Well water in Mason County, Washington
In Mason County, none of 95 tested well samples exceeded the EPA limit for nitrate, based on public data from USGS Water Quality Portal.
The strongest signal is coliform bacteria: Recommended for every private well, every year, too few samples to carry the headline but first on the test list below.
707 test results across 13 contaminants from at least 249 wells, 1947 to 2016; chloride has the most at 337. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 21,628 people in Mason County get their drinking water from a private well, per USGS 2015 county water use. That is about 35% of residents.
Separately, the USGS national arsenic model estimates that 2% of the 21,628 people on private wells in Mason County draw water above 10 ug/L (the model's range is 1% to 2%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Mason 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 |
|---|---|---|---|---|---|---|
| Nitrate 1960 to 2016 | 95 | 79% | 0.066 | 4.1 | 10 mg/L EPA limit | |
| Fluoride 1960 to 2016 | 57 | 77% | 0.1 | 0.2 | 4 mg/L EPA limit | |
| Chloride 1947 to 2016 | no limit | 337 | 100% | 2 | 870 | 250 mg/L nuisance level |
| Iron 1960 to 2016 | no limit | 72 | 64% | 29.9 | 4,000 | 300 ug/L nuisance level |
| Sodium 1960 to 2016 | no limit | 58 | 100% | 3.8 | 60 | 20 mg/L health guideline |
| Manganese 1965 to 2016 | no limit | 53 | 79% | 20 | 900 | 300 ug/L health guideline |
Sampled fewer than 10 times, too few to summarize: Copper (9), Lead (7), Radium (226 + 228) (6), Arsenic (5), E. coli (4), Uranium (3), Radon (1).
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 |
|---|---|---|---|---|
| Nitrate | 0%63 samples | 0%26 samples | n/a | n/a |
| Fluoride | 0%41 samples | 0%13 samples | n/a | n/a |
Nitrate has 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 addressWhat this means
Among contaminants with a federal limit, no tested sample in Mason County came back above it. That is reassuring, but sampling is uneven: wells that were never tested are not in this data.
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 Mason County
- 1Coliform bacteriaRecommended for every private well, every year.
- 2RadonMedian local reading is 310 pCi/L.
- 3NitrateRecommended for every private well, especially near farms or septic systems.
- 4ChlorideDetected in 100% of local samples.
- 5ManganeseDetected in 79% of local samples.
- 6FluorideDetected in 77% of local samples.
- 7CopperDetected in 67% of local samples.
- 8Volatile organic compounds (VOCs)7 reported spills and releases on record locally.
Test your Mason County well for coliform bacteria, radon 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 $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 Washington: certified labs, state programs and prices.
Already have results back? Get your lab report read against the Mason County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
Wells in Mason County
5,563 domestic wells in Mason County are on record with the state since 1989. the typical well is 129 feet deep (one in ten goes past 273 feet).
In a typical Mason County well, the median yield is 28 gallons per minute, 10% of wells yield under 3 gallons per minute and usually need a storage tank.
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 Mason 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 Mason County
Washington Ecology facility interactions records 7 reported spills and releases in Mason County, among 444 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.
- 7
- reported spills and releasesMost spills are fuel; a few are solvents or chemicals that reach the water table.
- 397
- other remediation projects
- 40
- other solid waste facilities
Treatment that fits the local profile
Manganese
Oxidizing filter (greensand, birm) whole-house; Water softener (low levels); Point-of-use reverse osmosis
Iron
Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration
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 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 Mason County
No water treatment company serves Mason 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 Mason County, and you get an email naming it first.
Common questions
How deep are wells in Mason County?
The typical domestic well in Mason County is 129 feet deep, based on 5,563 wells reported to the state since 1989. One in ten is deeper than 273 feet.
How often should I test a private well in Mason 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 Mason 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: 707 samples in Mason 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 21,628 people on private wells in Mason 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: 5,568 well records in Mason County, 5,563 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: 444 regulated sites located in Mason 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.