What Arsenic Is and Why It Matters in Well Water

Arsenic is a naturally occurring metalloid found in the earth's crust. In residential well water, almost all arsenic contamination is geological, not man-made. Groundwater moving slowly through arsenic-bearing bedrock dissolves trace amounts of the element, which then travel up the well casing into the home's plumbing.

The element has been used historically in pesticides, wood preservatives (CCA-treated lumber), and industrial processes, and those sources can contribute to local groundwater arsenic in agricultural areas. But for the typical private well in New England, the Mid-Atlantic, the Upper Midwest, or the Southwest, the arsenic in your water came out of the rock, not out of a factory.

What makes arsenic uniquely dangerous as a drinking water contaminant is its complete absence of sensory cues. Iron stains your laundry orange. Sulfur smells like rotten eggs. Hardness leaves white scale on your faucets. Arsenic gives you nothing. A glass of water at 50 ppb arsenic looks, smells, and tastes identical to a glass of bottled spring water. The only way to know is to test.

The Two Forms: Arsenite (As III) and Arsenate (As V)

Arsenic in groundwater exists in two main inorganic forms, and the difference matters for treatment:

• Arsenate, As(V): the oxidized, negatively charged form. This is what most arsenic-removal systems are designed to capture. It is the dominant form in oxygen-rich (aerobic) groundwater.
• Arsenite, As(III): the reduced, neutral form. Common in oxygen-poor (anaerobic) deep wells, especially when iron, manganese, or sulfide are also present. As(III) is more toxic and harder to remove because it carries no electrical charge for an ion exchange resin to grab onto.

If your lab test shows total arsenic above the EPA limit and your well is also iron-bearing, sulfur-smelling, or pulled from a deep aquifer, assume some of that arsenic is As(III) and plan on a pre-oxidation step. The EPA's arsenic treatment design manual covers oxidation in detail; in practical residential terms, an upstream iron filter using oxidative media handles the conversion of As(III) to As(V) while also removing the iron that would otherwise foul the arsenic resin.

This is why the order of your treatment matters and why we always ask for a full water test, not just a number for arsenic.

Health Risks at Different Exposure Levels

Health effects from arsenic in drinking water are dose-dependent and accumulate with long-term exposure. The numbers below are summarized from the EPA Chemical Contaminant Rules, the ATSDR/CDC Toxicological Profile for Arsenic, the IARC monograph, and state public health guidance from Massachusetts, New Jersey, New Hampshire, and Minnesota.

The EPA Maximum Contaminant Level: 10 ppb

In 2001 the EPA lowered the federal MCL for arsenic in public drinking water from 50 ppb to 10 ppb (10 micrograms per liter), with full compliance required by 2006. The 10 ppb limit reflects a balance between health protection and treatment feasibility for water utilities. The MCL is not a safety threshold; it is a regulatory ceiling. The EPA's Maximum Contaminant Level Goal (MCLG) for arsenic is zero, meaning no exposure is considered risk-free.

Several states recommend or enforce a lower limit for private wells:

• New Jersey: drinking water standard of 5 ppb, half the federal MCL.
• New Hampshire: state MCL lowered to 5 ppb in 2021.

Long-Term Health Effects (Chronic Exposure)

Long-term ingestion of arsenic above 10 ppb is associated with the following, per the CDC, IARC, and peer-reviewed research:

• Cancers: bladder, skin (non-melanoma), and lung cancer have the strongest evidence. Liver, kidney, and prostate cancer are also linked.
• Cardiovascular disease: increased risk of hypertension, atherosclerosis, and ischemic heart disease.
• Type 2 diabetes: meta-analyses of US populations show measurable increased risk at chronic exposures above the MCL.
• Skin changes: hyperkeratosis (thickening), pigmentation changes, dark spots on palms and soles. Often the first visible clinical sign.
• Neurological and developmental effects in children: reduced IQ, motor function deficits, and impaired memory in children chronically exposed during development.
• Reproductive effects: low birth weight and increased infant mortality in populations with high exposure.

Acute (Short-Term) Effects at Very High Levels

At very high exposures (typically thousands of ppb, well above what most US wells produce), acute symptoms can appear within hours: nausea, vomiting, abdominal pain, severe diarrhea, numbness in extremities, and muscle cramping. Acute arsenic poisoning from US well water is rare but documented in heavily contaminated areas.

The Practical Risk Scale

Note: The 0-10 ppb range is "within the EPA limit" but not "zero risk." The MCLG is 0. Some states (NJ, NH) treat 5 ppb as the action level. We treat any detectable arsenic above 5 ppb as worth filtering, especially if children are in the home.

How Arsenic Gets Into Well Water

For private wells in the United States, the dominant source of arsenic is geological. Specific pathways include:

• Weathering of arsenic-bearing bedrock. Granite, schist, and certain sedimentary rocks contain naturally elevated arsenic. As groundwater moves through fractures, it dissolves arsenic from the rock surface and carries it into the well.
• Reductive dissolution from iron oxides. In oxygen-poor aquifers, naturally occurring iron oxides that hold arsenic in place can dissolve, releasing arsenic into the water. This is the mechanism behind much of the arsenic in the Upper Midwest and parts of New England.
• Volcanic and geothermal sources. Common in parts of the Western US, where geothermal activity mobilizes arsenic into shallow aquifers.
• Legacy industrial and agricultural contamination. Historic use of arsenical pesticides on apple orchards, lead-arsenate insecticides on cotton, and CCA-treated wood can contribute. Modern mining and smelting operations can locally elevate arsenic. These sources matter regionally but are minor compared to natural sources for most wells.

The takeaway: arsenic in your well is almost always something that was already in the ground when the well was drilled. It is not caused by anything you did, and there is generally nothing you can do at the well head to remove it. The fix is whole-house treatment after the pressure tank.

Geographic Hotspots in the United States

The USGS estimates roughly 2.1 million Americans on private wells drink water above the 10 ppb MCL. Risk is not uniform across the country. The highest-risk regions are:

• New England: New Hampshire and Maine have the densest concentration of high-arsenic wells in the country, especially in granite-bedrock areas. NH DES estimates roughly 1 in 5 private wells in the state exceed the federal MCL. Massachusetts and Vermont also have widespread elevated arsenic.
• Mid-Atlantic: parts of New Jersey (especially the Highlands and Piedmont), eastern Pennsylvania, and Delmarva show pockets of elevated arsenic. Less uniform than New England but real.
• Upper Midwest: Wisconsin, Michigan, and Minnesota have widespread arsenic from glacial aquifers. Wisconsin's Fox River Valley and the Minnesota glacial outwash plains are well-documented hotspots.
• Southwest: Arizona, Nevada, and Utah have elevated arsenic from geothermal and volcanic sources. New Mexico's Albuquerque basin is another known area.
• Pacific Northwest: parts of Oregon, Washington, and Idaho show elevated arsenic from volcanic geology.

That said, geography is a starting point, not a substitute for testing. Two wells on adjacent properties can produce wildly different arsenic concentrations because groundwater flow paths through fractured bedrock are localized. We have seen homes in Pennsylvania at 40 ppb next door to a well at 2 ppb on the same street. Test your specific well. For a state-by-state breakdown of where arsenic risk is highest and why, see our arsenic in well water by state guide.

Do You Need an Arsenic Filter?

Use this quick diagnostic to figure out where you stand. Enter what your most recent test showed and the household details. The tool gives you the right next step, whether that is "do not treat," "test now," or "install a system today."

How to Test for Arsenic

Arsenic is one of the few well water contaminants where home test strips are not a substitute for a lab. The chemistry is too sensitive at the parts-per-billion level for color-match strips to be reliable in the 0-25 ppb range that determines whether you need a filter.

The Three Practical Options

For sizing an arsenic system you need more than just the arsenic number. Iron above 0.3 ppm fouls the resin. pH outside 4-8 reduces capacity. High silica, sulfate, or phosphate compete for binding sites. The full panel pays for itself by preventing a wrong-sized system. For a step-by-step walkthrough of how to test for arsenic in well water, see our dedicated arsenic testing guide. For broader testing methodology, see our complete guide to testing well water and how to read your results.

All Treatment Methods Compared

Five technologies can reduce arsenic in residential water. Each has a place. The right one depends on whether you need point-of-use (one faucet) or whole-house treatment, and on the rest of your water chemistry.

The reason RO and distillation are limited to point-of-use is practical: a whole-house RO system that produces 10 GPM of treated water for a family would require an enormous storage tank, a high-capacity booster pump, and would discharge thousands of gallons of reject water every month. For a single contaminant like arsenic, dedicated whole-house arsenic media is the correct architecture.

For a comparison of how arsenic treatment fits alongside other well water contaminant treatment, see our complete well water filtration guide.

Why We Use Arsenic-Selective Ion Exchange Resin

Both of our whole-house systems use the same media: a hybrid anion exchange resin with hydrated iron oxide monoatomically dispersed throughout the polymer beads. It is WQA Gold Seal certified for potable water contact and is the only arsenic-removal media we have ever sold. Here is why.

The Arsenic Dumping Problem

Most low-cost whole-house arsenic filters use granular adsorptive media: granular ferric hydroxide, granular ferric oxide, titanium-based media, or iron-coated activated alumina. These work by physically adsorbing arsenic onto the surface of the media particles. They are inexpensive and effective, until they are not.

When adsorptive media reaches its capacity, it does not just stop removing arsenic. It can release the arsenic it has accumulated back into the treated water at concentrations higher than the source water. This phenomenon is well-documented in the engineering literature and is the single biggest reason adsorptive media is not the right choice for residential whole-house treatment without active monitoring.

If you are not personally lab-testing your treated water every 3-6 months, you will not know when the media is exhausted. The first sign you notice may be a doctor's question about long-term arsenic exposure.

Why Ion Exchange Is Different

Arsenic-selective ion exchange resin uses a hybrid mechanism: anion exchange sites in the polymer matrix combined with hydrated iron oxide nanoparticles dispersed inside each bead. Arsenic is bound through both ionic exchange and chemical sorption to the iron oxide.

The critical difference: when the resin reaches its working capacity, effluent arsenic levels never exceed influent levels. There is no dumping. The system fails gracefully. Arsenic concentration in the treated water gradually rises toward the source concentration, but it never exceeds it. That single property is why this media is the WQA Gold Seal residential standard and why we use it.

Performance Specs (Per Manufacturer)

• Reduces arsenate (As V) from 50 ppb down to less than 10 ppb for 500,000+ gallons per cubic foot of media under typical residential conditions.
• Operating pH range: 4 to 8 (covers nearly all private well water in the US).
• Performance is largely unaffected by chlorides, bicarbonates, and sulfates at typical residential concentrations, the most common competing anions.
• Capacity reduced by very high silica, very high sulfate, or pH outside 4-8. This is why the full water test matters before sizing.

The Two Systems We Sell

Both systems use the same WQA Gold Seal arsenic-selective resin. The difference is the valve and the maintenance model. Most homes need the metered version. Cabins, off-grid setups, and seasonal properties take the non-backwashing version. For a head-to-head comparison of these two systems against the rest of the market, see our best arsenic water filter for well water buyer's guide.

Both systems ship from our warehouse with full DIY-friendly install instructions. Shipping is free to the lower 48. Aidan personally reviews every order before it ships and will call you if anything in your water profile suggests a different configuration. 800-460-5810 is the line.

Where the Arsenic System Fits in Your Treatment Train

Arsenic media is one of the more expensive components in a whole-house treatment train, so you want to protect it. Sediment, iron, and pH adjustment all happen upstream. The arsenic system is the polishing step at the end before water enters the home.

Why this order:

• Sediment first protects every downstream component from grit and turbidity that would otherwise clog beds and shorten media life.
• Iron filter second for two reasons: iron above 0.3 ppm fouls arsenic resin and shortens its life dramatically, and the oxidation step in an iron filter converts As(III) to As(V), which is the form the arsenic resin removes most effectively.
• Acid neutralizer third if pH is below 6.5. Arsenic resin works best in the pH 4-8 range, but very low pH water may also be corroding plumbing in ways unrelated to arsenic.
• Softener fourth if you have hardness above 7 grains. Hardness does not directly affect arsenic removal, but the softener belongs upstream of the arsenic vessel for plumbing reasons.
• Arsenic system last as the final polishing stage. By this point the water is clear, low in iron, properly pH-balanced, and softened. The arsenic resin sees clean water and lasts much longer.

For the long-form version of why this sequence matters and how to plumb it, see our guide to the correct order of well water treatment systems.

Cost Overview

The all-in cost of solving an arsenic problem in a private well runs $200 to $5,500 depending on what you need. The variables are testing, the system itself, install, and a confirmation test 30 days after startup.

For the full cost breakdown including 5-year and 10-year ownership math, see our well water treatment system cost guide. For a dedicated breakdown specific to arsenic, including media replacement and pre-treatment costs, see our arsenic filter cost guide.

Frequently Asked Questions

Written by Aidan Walsh, owner of Mid Atlantic Water. 32+ years installing well water treatment systems across Pennsylvania, Maryland, Delaware, New Jersey, Virginia, and the Mid-Atlantic. Article reviewed April 2026. Health information sourced from EPA, CDC, IARC, USGS, and state public health agencies. This article is for educational purposes and is not a substitute for medical advice; if you are concerned about a specific health symptom, consult a physician.