Well water in King County, Washington

1in6

Testing recommended

tested well samples in King County exceeded the EPA limit for arsenic (10 ug/L), based on public data from USGS Water Quality Portal.

4,462 test results across 13 contaminants from at least 528 wells, 1948 to 2016; chloride has the most at 864. Data refreshed September 15, 2026.

Compiled by Nick at Well Water Nerd from public records. How these pages are built

An estimated 128,846 people in King County get their drinking water from a private well, per USGS 2015 county water use. That is about 6% of residents.

Separately, the USGS national arsenic model estimates that <1% of the 128,846 people on private wells in King County draw water above 10 ug/L (the model's range is <1% to 1%), from geology and well depth rather than local tests.

What individual samples found

Each row summarizes every public sample for that contaminant inside King County. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.

Contaminant	Above limit	Samples	Detected	Median	Highest	Limit
Arsenic 1981 to 2016	18%	266	77%	2.15	389	10 ug/L EPA limit
Lead 1970 to 2016	<1%	239	23%	0	70	15 ug/L EPA limit
Nitrate 1948 to 2016	<1%	582	59%	0.064	37	10 mg/L EPA limit
Fluoride 1951 to 2016	0%	621	88%	0.1	2.5	4 mg/L EPA limit
Copper 1962 to 2016	0%	264	64%	0.001	0.8	1.3 mg/L EPA limit
E. coli 2005 to 2016	0%	11	0%	0	0	0 MPN/100mL EPA limit
Radium (226 + 228) 2016	0%	10	50%	0.015	0.43	5 pCi/L EPA limit
Chloride 1948 to 2016	no limit	864	100%	3.7	5,300	250 mg/L nuisance level
Sodium 1951 to 2016	no limit	560	100%	6.7	4,300	20 mg/L health guideline
Iron 1948 to 2016	no limit	540	94%	55	85,000	300 ug/L nuisance level
Manganese 1951 to 2016	no limit	479	83%	40	4,510	300 ug/L health guideline
Radon 1991 to 1996	no limit	21	90%	250	530	2,000 pCi/L health guideline

Sampled fewer than 10 times, too few to summarize: Uranium (5).

Over time

Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.

Contaminant	before 2000	2000s	2010s	2020s
Fluoride	0%497 samples	0%119 samples	n/a	n/a
Nitrate	<1%525 samples	0%47 samples	0%10 samples	n/a
Arsenic	16%223 samples	32%38 samples	n/a	n/a
Copper	0%237 samples	0%22 samples	n/a	n/a
Lead	<1%212 samples	0%22 samples	n/a	n/a

Arsenic above the limit in King County rose from 16% of 223 samples before 2000 to 32% of 38 in the 2000s.

Fluoride, nitrate, copper, lead have held within a few points between the two most recent periods with enough samples.

Closer than the county

See the sampled wells near your address

The same records, drawn around one point: which wells within a few miles were sampled and what they found, how deep the wells next to you were drilled, and any leaking tank or cleanup site within a mile.

Check my address

What this means

The contaminants most often above their limit here are arsenic (18% of 266 samples), lead (<1% of 239 samples), and nitrate (<1% of 582 samples). A well next door can differ sharply from yours because bedrock fractures, well depth and plumbing all matter, so these rates describe the area, not your tap.

Chloride, Iron, and Manganese are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.

What to test for in King County

1
Arsenic
18% of 266 local samples were above the limit.
2
Lead
<1% of 239 local samples were above the limit.
3
Nitrate
<1% of 582 local samples were above the limit.
4
Coliform bacteria
Recommended for every private well, every year.
5
Radon
Median local reading is 250 pCi/L.
6
Chloride
Detected in 100% of local samples.
7
Iron
Detected in 94% of local samples.
8
Volatile organic compounds (VOCs)
244 reported spills and releases on record locally.

Test your King County well for arsenic, lead and nitrate

Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.

Order the Tap Score well water test kit, from $209 Already tested? Upload the report

Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.

Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.

Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.

Where to get a well tested in Washington: certified labs, state programs and prices.

Already have results back? Get your lab report read against the King County numbers, $29. Every line explained, ranked by what matters, with the fix for each.

Wells in King County

4,689 domestic wells in King County are on record with the state since 1990. the typical well is 141 feet deep (one in ten goes past 342 feet).

In a typical King County well, the median yield is 30 gallons per minute.

1,317

1990s

1,869

2000s

983

2010s

507

2020s

Find your own well's report

The state keeps the driller's report for most wells drilled since 1984. It tells you how deep your well is, where bedrock starts, and how much water it makes, which decides what treatment can work.

Open Ecology well report search and choose Water Well.
It lists depth, casing, static water level, the yield test and the driller.

Washington has required well reports since 1971; reports are located only to the quarter-quarter section, so check the address on the scan.

The rock under King County

Puget Sound aquifer system

Unconsolidated sand and gravel. Wells in sand and gravel are usually shallow and productive, and they recharge quickly from the surface. That makes them the most exposed to nitrate, bacteria and anything spilled nearby, and the least likely to carry arsenic or radon from the rock.

Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the county centroid. A single well can sit in a different unit, especially near a boundary.

Official help in Washington

Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.

Drinking Water, Washington State Department of Health
Environmental lab accreditation, Washington State Department of Ecology

Contamination sources on record in King County

Washington Ecology facility interactions records 244 reported spills and releases in King County, among 11,863 regulated sites in total. Groundwater does not follow property lines, so a well within a few hundred feet of one of these is worth testing for what that site could release.

244: reported spills and releasesMost spills are fuel; a few are solvents or chemicals that reach the water table.
11,509: other remediation projects
110: other solid waste facilities

Treatment that fits the local profile

Arsenic

Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange

Lead

Point-of-use reverse osmosis; Certified lead-removal carbon filter; Neutralizing filter to raise pH and stop leaching

Nitrate

Point-of-use reverse osmosis; Anion exchange; Distillation

Iron

Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration

Manganese

Oxidizing filter (greensand, birm) whole-house; Water softener (low levels); Point-of-use reverse osmosis

Systems we would look at first

Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.

For arsenic

Under-sink reverse osmosis system

under-sink or countertop, about $300

Look for NSF/ANSI 58 certification listing the contaminant you need removed.

For arsenic

Whole-house arsenic and uranium filter

whole-house, at the point of entry, about $2,500

Adsorptive iron-based media; sized to your well yield.

For arsenic

Aquasana SmartFlow reverse osmosis

under-sink or countertop, about $225

Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.

For lead

Aquasana Claryum 3-Stage under-sink filter

under-sink or countertop, about $175

Certified to NSF/ANSI 53 to reduce 99% of lead and 99.7% of PFOA and PFOS at the tap. Not a fix for arsenic, nitrate or bacteria.

For lead

Acid neutralizing filter

whole-house, at the point of entry, about $1,200

Raises pH so plumbing stops leaching lead and copper.

For iron

Iron and manganese oxidizing filter

whole-house, at the point of entry, about $1,800

Air-injection or greensand media; fixes staining at the source.

Free, no obligation

Get on the list for a free in-home water test in King County

No water treatment company serves King County through Well Water Nerd yet. We are signing licensed companies on county by county: they test at your tap for free, walk you through the numbers, and quote treatment only if something needs fixing. Leave your details and you are first in line. Nothing is shared until a company joins for King County, and you get an email naming it first.

Common questions

How deep are wells in King County?

The typical domestic well in King County is 141 feet deep, based on 4,689 wells reported to the state since 1990. One in ten is deeper than 342 feet.

Is there arsenic in well water in King County?

Arsenic was detected in 77% of 266 public well samples in King County, and 18% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.

How high is radon in well water in King County?

The median radon level across 21 public samples in King County was 250 pCi/L, with a high of 530 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.

How often should I test a private well in King County?

Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.

Counties near King County

Closest first. Groundwater does not stop at a line on a map, so the next county over is a fair second opinion.

Want the numbers themselves? Download the Washington dataset as CSV, every county and town, with the sources named.

Sources

USGS Water Quality Portal: 4,462 samples in King County. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
USGS national arsenic model (Ayotte 2017): estimated share of 128,846 people on private wells in King County above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 2026.
Washington Ecology well construction reports: 4,691 well records in King County, 4,689 domestic. Well construction reports filed with the Washington State Department of Ecology, water wells only, located at the quarter-quarter section. Only aggregates are shown; owner names and addresses are never stored. Retrieved September 17, 2026.
Washington Ecology facility interactions: 11,863 regulated sites located in King County. Toxics cleanup sites, spills and solid waste facilities from the Washington State Department of Ecology Facility/Site register. Retrieved September 17, 2026.

Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.