Well water in Lake County, Montana

Name: Private well water quality in Lake County, Montana
Creator: Well Water Nerd
License: Public domain (US federal government data)

1in7

Testing recommended

tested well samples in Lake County exceeded the EPA limit for fluoride (4 mg/L), based on public data from USGS Water Quality Portal.

The strongest signal is arsenic: 35% of 17 local samples were above the limit, too few samples to carry the headline but first on the test list below.

717 test results across 15 contaminants from at least 82 wells, 1974 to 2024; nitrate has the most at 107. Data refreshed September 15, 2026.

Compiled by Nick at Well Water Nerd from public records. How these pages are built

An estimated 13,280 people in Lake County get their drinking water from a private well, per USGS 2015 county water use. That is about 45% of residents.

Separately, the USGS national arsenic model estimates that 4% of the 13,280 people on private wells in Lake County draw water above 10 ug/L (the model's range is 3% to 6%), from geology and well depth rather than local tests.

What individual samples found

Each row summarizes every public sample for that contaminant inside Lake County. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.

Contaminant	Above limit	Samples	Detected	Median	Highest	Limit
Arsenic 1978 to 2024	35%	17	100%	3.3	100	10 ug/L EPA limit
Fluoride 1974 to 2024	15%	106	77%	0.2	7.8	4 mg/L EPA limit
Nitrate 1974 to 2024	2%	107	100%	0.4	19.1	10 mg/L EPA limit
Copper 1983 to 2024	0%	42	40%	0	0.02	1.3 mg/L EPA limit
Lead 1983 to 2024	0%	12	8%	0	0.148	15 ug/L EPA limit
Sodium 1974 to 2024	no limit	106	100%	21.5	220	20 mg/L health guideline
Manganese 1974 to 2024	no limit	106	50%	0	1,310	300 ug/L health guideline
Iron 1974 to 2024	no limit	106	56%	15	2,000	300 ug/L nuisance level
Chloride 1974 to 2024	no limit	106	99%	2.95	90	250 mg/L nuisance level

Sampled fewer than 10 times, too few to summarize: Uranium (2), Radium (226 + 228) (2), PFOA (1), E. coli (1), PFOS (1), Radon (2).

Closer than the county

See the sampled wells near your address

The same records, drawn around one point: which wells within a few miles were sampled and what they found, how deep the wells next to you were drilled, and any leaking tank or cleanup site within a mile.

Check my address

What this means

The contaminants most often above their limit here are arsenic (35% of 17 samples), fluoride (15% of 106 samples), and nitrate (2% of 107 samples). A well next door can differ sharply from yours because bedrock fractures, well depth and plumbing all matter, so these rates describe the area, not your tap.

Iron and Chloride are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.

What to test for in Lake County

1
Arsenic
35% of 17 local samples were above the limit.
2
Fluoride
15% of 106 local samples were above the limit.
3
Nitrate
2% of 107 local samples were above the limit.
4
Coliform bacteria
Recommended for every private well, every year.
5
Radon
Median local reading is 655 pCi/L.
6
Chloride
Detected in 99% of local samples.
7
Iron
Detected in 56% of local samples.
8
Volatile organic compounds (VOCs)
140 leaking underground storage tank sites on record locally.

Test your Lake County well for arsenic, fluoride and nitrate

Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.

Order the Tap Score well water test kit, from $209 Already tested? Upload the report

Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.

Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.

Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.

Where to get a well tested in Montana: certified labs, state programs and prices.

Already have results back? Get your lab report read against the Lake County numbers, $29. Every line explained, ranked by what matters, with the fix for each.

The rock under Lake County

Not a principal aquifer

The USGS map places this area outside any principal aquifer, which usually means wells here draw from fractured bedrock or thin local deposits rather than a large regional aquifer. Bedrock wells of that kind are where arsenic, radon and uranium turn up, and yields vary from well to well.

Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the county centroid. A single well can sit in a different unit, especially near a boundary.

Official help in Montana

Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.

Water Resources, Montana DEQ
Water Laboratory Certification Program, Montana DPHHS

Contamination sources on record in Lake County

Montana DEQ records 140 leaking underground storage tank sites in Lake County, among 182 regulated sites in total. Groundwater does not follow property lines, so a well within a few hundred feet of one of these is worth testing for what that site could release.

140: leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
34: registered underground storage tank sitesRegistered tanks are regulated and inspected; they matter most when they are old or near a shallow well.
2: lined landfills and transfer facilities
6: other solid waste facilities

Treatment that fits the local profile

Arsenic

Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange

Fluoride

Point-of-use reverse osmosis; Activated alumina; Distillation

Nitrate

Point-of-use reverse osmosis; Anion exchange; Distillation

Iron

Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration

Manganese

Oxidizing filter (greensand, birm) whole-house; Water softener (low levels); Point-of-use reverse osmosis

Systems we would look at first

Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.

For arsenic

Under-sink reverse osmosis system

under-sink or countertop, about $300

Look for NSF/ANSI 58 certification listing the contaminant you need removed.

For arsenic

Whole-house arsenic and uranium filter

whole-house, at the point of entry, about $2,500

Adsorptive iron-based media; sized to your well yield.

For arsenic

Aquasana SmartFlow reverse osmosis

under-sink or countertop, about $225

Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.

For manganese

Iron and manganese oxidizing filter

whole-house, at the point of entry, about $1,800

Air-injection or greensand media; fixes staining at the source.

Free, no obligation

Get on the list for a free in-home water test in Lake County

No water treatment company serves Lake County through Well Water Nerd yet. We are signing licensed companies on county by county: they test at your tap for free, walk you through the numbers, and quote treatment only if something needs fixing. Leave your details and you are first in line. Nothing is shared until a company joins for Lake County, and you get an email naming it first.

Common questions

Is there arsenic in well water in Lake County?

Arsenic was detected in 100% of 17 public well samples in Lake County, and 35% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.

How often should I test a private well in Lake County?

Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.

Counties near Lake County

Closest first. Groundwater does not stop at a line on a map, so the next county over is a fair second opinion.

Want the numbers themselves? Download the Montana dataset as CSV, every county and town, with the sources named.

Sources

USGS Water Quality Portal: 717 samples in Lake County. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
USGS national arsenic model (Ayotte 2017): estimated share of 13,280 people on private wells in Lake County above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 2026.
Montana DEQ regulated sites: 182 regulated sites located in Lake County. Petroleum tank releases, underground storage tank facilities and licensed solid waste facilities from the Montana Department of Environmental Quality. Retrieved September 18, 2026.

Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.