Arsenic is worth checking in St. Mary's County wells.

1 worth checking, 5 rare out of 6 health checks.

From 26 wells tested in St. Mary's County since 2000; your own well needs its own test. How to read this

Tested wells over the limit St. Mary'sMaryland

Arsenic12%3 of 25 Manganese0%0 of 24 Nitrate0%0 of 23 Lead0%0 of 15 Fluoride0%0 of 24 Uranium0%0 of 15

StainsNot a health risk

Iron stains12%3 of 24

Not charted, because fewer than 10 wells were tested: radon (7).

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

Lab results: Water Quality Portal (USGS and EPA)

Show iron

Who drinks from a well here

6 in 10 people 67,718 people

Nobody else tests these wells. Only you do.

People on wells: EPA researchers' 2020 estimate

How deep wells go here

Deep wells: fewer surface germs, more minerals from the rock.

Well depths: US Groundwater Well Database

Radon risk (a gas from rock)

LowMiddleHigh

Possible here. A home air test is inexpensive.

Radon zones: EPA map

Here's how St. Mary's County compares with Maryland and the counties around it.

Each result puts St. Mary's County next to the 6 counties that border it, on the chart's 0 to 50% scale.

This one is worth checking in St. Mary's County.

Arsenic3 of 25 wells over

3 of 25 wells tested here from 2000 to 2022 were over the EPA limit of 10 µg/L, against about 1 in 12 across Maryland. At least half the results were too low for the lab to measure, and the top 1 in 10 were 11 µg/L or more. That's a bigger share than in 12 of the 15 other Maryland counties with 10 or more tested wells.

St. Mary's and its neighbors

Dorchester37%

St. Mary's12%

Calvert0%

Charles0%

Too few tested to show: Northumberland, VA, Somerset, Westmoreland, VA.

Arsenic guide

These 5 are rare in St. Mary's County.

Manganese0 of 24 wells over

None of the 24 wells tested here from 2000 to 2022 were over the EPA health advisory of 0.3 mg/L, against about 2 in 10 across Maryland. Half the results were under 0.0035 mg/L, and the top 1 in 10 were 0.011 mg/L or more. St. Mary's County had none, while 12 of the 17 other Maryland counties with 10 or more tested wells had at least one.

St. Mary's and its neighbors

Dorchester27%

Charles5%

Calvert0%

Northumberland, VA0%

St. Mary's0%

Too few tested to show: Somerset, Westmoreland, VA.

Manganese guide

Nitrate0 of 23 wells over

None of the 23 wells tested here from 2000 to 2022 were over the EPA limit of 10 mg/L, against about 1 in 10 across Maryland. St. Mary's County had none, while 11 of the 17 other Maryland counties with 10 or more tested wells had at least one, leaving out 1 county whose results are likely overstated.

St. Mary's and its neighbors

Dorchester7%

Calvert0%

Charles0%

St. Mary's0%

Too few tested to show: Northumberland, VA, Somerset, Westmoreland, VA.

Nitrate guide

Lead0 of 15 wells over

None of the 15 wells tested here from 2000 to 2022 were over the EPA action level of 15 µg/L, against under 1 in 100 across Maryland. St. Mary's County had none, while 1 of the 13 other Maryland counties with 10 or more tested wells had at least one.

St. Mary's and its neighbors

Charles0%

Dorchester0%

St. Mary's0%

Too few tested to show: Calvert, Northumberland, VA, Somerset, Westmoreland, VA.

Lead guide

Fluoride0 of 24 wells over

None of the 24 wells tested here from 2000 to 2022 were over the EPA limit of 4 mg/L, against under 1 in 100 across Maryland. Half the results were under 0.5 mg/L, and the top 1 in 10 were 0.85 mg/L or more. St. Mary's County had none, while 1 of the 17 other Maryland counties with 10 or more tested wells had at least one.

St. Mary's and its neighbors

Northumberland, VA7%

Dorchester3%

Calvert0%

Charles0%

St. Mary's0%

Too few tested to show: Somerset, Westmoreland, VA.

Uranium0 of 15 wells over

None of the 15 wells tested here from 2000 to 2022 were over the EPA limit of 30 µg/L, against none across Maryland. St. Mary's County had none, and neither did any of the 9 other Maryland counties with 10 or more tested wells.

St. Mary's and its neighbors

Charles0%

Dorchester0%

St. Mary's0%

Too few tested to show: Calvert, Northumberland, VA, Somerset, Westmoreland, VA.

Uranium guide

Too few St. Mary's County wells were tested for this one.

Radon in water7 wells tested

Only 7 wells here were tested from 2002 to 2012, too few to say how common it is. Across Maryland it's about 1 in 10.

Radon guide

Every well needs a bacteria test every year.

No bacteria tests from St. Mary's County wells are in the public data. Test every year anyway.

Iron reached staining levels in 3 of 24 tested wells, and it isn't a health risk.

Iron stains3 of 24 wells had it

3 of 24 wells tested here from 2000 to 2022 had enough iron to stain (0.3 mg/L), against about 4 in 10 across Maryland.

St. Mary's and its neighbors

Charles63%

Dorchester49%

Calvert40%

St. Mary's12%

Northumberland, VA9%

Too few tested to show: Somerset, Westmoreland, VA.

Iron guide

Most of the 26 wells tested in St. Mary's County were last tested before 2010.

14 wells had a test in the 2000s, 13 in the 2010s and 1 since 2020.

An old result shows the water as it was then, so treat these 26 wells as a starting point and test your own.

Most St. Mary's 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 St. Mary's County.

Bacteria
Nitrate
Arsenic

What Maryland offers well owners

Free testingWe didn't find a free program for all of Maryland. Maryland's private well page
Certified labsMaryland's list of certified labs
Well recordsWe didn't find an online search.
New wellsSince October 1, 2024, Md. Code, Real Property 10-713 requires a contract for sale of property with a private well to make well water quality testing a condition of sale, with both parties certifying they reviewed the results.

Half the household wells here are shallower than 370 feet.

That comes from 25,059 household wells in Maryland's drilling records. Most wells of every kind here are 320 to 430 feet deep. Wells as deep as St. Mary's County's are usually better protected from fertilizer and bacteria at the surface, but they can pick up more of what's in the rock, like arsenic and uranium. Typical household wells here go deeper than in 20 of the 23 other Maryland counties with enough drilling records.

How to find your own well's depth

Well depths: US Groundwater Well Database

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

St. Mary's County is in the EPA's middle radon zone. An indoor air test is cheap and tells you whether to look at your water. Radon guide

Between 2023 and 2025 the EPA had 5 town water systems in St. Mary's County test for PFAS. None found any. No well in St. Mary's 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 St. Mary's County

Calvert Charles

Elsewhere in Maryland

Anne Arundel Baltimore Carroll Harford Frederick Montgomery

All of Maryland

Questions people ask about well water in St. Mary's County

Is well water in St. Mary's County safe to drink?

3 of 25 wells tested in St. Mary's County had arsenic over the limit, about 1 in 8. Across Maryland it's about 1 in 12. That's a bigger share than in 12 of the 15 other Maryland counties with 10 or more tested wells. So test your own well for arsenic before you count on it.

Where can I get my well tested in St. Mary's County?

Use a lab Maryland has certified for drinking water. Maryland's list is linked above, in the box about testing. Ask for bacteria, nitrate and arsenic, the tests that matter most in St. Mary's County.

How deep are wells in St. Mary's County?

Half the household wells here are shallower than 370 feet. Typical household wells here go deeper than in 20 of the 23 other Maryland counties with enough drilling records.

How does St. Mary's County compare with the counties around it?

For arsenic, the share of tested wells over the limit: 1 of the 3 neighboring counties with data had a higher share than St. Mary's County, at 12%. Dorchester had the most, at 37%. Groundwater doesn't stop at county lines, so Dorchester's 37% is worth knowing about if you live near it.

How often should I test my well in St. Mary's County?

St. Mary's County has no public bacteria results for wells, so follow the CDC's advice: 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 Maryland Department of the EnvironmentUpdated November 8, 2026

If you buy something through our links, we may earn a commission. We'd recommend the same thing either way, and we'll tell you when a free option exists. How we make money

Where the St. Mary's County numbers come from

People on wells: 67,718 in St. Mary's County, from EPA estimates built on the 2020 Census (Murray et al.).
Lab tests: 174 well water samples from 26 wells in the Water Quality Portal, run by the USGS and EPA, 2000 to 2022.
Well depths: 25,059 Maryland drilling records for St. Mary's County, gathered in the US Groundwater Well Database (Lin and others, 2024).
Neighboring counties: the 6 counties that share a border with St. Mary's County, from the Census Bureau's county adjacency file.
PFAS: EPA tests of 5 town water systems in St. Mary's County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
Radon: St. Mary's County is zone 2 on the EPA Map of Radon Zones.
State programs and links: Maryland Department of the Environment, checked November 1, 2026.

Download this county's data (CSV)How we read these numbers Report an error