Arsenic in Drinking Water in Montana
What residents of Montana 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, Montana Department of Environmental Quality, USGS · Last reviewed: 2025-01-01
Quick Answer
Is arsenic in drinking water a concern in Montana?
Yes. Montana has significant arsenic occurrence due to its volcanic geology, geothermal activity (Yellowstone region), and extensive copper, silver, gold, and coal mining history. The Butte-Anaconda mining district — the site of one of the most heavily mined copper deposits in history — has left massive arsenic contamination in soils, surface water, and groundwater across a broad area of southwestern Montana.
Where does arsenic come from in Montana's water?
Three primary pathways exist in Montana: (1) Natural volcanic and geothermal geology (Yellowstone hot spot influence in southwest Montana), (2) Legacy copper/silver/gold mining in the Butte-Anaconda-Helena corridor, and (3) Naturally elevated arsenic in alluvial aquifers of the Gallatin, Madison, and Jefferson river valleys from volcanic geology inputs. Private well owners near mining districts face the highest risk.
What should Montana residents know?
Butte-Anaconda area residents face some of the highest documented arsenic exposure from mining legacy of any U.S. community. The Clark Fork River basin, downstream from Butte, has documented arsenic loading from historic tailings. Residents near Warm Springs Ponds, the Clark Fork Superfund site, and private well users in Silver Bow, Deer Lodge, and Powell counties face direct arsenic risk. Montana DEQ actively monitors this region.
Key Facts
| EPA MCL | 10 µg/L (10 ppb) |
| MCLG | Zero |
| Primary source | Butte-Anaconda copper/silver mining legacy (Clark Fork Superfund); volcanic/geothermal geology (SW Montana); natural arsenic in alluvial aquifers |
| Superfund concern | Clark Fork Superfund site — one of largest U.S. sites by area; arsenic-bearing tailings over wide area |
| State regulator | Montana Department of Environmental Quality (DEQ) |
| Health effects | Bladder, lung, skin cancer; cardiovascular; diabetes risk |
| Effective treatment | Reverse osmosis or activated alumina; iron/manganese oxidation also useful for mining-influenced water |
Why Arsenic Matters in Montana
Montana's Butte-Anaconda mining district is one of the most contaminated areas in the United States. Over 100 years of copper mining generated massive quantities of acid mine drainage and arsenic-bearing tailings that have contaminated the Clark Fork River watershed for hundreds of miles. The Clark Fork Superfund site is among the largest in the U.S. by area. Beyond the mining legacy, Montana's volcanic and geothermal geology contributes naturally elevated arsenic to groundwater in the Yellowstone region and adjacent areas of southwestern Montana. Rural private well users across the state face arsenic risk from multiple geological sources.
Montana Arsenic Program
Montana DEQ monitors arsenic under federal SDWIS and recognizes Montana as a high-arsenic state due to volcanic and sedimentary geology. The Crazy Mountains, Bearpaw Mountains, and areas near volcanic centers have elevated natural arsenic. Historic gold and copper mining throughout Montana has also contributed arsenic to surface and groundwater in mining districts. Private well owners should test.
Montana 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.
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
Butte, Anaconda, and Silver Bow and Deer Lodge county residents near the Butte-Anaconda mining complex, communities along the Clark Fork River downstream from Butte, and private well users in the Yellowstone region's geothermal zones face the most serious arsenic risk.
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 Montana
- 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 Montana Department of Environmental Quality-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 Montana, 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
Montana State Overview
All utilities and water quality data
Montana 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.
Find Your Utility
State Regulator
Montana Department of Environmental Quality ↗