privatewelldata / Reports / 2026-10-19

Well water data summary, week of October 19, 2026: arsenic and nitrate in 564 US counties, Quebec well results

3,114 of 27,614 sampled wells (11.3%) had a latest arsenic result above the EPA limit of 10 µg/L, and 3,448 of 55,432 (6.2%) were above the nitrate limit of 10 mg/L as N, across 564 counties in 13 states. Data downloaded 2026-10-18 to 2026-10-19 from the Water Quality Portal (USGS, EPA, NWQMC); Quebec results from Données Québec (PACES), downloaded 2026-10-17. Published 2026-10-19.

Key findings

• Coverage roughly doubled this week: 278 → 564 US counties and 6 → 13 states; Quebec 59 → 74 municipalities.
• Arsenic: Maine had the highest share of sampled wells above the limit (59.4% of 261). Among states with thousands of wells, Minnesota stood out at 18.6% of 5,914.
• Nitrate: California (9.6% of 16,344) and Pennsylvania (8.3%) had the highest shares. Lancaster County, Pennsylvania led all counties with 30+ wells at 39.5%.
• Quebec: manganese was the most frequent issue, above 0.12 mg/L in 27.1% of 1,295 sampled wells across 70 municipalities.
• Many counties had no wells above the limit in this data: 129 of 392 for arsenic and 260 of 558 for nitrate. That is not a guarantee for any single well.

Arsenic by state

Maine's figure rests on 261 wells in 9 counties. Wells in this data were not chosen at random, so treat a high share as a reason to test, not as a statewide rate. Arsenic comes from rock, has no taste or smell, and matters over years of drinking; see what to do if arsenic is high.

Nitrate by state

Nitrate usually comes from fertiliser, manure or septic systems and can change from season to season, which is why health agencies suggest testing for it every year. It is most urgent for homes with babies: see what to do if nitrate is high.

All states

Counties with the highest shares (30+ wells)

Quebec private wells

The PACES groundwater studies sampled 4,899 well results across 74 municipalities between 1969 and 2019. Manganese was above 0.12 mg/L most often; it matters most for babies. Highest shares (20+ wells): Rouyn-Noranda (46.2%), Val-d'Or (44.4%), Lévis (44.3%), Saint-Mathieu-d'Harricana (40.9%), Gatineau (37.5%). See what to do if manganese is high and the French version of the Quebec pages.

What this means for you

1. Look up your area: US county pages or Quebec municipalities. A high local share is a reason to test, a low one is not a clean bill of health.
2. Test every year for bacteria and nitrate, and at least once for arsenic. Build your test list.
3. Use a certified lab: find a lab or start from your state's official list.
4. If a result is high, read the guides before buying treatment, and check the salesperson checklist.

How we calculated this

For each well we keep its latest result (if a well was sampled several times on that day, the highest value), convert units (arsenic to µg/L, nitrate to mg/L as nitrogen), and count it as above the limit when that latest result is higher than the EPA maximum contaminant level. A county is published only with at least 10 wells; state figures are sums over published counties. The data mixes private, monitoring and research wells sampled between 1901 and 2026, and sampling was not random. Full rules: data rules. Sources and licences: data sources.

Download the data

• County summary (CSV)950 rows: state, county, analyte, wells, wells above limit, share, median, 90th percentile, dates, source URL
• State summary (CSV)13 states, arsenic and nitrate

Free to reuse with credit to privatewelldata.com and the Water Quality Portal (USGS, EPA, NWQMC). Quebec figures: Données Québec, CC BY 4.0.

Questions

All reports · A new summary is published every Monday.