Guide · Updated October 27, 2026

Arsenic in Private Well Water: The Invisible Risk Underground

Arsenic in groundwater is often naturally occurring. It is usually invisible, tasteless, and odorless. USGS research estimates that millions of Americans who use private wells may be drinking water above EPA’s arsenic standard — and private wells are not required to meet that standard.

There is no sensory warning. You cannot tell arsenic is in your water by looking at the glass. EPA’s Maximum Contaminant Level for arsenic in public drinking water is 10 micrograms per liter (µg/L). That limit does not legally apply to most private wells. Testing with a certified lab is the only reliable way to know your level.

How common is high arsenic in private wells?

A USGS and CDC modeling study estimated that about 2.1 million people in the contiguous United States use domestic wells with predicted arsenic above 10 µg/L — roughly 4.8% of domestic-well users in that analysis. Later USGS work raised the estimate under normal conditions and found that drought can push the at-risk population even higher (about 4.1 million in one drought scenario).

USGS has also found arsenic detections across many principal aquifers. Concentrations tend to be higher in parts of the West and Southwest, but elevated arsenic shows up in glacial aquifers in the North, crystalline-rock aquifers in the East, and other regions as well. “We’re not in a famous arsenic hotspot” is not a reason to skip testing.

Where arsenic comes from

Most arsenic in U.S. groundwater comes from natural geologic sources — rocks and sediments that release arsenic under certain chemical conditions. Human activities can add to the problem: mining, some industrial uses, historical pesticides, and wood preservatives.

Because the source is often the aquifer itself, a well can have arsenic even when the property looks rural, clean, and far from industry. Depth, geology, pH, and drought conditions can all matter. USGS has warned that drought may elevate arsenic levels in some private wells.

Who is at risk?

Prioritize arsenic testing if:

Your state or county has mapped elevated groundwater arsenic
Neighbors’ wells have tested high
Your well is in a region USGS or your health department flags for arsenic
Anyone in the home is pregnant, nursing, elderly, or a young child
You have never tested the well for metals — only for bacteria

Long-term arsenic exposure is linked to serious health risks, including certain cancers. Short-term, high exposures can cause gastrointestinal and other symptoms. EPA lowered the public-system MCL from 50 µg/L to 10 µg/L in 2001 because of those health concerns. Private-well owners were left to adopt that benchmark on their own.

What “safe enough” looks like in practice

Public water systems must monitor arsenic and treat or blend if they exceed the MCL. Your private well does not get that automatic check. CDC’s well-testing guidance lists arsenic among the chemicals to ask your health department about based on where you live — on top of the annual basics (coliform, nitrates, TDS, pH).

If your result is above 10 µg/L, contact your local health or environmental department. They can help interpret the number and point you toward treatment options or alternate water for drinking and cooking. Do not assume a water softener alone removes arsenic — many do not. Confirm any treatment device is appropriate for arsenic and have treated water retested.

Steps to take this month

Call a state-certified lab and ask specifically for arsenic in drinking water (and whether they accept private-well samples).
Ask your health department whether arsenic is a known local groundwater issue and whether any free or low-cost testing clinics are available.
Keep the lab report. A baseline helps you see changes after drought, a new well pump, or nearby land-use changes.
If levels are high, use an alternate drinking-water source while you work with local officials on treatment or other options.

Find a certified lab for arsenic testing

Browse official state certified-lab lists we maintain for private well owners.

Browse labs by state Minnesota Vermont Texas

Disclaimer: This page is informational only and is not medical advice. Discuss symptoms and exposure concerns with a healthcare professional. Use your state or local health department for guidance on results and treatment.