Arsenic in Drinking Water: A Scientist's Guide For Your Home

Eric Roy, PhD | Chief Scientist

Arsenic is one of the contaminants I worry about most, not because it makes headlines, but because of how quietly it harms people. Unlike PFAS or lead, which usually enter water through human activity, arsenic comes from the earth itself. It is naturally present in rock and soil formations across the United States, and millions of private wells tap directly into those formations.

If you rely on a private well, arsenic is not a hypothetical risk. It is something you should test for and understand. This guide explains what arsenic is, how it enters water, what the science says about long-term health effects, and how to remove it reliably at home.

What Is Arsenic?

Arsenic is a naturally occurring element found in bedrock, soil, groundwater aquifers, and sediments. In drinking water, arsenic appears in two primary forms:

1. Arsenic III (Arsenite)
2. Arsenic V (Arsenate)

Both forms occur naturally, both are toxic, but they behave differently in treatment systems.

Why Speciation Matters

• Arsenic V carries a charge and is easier to remove with filtration media.  If you are on city water that uses chlorine, arsenic III is oxidized to Arsenic V.
• Arsenic III is neutrally charged, more difficult to capture, and unless you're using highly specialized active media, often must be oxidized into Arsenic V before it can be removed effectively.

Important: You cannot see, smell, or taste arsenic in water. Only laboratory testing can detect it.

How Arsenic Enters Drinking Water

Unlike lead, which comes from plumbing, arsenic usually comes from the aquifer itself.

1. Geological Sources Many U.S. regions contain arsenic-rich bedrock. Groundwater dissolves arsenic from these formations and carries it into private wells. This is common in parts of:

• New England
• The Upper Midwest
• The Southwest
• The Pacific Northwest

The "Neighbor Paradox" Two homes on the same road, tapping the same general aquifer, can have arsenic levels that differ by orders of magnitude. This happens because micro-geology, fracture pathways, and well construction vary from property to property. You cannot rely on your neighbor's test results.

2. Historic Agricultural Use In some regions, arsenic-based pesticides were used widely in the early and mid-1900s. Residual arsenic persists in soil and can leach into groundwater.

3. Industrial Sources Mining, smelting, and certain manufacturing processes can elevate arsenic in localized hotspots, though this is far less common than natural geological sources.

4. Municipal Water vs. Private Wells Most municipal utilities monitor and treat for arsenic, so city water is generally low-risk. Private wells are unregulated and represent the greatest vulnerability.

Health Risks of Arsenic Exposure

Arsenic is one of the most well-studied toxins in environmental health. Public health agencies classify it as a Group 1 Carcinogen, meaning there is strong evidence that it causes cancer in humans.

Cancer Risk Health risk increases with both concentration and duration of exposure. Arsenic is linked to:

• Bladder cancer
• Lung cancer
• Skin cancer
• Kidney cancer

Cardiovascular and Metabolic Effects

• High blood pressure
• Cardiovascular disease
• Type 2 diabetes
• Vascular injury

Neurological and Cognitive Effects

• Cognitive impairment in children
• Lower IQ
• Developmental delays
• Peripheral neuropathy

The Dose and Duration Problem Arsenic-related health effects develop slowly and silently over years. Most people have no symptoms until long after exposure has already occurred. Testing and filtration are the only reliable protection.

How Arsenic Is Regulated

The EPA regulates arsenic in public water systems with a Maximum Contaminant Level (MCL) of 10 parts per billion (ppb).

1. The 10 ppb limit is not a “safe” level Toxicologists agree that there is no truly safe level of arsenic. The World Health Organization notes that health risks exist below 10 ppb. The EPA limit reflects what was considered achievable nationwide, not a threshold of zero risk.

2. Private Wells Are Not Regulated If you rely on a private well, the federal arsenic standard does not apply. There is no requirement for anyone to test, monitor, or notify you of contamination. You are responsible for your own water quality.

How To Test for Arsenic In Drinking Water

Testing is simple and inexpensive compared to the consequences of not knowing.

Testing Protocol

• Use a certified laboratory.
• Request "Total Arsenic." via EPA Method 200.8 or 200.9
• Advanced: If arsenic is detected, ask whether the lab can speciate it (distinguish Arsenic III vs Arsenic V).

Testing Frequency

• If you have never tested: Test immediately.
• If you live in an arsenic-prone region: Test every 1 to 3 years.
• If levels are close to or above 10 ppb: Test more frequently.

How To Remove Arsenic From Drinking Water

This part is simple. You need to filter your water with a product that has been proven to reduce Arsenic.

The only way to know if a product works is to look at third-party certifications, specifically NSF/ANSI Standard 53 or 58, specifically for Arsenic reduction.. Do not overthink the technology (Reverse Osmosis vs. Carbon).

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What Does Not Work

• Boiling Water: This is dangerous. It evaporates the water and concentrates the arsenic.
• Standard Carbon Pitchers: Most do not remove arsenic reliably.
• Water Softeners: Designed for hardness, not toxicity. They can suffer from "chromatographic dumping," where they release captured arsenic back into the water all at once.

Frequently Asked Questions

Can I boil arsenic out of water? No. Never boil water to remove arsenic.

Does my neighbor's safe test mean I am safe? No. Arsenic levels can vary dramatically over short distances due to micro-geology. You must test your own well.

Does bottled water contain arsenic? Generally no, but some brands bottled near arsenic-rich geological sources have had recalls. Filtered water is a more consistent choice.

What To Do Next

If you rely on a private well:

1. Test your water for Total Arsenic immediately.
2. Re-test every 1-3 years.
3. Install a Point-of-Use RO system (NSF 58) if any arsenic is detected.

If you are on city water:

1. Review your utility’s Water Quality Report.
2. If you are in a high-arsenic region, consider Point-of-Use RO for extra protection.

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