What's in St. Mary's County, MD tap water?
St. Mary's County sits about 55 miles from our Arlington base and is served primarily by Lexington Park, which provides water to roughly 43,000 people. It draws from the Coastal-plain aquifers. Maryland's Eastern Shore and Southern Maryland sit on coastal-plain aquifers. That groundwater commonly carries naturally occurring arsenic and radionuclides, iron and manganese, and nitrate from farming, with saltwater intrusion a growing concern near the Chesapeake Bay.
Find a local installerLexington Park water quality report, explained
What testing of Lexington Park water shows for St. Mary's County, compared with EWG's health guidelines. Being above a guideline is not a legal violation.
Primary water system (EPA)
Lexington Park
Serves about 43,000 people
Contaminants EWG most often flags above its health guidelines for the Coastal-plain aquifers:
Larger areas have more than one water system. Enter your exact zip code below for your utility's own EPA record and EWG report.
Measured by the EPA · St. Mary's County, MD
St. Mary's County, MD tap water is fully legal.
It is also 70× the health standard for lead.
St. Mary's County is on groundwater — wells rather than a river intake — serving roughly 43,030 people through 15,820 connections. That is unusual for this area, where most supplies come off the Potomac, and it changes which contaminants are worth worrying about.
Groundwater does not carry the chlorination byproducts that dominate river supplies, because it needs far less disinfection. What it carries instead comes out of the rock: arsenic, radium, manganese, and nitrate where there is farming above the aquifer. Boiling concentrates all of them rather than removing them.
Every federal limit that applies here is met. Those limits weigh what removal costs. EWG says its health standards are “based only on protecting health and does not consider technical feasibility or the cost of water treatment”. Here is how far apart the two are.
Harm to the brain and nervous system · EPA limit 10 ppb (action level, lowered from 15 ppb, in force November 2027)— also above this
Lead was measured above the federal limit too. Compliance is judged on a running annual average rather than a single sample, so this is not a violation — but it is the number EPA itself set as the line.
It was tested for 29 PFAS compounds in the EPA's latest round and none came back above reporting limits — genuinely good news, and a single window in time. Lead sampling put the ninetieth percentile at 14 ppb — meaning nine in ten homes tested below that, and one in ten above it. It carries 5 violations on file, none health-based — monitoring and reporting lapses rather than contamination.
Lexington Park, on the EPA's record
- Source
- groundwater
- Connections served
- 15,820
- People served
- 43,030
- Violations on file
- 5, none health-based
How this water is treated before it reaches you
Treatment makes water safe to distribute. It is not designed to remove PFAS, and chlorine-based disinfection creates byproducts of its own.
Levels: EPA Fifth Unregulated Contaminant Monitoring Rule (UCMR5, 2023–2025) for PFAS, and the Lead and Copper Rule 90th-percentile result for lead and copper — the highest reported for Lexington Park. Health standards: EWG's published drinking water standards, updated February 2025. Levels vary by sampling point and date, and being above a health standard is not a legal violation.
9 contaminants above EWG's health guidelines
Everything EWG found above its guideline in St. Mary's County's water, from testing in 2015-2024. A level can be under the legal limit and still be listed here: EWG's guidelines are set to protect health, not to be easy to meet.
| Contaminant | In this water | EWG guideline | Times over |
|---|---|---|---|
| Arsenic | 4.70 ppb | 0.004 ppb | 1,174× |
| Haloacetic acids (HAA9) | 2.42 ppb | 0.06 ppb | 40× |
| Total trihalomethanes (TTHMs) | 5.69 ppb | 0.15 ppb | 38× |
| Bromodichloromethane | 1.08 ppb | 0.06 ppb | 18× |
| Haloacetic acids (HAA5) | 1.58 ppb | 0.1 ppb | 16× |
| Dibromochloromethane | 0.850 ppb | 0.1 ppb | 8.5× |
| Radium, combined (-226 and -228) | 0.30 pCi/L | 0.05 pCi/L | 6× |
| Chloroform | 1.89 ppb | 0.4 ppb | 4.7× |
| Bromochloromethane | 0.145 ppb | 0.06 ppb | 2.4× |
Source: EWG Tap Water Database, Lexington Park (MD0180007).
The fix for St. Mary's County water
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Reverse osmosis under your sink, for the contaminants EWG most often flags in the Coastal-plain aquifers.
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Common in U.S. tap water
St. Mary's County water quality FAQ
Is St. Mary's County, MD tap water safe to drink?
Public water in St. Mary's County meets federal EPA legal limits. But "legal" is not the same as "clean": across the Coastal-plain aquifers, independent testing routinely finds contaminants like arsenic (naturally occurring) and radium & radionuclides (naturally occurring) above EWG's stricter health guidelines. Enter your zip code above to see your exact utility's record.
What contaminants are common in St. Mary's County water?
For the Coastal-plain aquifers, the contaminants that most often exceed health guidelines are arsenic (naturally occurring), radium & radionuclides (naturally occurring), nitrates (agricultural runoff), PFAS (forever chemicals) and total trihalomethanes (where chlorinated). Your specific levels depend on your utility, which you can look up by zip code above.
Does Sanavia install reverse osmosis systems in St. Mary's County?
Yes. St. Mary's County is roughly 55 miles from our Arlington base and inside our service area. A Sanavia Hydra reverse osmosis system installs under your sink and reduces every contaminant class above, and we'll mail you a free water test kit.
Where does St. Mary's County tap water come from?
St. Mary's County's primary public water system is Lexington Park, which serves roughly 43,000 people. It sits in the Coastal-plain aquifers, so its water shares that region's contaminant profile. Larger areas may have more than one utility, so enter your exact zip code above for your own system's EPA record.
Is well water in St. Mary's County tested for these contaminants?
No. Private wells are not covered by municipal testing, so the responsibility falls on the homeowner. If your St. Mary's County home is on a well, nitrate and naturally occurring contaminants are worth testing for specifically, and a reverse osmosis system reduces them at the tap. We'll mail you a free water test kit either way.
Contaminants common in the Coastal-plain aquifers
Each of these is a plain-English guide to a contaminant class common in St. Mary's County and the surrounding Coastal-plain aquifers — what it is, the health context, and what actually removes it.
Reverse osmosis in St. Mary's County, MD
Lexington Park treats this water to the federal standard, yet the contaminants listed above still exceed EWG’s stricter health guidelines — Arsenic (naturally occurring), Radium & radionuclides (naturally occurring), Nitrates (agricultural runoff) and 2 more. Most of them either pass straight through treatment or form after the plant, in the pipes between it and the tap, which is why the fix has to be at the tap as well.
PFAS is the reason a carbon pitcher is not enough here. The compounds do not break down, they are measured in parts per trillion, and the federal limit that covers them is not enforceable until 2029. A reverse osmosis membrane is one of the few things at household scale that reduces them rather than passing them through.
Fit it yourself
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Full testing record for St. Mary's County · Find a local installer
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