Arsenic

Naturally occurring, slow-acting, and present in well water across roughly half the United States

EPA MCL

10 μg/L (parts per billion); lowered from 50 μg/L in 2001

Health concern

Bladder, lung, and skin cancer; cardiovascular disease; skin lesions; cognitive effects in children

Testing method

ICP-MS or graphite furnace atomic absorption; $20-40 mail-in

Arsenic is a naturally occurring metalloid present in rocks and soils throughout the world. It's also one of the most consequential drinking-water contaminants on Earth — chronic exposure to even modest levels causes cancer, heart disease, and developmental problems. The most catastrophic case is Bangladesh, where tens of millions of people drink arsenic-contaminated well water and the resulting cancer rate has been called the largest mass poisoning in human history. The United States has its own version of this problem, just at lower concentrations and in less concentrated populations.

Where it comes from

Arsenic in US groundwater is overwhelmingly geologic, not industrial. It leaches from arsenic-bearing minerals — pyrite, arsenopyrite, arsenian iron oxides — under specific water-chemistry conditions:

• Volcanic terrain releases arsenic as glasses and minerals weather. Most of the western US fits this — the Basin and Range provinces of Nevada, Utah, Arizona, southwestern New Mexico, and the volcanic Pacific Northwest all have widespread arsenic.
• Glacial sediments in New England, eastern Canada, and the upper Midwest carry arsenic from old crystalline-rock source areas. New Hampshire and Maine have the worst US East Coast arsenic problem.
• Reducing groundwater conditions — low oxygen, high organic matter — mobilize arsenic that's locked up in iron oxides under oxidizing conditions. This is the mechanism behind much of the Mississippi Alluvial and Delta arsenic.
• Closed basins like Colorado's San Luis Valley accumulate arsenic from volcanic source rocks; drought concentrates it further.

Industrial sources exist (smelter emissions, chromated-copper-arsenate wood treatment, historical pesticides) but they're localized exceptions. If you have arsenic in your well, geology is the most likely cause.

The numbers

The USGS estimates that about 7% of US private wells exceed the EPA's 10 μg/L MCL for arsenic. That's roughly 3 million Americans drinking water above the legal standard — and many more drinking water at levels (1-10 μg/L) that recent epidemiology suggests still carry meaningful risk.

Hot zones in the US:

• San Luis Valley, Colorado — 25% of private wells exceed 10 μg/L; some wells exceed 100 μg/L. Arsenic levels are increasing as drought concentrates groundwater.
• New Hampshire — about 12% of wells exceed; concentrated in the southeastern part of the state where bedrock geology is most arsenic-rich.
• Maine — 10-12% of wells statewide; up to 30% in some southern Maine towns.
• Nevada / Arizona / Utah Basin and Range — widespread elevated arsenic, with many community water systems struggling to meet the MCL.
• Texas High Plains and eastern New Mexico — pockets of high arsenic in the Ogallala and Pecos Valley aquifers.
• Mississippi Alluvial Valley — reducing-condition arsenic, concentrated in private wells in Louisiana, Mississippi, and Arkansas.

Health effects

Arsenic is a confirmed human carcinogen (IARC Group 1). The cancers it's been shown to cause at chronic exposure: bladder, lung, skin, kidney, and liver. Beyond cancer, it causes cardiovascular disease, peripheral neuropathy, skin lesions (the classic clinical sign of chronic arsenic toxicity), diabetes, and developmental and cognitive effects in children.

The dose-response curve is what makes arsenic difficult. There's no obvious "safe" threshold below 10 μg/L — recent meta-analyses suggest measurable bladder cancer risk at 1-10 μg/L. The WHO and EPA standards are not zero-risk levels; they're regulatory compromises between health protection and treatment cost.

Acute arsenic poisoning happens at much higher doses than you'd ever see from drinking water. The well-water concern is chronic exposure measured in years and decades.

Testing

Arsenic doesn't smell, taste, or color the water. The only way to know is to test.

• Method: ICP-MS (inductively coupled plasma mass spectrometry) or graphite furnace AA. Both are standard at certified labs.
• Cost: $20-40 for a single arsenic test by mail-in kit; usually included in standard "well water panel" tests for $80-150.
• Frequency: at least once when you move into a property, and every 3-5 years after. More often if you're in a known hot zone or your water source has changed (drought, new well, increased pumping nearby).
• Speciation: arsenic in groundwater exists as As(III) (arsenite) or As(V) (arsenate). They behave differently in treatment systems. Most basic tests give you total arsenic; if you need to design treatment, ask the lab to speciate.

Treatment

Several treatment methods are well-established. Pick based on your concentration, your water chemistry, and your budget:

• Reverse osmosis (RO) — removes 90%+ of As(V); less effective on As(III) without pre-oxidation. Best for point-of-use (drinking and cooking taps). Whole-house RO is rare and expensive.
• Adsorptive media (iron-based, alumina-based) — removes both species effectively, can be installed point-of-entry to treat the whole house. Media needs replacement every 1-3 years depending on load.
• Anion exchange — works for As(V), not As(III). Sulfate competes; high-sulfate water reduces effectiveness.
• Coagulation/filtration — works at scale (community water systems) but rare for residential.

If your test shows mostly As(III), the treatment system needs to oxidize it to As(V) first (chlorine, ozone, or specific media). Otherwise you'll get poor removal.

If you've been drinking high-arsenic water

The damage from chronic arsenic exposure is cumulative and partially irreversible — past exposure can't be reversed. But future exposure can be eliminated. If you've been drinking elevated-arsenic water for years, talk to your doctor about appropriate cancer screening (especially bladder cancer surveillance) and any skin findings. Arsenic is one of the contaminants where the medical history actually changes the screening recommendations.

Aquifers where this is a concern