How Much Arsenic Is Safe in Well Water?

The regulatory answer is 10 micrograms per liter (μg/L), which is the maximum contaminant level set by both the U.S. Environmental Protection Agency and the World Health Organization. But a growing body of research suggests that this threshold does not guarantee safety, with studies linking cancer risk and cognitive harm in children to concentrations below that number. If you rely on a private well, the situation is more complicated still: no federal agency monitors or enforces water quality standards for your supply, so knowing what is in your water and what to do about it falls entirely on you.

Where the 10 μg/L Standard Comes From and Why It May Not Be Enough

In 2001, the EPA published its Final Arsenic Rule, dropping the maximum contaminant level for public drinking water from 50 μg/L down to 10 μg/L.¹ That revision was a significant improvement, but the new limit was partly a compromise between health protection and the cost of compliance for water systems. The WHO adopted the same 10 μg/L guideline, acknowledging it was a practical target rather than a level at which all risk disappears.

Research published since then has reinforced those concerns. A review of U.S. studies found an association between arsenic-contaminated drinking water and skin cancer at concentrations below 10 μg/L.² A systematic review covering 35 years of evidence concluded that people exposed to even low-to-moderate arsenic levels (under 150 μg/L) faced elevated risk of developing or dying from lung cancer, with a clear dose-response pattern running well into the low range.³ New Jersey has set its own standard at 5 μg/L, half the federal limit, which hints at where the science is heading even if national policy has not caught up.

What Arsenic Does to Your Body Over Time

Arsenic in drinking water is not the kind of poison that makes you sick the same day you swallow it. At the concentrations found in most contaminated wells, the harm builds up gradually over years. The health effects span an unsettling range of organ systems.

Cancer is the most studied outcome. Arsenic in U.S. water supplies has been linked to cancers of the skin, lung, bladder, kidney, and liver, with population-level cancer risks estimated to be comparable to those from secondhand tobacco smoke and radon in homes.⁴ Skin changes often appear first: patches of darkened or lightened skin and rough, thickened spots called keratoses, sometimes in a distinctive “raindrop” pattern scattered across the torso.⁵ These skin lesions can be early warning signs of broader arsenic toxicity, and large cross-sectional studies in Bangladesh and West Bengal have documented them across hundreds of affected individuals even at moderate exposure levels.⁶

Beyond cancer and skin disease, chronic arsenic exposure is tied to cardiovascular problems and metabolic disorders. Epidemiological evidence shows links between how the body metabolizes arsenic and risk of carotid atherosclerosis, cardiovascular disease, metabolic syndrome, and diabetes.⁷ A separate review highlighted diabetes as a particular concern, noting it as one of the most prevalent chronic diseases associated with arsenic in drinking water.⁸

Children Are Especially Vulnerable

The evidence on children is some of the most troubling in the arsenic literature, because the effects show up at concentrations that would pass most regulatory screenings. A study of schoolchildren in Maine found that kids whose well water contained 5 μg/L or more of arsenic scored roughly 5 to 6 points lower on IQ tests compared to those with lower exposure, even after adjusting for the mother’s IQ, education level, and home environment.⁹ That 5 μg/L cutoff is half the EPA’s maximum contaminant level, which means the standard designed to protect public health may not adequately protect developing brains.

The timing of exposure matters. A prospective study following children from prenatal life to age 10 found that both early-pregnancy exposure and childhood exposure were independently and inversely associated with cognitive ability, with effects on verbal comprehension, perceptual reasoning, and processing speed appearing at urinary arsenic concentrations as low as about 50 μg/L.¹⁰ A population-based cohort study found that girls appeared more sensitive than boys, with arsenic exposure during pregnancy and early childhood associated with reductions in verbal and full-scale IQ scores in girls but not consistently in boys.¹¹

Children drink more water per pound of body weight than adults do, and their brains are still developing, which means the same arsenic concentration in a well poses a disproportionate risk to a six-year-old compared to a 40-year-old. If you have young children and your household relies on well water, the practical takeaway is that the 10 μg/L standard may not be protective enough, and testing with a goal of getting well below that number is worth the effort.

Where High-Arsenic Wells Are Most Common in the United States

Arsenic in groundwater is a geological problem, not an industrial one. It leaches out of naturally occurring minerals in rock and sediment, and its presence depends heavily on local geology and climate. In the continental U.S., researchers have mapped the areas where domestic wells are most likely to exceed 10 μg/L. The hotspots include New England (particularly Maine and New Hampshire), a band across the upper Midwest, the Southwest (Nevada, southern Arizona, southern and central California), and southern Texas.¹² Climate turns out to be one of the strongest predictors: drier regions tend to have higher arsenic concentrations, likely because less rainfall means less dilution and flushing of groundwater.

Well construction also plays a role. In the upper Midwest, arsenic is more common in domestic wells than in public water system wells, and it tends to be higher in wells with short screens set near a confining layer like glacial till.¹³ The depth and geology your well taps into matter a great deal, so two neighbors with wells drilled to different depths can have very different arsenic levels.

Globally, the problem is most severe in South Asia. In Bangladesh, parts of India, Vietnam, and Myanmar, arsenic-bearing sediments deposited by the Himalayan river systems can push groundwater concentrations above 1,000 μg/L, a hundred times the WHO guideline.¹⁴ The mechanism involves arsenic-bearing iron-oxide minerals in river sediments reacting with buried organic carbon, releasing arsenic into aquifers. Tens of millions of people in this region rely on wells tapping these contaminated aquifers.

Nobody Is Watching Your Private Well

If your water comes from a municipal system, it is tested regularly and must comply with the EPA’s 10 μg/L limit. If your water comes from a private well, no such requirement exists. The Safe Drinking Water Act, passed in 1974, regulates more than 170,000 public water systems but explicitly excludes the more than 13 million private wells in the United States.¹⁵ No federal policies exist regarding private wells; the authority is left entirely to the states.¹⁶

In practice, state and local requirements for private well testing are rare and inconsistent. Some states require a test at the time of a real estate transaction, but ongoing monitoring is almost unheard of. The responsibility to ensure your water is safe falls on you. This is not a hypothetical problem: an estimated 3 million people in the U.S. drink well water with arsenic above the EPA limit, and many of them do not know it.

How to Test Your Well

The gold standard for arsenic testing is a certified laboratory analysis. You collect a sample in a provided container, mail or deliver it, and receive results typically within a week or two. State health departments often maintain lists of certified labs, and some offer subsidized or free testing programs. This is the approach you should use at least once, and ideally every few years, because arsenic levels in a well are not static.

If you want a quicker read, field test kits exist. Their accuracy varies considerably. An evaluation of eight commercially available kits found that only two (the LaMotte and Quick II kits) provided both accurate and precise estimates of arsenic, while four others were either accurate or precise but not both, and two were neither.¹⁷ An earlier evaluation of two kits found strong overall correlation with laboratory measurements, with both kits placing samples in the correct concentration category or within one category roughly 97 to 99 percent of the time.¹⁸ The catch is that field kits tend to be least reliable right around the 10 μg/L boundary where accuracy matters most. In Bangladesh, one widely used kit correctly classified about 89 percent of wells relative to the WHO guideline, but all of its misclassifications at that cutoff were underestimates — meaning it called contaminated wells safe.¹⁹

A field kit can be useful as a screening tool, especially if you are testing many wells or want a rough check between lab tests. But if you are making decisions about treatment or about whether your family’s water is safe, a lab test is the only way to be confident.

Arsenic Levels in Your Well Can Change Over Time

One test, even a lab test, gives you a snapshot. Arsenic concentrations in groundwater fluctuate with the seasons and over longer periods. In New England, a U.S.-focused study found a statistically significant seasonal pattern: arsenic tended to be lower in the first half of the year (January through June) and higher in the second half (July through December), likely related to changes in groundwater levels, pumping stress, and the mixing of recharge water during wetter months.²⁰ Research from other parts of the world has found similar patterns, with dry-season concentrations running higher than wet-season concentrations.²¹

The implication is straightforward: a well that tests at 8 μg/L in March might test at 14 μg/L in September. If your result comes back close to the 10 μg/L limit, testing again in a different season gives you a more complete picture. And if you are in a region known for arsenic, periodic retesting every few years makes sense even if your initial result was reassuring.

Treatment Options and Why They Sometimes Fail

If your well tests high, several treatment technologies can bring arsenic down. The main approaches for home use are reverse osmosis, adsorptive media filters, and anion exchange systems. These can be installed as point-of-entry (POE) systems that treat all the water entering your house or as point-of-use (POU) systems that treat water at a single tap, typically the kitchen faucet.

Among simpler options, most tabletop pitcher filters do not remove arsenic effectively. Testing of several common pitcher brands found that only one, ZeroWater, reduced arsenic from 1,000 μg/L to below 3 μg/L, and it continued to perform even when competing dissolved solids were present.²² Standard Brita-style carbon filters will not do anything meaningful for arsenic.

The more sobering finding is that even dedicated treatment systems fail at meaningful rates. A study of private well households in Maine and New Jersey found treatment failure rates of 19 percent in Maine (water still above the 10 μg/L federal limit after treatment) and 16 percent in New Jersey (above that state’s stricter 5 μg/L limit). Failures were more common when the untreated water had higher arsenic concentrations or a higher proportion of arsenite (the trivalent form, which is harder to remove). Point-of-entry systems failed less often than point-of-use systems, and systems installed and maintained by professional vendors outperformed those handled by homeowners themselves.²³

That last point deserves emphasis. A treatment system is not a set-it-and-forget-it solution. Filters need to be replaced, media beds need regeneration, and the treated water should be retested periodically to confirm the system is still working. If you install a system yourself and never check it again, there is roughly a one-in-five chance it is no longer doing its job.

Arsenic From Food Can Add to What You Get From Water

Drinking water gets most of the attention, but it is not the only route of arsenic exposure, and ignoring the dietary piece can undermine even successful water treatment. Rice is the biggest culprit. Rice plants are unusually efficient at absorbing arsenic from soil and irrigation water, and the grain retains it through cooking.

In rural Bengal, where rice is a dietary staple, researchers found that a significant number of households drank arsenic-safe water but used arsenic-rich water to cook rice. As a result, roughly two-thirds of participants who cooked with contaminated water exceeded the threshold for cancer risk from rice consumption alone, compared to about 29 percent of those who cooked with clean water.²⁴ The cancer risk from arsenic in cooked rice was estimated to be an order of magnitude higher than the risk from drinking water alone. A separate study found that even when people drank water meeting the 10 μg/L standard, over a third exceeded the WHO’s provisional tolerable daily intake for inorganic arsenic because of their rice consumption.²⁵

This is primarily a concern for populations that eat large quantities of rice daily, which is more typical in South and Southeast Asia than in the U.S. or Europe. But even in the U.S., rice and rice products (cereal, rice milk, rice-based baby food) contribute to total arsenic intake, and if your well water is already pushing toward the limit, the dietary contribution could push your total exposure into a zone that matters. Using clean water for cooking, rinsing rice thoroughly, and cooking it in excess water that you drain off can reduce its arsenic content substantially.

Why the Same Well Can Affect Two People Differently

Not everyone who drinks arsenic-contaminated water develops the same health problems at the same rate. Part of this comes down to genetics. Your body processes arsenic through a methylation pathway involving an enzyme encoded by the AS3MT gene, and inherited genetic variation in the region of the genome containing this gene affects how efficiently you clear arsenic from your system.²⁶ People who metabolize arsenic less efficiently tend to retain higher levels of more toxic intermediate metabolites, which is associated with greater risk of cardiovascular disease.⁷

Nutritional status also plays a role. Folate, which is involved in methylation reactions throughout the body, appears to influence arsenic metabolism. People with adequate folate intake tend to convert arsenic to its less harmful forms more efficiently. Smoking and alcohol use can compound the damage. And as discussed earlier, children and pregnant women face heightened vulnerability for reasons related to body weight, developmental biology, and the timing of exposure during critical windows. Two people drinking from the same well for the same number of years can end up with very different outcomes, which is part of why setting a single “safe” number is so difficult.

The Two Forms of Arsenic in Your Water

Arsenic in groundwater exists in two main chemical forms, and which one you have affects both the health risk and how easy it is to remove. Arsenate is the oxidized form, and arsenite is the reduced form. Arsenite is considered the more toxic of the two and is also harder to filter out of water.²⁷ Many treatment technologies work by first converting arsenite to arsenate through an oxidation step, then removing the arsenate. If your treatment system does not include this pre-oxidation step and your water is dominated by arsenite, it may underperform. This is one of the reasons the Maine and New Jersey study found that wells with a higher arsenite proportion had higher treatment failure rates.

A standard water test from a certified lab will typically report total arsenic but may not distinguish between the two forms. If your total arsenic is elevated and you are shopping for a treatment system, asking the lab to speciate the arsenic (that is, measure arsenite and arsenate separately) can help you choose the right technology. Some systems handle both forms well; others are designed primarily for arsenate and will struggle with arsenite-dominated water.