Quick answer
About 679,000 people in Washington (10.1% of residents) got their water from private wells in 2010, according to USGS estimates. Washington State Department of Health (well construction: Department of Ecology) runs the state’s private well guidance. Washington DOH says households within a specific boundary of the Lower Yakima Valley or Western Benton County may qualify for free nitrate and coliform testing.
- People on private wells
- 678,894
- Share of residents
- 10.1%
- Rank among states
- 20th
- Well users modeled above 10 µg/L arsenic
- 11.1%
Where to get your well tested in Washington
- Washington State Department of Health (well construction: Department of Ecology): Private Wells: Information for Owners, the state’s guidance for private well owners.
- Well water testing in Washington: what to test for and how to collect a sample.
- Certified laboratories in Washington. Use a lab certified for drinking water, so results are reliable.
Free or low-cost testing
Washington DOH says households within a specific boundary of the Lower Yakima Valley or Western Benton County may qualify for free nitrate and coliform testing. (source)
Mail-in lab kits are another option when no local lab is convenient. See well water test kits for what home kits can and can’t tell you.
What to test for
- Every year: total coliform bacteria and nitrate.
- Before buying equipment: pH, hardness, iron, manganese and, if there’s a smell, hydrogen sulfide.
- Based on location: arsenic, uranium and radon, PFAS, and lead in older plumbing.
USGS modeling estimates that about 11.1% of Washington’s private well users have water above the 10 µg/L arsenic limit. Arsenic has no taste or smell, so include it in your first test.
Private wells by county
Estimated people relying on private wells in each Washington county in 2010, the share of residents that represents, and the modeled share of well users with arsenic above 10 µg/L. These are USGS model estimates (2010 population), not measurements of any well. Arsenic is hidden for counties with fewer than 500 well users.
| County | People on wells | Share | Arsenic >10 µg/L |
|---|---|---|---|
| Yakima County | 72,901 | 30.0% | 17.0% |
| Spokane County | 48,527 | 10.3% | 14.5% |
| Pierce County | 43,992 | 5.5% | 5.2% |
| Thurston County | 43,575 | 17.3% | 3.6% |
| King County | 40,708 | 2.1% | 7.2% |
| Snohomish County | 39,386 | 5.5% | 8.3% |
| Clark County | 38,388 | 9.0% | 8.1% |
| Kitsap County | 32,594 | 13.0% | 7.7% |
| Lewis County | 29,649 | 39.3% | 2.6% |
| Grant County | 24,067 | 27.0% | 29.1% |
| Whatcom County | 20,884 | 10.4% | 4.3% |
| Benton County | 20,575 | 11.7% | 41.2% |
Show all 39 counties
| Stevens County | 19,910 | 45.7% | 6.1% |
| Okanogan County | 16,700 | 40.6% | 17.5% |
| Skagit County | 16,656 | 14.2% | 5.5% |
| Grays Harbor County | 16,535 | 22.7% | 1.4% |
| Cowlitz County | 16,154 | 15.8% | 4.9% |
| Mason County | 15,960 | 26.3% | 3.5% |
| Kittitas County | 12,323 | 30.1% | 16.0% |
| Clallam County | 11,559 | 16.2% | 4.3% |
| Chelan County | 11,447 | 15.8% | 9.7% |
| Franklin County | 10,652 | 13.6% | 38.3% |
| Island County | 10,167 | 13.0% | 8.3% |
| Walla Walla County | 9,501 | 16.2% | 16.8% |
| Klickitat County | 6,802 | 33.5% | 7.3% |
| Pend Oreille County | 6,244 | 48.0% | 3.8% |
| Jefferson County | 5,709 | 19.1% | 5.9% |
| Adams County | 5,286 | 28.2% | 32.4% |
| San Juan County | 5,165 | 32.8% | 8.8% |
| Whitman County | 5,153 | 11.5% | 12.5% |
| Douglas County | 4,993 | 13.0% | 25.5% |
| Pacific County | 3,891 | 18.6% | 2.4% |
| Ferry County | 3,485 | 46.2% | 6.4% |
| Lincoln County | 3,222 | 30.5% | 17.8% |
| Skamania County | 2,923 | 26.4% | 4.3% |
| Asotin County | 1,053 | 4.9% | 10.1% |
| Wahkiakum County | 862 | 21.7% | 2.0% |
| Columbia County | 844 | 20.7% | 9.0% |
| Garfield County | 452 | 19.9% | – |
Counties with the highest modeled arsenic share: Benton County (41.2%), Franklin County (38.3%), Adams County (32.4%), Grant County (29.1%), Douglas County (25.5%).
After you get results
Enter your numbers in the test results tool to see what needs treating and in what order. The guide to reading a well water test explains units and limits.
Common questions
Does Washington offer free well water testing?
Washington DOH says households within a specific boundary of the Lower Yakima Valley or Western Benton County may qualify for free nitrate and coliform testing.
Who oversees private wells in Washington?
Washington State Department of Health (well construction: Department of Ecology) (Private Wells: Information for Owners) publishes guidance for private well owners. Private wells aren’t covered by the federal Safe Drinking Water Act, so testing and treatment are up to the owner.
How many people use private wells in Washington?
USGS estimates about 679,000 people, or 10.1% of the population in 2010. That ranks 20th of the states covered by the estimate.
What should I test my well for in Washington?
Test every year for total coliform bacteria and nitrate, plus pH, hardness, iron and manganese before buying equipment. Arsenic is a known concern in parts of Washington, so add it to the first test.
Sources
- Washington State Department of Health (well construction: Department of Ecology): Private Wells: Information for Owners
- Washington State Department of Health (well construction: Department of Ecology): well water testing
- Washington certified laboratories
- Washington free or low-cost testing
- USGS data release: domestic well locations and populations served, 2000 and 2010 (Johnson, Belitz and Lombard, 2019)
- CDC Environmental Public Health Tracking: private well water data (from USGS)