Washington well water by county: arsenic and nitrate
Modelled arsenic and nitrate for private-well users in every Washington county, what to test for, and where to find a certified lab. Private wells aren’t covered by the Safe Drinking Water Act.
All 39 counties
- Yakima CountyWell users 72,900Arsenic about 17 in 100Nitrate 2.9 mg/L
- Spokane CountyWell users 48,500Arsenic about 15 in 100Nitrate 0.5 mg/L
- Pierce CountyWell users 44,000Arsenic about 5 in 100Nitrate 0.3 mg/L
- Thurston CountyWell users 43,600Arsenic about 3 in 100Nitrate 0.5 mg/L
- King CountyWell users 40,700Arsenic about 7 in 100Nitrate 0.2 mg/L
- Snohomish CountyWell users 39,400Arsenic about 8 in 100Nitrate 0.2 mg/L
- Clark CountyWell users 38,400Arsenic about 8 in 100Nitrate 0.4 mg/L
- Kitsap CountyWell users 32,600Arsenic about 8 in 100Nitrate 0.1 mg/L
- Lewis CountyWell users 29,600Arsenic about 2 in 100Nitrate 0.1 mg/L
- Grant CountyWell users 24,100Arsenic about 29 in 100Nitrate 2.9 mg/L
- Whatcom CountyWell users 20,900Arsenic about 4 in 100Nitrate 2.0 mg/L
- Benton CountyWell users 20,600Arsenic about 41 in 100Nitrate 2.3 mg/L
- Stevens CountyWell users 19,900Arsenic about 6 in 100Nitrate 0.2 mg/L
- Okanogan CountyWell users 16,700Arsenic about 18 in 100Nitrate 0.8 mg/L
- Skagit CountyWell users 16,700Arsenic about 5 in 100Nitrate 0.1 mg/L
- Grays Harbor CountyWell users 16,500Arsenic about 2 in 100Nitrate 0.1 mg/L
- Cowlitz CountyWell users 16,200Arsenic about 5 in 100Nitrate 0.1 mg/L
- Mason CountyWell users 16,000Arsenic about 3 in 100Nitrate 0.1 mg/L
- Kittitas CountyWell users 12,300Arsenic about 16 in 100Nitrate 0.4 mg/L
- Clallam CountyWell users 11,600Arsenic about 4 in 100Nitrate 0.2 mg/L
- Chelan CountyWell users 11,400Arsenic about 10 in 100Nitrate 0.5 mg/L
- Franklin CountyWell users 10,700Arsenic about 38 in 100Nitrate 5.7 mg/L
- Island CountyWell users 10,200Arsenic about 8 in 100Nitrate 0.0 mg/L
- Walla Walla CountyWell users 9,500Arsenic about 17 in 100Nitrate 1.6 mg/L
- Klickitat CountyWell users 6,800Arsenic about 8 in 100Nitrate 0.3 mg/L
- Pend Oreille CountyWell users 6,200Arsenic about 4 in 100Nitrate 0.1 mg/L
- Jefferson CountyWell users 5,700Arsenic about 6 in 100Nitrate 0.1 mg/L
- Adams CountyWell users 5,300Arsenic about 32 in 100Nitrate 2.2 mg/L
- San Juan CountyWell users 5,200Arsenic about 9 in 100Nitrate 0.0 mg/L
- Whitman CountyWell users 5,200Arsenic about 13 in 100Nitrate 0.9 mg/L
- Douglas CountyWell users 5,000Arsenic about 26 in 100Nitrate 1.1 mg/L
- Pacific CountyWell users 3,900Arsenic about 3 in 100Nitrate 0.3 mg/L
- Ferry CountyWell users 3,500Arsenic about 6 in 100Nitrate 0.2 mg/L
- Lincoln CountyWell users 3,200Arsenic about 18 in 100Nitrate 2.2 mg/L
- Skamania CountyWell users 2,900Arsenic about 4 in 100Nitrate 0.1 mg/L
- Asotin CountyWell users 1,100Arsenic about 10 in 100Nitrate 0.4 mg/L
- Wahkiakum CountyWell users 860Arsenic about 2 in 100Nitrate 0.1 mg/L
- Columbia CountyWell users 840Arsenic about 9 in 100Nitrate 1.1 mg/L
- Garfield CountyWell users 450Arsenic about 16 in 100Nitrate 1.0 mg/L
Each mark = 1 in 100 well users the model places above 10 µg/L arsenic (scale 0–10)10 mg/L nitrate: the federal limit (MCL) for public systems
- These figures describe the area, not your well. Only a lab test shows what’s in your water.
- CDC says the Well Water Index figures “do not describe risk to individual wells.” (CDC Tracking Network)
- The arsenic model is better at spotting low-arsenic areas than at confirming high ones.
- Well-user counts are 2010 USGS estimates; tract values are averages over each census tract.
- Total coliform bacteriaEvery year
- NitrateEvery year
- Total dissolved solidsEvery year
- pHEvery year
- ArsenicAsk your local health department
EPA: Private Drinking Water Wells · CDC: Guidelines for Testing Well Water
Use a state-certified lab
Washington State Department of Ecology lists the laboratories certified to test drinking water in Washington.
Screening tells you if something is worth a closer look
A lab result is the number you can act on.
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Lab-certified results
ETR Labs Basic Water Quality Test Kit
ETR Laboratories
50+ substances, including lead, copper, arsenic, nitrate, hardness and coliform bacteria
- Analyzed by Environmental Testing and Research Laboratories (ETR Labs), Leominster, MA · ISO/IEC 17025:2017, ANAB certificate AT-2638
- Turnaround: 2–4 business days after the lab receives it
What it can't tell you: It does not test PFAS, and not every result on the panel falls under the lab's accredited scope.
Screening: quick check, not lab-grade
Watersafe Drinking Water Test Kit
Silver Lake Research
Lead, bacteria, nitrate, copper, iron, chlorine, hardness and pH
- Turnaround: Minutes; the bacteria test can take up to 48 hours
What it can't tell you: Each test gives a yes/no or a rough range, not a lab number, and it cannot detect PFAS or arsenic.
A mail-in kit’s lab is accredited (ISO/IEC 17025) but may not be certified by your state. For a home sale, a loan, or a health-department follow-up, use a state-certified lab.
Washington well water
Sources: CDC Well Water Index (2024) · USGS arsenic and nitrate models · USGS domestic well populations · EPA and CDC testing guidance. Data last updated 2026-10-08. All sources →
How many people use private wells in Washington?
About 678,900 people across 39 Washington counties, by the USGS 2010 estimate. USGS models place about 75,200 of them in areas where wells are likely to be above the 10 µg/L arsenic limit. These are area estimates, not well results.
Are private wells regulated in Washington?
The quality and safety of drinking water from private domestic wells are not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws. (EPA) Testing and treatment are the owner's responsibility.
Where can I get my well tested in Washington?
Washington State Department of Ecology keeps the list of laboratories certified to test drinking water in Washington. Only use laboratories that are certified to do drinking water testing. (EPA)