Well water testing in King County, Washington
What official data says about private wells in King County, what to test for, and the state-certified labs nearby.
About 6% of the county's 2,117,125 residents. Rank 1 of 39 in Washington.
95% range 739 to 1,392 private-well users (2010 base year). Rank 13 of 39 in Washington.
Low potential (average indoor radon may be less than 2 pCi/L). EPA says the map should not be used to decide whether an individual home needs testing.
Labs with an address in this county, from the official state lists on this site, matched by ZIP code.
What to test for in King County
Private wells are not regulated like public water: EPA advises owners to test their well every year for total coliform bacteria, nitrates, total dissolved solids and pH. Your state's own schedule:
- Coliform bacteria and nitrate every year. If nitrate is 5 mg/L or higher, the state says you may want to re-test in six months. It also suggests testing arsenic at least twice while you own the home, once in summer and once in winter. source
Arsenic. USGS estimates that about 1,031 people using private wells in King County had arsenic above 10 µg/L, EPA's limit for public water (2010 base year; 95% range 739 to 1,392). USGS describes this kind of estimate as useful "for broad screening of the population at risk", so it says nothing certain about any one well. Arsenic has no taste or smell: test for it by name.
Radon. EPA puts King County in Zone 3: low potential (average indoor radon may be less than 2 pCi/L). That map is about radon in indoor air, and EPA says it should not be used to decide whether an individual home needs testing. If radon is found in your air and you have a well, EPA suggests testing the water too: see radon in well water.
For anything specific to your home (farms nearby, old plumbing, a new baby, a smell or stain), use the which-tests checklist. When the results arrive, check your numbers against EPA's limits.
Show the numbers
| Item | Value | Note |
|---|---|---|
| Yakima County | 6,940 people (2010 base year) | |
| Snohomish County | 6,178 people (2010 base year) | |
| Benton County | 5,218 people (2010 base year) | |
| Clark County | 4,768 people (2010 base year) | |
| Grant County | 4,097 people (2010 base year) | |
| King County (this county) | 1,031 people (2010 base year) |
USGS model estimate for broad screening, not a measurement of any well. Only a test tells you about your own well. Source: USGS, Estimated county level domestic well population with arsenic greater than 10 micrograms per liter (Ayotte, Medalie and Qi, 2017), accessed 2026-11-01.
PFAS in public water near King County (public supply, not your well)
This is public water data, not your well. A private well draws its own groundwater, which can have more or less PFAS than the public supply nearby. Only a test of your well tells you.
Under EPA’s fifth Unregulated Contaminant Monitoring Rule (UCMR 5), public water systems sampled their water for 29 PFAS between Jan 3, 2023 and Jun 17, 2026. 48 public systems that serve ZIP codes mainly in King County reported results. 4 of them had at least one sample with PFOA or PFOS at or above 4.0 ng/L, and 11 had at least one of the 29 PFAS measured at or above its reporting level.
| Public water system | System ID | Highest PFOA (ng/L) | Highest PFOS (ng/L) |
|---|---|---|---|
| Sammamish Plateau Water & Sewer | WA5340900 | 11.1 | |
| Covington Water District | WA5341650 | 8.6 | |
| Redmond Water System City Of | WA5371650 | 5 | |
| Lakehaven Water And Sewer District | WA5341997 | 4.1 |
4.0 ng/L (parts per trillion) is both EPA’s UCMR 5 reporting level for PFOA and PFOS and the number in EPA’s 2024 limit for public water. That limit is judged on a running annual average, so one sample at that level is not a violation. EPA proposed changes to its PFAS rules in May 2026: see PFAS in well water for the current status and who should test a well. To test your own well, find a lab certified for PFAS.
Method: a system counts for this county when it reports serving a ZIP code whose largest land part is in this county, so a system serving several counties counts in each. Source: U.S. EPA, UCMR 5 occurrence data (by state), file dated Oct 15, 2026; downloaded Nov 1, 2026.
Testing programs and help in Washington
- Free nitrate and coliform testing for private wells in the Lower Yakima Valley and Western Benton County. source
Buying a home with a well in King County
Loan test (FHA, VA, USDA) in King County, WA
- VA. Test results are valid for 90 days from the date the local health authority certifies them, unless the local authority says otherwise. VA’s chapter names no list of contaminants: the water must meet the local health authority’s requirements, or EPA guidelines where it has none. Your lender may ask for specific tests.
- FHA. A well test is required in the situations FHA’s handbook lists, among others, and the report may not be more than 180 days old at disbursement.
- USDA. Guaranteed loans need a water quality test for a private well, done by the local health authority or a state-certified lab, and no more than 180 days old at closing.
Lenders can ask for more (extra tests, a particular lab, who takes the sample): ask your lender before you book. Federal rules as read on Nov 3, 2026 source 1 source 2 source 3. Full comparison: VA, FHA and USDA well water test requirements. Loan test kits sold by mail, with price and days to results: kits compared.
State-certified labs in King County
Alliance Technical Group, LLC-Seattle
3600 Fremont Ave N, Seattle, WAPublic samples: yes (per state)AmTest Laboratories
1131 SW 7th Street, Renton, WAPublic samples: yes (per state)Analytical Resources, LLC
4611 S 134th Place, Tukwila, WAPublic samples: yes (per state)Aqua Test, Incorporated
28620 Maple Valley Road SE, Maple Valley, WAPublic samples: yes (per state)Brooks Applied Labs, LLC
13751 Lake City Way NE, Seattle, WAPublic samples: yes (per state)Eurofins EPK Built Environment Testing, LLC
7619 6th Avenue NW, Seattle, WAPublic samples: yes (per state)Friedman & Bruya, Inc.
5500 4th AVE S, Seattle, WAPublic samples: yes (per state)IEH Analytical Laboratories
13751 Lake City Way NE, Seattle, WAPublic samples: yes (per state)MCL Food Testing
711 6th Ave N, Seattle, WAPublic samples: yes (per state)NVL Laboratories, Inc
4708 Aurora Ave N, Seattle, WAPublic samples: yes (per state)OnSite Environmental, Inc.
14648 NE 95th Street, Redmond, WAPublic samples: yes (per state)Seattle-King Co Dept of Public Health Lab
325 Ninth Ave, Seattle, WAPublic samples: yes (per state)Labs on the Washington list whose own websites say they take homeowner samples: Bureau Veritas Canada (2019) Inc. (Mississauga). Use Check my ZIP to sort labs by distance.
See all Washington labs in the finder and filter by the test you need and by labs that take homeowner samples.
- USGS, Estimated use of water in the United States, county-level data for 2015 (Dieter et al., ver. 2.0, 2018)
- USGS, Estimated county level domestic well population with arsenic greater than 10 micrograms per liter (Ayotte, Medalie and Qi, 2017)
- U.S. EPA, Map of Radon Zones, county list (EPA 402/A-16/001)
- U.S. Census Bureau, 2020 ZCTA to county relationship file (lab ZIP codes to counties)
- WA Ecology - Environmental Laboratories Accredited for Drinking Water Analyses (with scopes) (Information current as of 11/13/2026)
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