Arsenic in Drinking Water
The Short Answer
Arsenic is a naturally occurring element that dissolves into groundwater straight out of the rock and soil it sits in. It’s a known human carcinogen, and the unsettling part is that it gives you nothing to go on: it has no taste, no smell, and no color in water, even at levels well above what’s considered safe. You cannot find it without a laboratory test.
It matters most if you’re on a private well, because arsenic comes from geology, not from pipes or pollution you can point to — and no one is testing your well but you. Whole regions of the US sit over arsenic-bearing bedrock: much of the West, parts of the Upper Midwest, and a good deal of New England. A well in one of those areas can be over the limit while the well down the road is fine.
If your water has arsenic, the fixes are real but specific. Reverse osmosis, distillation, certain certified adsorptive filters, and anion exchange can all remove it. A carbon filter — the kind in most pitchers and faucet units — does essentially nothing for arsenic. And there’s a catch most filter ads won’t mention: arsenic comes in two chemical forms, and the harder-to-remove one is exactly the form most likely to be in well water. So the answer is: test first, then match the fix to what you actually have.
The Full Picture
Where it comes from
Most arsenic in American drinking water isn’t pollution in the usual sense. It’s geological. Arsenic is a common element in the earth’s crust, and in many places it’s bound up in the bedrock and sediment that aquifers run through. Under the right underground chemistry, it dissolves out of the rock and into the groundwater — the same groundwater a well pulls from. That’s why arsenic doesn’t behave like a contaminant with a visible source: there’s no spill, no factory, no corroding pipe to blame. The source is the ground itself.
Most arsenic in American drinking water isn’t pollution in the usual sense. It’s geological. Arsenic is a common element in the earth’s crust, and in many places it’s bound up in the bedrock and sediment that aquifers run through. Under the right underground chemistry, it dissolves out of the rock and into the groundwater — the same groundwater a well pulls from. That’s why arsenic doesn’t behave like a contaminant with a visible source: there’s no spill, no factory, no corroding pipe to blame. The source is the ground itself.
This is the fact that catches well owners off guard. The instinct that rural water is clean water assumes contamination has to come from something nearby. Arsenic doesn’t. It can be high in a pristine, remote area with no industry for miles, simply because of what the local rock is made of. (Human activity can add to it — historic pesticides, mining, and some industrial processes have left arsenic behind in places — but across the US, natural geology is the dominant source in drinking water.)
Because it’s tied to geology, arsenic is intensely local. Two wells on the same street, drilled to different depths or into slightly different rock, can test very differently. There’s no way to reason your way to whether your water has it. There’s only testing.
The complication nobody mentions: arsenic has two forms
Here’s the part that makes arsenic genuinely different from most contaminants, and it’s worth understanding because it changes which fixes work.
In water, inorganic arsenic shows up in two chemical forms: arsenite, written As(III) or “trivalent,” and arsenate, written As(V) or “pentavalent.” They are the same element, but they behave like different problems. At the near-neutral pH of normal drinking water, arsenate carries a negative electrical charge, which makes it relatively easy for treatment systems to grab. Arsenite carries no charge at all — it drifts through water as a neutral molecule, which makes it much harder to catch. As a bonus insult, the uncharged, harder-to-remove form is also the more toxic of the two.
Now the cruel twist for well owners. In chlorinated city water, the chlorine tends to convert arsenite into the easier-to-remove arsenate automatically. In a private well — where the water is often “reducing,” meaning low in oxygen — arsenic is more likely to stay in the stubborn arsenite form. So the exact people most exposed to arsenic (well owners) are also the ones most likely to have the form that’s hardest to filter. This is why a filter that performs beautifully on city water can quietly underperform on a well, and why “certified to remove arsenic” is a claim you have to read closely.
The health picture — what’s actually established
Arsenic is not a maybe. The International Agency for Research on Cancer classifies it as a Group 1 human carcinogen — its highest, most certain category — and it’s classified as carcinogenic specifically in drinking water. That puts it in a different evidentiary place than contaminants where the science is still arguing about whether harm is real.
Long-term exposure is most strongly linked to cancers of the bladder, lung, and skin; there’s also evidence connecting it to kidney, liver, and prostate cancers. Those bladder and lung cancer findings are the basis for the current legal limit. Beyond cancer, chronic arsenic exposure is associated with cardiovascular disease (high blood pressure and heart disease), type 2 diabetes, skin changes (discoloration, thickening, and characteristic lesions), respiratory problems, and nerve damage. Exposure during pregnancy and early childhood has been linked to developmental and cognitive effects in children.
Two honest qualifiers. First, these are effects of long-term exposure — years, usually — not a single glass of water. Arsenic’s danger is the slow kind: most people drinking elevated arsenic feel completely fine, which is exactly why it goes unaddressed. Second, the strength of evidence varies by outcome. The cancer link is about as settled as environmental health gets; some of the cardiovascular, diabetes, and developmental associations are strong and consistent but still being refined. The honest summary is that arsenic is one of the better-established serious contaminants in drinking water — there’s no real scientific debate about whether it’s dangerous, only about the precise size of the risk at low doses.
Can You DIY This?
Partly — and arsenic is a good example of where “can I do it myself” has an honest ceiling.
The treatment hardware itself is DIY-friendly. An under-sink reverse osmosis system is a weekend install with no soldering, and certified arsenic cartridge filters drop into standard housings. If you’re handling just the drinking and cooking water at one tap, this is well within reach for a confident homeowner.
What is not a DIY judgment call is figuring out what you’re dealing with. Two things have to be done right, and getting them wrong is how people end up with a filter that doesn’t actually protect them:
First, test at a certified lab, not with a strip. Arsenic matters at parts per billion, and home test strips can’t reliably measure it. A proper lab test is the only thing that tells you whether you have a problem and how big it is.
Second, know your form, or treat as if you have the hard one. Because well water often contains the stubborn arsenite, you either need your lab to report the arsenic species, or you need a system rated to handle both forms — which, for adsorptive media and some setups, can mean adding a pre-oxidation step to convert the arsenite into the removable form. This is the part that trips up DIYers: a perfectly good filter sized for the easy form will let the hard form slip through, and you’d never know, because arsenic is invisible.
So the honest framing: the installation is DIY. The decision of which system, and the testing that drives it, is where you should not guess. Match the certified system to a real lab result, and re-test your treated water afterward to confirm it’s actually working.
What Actually Removes It
Four approaches genuinely work, and one popular one doesn’t.
Reverse osmosis (RO) is the most common point-of-use answer. It pushes water through a dense membrane that rejects dissolved arsenic, and it’s effective — especially on arsenate, the charged form. It removes arsenite too, just less efficiently, so pre-oxidation can help. RO is a strong choice when you have other issues to fix at the same time (high dissolved solids, for instance), at the cost of some water waste.
Adsorptive media filters — typically iron-based media such as granular ferric oxide/hydroxide, or activated alumina — work by chemically binding arsenic onto their surface as water passes through. The good ones remove both forms (better on arsenate), come in cartridge form for under-sink or whole-house use, and are a clean fit for a well. The media is used up over time and must be replaced on schedule; an exhausted cartridge stops protecting you.
Anion exchange swaps arsenic out of the water for harmless ions on a resin. It’s effective but with an important limit: it only removes the charged form, arsenate. On well water with the uncharged arsenite, it needs a pre-oxidation step first. It’s also sensitive to competing ions in the water (sulfate and others), which can shorten its effective life.
Distillation boils the water and condenses the steam, leaving arsenic behind. It works on both forms and needs no chemistry knowledge, but it’s slow, energy-hungry, and produces small batches — more of a niche solution.
What doesn’t work: carbon. This is the one to internalize, because it’s the most common filter people already own. Standard activated carbon — the media in most pitcher filters, faucet-mount units, and refrigerator filters — does essentially nothing for arsenic. Arsenic isn’t the kind of molecule carbon grabs. If your water has arsenic and you’re “filtering” it with a carbon pitcher, you are not treating it.
The phrase to look for when buying is NSF/ANSI certification for arsenic reduction — Standard 58 for reverse osmosis systems, Standard 53 for adsorptive/cartridge filters. And read the fine print: some products are certified to reduce pentavalent arsenic only. On a well, where the trivalent form is common, an “arsenic-certified” filter that only handles pentavalent can leave you exposed. Look for certification that covers both forms, or pair it with the pre-oxidation the manufacturer specifies. Without certification, an arsenic-removal claim is just marketing.
What the Rules Say — and What They Don’t
The federal limit for arsenic in drinking water is 10 parts per billion (ppb), set by the EPA. Public water systems have had to meet it since 2006. If you’re on city water, your utility is required to test for arsenic and report it; ask for your annual Consumer Confidence Report (CCR) or look it up online. If you’re on a private well, there is no legal standard that applies to you and no one testing it — the responsibility is entirely yours.
Now the part the number doesn’t tell you. That 10 ppb limit replaced an older standard of 50 ppb in 2001 — a fivefold tightening that happened only because the science caught up to how dangerous arsenic actually is. And even 10 ppb was not chosen as a “safe” level. The EPA’s health goal for arsenic — the level with no known risk — is zero. Ten was the number that balanced health against what it would cost water systems to get there. Some states have gone lower: California’s public-health goal for arsenic is 0.004 ppb, effectively zero, and it has been weighing whether to lower its enforceable limit below the federal 10.
This is the honest thread that runs through every contaminant on this site: the legal limit is drawn around the danger best understood at the time, tempered by cost — and the science keeps moving. For arsenic, an EPA review in 2025 reinforced that exposure at 10 ppb is still associated with increased bladder and lung cancer, heart disease, and diabetes. In other words, meeting the legal limit lowers your risk; it doesn’t zero it out. “Under 10” means “legal,” not “nothing to think about” — especially for the most vulnerable, like infants and pregnant women. If your water is in the single digits below 10, that’s a genuinely reasonable thing to want to reduce further, and you’re not being paranoid for doing so.
Around the World
Arsenic is the cause of what the World Health Organization has called the largest mass poisoning of a population in history — and it happened, tragically, because of a public-health success gone wrong.
In Bangladesh and parts of neighboring India, millions of people once relied on surface water that carried cholera and other deadly waterborne diseases. Starting in the 1970s, a massive effort drilled millions of shallow tube wells to give people “clean” groundwater instead. It worked against the microbes — but no one had tested for arsenic, and the sediments those wells tapped were naturally rich in it. The result was tens of millions of people exposed to arsenic over decades, with the cancers and skin lesions to show for it. It stands as the definitive proof that groundwater is not automatically safe water, and that the threat you can’t see is the one that hurts you.
Arsenic also travels through food, not just water — most notably rice, which takes up arsenic from soil and irrigation water far more readily than most crops. This is a live issue in arsenic-affected regions worldwide and a reason that, in places with high arsenic, the contamination problem extends well past the drinking glass.
Beyond the Kitchen Tap
A useful and slightly counterintuitive fact: your skin does not absorb arsenic in any meaningful amount. That means arsenic-contaminated water is safe for showering, bathing, washing dishes, and doing laundry. The danger is ingestion — drinking it, cooking with it, and making coffee, tea, ice, or baby formula with it. So unlike some contaminants, a point-of-use filter at the kitchen tap genuinely addresses most of your exposure; you don’t necessarily need to treat every faucet in the house.
The big exception is food you grow. If you irrigate a vegetable garden from an arsenic-contaminated well, plants can take it up from the water and soil — leafy greens and some root vegetables more than others, and rice most of all. For homesteaders growing their own food on a well that tests high for arsenic, the contamination isn’t just a drinking-water question; it reaches the garden and, through it, the dinner table. That’s worth knowing before you plan where your irrigation water comes from.
For livestock, the same well water you’d treat for yourself is worth thinking about for animals you raise for food or milk, though the specifics depend on levels and species. The throughline for anyone who is their own water utility: arsenic in the well is a whole-property fact, and “what do I drink” is only the first of the questions it raises.
The Deep End
For the chemically curious, arsenic’s behavior comes down to charge and oxidation state — and it’s a clean illustration of why “what’s in the water” and “what removes it” are different questions.
Inorganic arsenic in groundwater exists mainly in two oxidation states. Arsenite, As(III), exists at normal drinking-water pH (roughly 6.5–8.5) as arsenious acid, H₃AsO₃ — a molecule that stays electrically neutral. Arsenate, As(V), exists as the anions H₂AsO₄⁻ and HAsO₄²⁻ — negatively charged. That charge is everything for treatment. Anion exchange resins and many adsorptive media work by electrostatic attraction: they grab negatively charged ions. The charged arsenate sticks readily; the uncharged arsenite drifts past as if the filter weren’t there. This is the entire reason arsenite is the harder form to remove, and why the standard fix is a pre-oxidation step — adding an oxidant such as chlorine — that converts As(III) into As(V) so the rest of the system can catch it.
The geology behind which form you get is equally tidy. Oxygen-rich (oxidizing) water tends to hold arsenic as arsenate; oxygen-poor (reducing) water — common in deeper wells and certain aquifers — tends to hold it as arsenite. Chlorinated municipal water is oxidizing by design, so it leans toward the easy form. A reducing well leans toward the hard one. Geology and plumbing, not luck, decide which problem you have.
Two more wrinkles for the genuinely deep divers. Treatment by adsorption and anion exchange is degraded by competing ions — sulfate, phosphate, and silica all jockey for the same binding sites and can shorten a system’s life or reduce its effectiveness, which is why real-world performance depends on your whole water chemistry, not just the arsenic number. And at the biological level, arsenic’s carcinogenicity is thought to involve its interference with DNA repair and with the body’s methylation processes — part of why low-dose, long-term exposure can do damage that takes years to surface. None of this changes what you do about it. It just explains why testing for the specific situation, and matching a certified system to it, beats grabbing whatever filter is on the shelf.
Not sure if arsenic is in your water? You can’t taste it, smell it, or see it — and if you’re on a well, no one is checking but you. The only way to know is a certified lab test. → Test Your Water