Private wells
Washington well water: who's on a well and what to test for
People on private wells
715,300
Homes on wells
9.8%
Est. above arsenic limit
52,200
Every Washington county
Sorted by people on private wells. Arsenic and nitrate are modeled for typical wells in the county, not measured at yours; depth is the median of domestic well records.
| Yakima County | 70,600 | 27% | 16.9% | 2.9 | 63 ft |
| Thurston County | 48,700 | 16% | 3.5% | 0.5 | 93 ft |
| Mason County | 48,700 | 73% | 3.3% | 0.1 | 70 ft |
| Pierce County | 40,400 | 5% | 5.1% | 0.3 | 97 ft |
| Spokane County | 32,800 | 6% | 14.5% | 0.5 | 120 ft |
| Snohomish County | 32,200 | 4% | 8.3% | 0.2 | 100 ft |
| Lewis County | 32,000 | 39% | 2.5% | 0.1 | 50 ft |
| Stevens County | 30,500 | 68% | 6.0% | 0.2 | 165 ft |
| Benton County | 26,200 | 11% | 41.1% | 2.3 | 125 ft |
| Kitsap County | 25,000 | 9% | 7.7% | 0.1 | 116 ft |
| Cowlitz County | 24,900 | 22% | 5.1% | 0.1 | 124 ft |
| Grant County | 24,600 | 24% | 28.9% | 2.9 | 157 ft |
| Whatcom County | 24,500 | 10% | 4.3% | 2.0 | 48 ft |
| Okanogan County | 23,600 | 59% | 17.5% | 0.8 | 75 ft |
| Kittitas County | 23,400 | 52% | 16.1% | 0.4 | 290 ft |
| Clallam County | 17,700 | 22% | 4.2% | 0.2 | 68 ft |
| King County | 17,400 | 1% | 7.2% | 0.2 | 112 ft |
| Grays Harbor County | 15,900 | 18% | 1.5% | 0.1 | 46 ft |
| Franklin County | 15,500 | 13% | 38.3% | 5.7 | 170 ft |
| Pend Oreille County | 12,900 | 94% | 3.9% | 0.1 | n/a |
| Island County | 12,800 | 15% | 8.3% | 0.0 | 138 ft |
| Klickitat County | 12,500 | 55% | 7.7% | 0.3 | 120 ft |
| Chelan County | 12,100 | 20% | 9.7% | 0.5 | n/a |
| Walla Walla County | 11,500 | 18% | 16.8% | 1.6 | n/a |
| Skagit County | 10,900 | 11% | 5.4% | 0.1 | 108 ft |
| San Juan County | 8,600 | 53% | 8.6% | 0.0 | 180 ft |
| Whitman County | 7,900 | 18% | 12.9% | 0.9 | 135 ft |
| Douglas County | 7,400 | 22% | 25.8% | 1.1 | n/a |
| Jefferson County | 7,400 | 24% | 5.9% | 0.1 | 92 ft |
| Lincoln County | 7,400 | 72% | 17.6% | 2.1 | n/a |
| Adams County | 6,900 | 32% | 32.4% | 2.1 | 304 ft |
| Ferry County | 5,800 | 83% | 6.1% | 0.2 | n/a |
| Pacific County | 5,400 | 20% | 2.6% | 0.3 | 25 ft |
| Skamania County | 5,200 | 47% | 4.4% | 0.1 | n/a |
| Garfield County | 2,300 | 100% | 15.6% | 1.0 | n/a |
| Wahkiakum County | 1,600 | 38% | 1.7% | 0.1 | n/a |
| Columbia County | 1,200 | 37% | 9.1% | 1.1 | n/a |
| Asotin County | 1,000 | 6% | 10.0% | 0.4 | n/a |
| Clark County | 0 | 0% | 8.1% | 0.4 | 158 ft |
Shading: red is among the highest counties nationally, amber the middle, green the lower range. Click a column to sort.
Most well users above the arsenic limit
- Yakima County: about 6,900 people
- Snohomish County: about 6,200 people
- Benton County: about 5,200 people
- Clark County: about 4,800 people
- Grant County: about 4,100 people
Highest modeled nitrate
- Franklin County: 5.7 mg/L average (limit 10)
Test first, then treat what the test finds
We may earn a commission from links below, at no cost to you. Picks follow the testing order, not who pays. How we make money.
1. Certified lab well test
Start here, every year
A mail-in kit analyzed by a certified lab measures bacteria, nitrate, arsenic, metals and more. Test strips cannot measure arsenic or uranium reliably.
Mail-in certified lab well water test
Mid-range2. Sediment pre-filter
Sand, silt or cloudy water
A 5-micron cartridge after the pressure tank catches sand, silt and rust before they reach fixtures or other filters. Cartridges are cheap and swap every few months.
5-micron whole-house sediment filter
Budget3. Iron and sulfur filter
Orange stains or rotten-egg smell
Air-injection or oxidizing-media filters remove orange staining and rotten-egg smell. Size one from a lab result for iron, manganese and hydrogen sulfide.
Air-injection iron and sulfur filter
Premium4. UV disinfection system
Coliform bacteria in a test
Inactivates bacteria and viruses; usually for private wells or after boil notices.
12 GPM whole-house UV purifier
Premium5. Under-sink reverse osmosis
Arsenic, nitrate or uranium in a test
The most thorough option for drinking water: PFAS, lead, nitrate, arsenic and more. Look for NSF/ANSI 58.
Waterdrop G3P600 tankless RO
Price at Waterdrop, checked Oct 5, 2026or compare on Amazon
iSpring RCC7AK reverse osmosis
Mid-range6. Whole-house well filter
Several problems at once
Multi-stage sediment and carbon for every tap. Add UV if bacteria shows up, and choose stages from your lab results, not a guess.
Waterdrop WHF21 2-stage whole-house filter
Mid-rangeQuestions
Which Washington counties rely most on wells?+
By share of homes: Garfield County (100%), Pend Oreille County (94%), Ferry County (83%), Mason County (73%), Lincoln County (72%). By number of people: Yakima County (70,600), Thurston County (48,700), Mason County (48,700).
Is arsenic a problem in Washington wells?+
USGS estimates about 52,200 Washington well users have water above the 10 microgram per liter arsenic limit that applies to public systems, the largest numbers in Yakima County, Snohomish County, Benton County. Arsenic has no taste or smell, so only a lab test finds it.
Do I have to test my well when I sell a house in Washington?+
We could not confirm a statewide rule for Washington. Check with your county health department. Buyers can still make a lab test a condition of the offer.
How often should I test my well?+
No federal law covers private wells. EPA's drinking water limits apply to public water systems, so testing and treating a private well is up to the owner. EPA and CDC recommend testing at least once a year for coliform bacteria, nitrate, pH and total dissolved solids, and testing for arsenic, uranium, radon and other local concerns based on where you live.
Well-user counts, arsenic and nitrate figures are each a modeled estimate built from national datasets, not a count of tested wells, and well depths summarize drilling records rather than every well. Your own well can be very different from the county average, which is why the only answer for your home is a lab test. Sources: EPA Office of Research and Development, Water Source 2020 (private well estimates by census block); CDC Environmental Public Health Tracking, Well Water Index (from USGS nitrate and arsenic models); USGS, county-level domestic well population with arsenic above 10 micrograms per liter (Ayotte and others, 2017); USGS Synthesized Domestic Well Database for the Conterminous United States (SynDWell-US), McCarthy and others, 2025 (depth, where a county has 30+ usable records).