Arsenic in Drinking Water: Risk, Sources, and Solutions

Arsenic in drinking water kills more people than almost any other water contaminant — not with dramatic acute poisoning, but through a slow accumulation of cancer risk over years of drinking water that looks and tastes perfectly normal.

The mechanism is geological. Arsenic is naturally present in rock and soil across large portions of the globe, and as groundwater moves through arsenic-bearing geological formations, it dissolves arsenic and carries it to the wells people drill to drink from. It’s not a story of industrial pollution or regulatory failure in most cases — it’s chemistry that has been happening in the Earth’s crust for millions of years, intersecting with human water supplies.

The United States is not a low-arsenic country. While the dramatic cases — Bangladesh, where tens of millions of people drink water with arsenic levels far above safe thresholds — get more global attention, significant arsenic contamination of private wells and some small public water systems affects communities across the American West, upper Midwest, and New England. An estimated 2 million Americans drink water with arsenic above the EPA’s 10 ppb limit.[2] Many more drink water with arsenic at lower but still potentially concerning levels.

The health consequences, established over decades of research across multiple continents, are serious. Understanding who is exposed and what to do about it is straightforward once you know where to look.

Arsenic is classified as a Group 1 carcinogen by the International Agency for Research on Cancer — meaning there is sufficient evidence that it causes cancer in humans.[1] The evidence is not from animal studies or mechanistic inference; it comes from epidemiological studies of populations drinking high-arsenic water, where cancer rates are directly and clearly elevated.

The strongest and most consistent cancer association is bladder cancer. Populations with high arsenic exposure — in Bangladesh, Chile, Argentina, Taiwan — show bladder cancer rates two to ten times higher than unexposed populations. Dose-response relationships are clear: more arsenic, more cancer. Lung cancer and arsenical skin lesions that can progress to squamous cell carcinoma complete the IARC Group 1 picture.

The EPA’s MCL of 10 ppb for arsenic was set in 2001, reduced from the previous limit of 50 ppb. The MCLG is zero — there is no known safe level.[2] Studies have found cancer risk associations at concentrations approaching current US tap water levels, which is why the EPA’s science advisors recommended an even lower limit. Cost and feasibility led to the 10 ppb standard, not evidence of safety at that level.

Arsenic exposure in the United States is geographically concentrated — but within those regions, it’s widespread enough that testing is non-negotiable for anyone on a private well.[3]

The highest-risk regions are the American West (Nevada, Arizona, Utah, Montana, and parts of California), the upper Midwest (Wisconsin, Michigan, Minnesota), New England (parts of New Hampshire, Maine, and Vermont), and the Southwest (New Mexico and parts of Texas).

Public water systems serving more than 25 people are regulated under the Safe Drinking Water Act and must meet the 10 ppb standard. Private wells — serving 43 million Americans — are entirely self-monitored. No federal agency requires private well owners to test for arsenic. Many never do.

The relationship between arsenic and well depth varies by local geology and can’t be generalised. Testing your specific well is the only way to know.

If you’re on a private well, arsenic testing is straightforward, affordable, and arguably the single most important water quality test you can do.

Use a state-certified laboratory — a list is available through your state health department. A basic arsenic test costs $15–30. Collect the sample as directed; typically a first-draw cold water sample after water has been stagnant for at least 6 hours.[2]

Reverse osmosis removes 90–95% of arsenic and is the most practical home treatment for drinking water. Standard activated carbon filters provide little protection. Iron-based adsorption media certified to NSF/ANSI 58 are highly effective, particularly for pentavalent arsenic — the most common form in groundwater. Only NSF/ANSI 58-certified systems are verified to reduce arsenic to safe levels; standard pitcher and refrigerator filters provide essentially no protection.