Is there arsenic in Arizona drinking water?

Arsenic is a natural element in the rock and soil across the Southwest, not something a factory dumped. As rain and snowmelt move through volcanic, granitic, and basin-fill formations, they dissolve small amounts of arsenic and carry it into the aquifers that feed wells and some city supplies. This process has gone on for thousands of years, long before any people lived here.

Arizona sits in a part of the country where this happens more than almost anywhere else. The U.S. Geological Survey, which has mapped arsenic in wells across the nation, lists Arizona and Nevada among the states with the highest arsenic levels. The USGS notes that "arsenic is a semi-metallic element that is odorless and tasteless," which is exactly why you cannot tell it is there by looking at or sipping your water.

Levels also change from one spot to the next. Two wells on neighboring properties can test very differently, because the rock and the depth of the water table are not the same underground. A deeper well may pull from a different layer of rock than a shallow one nearby, and that alone can shift the arsenic reading. That variation is a big reason testing matters so much for well owners, a point we come back to below.

It helps to know that the arsenic in Arizona groundwater is mostly inorganic, the form tied to long-term health risk. This is the type the EPA regulates in drinking water. Because it comes from the rock itself rather than a spill or a pipe, there is no single source to shut off. The element is simply part of the geology, which is why the answer is treatment and monitoring rather than tracking down a leak.

The Environmental Protection Agency sets a national ceiling called a Maximum Contaminant Level, or MCL. For arsenic, that limit is 10 parts per billion (0.010 mg/L). Public water systems across the country, including the ones serving Phoenix, must keep treated water at or below this number.

The 10 ppb limit is fairly strict, and it was not always this low. The EPA lowered the standard from the old level of 50 ppb down to 10 ppb in 2001, with full compliance required by 2006. The agency made the change after reviewing the health science on long-term arsenic exposure. So a glass of water that would have passed under the old rule could fail today.

Parts per billion is a tiny unit. Ten parts per billion is roughly ten drops in an Olympic-size pool. Arsenic is harmful enough over a lifetime that the limit is set at this small level. The number is a lifetime exposure standard, built around drinking the water every day for years, not a single bad glass.

One result above 10 ppb is not a reason to panic, but it is a reason to act. The health risk comes from steady exposure over a long time, so a single high reading on a well means you should plan treatment, not that you have already been harmed. The point of the limit is to keep your average exposure low across the years you live with that water.

Yes. If you drink water from the City of Phoenix or another regulated municipal utility in the metro area, it is treated and monitored to stay at or below the 10 ppb arsenic limit. Public systems test their water regularly and report the results, so arsenic is one of the contaminants they are required to control. City tap water is safe to drink for arsenic.

This is the core split most people miss. Public utilities fall under the federal National Primary Drinking Water Regulations, which set enforceable limits and require routine testing for arsenic and dozens of other contaminants. A large system treating millions of gallons a day has the equipment and the legal duty to remove arsenic before the water reaches your tap.

You can confirm your own city's numbers without guessing. Every public water provider publishes an annual water quality report, often called a Consumer Confidence Report, that lists arsenic results among other measured contaminants. If you want to see the exact arsenic level in your treated supply, that report is the place to look. (For more on city water, see our page on whether Phoenix tap water is safe to drink.)

A private well is not covered by the federal drinking water rules. No city or state agency treats the water, tests it on a schedule, or sends you a report. The owner is fully responsible. Some private wells and a few small water systems in Arizona draw from groundwater that exceeds the 10 ppb limit, sometimes by a wide margin, and the owner may have no idea.

This is where the natural geology meets a gap in oversight. The same volcanic and basin-fill rock that puts arsenic into the ground does not care whether a public utility or a single household is pumping the water. The difference is that the utility filters and checks it, while a well does neither on its own. Arsenic has no color, taste, or smell, so a well can test high while the water looks perfectly clean.

If you own a well, the Arizona Department of Health Services recommends a clear routine. Test your well water for arsenic, and then retest about every five years, or sooner if you notice a change in taste, color, or the water table. ADHS keeps guidance and a list of certified labs for exactly this. A test usually means collecting a sample in a lab-supplied bottle and mailing it in. (See our page on well water system basics in Phoenix for the wider maintenance picture.)

The reason the limit exists is long-term health risk, not a quick illness. The EPA links chronic exposure to high arsenic in drinking water with several cancers, including bladder, lung, and skin cancer, along with kidney and other effects. These risks build over years of drinking water above the safe level, which is why a low lifetime limit and regular well testing both matter.

If a test comes back above 10 ppb, the fix is treatment, and several proven options exist:

• Reverse osmosis (RO). A point-of-use RO system, usually installed under the kitchen sink, pushes water through a fine membrane that removes arsenic along with many other contaminants. The EPA notes RO is effective for arsenic. It treats water at one tap rather than the whole house, and it does send some water to the drain as reject.
• Distillation. A countertop or plumbed distiller boils water and re-condenses the steam, leaving arsenic and other minerals behind. It is slow and uses energy, but it is thorough for a single drinking-water tap.
• Adsorptive media. Certain specialty filters use media such as iron-based or activated alumina materials that bind arsenic as water flows through. These can be sized for a single tap or, in some cases, a whole well system, and the media is replaced on a schedule.

Whichever route you choose, match the system to your tested arsenic level and the form of arsenic present, and follow the maintenance schedule so it keeps working. A treatment device only protects you if its filter or media is changed on time. (If you are weighing an under-sink option, our page on whether reverse osmosis is worth it goes deeper.)

If you have a well and have never tested it, that one step matters more than any other on this page. It tells you whether you have a problem at all, and it is the only way to know, since arsenic gives no warning you can see or taste.