Well water in Washington County, Oregon
1in2
Testing strongly recommended
tested well samples in Washington County exceeded the EPA limit for arsenic (10 ug/L), based on public data from USGS Water Quality Portal.
1,124 test results across 11 contaminants from at least 130 wells, 1941 to 2025; chloride has the most at 205. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 50,433 people in Washington County get their drinking water from a private well, per USGS 2015 county water use. That is about 9% of residents.
Separately, the USGS national arsenic model estimates that 3% of the 50,433 people on private wells in Washington County draw water above 10 ug/L (the model's range is 2% to 4%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Washington 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 1975 to 2025 | 188 | 79% | 11.5 | 149 | 10 ug/L EPA limit | |
| Lead 1995 to 2020 | 34 | 100% | 2.17 | 22.5 | 15 ug/L EPA limit | |
| Uranium 1993 to 2020 | 19 | 100% | 0.28 | 35.7 | 30 ug/L EPA limit | |
| Nitrate 1975 to 2002 | 101 | 33% | 0 | 23.9 | 10 mg/L EPA limit | |
| Fluoride 1975 to 2002 | 62 | 97% | 0.2 | 0.96 | 4 mg/L EPA limit | |
| Copper 1997 to 2020 | 26 | 100% | 0.005 | 0.186 | 1.3 mg/L EPA limit | |
| Chloride 1941 to 2025 | no limit | 205 | 100% | 6 | 1,100 | 250 mg/L nuisance level |
| Manganese 1975 to 2025 | no limit | 166 | 97% | 720 | 15,700 | 300 ug/L health guideline |
| Iron 1975 to 2025 | no limit | 162 | 98% | 1,975 | 114,000 | 300 ug/L nuisance level |
| Sodium 1975 to 2025 | no limit | 159 | 100% | 15 | 283 | 20 mg/L health guideline |
Sampled fewer than 10 times, too few to summarize: Radon (2).
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 |
|---|---|---|---|---|
| Arsenic | 52%138 samples | 42%26 samples | 44%16 samples | n/a |
| Lead | n/a | 0%15 samples | 9%11 samples | n/a |
Arsenic above the limit in Washington County fell from 52% of 138 samples before 2000 to 42% of 26 in the 2000s.
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 addressWhat this means
The contaminants most often above their limit here are arsenic (51% of 188 samples), lead (6% of 34 samples), and uranium (5% of 19 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, Manganese, and Iron 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 Washington County
- 1Arsenic51% of 188 local samples were above the limit.
- 2Lead6% of 34 local samples were above the limit.
- 3Uranium5% of 19 local samples were above the limit.
- 4Nitrate<1% of 101 local samples were above the limit.
- 5Coliform bacteriaRecommended for every private well, every year.
- 6RadonMedian local reading is 930 pCi/L.
- 7CopperDetected in 100% of local samples.
Test your Washington County well for arsenic, lead and uranium
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 $209Already 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 Oregon: certified labs, state programs and prices.
Already have results back? Get your lab report read against the Washington County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Washington County
Willamette Lowland basin-fill aquifers
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 Oregon
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.
- Domestic Well Safety, Oregon Health Authority
- Oregon Environmental Laboratory Accreditation Program, Oregon Health Authority
Contamination sources on record in Washington County
Oregon DEQ active tank sites records 120 regulated sites in Washington County, among 120 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.
- 120
- registered underground storage tank sitesRegistered tanks are regulated and inspected; they matter most when they are old or near a shallow well.
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
Uranium
Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange
Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Chloride
Point-of-use reverse osmosis; Distillation
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 manganese
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 Washington County
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Common questions
Is there arsenic in well water in Washington County?
Arsenic was detected in 79% of 188 public well samples in Washington County, and 51% 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 often should I test a private well in Washington 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 Washington 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 Oregon dataset as CSV, every county and town, with the sources named.
Sources
- USGS Water Quality Portal: 1,124 samples in Washington 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 50,433 people on private wells in Washington 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.
- Oregon DEQ active tank sites: 120 regulated sites located in Washington County. Active underground storage tank sites from the Oregon Department of Environmental Quality. Retrieved September 18, 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.