Private wells
Maryland well water: who's on a well and what to test for
People on private wells
1,199,100
Homes on wells
19.0%
Est. above arsenic limit
41,300
Every Maryland county
Sorted by people on private wells. Arsenic and nitrate are modeled for typical wells in the county, not measured at yours; depth is the median of domestic well records.
| Anne Arundel County | 178,500 | 30% | 3.8% | 0.5 | 190 ft |
| Baltimore County | 98,300 | 11% | 2.7% | 4.0 | 250 ft |
| Carroll County | 89,100 | 51% | 2.8% | 9.0 | 200 ft |
| Harford County | 85,300 | 32% | 2.2% | 4.2 | 200 ft |
| Frederick County | 77,800 | 29% | 3.3% | 3.1 | 250 ft |
| St. Mary's County | 67,700 | 60% | 3.2% | 0.0 | 375 ft |
| Montgomery County | 65,400 | 6% | 3.5% | 2.6 | 210 ft |
| Calvert County | 62,300 | 66% | 3.3% | 0.0 | 325 ft |
| Cecil County | 58,200 | 55% | 2.6% | 1.7 | 180 ft |
| Charles County | 57,600 | 35% | 2.5% | 0.0 | 420 ft |
| Wicomico County | 55,600 | 54% | 2.1% | 0.7 | 90 ft |
| Howard County | 48,300 | 13% | 3.4% | 5.3 | 245 ft |
| Washington County | 47,700 | 27% | 2.7% | 5.0 | 225 ft |
| Prince George's County | 36,300 | 4% | 2.3% | 0.1 | 345 ft |
| Queen Anne's County | 36,000 | 72% | 4.3% | 0.0 | 250 ft |
| Caroline County | 21,700 | 65% | 2.9% | 0.1 | 260 ft |
| Garrett County | 20,200 | 72% | 3.9% | 0.1 | 200 ft |
| Worcester County | 19,700 | 18% | 3.1% | 0.1 | 104 ft |
| Talbot County | 17,900 | 51% | 4.2% | 0.0 | 362 ft |
| Allegany County | 17,200 | 26% | 3.4% | 0.1 | 178 ft |
| Dorchester County | 15,200 | 49% | 3.2% | 0.1 | 300 ft |
| Somerset County | 12,100 | 59% | 2.8% | 0.0 | 160 ft |
| Kent County | 10,900 | 61% | 3.4% | 0.4 | 125 ft |
| Baltimore city | 80 | 0% | 3.2% | 1.3 | n/a |
Shading: red is among the highest counties nationally, amber the middle, green the lower range. Click a column to sort.
Most well users above the arsenic limit
- Wicomico County: about 6,700 people
- Baltimore County: about 4,000 people
- Harford County: about 3,500 people
- Calvert County: about 3,000 people
- Anne Arundel County: about 2,600 people
Highest modeled nitrate
- Carroll County: 9.0 mg/L average (limit 10)
- Howard County: 5.3 mg/L average (limit 10)
- Washington County: 5.0 mg/L average (limit 10)
- Harford County: 4.2 mg/L average (limit 10)
- Baltimore County: 4.0 mg/L average (limit 10)
Test first, then treat what the test finds
We may earn a commission from links below, at no cost to you. Picks follow the testing order, not who pays. How we make money.
1. Certified lab well test
Start here, every year
A mail-in kit analyzed by a certified lab measures bacteria, nitrate, arsenic, metals and more. Test strips cannot measure arsenic or uranium reliably.
2. Sediment pre-filter
Sand, silt or cloudy water
A 5-micron cartridge after the pressure tank catches sand, silt and rust before they reach fixtures or other filters. Cartridges are cheap and swap every few months.
3. Iron and sulfur filter
Orange stains or rotten-egg smell
Air-injection or oxidizing-media filters remove orange staining and rotten-egg smell. Size one from a lab result for iron, manganese and hydrogen sulfide.
4. UV disinfection system
Coliform bacteria in a test
Inactivates bacteria and viruses; usually for private wells or after boil notices.
5. Under-sink reverse osmosis
Arsenic, nitrate or uranium in a test
The most thorough option for drinking water: PFAS, lead, nitrate, arsenic and more. Look for NSF/ANSI 58.
6. Whole-house well filter
Several problems at once
Multi-stage sediment and carbon for every tap. Add UV if bacteria shows up, and choose stages from your lab results, not a guess.
Questions
Which Maryland counties rely most on wells?+
By share of homes: Queen Anne's County (72%), Garrett County (72%), Calvert County (66%), Caroline County (65%), Kent County (61%). By number of people: Anne Arundel County (178,500), Baltimore County (98,300), Carroll County (89,100).
Is arsenic a problem in Maryland wells?+
USGS estimates about 41,300 Maryland well users have water above the 10 microgram per liter arsenic limit that applies to public systems, the largest numbers in Wicomico County, Baltimore County, Harford County. Arsenic has no taste or smell, so only a lab test finds it.
Do I have to test my well when I sell a house in Maryland?+
We could not confirm a statewide rule for Maryland. Check with your county health department. Buyers can still make a lab test a condition of the offer.
How often should I test my well?+
No federal law covers private wells. EPA's drinking water limits apply to public water systems, so testing and treating a private well is up to the owner. EPA and CDC recommend testing at least once a year for coliform bacteria, nitrate, pH and total dissolved solids, and testing for arsenic, uranium, radon and other local concerns based on where you live.
Well-user counts, arsenic and nitrate figures are each a modeled estimate built from national datasets, not a count of tested wells, and well depths summarize drilling records rather than every well. Your own well can be very different from the county average, which is why the only answer for your home is a lab test. Sources: EPA Office of Research and Development, Water Source 2020 (private well estimates by census block); CDC Environmental Public Health Tracking, Well Water Index (from USGS nitrate and arsenic models); USGS, county-level domestic well population with arsenic above 10 micrograms per liter (Ayotte and others, 2017); USGS Synthesized Domestic Well Database for the Conterminous United States (SynDWell-US), McCarthy and others, 2025 (depth, where a county has 30+ usable records).