Washington Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in Washington. They describe the area's modeled risk — not any single well.
Quick answer: Yes — arsenic occurs naturally in Washington groundwater. USGS modeling estimates about 52,249 Washington residents rely on private wells with arsenic above the EPA limit of 10 µg/L. The highest modeled exposure is in Yakima County (~6,940), Snohomish County (~6,178), Benton County (~5,218). For nitrate, about 2.2% of Washington's ~676,000 private-well residents are modeled above the 10 mg/L limit. These are area estimates, not a test of your well — arsenic has no taste or smell, so only a certified lab test tells you your level.
This is a modeled probability for the area, not a measurement of your well. USGS statistical models estimate how many private wells in this area are likely to exceed the federal limit — they cannot tell you what is in your water. Two neighbors can have very different results. The only way to know your well is to test it.
Nitrate
2.2%
of the private-well population is modeled above the 10 mg/L nitrate limit
~14,700
people on wells estimated above the limit
~676,000
total residents on private wells statewide
Source: USGS machine-learning nitrate model, Ransom et al. (2022), domestic-well depth zone.
Find out what's actually in your well
Washington has one of the higher modeled arsenic exposures in the country — but that number is an area estimate, not your well. Arsenic, nitrate and coliform bacteria are the three most common private-well problems, and none of them has a taste or smell. A certified lab test checks all of them at once; the CDC and EPA recommend testing every well at least once a year.
Basic screening only. Confirm arsenic, nitrate and bacteria with a certified lab.
Find a certified drinking-water labAmazon links are affiliate links — we may earn a commission at no extra cost to you.
Arsenic
Across Washington, an estimated ~52,249 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Yakima County (~6,940 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Yakima County | ~6,940 | 5,534–8,761 |
| Snohomish County | ~6,178 | 4,661–8,142 |
| Benton County | ~5,218 | 4,010–6,329 |
| Clark County | ~4,768 | 3,493–6,387 |
| Grant County | ~4,097 | 3,330–4,973 |
| Spokane County | ~3,271 | 2,513–4,228 |
| Franklin County | ~3,212 | 2,600–3,984 |
| Okanogan County | ~1,952 | 1,454–2,523 |
| Lewis County | ~1,825 | 1,299–2,583 |
| Pierce County | ~1,738 | 1,366–2,232 |
| Thurston County | ~1,692 | 1,268–2,286 |
| Kitsap County | ~1,374 | 1,078–1,735 |
| King County | ~1,031 | 739–1,392 |
| Cowlitz County | ~997 | 684–1,429 |
| Whatcom County | ~950 | 765–1,193 |
| Skagit County | ~878 | 651–1,173 |
| Kittitas County | ~841 | 652–1,091 |
| Clallam County | ~819 | 638–1,070 |
| Chelan County | ~656 | 482–910 |
| Adams County | ~632 | 524–758 |
| Walla Walla County | ~477 | 388–578 |
| Douglas County | ~429 | 339–541 |
| Stevens County | ~418 | 296–609 |
| Pend Oreille County | ~349 | 272–447 |
| Mason County | ~327 | 244–464 |
| Jefferson County | ~220 | 160–303 |
| Ferry County | ~207 | 162–266 |
| Klickitat County | ~202 | 159–264 |
| Whitman County | ~186 | 151–229 |
| Lincoln County | ~163 | 129–204 |
| Grays Harbor County | ~101 | 60–175 |
6 additional Washington counties have lower modeled arsenic exposure (fewer than 100 estimated well residents over the limit). Of these, 5 have measured USGS well samples: Asotin, Columbia, Garfield, Pacific, Skamania.
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Measured well tests by county
The USGS has published laboratory results from wells in 36 Washington counties (at least 5 wells each). These are real measurements of other wells — domestic, public-supply and monitoring wells — not a test of yours. The tables rank counties by the share of sampled wells over each federal threshold.
Nitrate (as N): counties with the most wells > 10 mg/L (EPA MCL)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Franklin County | 34% | 6.5 mg/L | 77 wells |
| Grant County | 21% | 4.6 mg/L | 76 wells |
| Whatcom County | 20% | 1.3 mg/L | 143 wells |
| Benton County | 20% | 0 mg/L | 5 wells |
| Adams County | 8% | 4.2 mg/L | 12 wells |
| Whitman County | 7% | 4.6 mg/L | 42 wells |
| Yakima County | 3% | 4.3 mg/L | 77 wells |
| Thurston County | 1% | 0.4 mg/L | 369 wells |
| Snohomish County | 1% | 0 mg/L | 296 wells |
| Pierce County | 1% | 0.2 mg/L | 85 wells |
Manganese: counties with the most wells > 300 µg/L (EPA health advisory)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Pend Oreille County | 30% | 150 µg/L | 10 wells |
| Kitsap County | 14% | 22 µg/L | 186 wells |
| Whatcom County | 13% | 24 µg/L | 175 wells |
| Skagit County | 11% | 120 µg/L | 37 wells |
| Stevens County | 10% | 10 µg/L | 52 wells |
| Pacific County | 8% | 3 µg/L | 26 wells |
| King County | 7% | 33 µg/L | 247 wells |
| Thurston County | 6% | 5 µg/L | 370 wells |
| Snohomish County | 6% | 28 µg/L | 291 wells |
| Pierce County | 5% | 4 µg/L | 148 wells |
Source: USGS ScienceBase 5f36baa782cee144fb3873e8 (manganese, pH, arsenic) and McMahon et al. (2019), Environ. Sci. Technol. 53:29–38 (nitrate, iron); aggregated by county with Census 2020 boundaries. USGS well samples; not a test of your well.
Acidic well water in Washington (low pH)
10% of 3,177 USGS-sampled wells in Washington had pH below 6.5. Acidic water corrodes copper pipes and fixtures and can leach lead and copper into your tap water — the usual fix is a whole-house acid neutralizer (calcite for mildly acidic water, soda-ash injection for very low pH). Counties with the highest share of acidic wells:
| County | Wells below pH 6.5 | Median pH | Sampled |
|---|---|---|---|
| Pacific County | 54% | 6.4 | 37 wells |
| Whatcom County | 44% | 6.6 | 216 wells |
| Pierce County | 25% | 6.9 | 182 wells |
| Chelan County | 20% | 7.2 | 5 wells |
| Mason County | 17% | 7.3 | 18 wells |
| Snohomish County | 13% | 7.7 | 317 wells |
| Thurston County | 11% | 7.1 | 381 wells |
| Kitsap County | 11% | 7.7 | 204 wells |
| Skagit County | 10% | 7.6 | 39 wells |
| King County | 7% | 7.6 | 309 wells |
Source: USGS ScienceBase 5f36baa782cee144fb3873e8 (pH). Measured other wells, not a test of yours.
Match the filter to your test result
Carbon pitchers and faucet filters do not remove arsenic or nitrate. Reverse osmosis is the point-of-use fix; a whole-house system handles the rest of the home.
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Under-sink units tested for arsenic reduction — the point-of-use fix for a well over the limit.
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Washington well water: FAQ
Is there arsenic in well water in Washington?
Yes. USGS modeling estimates about 52,249 people in Washington use private wells with arsenic above 10 µg/L, across 37 modeled counties. Arsenic comes from natural bedrock and varies well to well, even between neighbors, so a lab test is the only way to know your own level.
Which Washington counties have the most arsenic in well water?
By modeled number of well residents over the arsenic limit: Yakima County (~6,940), Snohomish County (~6,178), Benton County (~5,218). Counties with more private-well users rank higher, so a smaller county can still have a high share of affected wells.
How do I remove arsenic from well water in Washington?
Test first. If arsenic is present, a point-of-use reverse-osmosis system under the kitchen sink removes 90%+ of it; boiling does not remove arsenic (it concentrates it), and carbon pitchers and water softeners do not remove it either. Whole-house adsorptive arsenic media is used when levels are high. Re-test after installing and then annually.
Is nitrate a problem in Washington well water?
USGS models about 2.2% of Washington's private-well population (~14,700 people) above the 10 mg/L nitrate limit. Nitrate is highest in shallow wells near fertilized cropland, septic systems or livestock, and matters most for infants and pregnancy.
Do I need an acid neutralizer for my well in Washington?
Acidic well water turns up regularly in Washington. In USGS lab results from 3,177 wells across 26 Washington counties, 10% had pH below 6.5. The most acidic sampled counties are Pacific County (54% of 37 wells), Whatcom County (44% of 216 wells), Pierce County (25% of 182 wells). You only need a neutralizer if your own well tests below about 6.5 — signs include blue-green stains on fixtures and pinhole leaks in copper pipe. A calcite (or calcite/Corosex) neutralizer tank handles mildly acidic water (roughly pH 6–6.5); very acidic water usually needs soda-ash injection. Test first-draw lead and copper too, since acidic water dissolves both from plumbing.
High manganese in Washington well water — what should I do?
In USGS lab results from 2,006 wells across 22 Washington counties, 7% were above the EPA's 300 µg/L health advisory for manganese and 30% were above the 50 µg/L level where it stains fixtures and laundry black or brown. Sampled counties with the highest share over the health advisory: Pend Oreille County (30% of 10 wells), Kitsap County (14% of 186 wells), Whatcom County (13% of 175 wells). If your own test is over 300 µg/L, don't use the water for infant formula and use another source for drinking and cooking until it is treated — high manganese is linked to neurological effects, especially in babies and children. Boiling does not remove it. For the whole house, an oxidizing filter (greensand, Birm or air-injection) handles manganese, usually alongside iron; a water softener can remove low levels. For drinking water, a point-of-use reverse osmosis system reduces manganese at the kitchen tap. Retest after treatment to confirm it worked.