Purification Facility, imported water through the WISE partnership with Denver Water and Aurora Water (averaging around 6 percent of annual supply), and recycled water.

The Town is deliberately diversifying away from the nonrenewable Denver Basin, targeting roughly 75 percent renewable supply by 2050 and using aquifer storage and recovery to bank treated water underground.

The Aquifers Are Slowly Being Depleted

Castle Rock sits squarely in the Denver Basin bedrock footprint, and the Town's own groundwater page states plainly that "these aquifers are slowly being depleted." USGS monitoring specific to Douglas County (SIR 2014-5172 and its 2011–2019 update) found that increased pumping to meet rapid population growth has caused statistically significant, persistent water-level declines in the upper Dawson, lower Dawson, Denver, and Arapahoe aquifers — median declines on the order of 0.2 to 2.3 feet per year — while the deeper Laramie–Fox Hills showed no meaningful change over the same period.

This long-term mining of a nonrenewable resource is exactly why Castle Rock is shifting toward renewable supplies. For rural well owners in the surrounding area, it means watching your well's yield over time is as important as testing its quality.

Radium and Radon in the Bedrock Groundwater

The documented, load-bearing water-quality concern for Denver Basin bedrock wells here is naturally occurring radioactivity — radium (Ra-226 and Ra-228) and uranium hosted in the basin's sedimentary rocks. This is not theoretical in Douglas County:

Perry Park (unincorporated Douglas County, near Larkspur): Perry Park Water & Sanitation District wells tested 12.9 pCi/L combined radium in August 2024 — more than double the federal maximum contaminant level of 5 pCi/L — with follow-up samples of 5.6 and 3.4 pCi/L. The district flagged a radium violation and prioritized treatment, warning residents that boiling water does not remove radium. Louviers endured roughly six years of radium contamination before meeting safe-drinking-water standards, and Woodmoor reported radium increasing across its wells.

Castle Rock Water's own municipal supply tests radium "well below" the 5 pCi/L limit, with no health-based violations in the current reporting period — but these nearby cases are a clear reminder of what the regional geology can produce in wells and small systems. Radon-222, a decay product of the same radium and uranium in the rock, is a co-occurring bedrock-groundwater concern; Colorado is an EPA Radon Zone 1 area.

What Castle Rock Area Residents Should Know

If you're on Castle Rock Water: the municipal supply is blended and treated and tests below the radium limit; the longer-term issue is the aquifer depletion driving the Town's shift to renewable sources.

If you're on a private well or a small district in rural Douglas County (Larkspur, Perry Park, the county fringes): test for radium, radon, uranium, hardness, TDS, and bacteria, and monitor your well's production rate over time — the Denver Basin aquifers here are declining. See our testing guide and radon page.