privatewelldata / States / Pennsylvania / Nitrate
Nitrate in well water in Pennsylvania
8% of 6,478 sampled wells in Pennsylvania had nitrate above the EPA limit of 10 mg/L (as nitrogen) (latest result per well, 62 counties, samples 1901–2023). The highest county share was Lancaster County: 39% of 621 wells. Data updated 2026-10-18.
of sampled wells above 10 mg/L (as nitrogen)
wells with a result
counties with data
counties where 1 in 4+ wells were above
Nitrate in wells most often comes from fertilizer, manure and septic systems, so shallow wells near fields or septic fields are more exposed, and levels can change with the seasons. Only a lab test shows it. Federal limits apply to public water systems; a private well is the owner's responsibility to test (EPA).
% of sampled wells above the EPA limit (latest result per well; counties with 10+ wells). Source: USGS/EPA Water Quality Portal, 2026-10-18 · privatewelldata.com
Nitrate by county in Pennsylvania
| County | Above 10 mg/L (as nitrogen) | Median | 90th pct. | Years |
|---|---|---|---|---|
| Lancaster County | 39% of 621 | 8.27 mg/L as N | 20 | 1925–2022 |
| Blair County | 23% of 73 | 2.711 mg/L as N | 14.3 | 1933–2017 |
| Northampton County | 21% of 29 | 4.73 mg/L as N | 14.231 | 1925–2019 |
| Northumberland County | 17% of 69 | 2.7 mg/L as N | 11.8 | 1930–2023 |
| Cumberland County | 13% of 172 | 4.75 mg/L as N | 11.001 | 1925–2023 |
| Mifflin County | 13% of 46 | 0.86 mg/L as N | 10.8 | 1934–2018 |
| Philadelphia County | 11% of 195 | 0.8 mg/L as N | 12 | 1929–1986 |
| Chester County | 10% of 858 | 3.2 mg/L as N | 10.3 | 1925–2023 |
| Franklin County | 10% of 67 | 1.1 mg/L as N | 9.488 | 1925–2023 |
| York County | 10% of 201 | 3.614 mg/L as N | 10 | 1925–2023 |
| Lebanon County | 9% of 173 | 2.5 mg/L as N | 9.714 | 1925–2023 |
| Washington County | 8% of 12 | 0.124 mg/L as N | 0.66 | 1926–1997 |
| Berks County | 8% of 191 | 2.733 mg/L as N | 9.27 | 1925–2023 |
| Adams County | 7% of 98 | 2.711 mg/L as N | 8.8 | 1925–2023 |
| Fayette County | 7% of 28 | 0.2 mg/L as N | 4.518 | 1926–2010 |
| Clinton County | 6% of 82 | 0.555 mg/L as N | 5.941 | 1934–2023 |
| Bedford County | 6% of 83 | 0.407 mg/L as N | 7.906 | 1933–2023 |
| Fulton County | 5% of 21 | 0.07 mg/L as N | 6.2 | 1933–2023 |
| Erie County | 4% of 109 | 0.1 mg/L as N | 5.28 | 1927–2004 |
| Perry County | 4% of 85 | 1.107 mg/L as N | 6.35 | 1934–2018 |
| Montgomery County | 4% of 114 | 2.795 mg/L as N | 5.9 | 1925–2023 |
| Greene County | 3% of 58 | 0.315 mg/L as N | 5.6 | 1926–1996 |
| Luzerne County | 3% of 68 | 0.415 mg/L as N | 5.7 | 1930–2023 |
| Centre County | 3% of 77 | 2.8 mg/L as N | 7.38 | 1934–2023 |
| Lehigh County | 2% of 161 | 2.26 mg/L as N | 6.18 | 1925–2023 |
| Snyder County | 2% of 41 | 0.203 mg/L as N | 3.388 | 1934–2022 |
| Schuylkill County | 2% of 86 | 0.203 mg/L as N | 4.9 | 1931–2023 |
| Delaware County | 2% of 43 | 2.6 mg/L as N | 5.4 | 1925–2020 |
| Wyoming County | 2% of 47 | 0.361 mg/L as N | 3.3 | 1930–2023 |
| Bucks County | 2% of 244 | 1.804 mg/L as N | 5.65 | 1925–2023 |
| Clearfield County | 2% of 50 | 0.023 mg/L as N | 1.107 | 1901–2023 |
| Columbia County | 1% of 80 | 1.652 mg/L as N | 5.64 | 1930–2023 |
| Clarion County | 1% of 104 | 0 mg/L as N | 2.711 | 1970–2019 |
| Bradford County | 1% of 213 | 0.06 mg/L as N | 2.4 | 1935–2023 |
| Dauphin County | 1% of 121 | 1.8 mg/L as N | 6.054 | 1931–2023 |
| Lycoming County | 1% of 130 | 0.525 mg/L as N | 4.8 | 1935–2023 |
| Indiana County | 0% of 300 | 0.056 mg/L as N | 1.2 | 1964–2014 |
| Wayne County | 0% of 150 | 0.48 mg/L as N | 1.931 | 1930–2023 |
| Pike County | 0% of 149 | 0.09 mg/L as N | 1.3 | 1930–2023 |
| Susquehanna County | 0% of 119 | 0.23 mg/L as N | 1.2 | 1930–2023 |
| Mercer County | 0% of 100 | 0.09 mg/L as N | 0.93 | 1928–2023 |
| Potter County | 0% of 92 | 0.305 mg/L as N | 1.73 | 1935–2023 |
| Warren County | 0% of 90 | 0.11 mg/L as N | 1.2 | 1928–2023 |
| Tioga County | 0% of 76 | 0.17 mg/L as N | 1.98 | 1935–2023 |
| Huntingdon County | 0% of 58 | 0.09 mg/L as N | 5.5 | 1933–2018 |
| Jefferson County | 0% of 55 | 0.023 mg/L as N | 0.92 | 1929–1997 |
| Somerset County | 0% of 49 | 0.09 mg/L as N | 0.79 | 1933–2023 |
| Lackawanna County | 0% of 43 | 0.53 mg/L as N | 2.08 | 1930–2022 |
| Allegheny County | 0% of 35 | 0.16 mg/L as N | 1 | 1926–2023 |
| Montour County | 0% of 33 | 0.03 mg/L as N | 1.5 | 1930–2023 |
| Crawford County | 0% of 32 | 0.09 mg/L as N | 0.52 | 1928–2023 |
| Lawrence County | 0% of 29 | 0.05 mg/L as N | 0.68 | 1928–2023 |
| Sullivan County | 0% of 29 | 0.332 mg/L as N | 1.31 | 1935–2023 |
| Cambria County | 0% of 28 | 0.237 mg/L as N | 3 | 1933–2017 |
| Monroe County | 0% of 28 | 0.325 mg/L as N | 1.421 | 1930–2023 |
| Juniata County | 0% of 27 | 1.651 mg/L as N | 8.042 | 1934–2023 |
| Westmoreland County | 0% of 25 | 0.23 mg/L as N | 1.03 | 1926–2023 |
| Union County | 0% of 21 | 1.471 mg/L as N | 5.286 | 1934–2023 |
| Butler County | 0% of 17 | 0.07 mg/L as N | 0.294 | 1926–1997 |
| Beaver County | 0% of 16 | 0.735 mg/L as N | 3.84 | 1928–2019 |
| Cameron County | 0% of 16 | 0.034 mg/L as N | 0.59 | 1935–2018 |
| McKean County | 0% of 11 | 0.03 mg/L as N | 0.392 | 1935–2019 |
No rows match these filters. Try fewer words or clear the filters.
Download this table (CSV) · public-domain source data: Water Quality Portal (USGS, EPA, NWQMC). How we calculate these numbers.
How Pennsylvania compares
Share of sampled wells above the EPA nitrate limit in each state we cover (states with 100+ wells).
| # | State | Above 10 mg/L (as nitrogen) |
|---|---|---|
| 1 | California | 10% of 16,344 |
| 2 | Pennsylvania | 8% of 6,478 |
| 3 | Minnesota | 6% of 7,259 |
| 4 | North Carolina | 5% of 1,964 |
| 5 | New York | 5% of 6,236 |
| 6 | Massachusetts | 4% of 2,556 |
| 7 | New Jersey | 4% of 3,915 |
| 8 | Wisconsin | 4% of 3,666 |
| 9 | Virginia | 3% of 1,394 |
| 10 | Maine | 2% of 726 |
| 11 | Michigan | 2% of 1,970 |
| 12 | Ohio | 1% of 1,239 |
| 13 | New Hampshire | 0% of 1,685 |
Where to test: official lab list for Pennsylvania
About this data
- Each well counts once, using its latest nitrate result in the Water Quality Portal (siteType = Well), downloaded 2026-10-18.
- Results reported as "not detected" count as below the limit. Results far outside the plausible range are withheld for review, not shown.
- Sampled wells include monitoring and research wells, not only household wells, and are not a random sample, so the share is a guide to local risk, not a rate for all homes.
- The limit is the EPA maximum contaminant level for public water systems (EPA).
Questions
Is nitrate common in Pennsylvania well water?
In our data, 539 of 6,478 sampled wells in Pennsylvania (8%) had a latest nitrate result above the EPA limit of 10 mg/L (as nitrogen), across 62 counties. That ranks Pennsylvania 2 of 13 states we cover for this share.
Which Pennsylvania counties have the most wells above the nitrate limit?
Among counties with at least 10 sampled wells: Lancaster County (39%, 245 of 621 wells); Blair County (23%, 17 of 73 wells); Northampton County (21%, 6 of 29 wells).
Does this tell me whether my well is safe?
No. These are past results from wells sampled by USGS, EPA and state programs, including monitoring wells, and levels differ from well to well. The only way to know your water is to test your own well at a certified lab.
How do I test my well for nitrate in Pennsylvania?
Order a nitrate test from a lab on Pennsylvania's official certified-lab list (linked on this page) and ask for its bottles and sampling instructions. Our test helper suggests what else to test for at the same time.
What should I do if my nitrate result is above 10 mg/L (as nitrogen)?
Retest to confirm, use another water source for drinking and cooking while you sort it out, and look at certified treatment options. Our guide walks through each step with sources.