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Arsenic in Drinking Water: A Scientist's Guide For Your Home
January 8, 2025
Eric Roy, PhD | Chief Scientist
Arsenic is one of the contaminants I worry about most. It does not make headlines, and it harms people quietly, over years. Unlike PFAS or lead, which usually get into water through human activity, arsenic comes from the earth itself. It is naturally present in the rock and soil across much of the United States, and millions of private wells tap straight into those formations.
If you are on a private well, arsenic is something that you need to test for, because you cannot see, smell, or taste it. Private well owners are responsible for testing their water and treating it if problems are found.
Arsenic is a naturally occurring element found in bedrock, soil, and groundwater. In water it shows up in two forms, Arsenic III (arsenite) and Arsenic V (arsenate). Both are toxic, but they behave differently in a filter. Arsenic V carries a charge and is easier to capture; Arsenic III is neutral and harder, and usually has to be oxidized into Arsenic V before a filter can catch it (chlorinated city water does this oxidation on its own). That distinction matters when you choose a treatment system.
Arsenic contamination in drinking water almost always comes from the bedrock. Arsenic hot spots include New England, the Upper Midwest, the Southwest, and the Pacific Northwest. Old arsenic-based pesticides and some mining and smelting can add to it in specific areas, but geology is the main source.
The most important thing to understand about arsenic is how local it is. Two homes on the same road, drawing from the same general aquifer, can have arsenic levels that differ by orders of magnitude, because the micro-geology, the fracture pathways, and the well itself vary from property to property.
Arsenic is one of the most thoroughly studied toxins in environmental health, and the evidence is not in doubt: it is a Group 1 carcinogen, which means it is known to cause cancer in people. Long-term exposure is linked to cancers of the bladder, lung, skin, and kidney, and also to cardiovascular disease, high blood pressure, type 2 diabetes, and, in children, cognitive and developmental harm. The risk builds slowly and silently. Most people have no symptoms until long after the exposure, which is why testing and filtration are the only real protection.
The EPA sets a Maximum Contaminant Level for arsenic in public water of 10 parts per billion. That number is not a safe level. Toxicologists agree there is no truly safe level of arsenic, and the World Health Organization notes that real risk exists below 10 ppb. The limit reflects what was considered achievable nationwide, not a "health only" goal. And if you are on a private well, even that limit does not apply to you. No one is required to test your water, monitor it, or tell you if it is contaminated. You are on your own.
Testing is cheap next to the cost of not knowing. Use a certified laboratory and ask for "Total Arsenic" by an accredited EPA method. If arsenic is found in the test results, ask whether the lab can speciate it, meaning tell you how much is Arsenic III versus V, which affects how you'll approach it from a treatment standpoint. Test now if you never have, and every one to three years if you are in an arsenic-prone area or your levels are near 10 ppb.
You want to look for products that carry NSF/ANSI certification reduction claims for arsenic, which will take place under Standard 53 or 58. NSF, WQA, or IAPMO can issue these certifications.
Can I boil arsenic out of my water? No. Boiling water does not reduce dissolved chemical contaminants.
Does my neighbor's clean test mean I am safe? No. Arsenic can vary enormously over short distances because of the local geology. You have to test your own well.
Does bottled water contain arsenic? Usually not, though some brands bottled near arsenic-rich geology have been recalled.
If you are on a private well: test for Total Arsenic now, retest every one to three years, and install a point-of-use reverse osmosis system (NSF/ANSI 58) if any arsenic shows up.
If you are on city water: check your utility's water quality report, and if you are in a high-arsenic region, consider a point-of-use RO system for extra margin.