Arsenic in Well Water: What It Is and How to Remove It

Arsenic is one of the few well-water contaminants that gives you absolutely no warning. It has no color, no smell, and no taste, so a well can carry elevated arsenic for years without anyone noticing. Unlike nitrate or bacteria, arsenic usually isn't caused by anything you did — it occurs naturally in the ground. This guide explains where arsenic comes from, why testing is the only way to know you have it, what the EPA limit means, why the chemical form of arsenic affects treatment, and the two proven methods for removing it.

Where arsenic in well water comes from

Most arsenic in private wells is naturally occurring. It's a natural component of many rock formations, and as groundwater moves through arsenic-bearing bedrock and sediment, small amounts dissolve into the water. This is a geologic process, not pollution — which is why arsenic can appear even in a well far from any farm, factory, or road.

Because it's tied to local geology, arsenic tends to cluster regionally. Areas commonly associated with higher natural arsenic include parts of the Southwest, sections of New England, and the Upper Midwest. But geology varies well to well — two neighbors can have very different arsenic levels — so regional maps are only a starting point, never a substitute for testing your own water.

Why testing is the only way to know

This is the most important point on the page. Arsenic is colorless, odorless, and tasteless. There is no home symptom, no cloudy glass, no off flavor — nothing you can sense will tell you it's there. The only way to know whether your well has arsenic, and at what level, is a certified laboratory test. If you've never tested your well for arsenic, that's the first step, well before you consider any equipment.

Why arsenic matters — and the EPA limit

The U.S. EPA sets the Maximum Contaminant Level (MCL) for arsenic in drinking water at 10 ppb (parts per billion, equal to 0.010 mg/L). This is a standard designed around long-term exposure: the health concerns associated with arsenic come from drinking water above the limit consistently over many years, not from a single glass. The takeaway isn't panic — it's that if your test comes back above 10 ppb, treating your water is a sensible, permanent way to bring it into the safe range. Water at or below the limit meets the federal drinking water standard.

Arsenic III vs. arsenic V: why the form matters

Arsenic shows up in water in two main forms, and the difference affects treatment:

• Arsenic V (arsenate): The oxidized form. It carries an electrical charge that makes it relatively easy for treatment media and membranes to capture.
• Arsenic III (arsenite): The reduced form, common in oxygen-poor groundwater. It's uncharged, which makes it much harder to remove directly.

The practical fix is oxidation: converting arsenic III to arsenic V (for example with an oxidizing pre-treatment step) so the removal media can do its job effectively. Your lab test can often report speciation, or a water professional can advise whether a pre-oxidation stage is needed for your well. This is why sizing an arsenic system is best done from real test data rather than guesswork.

The two proven ways to remove arsenic

Two treatment approaches have a strong, well-documented record for arsenic. As with most well-water issues, the choice comes down to whole-house versus point-of-use drinking water.

1. Adsorptive arsenic-selective media (whole-house)

A whole-house tank filled with arsenic-selective adsorptive media (often iron-based) captures arsenic as water passes through, treating every tap in the home. These systems are effective and low-maintenance, but they work best on arsenic V — so if your water carries arsenic III, an oxidation step ahead of the media may be required. Media is eventually replaced once its capacity is used up, based on your arsenic level and water volume.

2. Reverse osmosis (point-of-use drinking water)

A reverse osmosis (RO) system forces water through a semipermeable membrane that rejects arsenic (particularly arsenic V) along with many other dissolved contaminants. Installed under the kitchen sink, RO is a focused, cost-effective way to make your drinking and cooking water safe without treating the whole house. Many households use RO at the kitchen tap in addition to, or instead of, a whole-house system. Explore our reverse osmosis drinking water systems →

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Frequently asked questions

Can I taste or smell arsenic in my water?

No. Arsenic is completely colorless, odorless, and tasteless. There is no way to detect it by sight, smell, or taste — a certified laboratory test is the only reliable way to know if it's present and at what level.

What arsenic level is considered safe?

The EPA drinking water standard for arsenic is 10 ppb (0.010 mg/L). Water at or below that level meets the federal standard. If your test comes back above it, treatment is recommended to bring it down into the safe range.

Does boiling remove arsenic?

No. Boiling does not remove arsenic and, because it evaporates water while leaving the arsenic behind, it can concentrate it. Arsenic requires dedicated treatment such as adsorptive media or reverse osmosis.

What is the difference between arsenic III and arsenic V?

Arsenic V (arsenate) is the oxidized, charged form that treatment media and RO membranes remove readily. Arsenic III (arsenite) is uncharged and harder to remove, so it often needs an oxidation step first to convert it to arsenic V. Your test can help determine which form you have.

Whole-house or under-sink for arsenic?

If your only concern is drinking and cooking water, an under-sink reverse osmosis system is the most economical option. If you want arsenic reduced at every tap, choose a whole-house adsorptive media system — and confirm whether pre-oxidation is needed based on your test.

This page is general educational information, not medical advice. For health concerns related to arsenic exposure, consult a healthcare provider, and rely on a certified laboratory test to determine your water's arsenic level.