Maryland Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in Maryland. They describe the area's modeled risk — not any single well.
This is a modeled probability for the area, not a measurement of your well. USGS statistical models estimate how many private wells in this area are likely to exceed the federal limit — they cannot tell you what is in your water. Two neighbors can have very different results. The only way to know your well is to test it.
Nitrate
6%
of the private-well population is modeled above the 10 mg/L nitrate limit
~43,300
people on wells estimated above the limit
~718,000
total residents on private wells statewide
Source: USGS machine-learning nitrate model, Ransom et al. (2022), domestic-well depth zone.
Arsenic
Across Maryland, an estimated ~41,277 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Wicomico County (~6,673 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Wicomico County | ~6,673 | 4,696–9,392 |
| Baltimore County | ~4,039 | 2,717–6,151 |
| Harford County | ~3,505 | 2,015–5,982 |
| Calvert County | ~3,020 | 1,942–4,651 |
| Anne Arundel County | ~2,558 | 1,935–3,424 |
| St. Mary's County | ~2,348 | 1,510–3,620 |
| Caroline County | ~2,017 | 1,337–3,004 |
| Charles County | ~1,789 | 1,200–2,667 |
| Cecil County | ~1,703 | 960–3,218 |
| Worcester County | ~1,671 | 1,171–2,356 |
| Queen Anne's County | ~1,421 | 930–2,130 |
| Talbot County | ~1,318 | 854–2,018 |
| Dorchester County | ~1,310 | 917–1,854 |
| Carroll County | ~1,227 | 848–1,855 |
| Allegany County | ~1,114 | 532–2,077 |
| Frederick County | ~1,098 | 774–1,637 |
| Somerset County | ~1,098 | 756–1,580 |
| Washington County | ~909 | 573–1,458 |
| Prince George's County | ~831 | 544–1,264 |
| Garrett County | ~698 | 326–1,440 |
| Howard County | ~541 | 376–794 |
| Kent County | ~249 | 191–325 |
| Montgomery County | ~140 | 96–209 |
1 additional Maryland county has lower modeled arsenic exposure (fewer than 100 estimated well residents over the limit).
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Treating nitrate in Maryland
Nitrate is not removed by carbon pitchers or faucet filters. Reverse osmosis is the treatment that brings it below the federal limit at your tap.
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