New Hampshire Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in New Hampshire. They describe the area's modeled risk — not any single well.
Quick answer: Yes — arsenic occurs naturally in New Hampshire groundwater. USGS modeling estimates about 60,962 New Hampshire residents rely on private wells with arsenic above the EPA limit of 10 µg/L. The highest modeled exposure is in Rockingham County (~21,388), Hillsborough County (~17,240), Merrimack County (~7,219). For nitrate, about 0% of New Hampshire's ~396,000 private-well residents are modeled above the 10 mg/L limit. These are area estimates, not a test of your well — arsenic has no taste or smell, so only a certified lab test tells you your level.
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
0%
of the private-well population is modeled above the 10 mg/L nitrate limit
~0
people on wells estimated above the limit
~396,000
total residents on private wells statewide
Source: USGS machine-learning nitrate model, Ransom et al. (2022), domestic-well depth zone.
Find out what's actually in your well
New Hampshire has one of the higher modeled arsenic exposures in the country — but that number is an area estimate, not your well. Arsenic, nitrate and coliform bacteria are the three most common private-well problems, and none of them has a taste or smell. A certified lab test checks all of them at once; the CDC and EPA recommend testing every well at least once a year.
Basic screening only. Confirm arsenic, nitrate and bacteria with a certified lab.
Find a certified drinking-water labAmazon links are affiliate links — we may earn a commission at no extra cost to you.
Arsenic
Across New Hampshire, an estimated ~60,962 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Rockingham County (~21,388 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Rockingham County | ~21,388 | 16,663–27,013 |
| Hillsborough County | ~17,240 | 12,990–22,650 |
| Merrimack County | ~7,219 | 5,172–9,917 |
| Strafford County | ~3,708 | 2,726–4,983 |
| Cheshire County | ~2,982 | 2,202–4,004 |
| Belknap County | ~2,952 | 2,015–4,173 |
| Sullivan County | ~2,186 | 1,565–2,970 |
| Grafton County | ~2,003 | 1,390–2,827 |
| Coos County | ~807 | 583–1,076 |
| Carroll County | ~477 | 337–662 |
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Measured well tests by county
The USGS has published laboratory results from wells in 10 New Hampshire counties (at least 5 wells each). These are real measurements of other wells — domestic, public-supply and monitoring wells — not a test of yours. The tables rank counties by the share of sampled wells over each federal threshold.
No New Hampshire county with ≥ 5 sampled wells had any well over the nitrate limit.
Manganese: counties with the most wells > 300 µg/L (EPA health advisory)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Hillsborough County | 32% | 60 µg/L | 22 wells |
| Sullivan County | 29% | 150 µg/L | 17 wells |
| Belknap County | 23% | 48 µg/L | 13 wells |
| Cheshire County | 20% | 110 µg/L | 15 wells |
| Rockingham County | 20% | 97 µg/L | 10 wells |
| Merrimack County | 17% | 71 µg/L | 18 wells |
| Grafton County | 15% | 19 µg/L | 41 wells |
| Coos County | 15% | 26 µg/L | 27 wells |
| Carroll County | 9% | 15 µg/L | 33 wells |
Source: USGS ScienceBase 5f36baa782cee144fb3873e8 (manganese, pH, arsenic) and McMahon et al. (2019), Environ. Sci. Technol. 53:29–38 (nitrate, iron); aggregated by county with Census 2020 boundaries. USGS well samples; not a test of your well.
Acidic well water in New Hampshire (low pH)
31% of 575 USGS-sampled wells in New Hampshire had pH below 6.5. Acidic water corrodes copper pipes and fixtures and can leach lead and copper into your tap water — the usual fix is a whole-house acid neutralizer (calcite for mildly acidic water, soda-ash injection for very low pH). Counties with the highest share of acidic wells:
| County | Wells below pH 6.5 | Median pH | Sampled |
|---|---|---|---|
| Hillsborough County | 53% | 6.4 | 89 wells |
| Coos County | 44% | 6.6 | 41 wells |
| Carroll County | 42% | 6.8 | 67 wells |
| Merrimack County | 41% | 7.2 | 61 wells |
| Sullivan County | 34% | 7.0 | 29 wells |
| Cheshire County | 31% | 6.9 | 29 wells |
| Belknap County | 24% | 7.5 | 49 wells |
| Grafton County | 20% | 7.6 | 90 wells |
| Rockingham County | 8% | 7.6 | 98 wells |
| Strafford County | 5% | 7.6 | 22 wells |
Source: USGS ScienceBase 5f36baa782cee144fb3873e8 (pH). Measured other wells, not a test of yours.
Match the filter to your test result
Carbon pitchers and faucet filters do not remove arsenic or nitrate. Reverse osmosis is the point-of-use fix; a whole-house system handles the rest of the home.
APEC ROES-50 5-Stage Reverse Osmosis System
5-stage RO, ★ 4.7 — removes arsenic, nitrate, lead, fluoride and PFAS.
Check price on Amazon →Whole-house well water system
Treats every tap, shower and appliance — the right scale for sediment, iron, sulfur and bacteria on a private well.
Compare on Amazon →Waterdrop arsenic-reduction filters
Under-sink units tested for arsenic reduction — the point-of-use fix for a well over the limit.
Shop on Waterdrop direct →Affiliate links — we may earn a commission at no extra cost to you. Re-test after installing to confirm the system is working.
New Hampshire well water: FAQ
Is there arsenic in well water in New Hampshire?
Yes. USGS modeling estimates about 60,962 people in New Hampshire use private wells with arsenic above 10 µg/L, across 10 modeled counties. Arsenic comes from natural bedrock and varies well to well, even between neighbors, so a lab test is the only way to know your own level.
Which New Hampshire counties have the most arsenic in well water?
By modeled number of well residents over the arsenic limit: Rockingham County (~21,388), Hillsborough County (~17,240), Merrimack County (~7,219). Counties with more private-well users rank higher, so a smaller county can still have a high share of affected wells.
How do I remove arsenic from well water in New Hampshire?
Test first. If arsenic is present, a point-of-use reverse-osmosis system under the kitchen sink removes 90%+ of it; boiling does not remove arsenic (it concentrates it), and carbon pitchers and water softeners do not remove it either. Whole-house adsorptive arsenic media is used when levels are high. Re-test after installing and then annually.
Is nitrate a problem in New Hampshire well water?
USGS models about 0% of New Hampshire's private-well population (~0 people) above the 10 mg/L nitrate limit. Nitrate is highest in shallow wells near fertilized cropland, septic systems or livestock, and matters most for infants and pregnancy.
Do I need an acid neutralizer for my well in New Hampshire?
Acidic well water is common in New Hampshire. In USGS lab results from 575 wells across 10 New Hampshire counties, 31% had pH below 6.5. The most acidic sampled counties are Hillsborough County (53% of 89 wells), Coos County (44% of 41 wells), Carroll County (42% of 67 wells). You only need a neutralizer if your own well tests below about 6.5 — signs include blue-green stains on fixtures and pinhole leaks in copper pipe. A calcite (or calcite/Corosex) neutralizer tank handles mildly acidic water (roughly pH 6–6.5); very acidic water usually needs soda-ash injection. Test first-draw lead and copper too, since acidic water dissolves both from plumbing.
High manganese in New Hampshire well water — what should I do?
In USGS lab results from 196 wells across 9 New Hampshire counties, 18% were above the EPA's 300 µg/L health advisory for manganese and 48% were above the 50 µg/L level where it stains fixtures and laundry black or brown. Sampled counties with the highest share over the health advisory: Hillsborough County (32% of 22 wells), Sullivan County (29% of 17 wells), Belknap County (23% of 13 wells). If your own test is over 300 µg/L, don't use the water for infant formula and use another source for drinking and cooking until it is treated — high manganese is linked to neurological effects, especially in babies and children. Boiling does not remove it. For the whole house, an oxidizing filter (greensand, Birm or air-injection) handles manganese, usually alongside iron; a water softener can remove low levels. For drinking water, a point-of-use reverse osmosis system reduces manganese at the kitchen tap. Retest after treatment to confirm it worked.