Arsenic in Drinking Water: Natural Occurrence, Cancer Risk, and What to Do

Arsenic is the 20th most abundant element in the Earth's crust. It occurs naturally in many types of rock, and when groundwater flows through arsenic-containing rock formations, it picks up dissolved arsenic compounds. This is called geogenic contamination — contamination from natural geological sources rather than human activity.

The regions with the highest naturally occurring arsenic in groundwater include: the western United States (particularly Nevada, Montana, Idaho, Wyoming, and parts of California, Oregon, and Arizona), New England (especially Maine, New Hampshire, and Vermont), the upper Midwest (Michigan, Wisconsin, Minnesota, South Dakota), and parts of the Southeast. But arsenic has been detected in wells across every region of the country.

Beyond natural sources, human activities have contributed arsenic to water supplies. Historically, arsenic compounds were widely used as pesticides in agriculture — particularly in orchards (apple, pear, cherry) and cotton fields. Decades of use left arsenic residues in soils that continue to leach into groundwater. Copper smelting, gold mining, and the manufacture of certain glass and wood preservatives (notably the pressure-treated wood formerly made with chromated copper arsenate, or CCA) have also been localized industrial sources.

The form of arsenic in water matters for treatment purposes. Arsenic exists in two main forms: arsenite (As(III)) and arsenate (As(V)). Arsenate is more common in oxygenated surface waters and is more easily removed by treatment. Arsenite is more common in deeper groundwater and is harder to remove without oxidation pretreatment. If you're investigating arsenic treatment for your well, knowing which form predominates in your water affects which treatment technology will work best.

Arsenic is a Group 1 human carcinogen — the highest classification assigned by the International Agency for Research on Cancer, meaning the evidence for cancer causation in humans is conclusive. Long-term consumption of arsenic in drinking water above safe levels is associated with increased risk of several cancers.

Bladder cancer has the most robust epidemiological evidence. Studies from Taiwan, Bangladesh, and Chile — where arsenic in drinking water reached very high levels — showed dramatic increases in bladder cancer risk with long-term arsenic exposure. Studies in the United States at lower arsenic levels also show statistically significant associations.

Lung cancer, skin cancer (particularly a form called Bowen's disease), kidney cancer, and prostate cancer are also associated with long-term arsenic exposure. The risk is dose-dependent — higher exposure levels produce higher cancer risk, and longer exposure duration compounds the risk.

The cancer mechanism involves arsenic disrupting DNA repair processes and interfering with cell signaling pathways. Arsenic is unusual in that it acts as a complete carcinogen — it can both initiate DNA damage and promote the development of established tumors.

Beyond cancer, chronic arsenic exposure is associated with cardiovascular disease, peripheral neuropathy (numbness and tingling in the extremities), skin changes (darkening and the growth of hard patches called keratoses), and adverse pregnancy outcomes.

I want to be direct about risk scale here: the cancer risk from arsenic in U.S. drinking water at typical detected levels is real but modest for most people. The EPA estimates that lifelong exposure to water at exactly the 10 ppb limit increases lifetime bladder cancer risk by approximately 3 to 7 cases per 10,000 people — not a trivial addition to baseline risk, but far from certain illness. For people with well water that significantly exceeds the limit, or who have consumed high-arsenic water for decades, the risk is more substantial.

The EPA set the current Maximum Contaminant Level for arsenic at 10 parts per billion (0.010 mg/L) in 2001, effective in 2006. This was a significant tightening from the previous standard of 50 ppb, which had been in place since 1975.

Even at 10 ppb, the EPA acknowledges residual cancer risk. The public health goal (MCLG) for arsenic is zero — meaning any detectable level of arsenic carries some theoretical risk. The 10 ppb standard was set at the lowest level achievable for large water systems using the best available treatment technology at a cost deemed feasible.

Several scientific and public health organizations, including the National Academy of Sciences, have recommended that the EPA consider lowering the standard further — potentially to 5 ppb or even 3 ppb. At 5 ppb, the estimated lifetime cancer risk drops by roughly half. But more stringent standards would require more water systems to install treatment, at significant cost, and the regulatory process for MCL revision is slow and politically complex.

This history matters because it affects how you should interpret data. A water system that reports arsenic at 8 ppb is technically in compliance with federal law. But that 8 ppb represents a meaningful lifetime cancer risk if you're drinking that water every day for decades. "In compliance" and "zero additional risk" are not the same thing for arsenic.

For private wells, there is no enforceable federal standard. If your well contains 15 ppb of arsenic, no government agency is going to require you to treat it. The decision is entirely yours — but it's a decision worth making with a clear understanding of the health implications.

For households on public water systems, arsenic data is available through the Consumer Confidence Report. Public systems serving more than 25 people are required to test for arsenic on a regular schedule (annually or more frequently if levels are elevated) and report the results in the CCR. You can also check WaterSafeCheck for your ZIP code's water quality data, which includes arsenic levels where available.

For private well owners, testing is the only way to know. Arsenic has no taste, no color, and no odor at the concentrations typically found in drinking water. You cannot detect it without laboratory analysis.

The good news is that arsenic testing is relatively inexpensive through a state-certified water testing laboratory — typically $20 to $40 for a basic arsenic test. Some states offer free or reduced-cost testing programs for well owners, particularly in regions known to have elevated arsenic. Contact your state health or environmental agency to find out what's available locally.

When should you test? If you're on a private well in one of the high-risk regions mentioned earlier — the West, New England, upper Midwest — testing at least once is important, and re-testing every few years makes sense since arsenic levels can change as the water table fluctuates. If you're in other regions, testing is still worthwhile for a baseline assessment, particularly if you have an older well or are in an area with historical agricultural or industrial activity.

For well owners who test positive for arsenic above 10 ppb, follow-up testing to determine the form of arsenic present (arsenite vs. arsenate) is helpful for selecting the most effective treatment technology.

Arsenic removal from drinking water is technically feasible, but the treatment that works best depends on what form of arsenic is present and whether you're treating the whole house or just your drinking water.

Reverse osmosis (RO) — highly effective for arsenate. An under-sink reverse osmosis system certified to NSF/ANSI Standard 58 removes 90–95% of arsenic, including both arsenate (As(V)) and, to a lesser extent, arsenite (As(III)). For most household applications, an RO system is the most practical solution. Cost: $200–$600 installed, with annual maintenance around $50–$150.

Activated alumina — effective for both forms. Activated alumina filters adsorb arsenic onto aluminum oxide media. They work well for both arsenite and arsenate, making them useful when the form of arsenic isn't known or when arsenite is the predominant species. Activated alumina units require periodic regeneration or media replacement. They're available as under-sink units or whole-house systems.

Oxidation plus filtration — required for arsenite. If your water contains primarily arsenite (As(III)), pre-oxidation using chlorine, permanganate, or air is necessary to convert it to arsenate before filtration or RO treatment. Without this step, arsenite passes through many treatment media more easily than arsenate. A water treatment professional can help determine if pre-oxidation is needed based on your specific water chemistry.

Anion exchange — effective for arsenate. Similar to the process used in water softeners, anion exchange resins can selectively remove arsenate. These systems require periodic regeneration and generate arsenic-concentrated brine that must be disposed of properly.

Standard carbon filters — not effective. Pitcher filters, under-sink carbon block filters, and refrigerator filters are generally not effective for arsenic removal. Do not rely on a standard NSF/ANSI 42 or 53 certified filter to address arsenic unless it specifically lists arsenic removal in its certification claims.

For public water systems, treatment is a utility responsibility. If your utility exceeds the MCL for arsenic, they are legally required to notify you and implement corrective action. In the meantime, use bottled or treated water for drinking and cooking.

The first step is knowing your current situation. Pull up your Consumer Confidence Report or check WaterSafeCheck for your ZIP code. If you're on a public water system, look for the arsenic entry in the CCR table and note the detected level. If it's below 5 ppb, your risk is low. If it's between 5 and 10 ppb, you're in compliance but some additional caution is reasonable — particularly for children and pregnant women. If it's above 10 ppb, your utility is in violation and required to take action.

If you have a private well and live in a high-arsenic region or simply haven't tested, arrange for testing through a state-certified lab. The cost is modest relative to the health information it provides. If results come back above 10 ppb — or even above 5 ppb if you have children — look into treatment options.

For renters on public water, review the CCR for your water system. If you're in an area with known elevated arsenic, consider an under-sink RO filter for your drinking water — it addresses not just arsenic but also lead, nitrates, PFAS, and other contaminants simultaneously. The $200–$400 investment provides comprehensive protection for the water you drink and cook with.

The arsenic issue is one where the geology of where you live matters enormously. Someone in Seattle or Portland has very different arsenic concerns than someone in rural Nevada or western Maine. Understanding the local risk context — which you can get from your CCR, from WaterSafeCheck, and from your state environmental agency's groundwater data — is the starting point for making sensible decisions.