Well water in Washington County, Idaho
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Testing strongly recommended
tested well samples in Washington County exceeded the EPA limit for arsenic (10 ug/L), based on public data from USGS Water Quality Portal and Idaho statewide ground water quality monitoring (IDWR EDMS).
3,302 test results across 14 contaminants from at least 206 wells, 1958 to 2025; nitrate has the most at 620. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 4,005 people in Washington County get their drinking water from a private well, per USGS 2015 county water use. That is about 40% of residents.
Separately, the USGS national arsenic model estimates that 12% of the 4,005 people on private wells in Washington County draw water above 10 ug/L (the model's range is 10% to 14%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Washington 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 1958 to 2025 | 297 | 94% | 11 | 950 | 10 ug/L EPA limit | |
| Nitrate 1958 to 2025 | 620 | 72% | 1.61 | 193 | 10 mg/L EPA limit | |
| Total coliform 2014 to 2019 | 118 | 15% | 0 | 219 | 0 MPN/100mL EPA limit | |
| Uranium 2016 to 2025 | 19 | 42% | 0 | 35 | 30 ug/L EPA limit | |
| Fluoride 1958 to 2025 | 346 | 95% | 0.39 | 5 | 4 mg/L EPA limit | |
| Nitrite 1991 to 2019 | 217 | 7% | 0 | 0.02 | 1 mg/L EPA limit | |
| Copper 1990 to 2019 | 189 | 47% | 0 | 0.075 | 1.3 mg/L EPA limit | |
| Lead 1990 to 2010 | 122 | 11% | 0 | 3 | 15 ug/L EPA limit | |
| E. coli 1991 to 2008 | 75 | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Chloride 1958 to 2025 | no limit | 352 | 100% | 8.33 | 220 | 250 mg/L nuisance level |
| Sodium 1958 to 2025 | no limit | 332 | 100% | 71.9 | 521 | 20 mg/L health guideline |
| Iron 1958 to 2025 | no limit | 311 | 68% | 37.1 | 4,400 | 300 ug/L nuisance level |
| Manganese 1958 to 2025 | no limit | 291 | 73% | 82.2 | 935 | 300 ug/L health guideline |
| Radon 1991 to 1995 | no limit | 13 | 100% | 610 | 860 | 2,000 pCi/L health guideline |
Over time
Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.
| Contaminant | before 2000 | 2000s | 2010s | 2020s |
|---|---|---|---|---|
| Nitrate | 15%209 samples | 24%203 samples | 43%192 samples | 31%16 samples |
| Fluoride | 1%140 samples | 0%129 samples | 0%61 samples | 0%16 samples |
| Arsenic | 54%119 samples | 53%121 samples | 48%44 samples | 54%13 samples |
| Nitrite | 0%32 samples | 0%43 samples | 0%142 samples | n/a |
| Copper | 0%97 samples | 0%49 samples | 0%43 samples | n/a |
| Lead | 0%97 samples | 0%22 samples | n/a | n/a |
Nitrate above the limit in Washington County rose from 24% of 203 samples in the 2000s to 43% of 192 in the 2010s.
Arsenic above the limit in Washington County fell from 53% of 121 samples in the 2000s to 48% of 44 in the 2010s.
Fluoride, nitrite, copper, lead have held within a few points between the two most recent periods with enough samples.
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 addressWhat this means
The contaminants most often above their limit here are arsenic (53% of 297 samples), nitrate (27% of 620 samples), and total coliform (15% of 118 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.
Chloride, Iron, and Manganese 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 Washington County
- 1Arsenic53% of 297 local samples were above the limit.
- 2Nitrate27% of 620 local samples were above the limit.
- 3Total coliform15% of 118 local samples were above the limit.
- 4Uranium5% of 19 local samples were above the limit.
- 5Fluoride<1% of 346 local samples were above the limit.
- 6Coliform bacteriaRecommended for every private well, every year.
- 7RadonMedian local reading is 610 pCi/L.
Test your Washington County well for arsenic, nitrate and total coliform
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 $209Already 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 Idaho: certified labs, state programs and prices.
Already have results back? Get your lab report read against the Washington County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Washington County
Pacific Northwest basaltic-rock aquifers
Igneous and metamorphic-rock. Granite and similar crystalline rock hold water only in fractures. Wells are deep and yields modest, and the rock itself is the source of arsenic, radon and uranium, which is why those three head the test list in bedrock country.
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 Idaho
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.
- Wells, Idaho DEQ
- Drinking Water Testing and Certification, Idaho Department of Health and Welfare
Treatment that fits the local profile
Arsenic
Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Total coliform
Inspect and repair the well cap and casing, then shock chlorinate and retest; UV disinfection whole-house; Chlorination system
Uranium
Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange
Fluoride
Point-of-use reverse osmosis; Activated alumina; Distillation
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 total coliform
Aquasana Rhino Well Water with UV
whole-house, at the point of entry, about $2,871
Whole-house system built for wells: sediment pre-filter, KDF and carbon, and a UV lamp rated to kill 99.99% of bacteria and viruses. It does not remove arsenic, nitrate, uranium or radon.
For iron
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
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Common questions
Is there arsenic in well water in Washington County?
Arsenic was detected in 94% of 297 public well samples in Washington County, and 53% 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 high is radon in well water in Washington County?
The median radon level across 13 public samples in Washington County was 610 pCi/L, with a high of 860 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Washington 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 Washington 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 Idaho dataset as CSV, every county and town, with the sources named.
Sources
- USGS Water Quality Portal: 1,689 samples in Washington 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.
- Idaho statewide ground water quality monitoring (IDWR EDMS): 1,613 samples in Washington County. Ground water quality results from the Idaho Department of Water Resources statewide monitoring program and partner agencies (ISDA, IDEQ, INL), EDMS bulk download. Retrieved September 9, 2026.
- USGS national arsenic model (Ayotte 2017): estimated share of 4,005 people on private wells in Washington 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.
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.