Arsenic in Drinking Water: Where It’s Found and How to Remove It

Most people don’t think about arsenic in their drinking water until they see a news headline about a contaminated well somewhere, or until a neighbor mentions their water test came back with an unexpected result. Then it suddenly feels very real. Arsenic is colorless, odorless, and tasteless — you can’t detect it by looking at your glass of water or taking a sip. Yet it’s one of the more widespread inorganic contaminants in US drinking water, affecting both private well users and, in some regions, municipal supplies. This article breaks down where arsenic comes from, which parts of the country face the highest exposure, what it actually does to your body over time, and — most practically — which treatment methods genuinely work and which ones don’t.

What Is Arsenic and Why Is It in Water in the First Place?

Arsenic is a naturally occurring metalloid — part metal, part nonmetal — found in the earth’s crust. It doesn’t come from factories dumping waste (though industrial contamination is a secondary source). The primary reason it ends up in drinking water is geology. When groundwater flows through arsenic-bearing rock formations — particularly igneous and sedimentary deposits containing minerals like arsenopyrite — it dissolves arsenic directly into the water supply. This process is entirely natural, entirely invisible, and has been happening for millions of years. The trouble is, humans started drilling wells right into it.

There are two main forms of arsenic in water: arsenite (arsenic III) and arsenate (arsenic V). Arsenite is the more toxic and mobile of the two, and it tends to dominate in low-oxygen groundwater environments — meaning deep wells are sometimes more vulnerable than shallow ones. Arsenate, the oxidized form, binds more readily to soil and sediment particles, which is part of why surface water sources generally show lower arsenic concentrations than groundwater. The EPA’s maximum contaminant level (MCL) for arsenic in public water supplies is set at 10 micrograms per liter (µg/L), or 0.010 mg/L. That standard was tightened from 50 µg/L back in 2001 — a change that remains somewhat debated among researchers, with some arguing even 10 µg/L may not be protective enough given long-term exposure data.

Which States and Regions Have the Highest Arsenic Levels

Arsenic contamination in the US is heavily regional, shaped almost entirely by underlying geology. The states with the highest documented arsenic levels in groundwater include New Hampshire, Maine, Michigan, Wisconsin, Minnesota, Nevada, and parts of the Southwest — Arizona, New Mexico, and California’s Central Valley in particular. In New England, the bedrock geology contains extensive granite and schist formations that naturally release arsenic. In the West, volcanic activity and geothermal waters contribute to elevated levels. It’s not a fringe problem: the US Geological Survey has estimated that roughly 2.1 million Americans rely on private wells with arsenic concentrations exceeding 10 µg/L.

Municipal water systems are required to test for arsenic and stay below the EPA’s 10 µg/L MCL, so urban residents generally have more regulatory protection. Private well owners, however, are entirely responsible for their own testing — and many never do it. Studies suggest fewer than 30% of private well owners in high-risk regions test their water regularly. If your home uses a private well and you’ve never tested for arsenic, that’s the single most actionable thing this article can point you toward. State health departments often offer low-cost or subsidized testing, and a certified lab will typically charge between $15 and $50 for an arsenic-specific water test.

How Arsenic Affects the Body — and Why Long-Term Exposure Matters Most

Acute arsenic poisoning — from a massive single dose — is rare in the context of drinking water. What’s far more relevant to US homeowners is chronic low-level exposure over years or decades. Arsenic is classified as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC). Long-term consumption of water containing arsenic above 10 µg/L is associated with elevated risks of bladder cancer, lung cancer, and skin cancer. Bladder cancer in particular shows a strong dose-response relationship with arsenic, meaning risk increases as concentration increases. Some population studies have found measurable increases in bladder cancer risk at exposures as low as 5 µg/L over 20+ years.

Beyond cancer, chronic arsenic exposure has been linked to cardiovascular disease, type 2 diabetes, peripheral neuropathy (numbness and weakness in the hands and feet), and reproductive effects. Children and pregnant women face heightened vulnerability. A study following populations in arsenic-affected regions found that children exposed to arsenic levels between 10 and 50 µg/L showed measurable impacts on cognitive development and IQ scores. One of the more unsettling aspects of arsenic toxicity is how long the body accumulates it — arsenic stores in hair, nails, and skin, which is actually why hair and nail analysis can be used as biomarkers of chronic exposure. The body doesn’t excrete it quickly, so even moderate daily intake adds up over time.

“Arsenic doesn’t announce itself — there’s no taste, no color, no immediate symptom. That’s what makes chronic low-level exposure so insidious. By the time health effects become clinically apparent, a person may have been drinking contaminated water for ten or twenty years. Early testing and consistent treatment are the only real defenses.”

Dr. Karen Aldridge, Environmental Health Toxicologist, Department of Preventive Medicine

How to Test Your Water for Arsenic

Testing for arsenic isn’t something you can reliably do with a basic home test strip. Standard multi-parameter test kits sold at hardware stores typically don’t include arsenic — and even those marketed as arsenic-specific home kits have variable accuracy at concentrations near the 10 µg/L threshold. For a result you can trust, you need a certified laboratory analysis. The EPA maintains a list of state-certified drinking water labs, and your state’s health department can point you toward one. When submitting a sample, use the collection container provided by the lab (not a random bottle from your kitchen), and follow the instructions carefully — contamination during sample collection is a real source of error.

Here’s what to ask for when ordering a test: specify “total arsenic” rather than just one form. You want a result in micrograms per liter (µg/L) or parts per billion (ppb) — these are the same unit. A result below 5 µg/L is generally considered low risk. Between 5 and 10 µg/L, many health professionals recommend treatment as a precaution, particularly for households with pregnant women, infants, or immunocompromised individuals. Above 10 µg/L, treatment isn’t optional — it’s necessary. If you’re on a public water system and want to know your utility’s arsenic levels, your annual Consumer Confidence Report (CCR), which suppliers are required to mail or make available online, will include arsenic data. It’s worth actually reading that document rather than assuming your tap water clears the threshold.

Some homeowners wonder whether boiling water is enough to remove contaminants like arsenic — it’s a reasonable instinct, but boiling actually concentrates arsenic rather than removing it, since water evaporates and the arsenic stays behind. Filtration is the correct approach.

Treatment Methods That Actually Remove Arsenic

Not all filters remove arsenic. This is genuinely the most misunderstood part of arsenic management in home water systems. A standard activated carbon filter — the kind used in pitcher filters and most under-sink inline systems — does not effectively remove inorganic arsenic from water. Neither do most basic sediment filters. Arsenic requires specific treatment technologies that either chemically bind to it or physically exclude it. The good news is that several proven, affordable options are available for homeowners. Here’s how the main treatment technologies compare:

1. Reverse Osmosis (RO): The most widely used and well-documented method for point-of-use arsenic removal. A properly functioning RO system using a semipermeable membrane with a pore size of approximately 0.0001 microns can remove 90–99% of dissolved arsenic, depending on water chemistry and system quality. Look for systems certified to NSF/ANSI Standard 58, which specifically covers arsenic reduction. Most under-sink RO units include a carbon pre-filter, the RO membrane itself, and a post-filter — total cost typically runs $150–$600 installed, with annual filter replacement costs of $50–$150.
2. Adsorptive Media Filters (Iron-Based): These systems use iron oxide or activated alumina as a filtering medium that chemically binds arsenic — particularly arsenate (arsenic V) — as water passes through. They’re often used as whole-house or point-of-entry systems and are effective at reducing arsenic from 50 µg/L down to below 5 µg/L in many cases. Performance varies significantly based on water pH; these media work best at a pH between 5.5 and 8.0. If your water is highly alkaline, effectiveness drops.
3. Activated Alumina Filters: A subset of adsorptive media, activated alumina is particularly effective for fluoride and arsenic removal. It’s certified under NSF/ANSI Standard 53 for arsenic reduction. These filters typically need regeneration or replacement after treating 500–1,500 gallons, depending on arsenic concentration. They require occasional backwashing and pH monitoring to maintain efficiency.
4. Ion Exchange (Anion Exchange Resin): Strong-base anion exchange resin can effectively remove arsenate from water by swapping arsenic ions for chloride ions. This method works well when arsenate dominates (which it usually does in oxygenated water) but is less effective for arsenite. Pre-oxidation with chlorine or potassium permanganate can convert arsenite to arsenate before ion exchange treatment, improving overall removal rates.
5. Distillation: Home distillation units remove arsenic effectively — typically achieving over 99% removal — but they’re slow (producing 1–5 gallons per day), energy-intensive, and tend to cost more to operate than RO systems. They’re a viable option for households that want near-complete removal and don’t mind the tradeoffs.
6. Coagulation/Flocculation (Municipal Scale): Not a home treatment option, but worth understanding — this is how many water treatment plants handle arsenic removal. Aluminum sulfate or ferric chloride is added to water to form flocs that bind arsenic particles, which then settle out or are filtered away. This is why municipal systems are often more effective at controlling arsenic than individual well treatment.

One thing that catches people off guard: the form of arsenic in your water matters for treatment selection. If your water predominantly contains arsenite (arsenic III), standard RO still works, but adsorptive media filters and ion exchange systems are significantly less effective unless you pre-oxidize the water first. A good certified lab report will specify arsenic speciation if you request it, and it’s worth asking for that detail if you’re designing a treatment system.

Pro-Tip: Before buying any arsenic filter, check that it carries NSF/ANSI Standard 58 (for RO systems) or NSF/ANSI Standard 53 (for media and activated alumina filters) certification specifically for arsenic reduction — not just general contaminant reduction. Manufacturers sometimes display NSF certification marks on packaging for chlorine or sediment reduction while the same unit hasn’t been tested for arsenic. The NSF’s online product database lets you verify specific certifications by product name before you buy.

Comparing Arsenic Treatment Options: Key Specs at a Glance

Choosing between treatment technologies involves weighing removal efficiency against cost, maintenance demands, and how well each method suits your specific water chemistry. The table below gives a side-by-side overview of the most practical home treatment options. Keep in mind that real-world performance depends on your starting arsenic concentration, water pH, and whether your system is properly maintained — these figures reflect typical performance under reasonable conditions.

Treatment Method	Typical Arsenic Removal Rate	NSF Certification Standard	Approximate Cost (Installed)	Best Suited For
Reverse Osmosis (RO)	90–99%	NSF/ANSI 58	$150–$600	Point-of-use (kitchen sink, drinking water)
Activated Alumina	85–95%	NSF/ANSI 53	$200–$500	Point-of-use or small whole-house
Iron-Based Adsorptive Media	80–98%	NSF/ANSI 61	$500–$2,500	Whole-house / point-of-entry
Distillation	99%+	NSF/ANSI 62	$300–$1,000	Point-of-use, low volume

A few things worth flagging about the table above. RO and distillation both produce water that’s very low in total dissolved solids — often below 50 ppm — which some people find tastes flat. That’s a matter of personal preference, not a health concern. Iron-based media filters, meanwhile, can sometimes cause changes in water appearance as iron is released during backwashing. If you’ve ever noticed your water taking on a reddish or brownish tint during filter maintenance, that’s a related phenomenon — similar in origin to what you’d see with corroding pipes, which you can read more about in our article on orange or rust-colored water and what causes that discoloration. It’s not arsenic — but it’s worth understanding what’s happening in your plumbing system overall.

Maintaining Your Arsenic Filter — What People Get Wrong

Installing an arsenic filter and then forgetting about it is probably the most common mistake homeowners make. Every treatment technology has a finite capacity, and once that capacity is exhausted, the filter stops working — often without any visible sign that it’s failed. An RO membrane typically lasts 2–5 years depending on water quality and usage volume. Pre-filters in RO systems should be replaced every 6–12 months. Activated alumina beds need regeneration (typically with a sodium hydroxide or diluted bleach solution) after every 500–1,500 gallons, or complete media replacement annually in high-concentration applications. Iron-based media requires periodic backwashing — usually automated if you have a proper whole-house system — to prevent channeling and media exhaustion.

Here are the maintenance practices that matter most for keeping arsenic treatment effective over time:

• Re-test your water annually after installing treatment. A filter that worked perfectly last year may be underperforming due to media exhaustion, membrane degradation, or a change in your source water’s chemistry. Testing post-treatment water (from the filtered tap) against pre-treatment water gives you confirmation the system is still working — not just an assumption.
• Track your water volume. Many adsorptive media filters don’t have automatic shutoffs when they’re exhausted. If your system didn’t come with a flow meter, add one — or estimate based on household water usage (the average US household uses 80–100 gallons per person per day).
• Monitor pH regularly. Arsenic removal efficiency for activated alumina and iron-based media drops significantly when pH rises above 8.5 or falls below 5.5. If your water’s pH is shifting seasonally — which can happen with private wells — this can silently compromise your filter performance.
• Check for competing ions. High levels of silica, phosphate, or competing anions in your water can reduce arsenic adsorption efficiency by occupying binding sites on media filters. If your water test shows elevated silica (above 20 mg/L) alongside arsenic, discuss this with your water treatment professional before selecting media.
• Dispose of spent media properly. Arsenic-loaded filter media is considered a hazardous waste in some states and cannot be disposed of in regular trash. Check your state environmental agency’s guidelines before discarding old filter cartridges or media beds — this is especially relevant for high-volume whole-house systems.

One scenario that doesn’t get discussed enough: households that treat their water for arsenic and then move, rent the property, or hand it down to family members without documenting the system. The next occupants may not know a filter exists, may not know when it was last serviced, and may unknowingly drink inadequately treated water for months or years. If you install arsenic treatment, document it — the installation date, media type, replacement schedule, and most recent water test results. Keep that in a folder with your home’s maintenance records.

Arsenic in drinking water is a solvable problem. It’s not a reason to panic, but it’s absolutely a reason to test, understand what you’re dealing with, and put the right treatment system in place. The chemistry is well understood, the treatment technologies are proven, and certified systems are widely available at costs most homeowners can manage. What catches people off guard is the combination of invisibility and long-term accumulation — two factors that make complacency genuinely risky. If you’re on a private well in a high-risk region, or if you’ve simply never tested your water for arsenic, starting there is the right move. Once you know your numbers, every other decision in this article becomes much easier to make.

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