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. 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. 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.

Bladder cancer The strongest and most consistent cancer association. 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 Inhalation of arsenic in occupational settings has been linked to lung cancer for over a century. Ingested arsenic from water appears to elevate lung cancer risk as well, with several large studies finding significant associations.

Skin cancer and lesions Arsenical keratoses and skin cancers — often among the earliest clinical signs of chronic arsenic exposure — occur at high rates in heavily exposed populations. They appear on the palms and soles and can progress to squamous cell carcinoma.

The low-dose question 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. Studies have found cancer risk associations at concentrations approaching current tap water levels in the US, 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.

The highest-risk states • The American West: Nevada, Arizona, Utah, Montana, and parts of California have widespread naturally occurring arsenic in groundwater • Upper Midwest: Wisconsin, Michigan, and Minnesota have significant arsenic in glacial aquifers • New England: parts of New Hampshire, Maine, and Vermont have elevated arsenic in bedrock wells • The Southwest: New Mexico and parts of Texas

Private wells vs. public systems 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.

Depth matters Shallow wells in some geologies are lower in arsenic than deep wells; in others, the reverse is true. 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.

Small community water systems Systems serving fewer than 25 people or fewer than 15 service connections are exempt from many Safe Drinking Water Act requirements, including arsenic monitoring in some states. These very small systems serve rural communities in some of the highest-arsenic regions of the country.

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

Getting your well tested • Use a state-certified laboratory — a list is available through your state health department • Collect the sample according to lab instructions; typically a first-draw cold water sample • Cost: $15–30 for a basic arsenic test; comprehensive panels covering multiple contaminants run $100–200 • When: test now if you've never tested, then annually or after any change in water quality or nearby land use

If results are above 10 ppb • Do not use well water for drinking or cooking until treatment is installed or verified • Use bottled water as a short-term measure • Consult your state's private well program — many states offer assistance with testing and treatment for low-income households

Effective treatment technologies • Reverse osmosis: Removes 90–95% of arsenic. The most practical home treatment for drinking water. • Activated alumina filters: Specifically designed for arsenic removal; highly effective, requires periodic regeneration • Ion exchange: Effective for arsenic, particularly pentavalent arsenic (the most common form in groundwater) • Oxidation + filtration: Some systems require oxidation of trivalent arsenic to pentavalent arsenic before filtration is effective — your water chemistry determines this

PollutionProfile's Water Quality feature can connect you to arsenic monitoring data for public systems in your area and guide you toward appropriate testing for private wells based on your region's geological risk profile.