Arsenic in Private Wells: What Maine and New Hampshire Homeowners Should Know

Students in Maine and New Hampshire school communities have spent the past several years carrying sample bottles home to their kitchen taps. The program is All About Arsenic, run out of the MDI Biological Laboratory in Bar Harbor, and the students collected 3,070 drinking water samples. When the results were published in Environmental Health Perspectives in 2024, one figure stood out. Among samples confirmed to come from dug, driven or drilled wells, 223 households (10.4 percent) landed between 5 and 10 parts per billion of arsenic. The authors called that band “levels deemed safe for drinking in one state but not another.”

Same bedrock. Sometimes the same aquifer. Different verdict, depending on which side of the Piscataqua the kitchen sink sits on.

Nobody Is Watching Your Well

Maine CDC says it in one sentence: “Private wells are not regulated; property owners are responsible for testing and treating the water from private wells.” EPA says the same at the federal level, noting that private well water is “not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws.” Around 15 percent of the country, more than 43 million people, drinks from one.

Here the share runs much higher. USGS reports that roughly half the population of Maine gets its drinking water from domestic wells, about 75 percent of them drilled into bedrock. NHDES says nearly half of New Hampshire residents use a private well at home. New Hampshire has no testing requirement either, though the agency notes that “mortgage lenders and some towns may require testing.”

A municipal customer in Portland or Manchester has an agency sampling the water and publishing what it finds. A neighbor two miles out on a dirt road has whatever they decide to pay for. That gap runs through plenty of household environmental exposures in Maine and New Hampshire, and it’s why arsenic can sit in a kitchen tap for decades unnoticed.

It’s in the Bedrock

The arsenic here isn’t a spill or a factory legacy. It’s a mineral in the rock the water moves through.

A 2003 study in Environmental Science and Technology mapped arsenic across eastern New England and found the highest concentrations in wells drawing from metasedimentary bedrock, a formation found primarily in Maine and New Hampshire. Nearly 30 percent of wells in those aquifers produced water above 10 micrograms per liter. The researchers estimated that about 103,000 people on private wells in the aquifers they studied could be drinking water above the federal limit.

A modeling study three years later put the regional figure at 20 to 30 percent of private wells in eastern New England above that standard. It also delivered the finding homeowners least want to hear. “Due to the complexity of the fractured bedrock aquifers in the region, well depth and related variables also are not significant predictors.” Going deeper isn’t a dependable fix, and a newer well isn’t automatically a cleaner one.

NHDES puts the practical version plainly: “due to our highly fractured bedrock, even wells within short distances can present very different water quality.” Your neighbor’s clean result tells you nothing reliable about your own water. Maine CDC is blunter still. “There are wells high in arsenic in all parts of Maine,” the agency writes, and “1 in 10 wells in Maine has too much arsenic.”

The Numbers, and Their Limits

How firm is that 1 in 10? Maine’s own data team supplies the caveat. The Maine Tracking Network warns that its well figures “are subject to selection bias, as they are based only on well owners who decide to test their wells,” and that in areas where arsenic clusters, testing rates run higher, so the share exceeding the state guideline “is likely to be biased upward.” Read the state figures as a map of where to look, not as a census.

New Hampshire’s two public numbers don’t match each other. NHDES estimates that 25 to 30 percent of private residential wells have arsenic at 5 ppb or higher, and that roughly 116,000 private well users statewide drink water above that level. NH DHHS Public Health Laboratories tells residents that “1 out of 5 wells in NH may contain dangerous amounts of ARSENIC” and that “4 out of 10 homes in NH get their water from private WELLS.” The gap is mostly threshold, 5 ppb against 10, and vintage.

The student samples land in the same neighborhood. Of the 3,070 collected, 453 (14.8 percent) exceeded the 10 ppb federal limit Maine uses and 752 (24.5 percent) exceeded New Hampshire’s 5 ppb limit. Those came from participating school communities, not a random draw, so it’s another self-selected set, though the town-level patterns tracked the state data.

What the Health Research Found

Two studies out of this region measure different things.

The first looked at children. Researchers tested home well water and gave IQ tests to 272 children in grades 3 through 5 across three Maine school districts. Children whose water measured at or above 5 micrograms per liter scored roughly 5 to 6 points lower on Full Scale IQ than children below that level, after adjusting for maternal IQ and education, home environment, school district and number of siblings. Those kids had lived in their current homes an average of 7.3 years, about three quarters of their lives.

The second looked at adults. Bladder cancer incidence in Maine, New Hampshire and Vermont runs about 20 percent higher than in the United States overall, and a case-control study of 1,213 cases and 1,418 controls across the three states set out to understand why. Participants in the highest cumulative arsenic exposure group carried roughly twice the risk of the lowest-exposure quartile, an odds ratio of 2.24 with a 95 percent confidence interval of 1.29 to 3.89. The trend ran strongest among people who had used shallow dug wells before 1960, when arsenical pesticides were widely used in the region.

That last detail is easy to misread. Shallow dug wells and modern drilled bedrock wells draw from different sources, and the 2006 modeling study tested variables for historic pesticide use and found they “do not improve the model, indicating that this source does not significantly contribute to current arsenic concentrations.” The arsenic in a bedrock well today is geologic. The old dug-well exposure is a separate chapter, not the same story told twice.

Getting a Well Tested

Maine CDC recommends testing a private well every year for coliform bacteria, E. coli, nitrates and nitrites, and every three to five years for arsenic along with radon, uranium, lead, manganese and fluoride. NHDES recommends the same three-to-five-year cycle, and NH DHHS Public Health Laboratories advises every three years for arsenic. All of those are recommendations. None is a requirement.

How many households follow them isn’t well documented. The 2024 Environmental Health Perspectives paper cites earlier research finding that 41 percent of surveyed Maine well owners had never tested for arsenic at all.

Two things are worth knowing going in. Maine’s Maximum Exposure Guideline for arsenic is 10 micrograms per liter and the state lab reports down to 0.5, so water well under the guideline still comes back as a number rather than a blank. And if a New Hampshire well tests above 5 ppb, NHDES recommends a follow-up test specifically for Arsenic-III, because the two chemical forms behave very differently in a treatment system.

Arsenic isn’t the only thing worth putting on the sample form. Maine groundwater has had its own reckoning with PFAS in drinking water, and the logic carries over. One result from your own tap beats any amount of inference about the neighborhood.

What Removes Arsenic, and What Doesn’t

Start with what doesn’t work. Dartmouth’s Toxic Metals Superfund Research Program states it without hedging. “Carbon-based water pitcher filters do not remove arsenic.” And “Boiling your water does not remove arsenic.” The pitcher in the fridge door is not doing this job.

Certified pitchers deserve a close read. NHDES warns that “even pitchers certified for arsenic removal by the National Sanitation Foundation are required only to reduce arsenic from 50 ppb down to below 10 ppb,” and says it “has identified only one model that reliably removes arsenic from high levels down to less than 5 ppb.” Dartmouth advises looking for certification specifically for arsenic, and a system rated to bring water to 5 ppb or lower.

Full treatment systems work, with an asterisk. Researchers followed 156 private-well households in southern-central Maine with treatment installed, mostly point-of-use reverse osmosis. Median arsenic fell from 71.7 to 0.8 micrograms per liter. But 19 percent of the Maine systems failed, meaning nearly one in five treated households still had water above the 10 microgram federal limit. Point-of-entry systems failed less often than point-of-use units, and systems installed and maintained by vendors failed less often than those left to homeowners. Treatment is equipment with a maintenance cycle, not a purchase you make once.

Reverse osmosis is also where the Arsenic-III question returns. NHDES reports that it removes about 95 percent of Arsenic-V but only about 60 percent of Arsenic-III, which is why the agency wants that second test after a result above 5 ppb.

One worry you can put down: the shower. NHDES, citing EPA, says that “dermal absorption (skin exposure) of arsenic is not a significant exposure path; therefore, washing and bathing do not pose a known risk to human health.” The exception it flags is untreated water above 100 ppb, where regularly swallowing even small amounts from a bathtub or bathroom tap can meaningfully raise risk.

Five Parts Per Billion, or Ten

The federal limit for arsenic in public drinking water was 50 ppb from 1975 until 2001. In January of that year, EPA set a health goal of zero arsenic and adopted an enforceable limit of 10 ppb, a figure it reached “based on balancing treatment costs in public water systems with public health benefits.” Ten was never a line between safe and unsafe. It was a negotiated number.

New Hampshire decided it could do better. A 2018 bill ordered a review, a 2019 bill directed adoption, and on July 1, 2021 a 5 ppb maximum contaminant level took effect, with the same figure adopted as an enforceable Ambient Groundwater Quality Standard. “New Hampshire is the second state, after New Jersey, to adopt an arsenic MCL that is more protective than the federal MCL of 10 ppb,” NHDES wrote at the time.

Then comes the catch. That 5 ppb standard governs public water systems. It doesn’t reach private wells, in New Hampshire any more than in Maine. Nearly half the state’s residents draw from a source it doesn’t touch.

Which brings it back to the students with the sample bottles, and the households whose water landed in the space between the two limits. Those families aren’t in a regulatory gray zone. There’s no regulation covering them at all. They’re in an informational one, and the only number that exists for a private well is the one somebody paid to go find.

Sources

• Maine CDC, Have you tested your well water for arsenic? (March 2020): maine.gov/dhhs/mecdc
• Maine CDC Drinking Water Program, Private Well Water: maine.gov/dhhs/mecdc
• Maine Tracking Network, Private Well Water and Data Limitations: data.mainepublichealth.gov/tracking
• USGS Data Series 1125, Culbertson et al. (2020), prepared with US CDC and Maine CDC: pubs.usgs.gov
• US EPA, Private Drinking Water Wells: epa.gov/privatewells
• NHDES, More Protective Arsenic Standard Will Reduce Risk for Many (March 2021): des.nh.gov
• NHDES fact sheet WD-DWGB-3-2, Arsenic in New Hampshire Well Water: des.nh.gov
• NHDES, Private Wells: des.nh.gov/water/drinking-water
• NH DHHS Public Health Laboratories, arsenic fact sheet: dhhs.nh.gov
• Ayotte et al., Arsenic in groundwater in eastern New England, Environmental Science and Technology 37(10) (2003): pubmed.ncbi.nlm.nih.gov
• Ayotte et al., Modeling the probability of arsenic in groundwater in New England as a tool for exposure assessment, Environmental Science and Technology 40(11) (2006): pubmed.ncbi.nlm.nih.gov
• Wasserman et al., A cross-sectional study of well water arsenic and child IQ in Maine schoolchildren, Environmental Health 13(1) (2014): pubmed.ncbi.nlm.nih.gov
• Baris et al., bladder cancer, drinking water and arsenic in northern New England, Journal of the National Cancer Institute 108(9) (2016): pubmed.ncbi.nlm.nih.gov
• Yang et al., Reduction in drinking water arsenic exposure and health risk through arsenic treatment among private well households in Maine and New Jersey, Science of the Total Environment 738 (2020): pubmed.ncbi.nlm.nih.gov
• Taylor et al., A Mixed Methods Approach to Understanding the Public Health Impact of a School-Based Citizen Science Program to Reduce Arsenic in Private Well Water, Environmental Health Perspectives 132(8) (2024): pmc.ncbi.nlm.nih.gov
• Dartmouth Toxic Metals Superfund Research Program, Arsenic Water Treatment: sites.dartmouth.edu/arsenicandyou

This article is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare provider before making any health decisions.