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Health problems are rare in tested Mason County wells.

All 3 health checks came back rare here.

From 17 wells tested in Mason County since 2005; your own well needs its own test. How to read this

Tested wells over the limit MasonWashington
Manganese0%0 of 17Nitrate0%0 of 17Fluoride0%0 of 17
GermsTest every year, whatever the chart says
Coliform bacteria67%2 of 3E. coli0%0 of 3
StainsNot a health risk
Iron stains0%0 of 17

Not charted, because fewer than 10 wells were tested: arsenic (3), uranium (3), lead (3).

Rare: under 5%Worth checking: 5 to 15%Widespread: 15%+

Lab results: Water Quality Portal (USGS and EPA)

Who drinks from a well here

7 in 10 people 48,663 people

Nobody else tests these wells. Only you do.

People on wells: EPA researchers' 2020 estimate

Radon risk (a gas from rock)

Less likely, but the EPA still says test your air.

Radon zones: EPA map

Here's how Mason County compares with Washington and the counties around it.

Each result puts Mason County next to the 5 counties that border it, on the chart's 0 to 50% scale.

All 3 health checks are rare in Mason County.

Manganese0 of 17 wells over

None of the 17 wells tested here from 2005 to 2016 were over the EPA health advisory of 0.3 mg/L, against about 2 in 10 across Washington. At least half the results were too low for the lab to measure, and the top 1 in 10 were 0.006 mg/L or more. Mason County had none, while 8 of the 15 other Washington counties with 10 or more tested wells had at least one, leaving out 1 county whose results are likely overstated.

Mason and its neighbors
Kitsap35%
Thurston9%
Pierce2%
Mason0%

Too few tested to show: Grays Harbor, Jefferson.

Manganese guide

Nitrate0 of 17 wells over

None of the 17 wells tested here from 2005 to 2016 were over the EPA limit of 10 mg/L, against about 2 in 10 across Washington. Half the results were under 0.07 mg/L, and the top 1 in 10 were 0.73 mg/L or more. Mason County had none, while 8 of the 15 other Washington counties with 10 or more tested wells had at least one, leaving out 1 county whose results are likely overstated.

Mason and its neighbors
Pierce6%
Thurston2%
Kitsap0%
Mason0%

Too few tested to show: Grays Harbor, Jefferson.

Nitrate guide

Fluoride0 of 17 wells over

None of the 17 wells tested here from 2005 to 2016 were over the EPA limit of 4 mg/L, against under 1 in 100 across Washington. Mason County had none, while 2 of the 16 other Washington counties with 10 or more tested wells had at least one.

Mason and its neighbors
Kitsap0%
Mason0%
Pierce0%
Thurston0%

Too few tested to show: Grays Harbor, Jefferson.

Too few Mason County wells were tested for these 3.

Arsenic3 wells tested

Only 3 wells here were tested in 2016, too few to say how common it is. Across Washington it's about 1 in 7.

Arsenic guide

Uranium3 wells tested

Only 3 wells here were tested in 2016, too few to say how common it is. Across Washington it's about 1 in 7.

Uranium guide

Lead3 wells tested

Only 3 wells here were tested in 2016, too few to say how common it is. Across Washington it's about 1 in 37.

Lead guide

Coliform bacteria turned up in 2 of 3 tested Mason County wells, so test for it every year.

Coliform bacteria2 of 3 wells had it

2 of 3 wells tested here in 2016 had coliform bacteria, against about 4 in 10 across Washington.

Bacteria guide

E. coli0 of 3 wells had it

None of the 3 wells tested here in 2016 had E. coli, against about 1 in 9 across Washington.

E. coli means human or animal waste is getting in, so if a test of your Mason County well finds it, boil drinking water until the well is fixed.

Bacteria guide

Iron reached staining levels in 0 of 17 tested wells, and it isn't a health risk.

Iron stains0 of 17 wells had it

None of the 17 wells tested here from 2005 to 2016 had enough iron to stain, against about 2 in 10 across Washington.

Mason and its neighbors
Thurston42%
Kitsap27%
Mason0%
Pierce0%

Too few tested to show: Grays Harbor, Jefferson.

Iron guide

Most of the 17 wells tested in Mason County were last tested before 2010.

14 wells had a test in the 2000s and 3 in the 2010s.

An old result shows the water as it was then, so treat these 17 wells as a starting point and test your own.

Most Mason County wells in these records were tested for research or because someone suspected a problem, which can push the shares above what a typical home well would show (how we read the numbers).

One mail-in lab test covers the 3 things to check in Mason County.

  • Bacteria
  • Nitrate
  • Arsenic

What Washington offers well owners

Here's what we know about radon and PFAS in Mason County.

Mason County is in the EPA's lowest radon zone, so radon from well water is unlikely to be a problem. Radon guide

Between 2023 and 2025 the EPA had 4 town water systems in Mason County test for PFAS. 1 found some PFAS, all below the new limits. No well in Mason County has public PFAS results, so the town systems are the best clue to what's in the groundwater nearby. PFAS guide

Radon zones: EPA Map of Radon Zones. PFAS: EPA tests of town water systems, 2023 to 2025

Next door to Mason County
ThurstonPierce
Elsewhere in Washington
YakimaSpokane

All of Washington

Get a yearly reminder to test your Mason County well

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Questions people ask about well water in Mason County

Is well water in Mason County safe to drink?

Most tested wells in Mason County came in under the limits. Of 3 health checks with enough tests, nitrate came closest, at 0 of 17 wells over. Every well is different, so test yours for bacteria, nitrate and arsenic.

Can I get my well tested for free in Mason County?

DOH offers free nitrate and coliform testing to eligible well owners in parts of the Lower Yakima Valley and Western Benton County. If not, use a lab Washington has certified for drinking water. Ask for bacteria, nitrate and arsenic, the tests that matter most in Mason County.

How does Mason County compare with the counties around it?

For manganese, the share of tested wells over the limit: 3 of the 3 neighboring counties with data had a higher share than Mason County, at 0%. Kitsap had the most, at 35%. Groundwater doesn't stop at county lines, so Kitsap's 35% is worth knowing about if you live near it.

How often should I test my well in Mason County?

In Mason County, 2 of 3 tested wells had coliform bacteria, so the CDC's advice matters here: test for bacteria and nitrate every year, and right away after a flood, a repair, or any change in taste or smell.

By the Tested Water teamData from the USGS, EPA and Department of Health (DOH); Department of Ecology for well records and lab accreditationUpdated November 8, 2026

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Where the Mason County numbers come from

  • People on wells: 48,663 in Mason County, from EPA estimates built on the 2020 Census (Murray et al.).
  • Lab tests: 83 well water samples from 17 wells in the Water Quality Portal, run by the USGS and EPA, 2005 to 2016.
  • Neighboring counties: the 5 counties that share a border with Mason County, from the Census Bureau's county adjacency file.
  • PFAS: EPA tests of 4 town water systems in Mason County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
  • Radon: Mason County is zone 3 on the EPA Map of Radon Zones.
  • State programs and links: Department of Health (DOH); Department of Ecology for well records and lab accreditation, checked November 1, 2026.
Download this county's data (CSV)How we read these numbersReport an error
Test for bacteria and nitrate every yearSee lab tests