Arsenic in Drinking Water in Colorado
What residents of Colorado need to know about arsenic in drinking water — including natural geological sources, private well risk, which utilities have documented violations, and how to remove arsenic from tap water.
Source: EPA SDWIS, Colorado Department of Public Health and Environment, USGS · Last reviewed: 2025-01-01
Quick Answer
Is arsenic in drinking water a concern in Colorado?
Yes. Colorado has documented arsenic concerns due to its volcanic geology, hydrothermal systems, and historic mining activity. The San Luis Valley — one of the highest valleys in the world — has documented elevated arsenic in groundwater from geothermal and volcanic sources. Colorado's extensive gold and silver mining history has also left arsenic-bearing waste in numerous watersheds.
Where does arsenic come from in Colorado's water?
Geothermal and volcanic geology in the San Luis Valley, Rio Grande Rift zone, and mountainous areas is the primary natural arsenic source. Historic mining operations throughout the Rockies — including the Leadville and Summitville mining districts — have introduced additional arsenic through acid mine drainage and tailings leaching. Communities relying on mountain groundwater and small rural water systems face elevated risk.
What should Colorado residents know?
San Luis Valley communities, rural southern Colorado residents, and small mountain community water systems near historic mining districts should test for arsenic. Colorado CDPHE monitors public systems but private well owners in high-risk geological areas have no regulatory protection and must test independently.
Key Facts
| EPA MCL | 10 µg/L (10 ppb) |
| MCLG | Zero |
| Primary source | Geothermal/volcanic geology (San Luis Valley, Rio Grande Rift); historic mine drainage (Leadville, Summitville) |
| San Luis Valley | Elevated naturally occurring arsenic in geothermally influenced aquifer |
| State regulator | Colorado Department of Public Health and Environment (CDPHE) |
| Health effects | Bladder, lung, skin cancer; cardiovascular; diabetes risk |
| Effective treatment | Reverse osmosis (NSF/ANSI 58) or activated alumina |
Why Arsenic Matters in Colorado
Colorado's Rio Grande Rift and associated volcanic geology creates elevated arsenic in the San Luis Valley and surrounding areas. The valley — home to significant Hispanic and agricultural communities — relies heavily on groundwater from geothermally influenced aquifers. Colorado's mining legacy also means arsenic-contaminated mine drainage affects streams and groundwater in many mountain communities. The Summitville mine site (Superfund) and Leadville area have documented arsenic loading in surface and groundwater systems.
Colorado Arsenic Program
Colorado CDPHE has an active arsenic compliance program for small systems, many of which serve volcanic-derived groundwater. The San Luis Valley in southern Colorado has documented elevated arsenic in groundwater. Mountain West geology means many Colorado private well owners in rural areas should test for arsenic.
Colorado Utilities With Arsenic Violation Records
The utilities listed below have at least one arsenic violation on record in EPA's SDWIS database. Violations may be open or resolved — see individual utility pages for current status and risk level.
City of Thornton
Thornton · 226,465 served
Town of Castle Rock
Colorado · 106,822 served
East Cherry Creek Valley Wsd
Colorado · 66,130 served
City of Fountain
Colorado · 25,130 served
Woodmoor Wsd
Colorado · 18,031 served
Town of Berthoud
Colorado · 14,211 served
Pinery Wwd
Colorado · 12,901 served
Castle Pines North Md
Colorado · 12,800 served
Cottonwood Wsd
Colorado · 10,867 served
How Does Arsenic Get Into Drinking Water?
Arsenic in drinking water is almost always naturally occurring — it leaches from arsenic-bearing rocks and minerals into groundwater over time. New England granite, Southwest volcanic geology, and Upper Midwest glacial aquifers are the primary high-risk formations. It has no taste, odor, or color.
Full arsenic overview — geology maps, health effects, all 50 statesWho Should Pay Closest Attention
San Luis Valley communities in Alamosa, Rio Grande, Conejos, and Costilla counties, small mountain community water systems near historic mining districts, and rural private well users throughout Colorado's mountainous regions should prioritize arsenic testing.
Private well owners near mining districts or agricultural areas
Residents in states with documented volcanic or geothermal geology
Long-term consumers of water from small groundwater systems
Households in homes built before 1960 with older well casings
Residents whose well water has never been tested for arsenic
Anyone living in a state where bedrock wells are common
How to Check Your Situation in Colorado
- 1
Identify your water source. If you use a public utility, use the ZIP lookup on this page to find your system and check its compliance record.
- 2
If on public water, review your utility's most recent Consumer Confidence Report (CCR) for arsenic monitoring data. The MCL is 10 ppb — your report should show recent test results.
- 3
If on a private well, order an arsenic test from a Colorado Department of Public Health and Environment-certified laboratory. A basic arsenic test costs $15–$40. The state agency website maintains a certified lab list.
- 4
Test your well at the tap — not just at the wellhead. The entire water distribution system within your home can affect water quality.
- 5
If your test shows arsenic above 5 ppb, install certified treatment immediately. If above 10 ppb, do not use the water for drinking or cooking until treatment is installed.
- 6
Retest after installing treatment to confirm it is working as certified. Replace filter media on the manufacturer's schedule — an exhausted filter may not perform as rated.
Treatment Options for Arsenic
Boiling does not remove arsenic — it concentrates it. Standard activated carbon filters (Brita, etc.) do not effectively remove arsenic. Certified treatment is required.
NSF/ANSI Standard 58 — Reverse Osmosis
RO systems certified to NSF/ANSI 58 remove 85–95% of arsenic. Under-sink installation. Most effective for removing multiple contaminants simultaneously. Replace membranes and pre-filters on schedule.
Activated Alumina Filters
Activated alumina is specifically designed for arsenic and fluoride removal. Point-of-use or whole-house options available. Must be certified by NSF International or WQA for arsenic reduction. Requires periodic media regeneration or replacement.
Iron/Manganese Oxidation Filters
Effective for arsenic in iron-rich well water, which is common in the Midwest and New England. Oxidation converts dissolved iron and arsenic to a form that can be filtered out. Best when arsenic is co-occurring with high iron levels.
What does NOT work for arsenic
Standard activated carbon filters (Brita, refrigerator filters, most pitcher filters) do NOT effectively remove arsenic. Boiling concentrates arsenic. Water softeners do not remove arsenic. Only use products with NSF certification specifically for arsenic reduction.
Take Action Now
If you use a private well in Colorado, test for arsenic — especially if you are in a region with granite, volcanic, or sedimentary geology. A basic arsenic test costs $15–$40 at a state-certified lab.
Public water users: check your utility's Consumer Confidence Report (CCR) for arsenic results. The EPA MCL is 10 ppb — any detection warrants attention.
If arsenic is detected above 10 ppb (or even below it, given MCLG is zero), install an NSF/ANSI 58-certified reverse osmosis system for drinking and cooking water.
Standard carbon filters do NOT remove arsenic — do not rely on a Brita or refrigerator filter for arsenic protection.
Frequently Asked Questions
Related Pages
Arsenic — National Overview
All U.S. utilities with arsenic records
Colorado State Overview
All utilities and water quality data
Colorado Well Water Guide
Testing, risks & certified labs for private wells
Reverse Osmosis Guide
Removes 85–95% of arsenic
Best Arsenic Filter Guide
What actually works (carbon doesn't)
PFAS in Drinking Water
The 'forever chemical' contamination overview
All Contaminants
Complete reference library
Data Sources & Provenance
All data on this page is sourced from official U.S. government or public datasets.
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State Regulator
Colorado Department of Public Health and Environment ↗