State Well Water Guide
Montana Private Well Water Guide
About 48% of Montanans rely on private wells — one of the highest rates in the nation — reflecting the state's vast rural landscape. Montana's groundwater quality is shaped by its geology: naturally occurring arsenic from volcanic and hydrothermal geology is the most pervasive concern, affecting wells in the Clark Fork watershed, Butte area, and numerous other regions. Historic hard rock mining — particularly copper mining around Butte — has created some of the nation's most contaminated groundwater. Uranium from granite and sedimentary geology, and nitrate from agricultural areas in the eastern plains, are additional concerns. MDEQ regulates well construction.
Testing Guidance
Montana DEQ recommends annual testing for coliform bacteria and nitrates. All well owners should test for arsenic given the state's widespread natural arsenic. Butte and Clark Fork watershed well owners should test for heavy metals (copper, lead, zinc, arsenic) from mining contamination. Eastern Montana agricultural area well owners should test for nitrates and uranium. All well owners should baseline test for TDS given Montana's high-mineral geology.
What to Test For in Montana
Total coliform bacteria and E. coli — annual minimum
Arsenic — widespread natural concern throughout Montana's volcanic and hydrothermal geology
Heavy metals (copper, lead, zinc, cadmium) — Clark Fork watershed and Butte area mining contamination
Uranium — granite and sedimentary geology throughout the state
Nitrates — eastern Montana agricultural and livestock areas
Iron and manganese — alluvial and sedimentary aquifer wells statewide
Fluoride — some Montana aquifer systems have naturally elevated fluoride
PFAS — near Malmstrom AFB and other Montana military facilities
Common Contamination Risks in Montana
Arsenic from volcanic and hydrothermal geology — Montana has widespread natural arsenic from hot spring deposits, volcanic rocks, and mineralized geological formations
Hard rock mining contamination — Butte's century of copper mining has created the Berkeley Pit Superfund site and extensive Clark Fork watershed contamination; one of the nation's most significant mining legacy contamination zones
Uranium from igneous and sedimentary geology — elevated in multiple Montana geological settings
Nitrate from eastern plains agriculture — cattle ranching and dry land wheat farming in eastern Montana
PFAS from Malmstrom AFB — the Great Falls area's military base is a documented PFAS source
Contaminant Guides Relevant to MT Wells
PFAS
Synthetic Chemicals
EPA limit: 4 ppt
Lead
Heavy Metals
EPA limit: 15 ppb (action level)
Nitrates
Agricultural Chemicals
EPA limit: 10 mg/L
DBPs
Disinfection Byproducts
EPA limit: 80 µg/L (TTHMs) / 60 µg/L (HAA5)
Arsenic
Heavy Metals
EPA limit: 10 ppb
Hard Water
Minerals
EPA limit: No federal limit
Chlorine
Disinfection Chemicals
EPA limit: 4 mg/L (MRDL)
Microplastics
Emerging Contaminants
EPA limit: No federal limit
Bacteria
Microbial Contamination
EPA limit: Zero E. coli / < 1 coliform per 100 mL
Fluoride
Minerals
EPA limit: 4 mg/L (MCL) / 2 mg/L (Secondary MCL)
Chromium-6
Heavy Metals
EPA limit: 100 ppb (total chromium)
Copper
Heavy Metals
EPA limit: 1.3 mg/L (action level)
Radon
Radioactive Contaminants
EPA limit: No finalized MCL
Iron & Manganese
Minerals
EPA limit: 0.3 mg/L iron / 0.05 mg/L manganese (aesthetic SMCLs)
Atrazine
Agricultural Chemicals
EPA limit: 3 ppb (0.003 mg/L)
Uranium
Radioactive Contaminants
EPA limit: 30 µg/L (30 ppb)
Perchlorate
Industrial Chemicals
EPA limit: 0.056 mg/L (56 ppb)
VOCs
Industrial Chemicals
EPA limit: Varies by compound: benzene 5 ppb; TCE 5 ppb; PCE 5 ppb; vinyl chloride 2 ppb
Radium
Radioactive Contaminants
EPA limit: 5 pCi/L (combined Ra-226 + Ra-228)
Crypto & Giardia
Microbial Contamination
EPA limit: Zero (treatment technique standard)
Hydrogen Sulfide
Naturally Occurring Compounds
EPA limit: No MCL; Secondary MCL (aesthetic) of 0.05 mg/L
Selenium
Minerals
EPA limit: 50 ppb (0.05 mg/L)
Cadmium
Heavy Metals
EPA limit: 5 ppb (0.005 mg/L)
Mercury
Heavy Metals
EPA limit: 2 ppb (0.002 mg/L)
Barium
Heavy Metals
EPA limit: 2 mg/L
Find a Certified Lab in Montana
Use the Montana state-certified laboratory program to find accredited labs for private well testing. Always verify current certification before submitting samples.
MT Certified Lab Directory ↗Montana Well Water FAQs
Related Pages
Data Sources & Provenance
All data on this page is sourced from official U.S. government or public datasets.
Quick Reference
State program
MT Certified Labs ↗EPA guidance
EPA Private Wells Program ↗State lab directory
MT Certified Labs ↗Minimum Annual Tests
Plus state-specific contaminants listed above
Other State Guides