Arsenic doesn't come from an industrial spill or a regulatory failure. It comes from the ground. Arsenic occurs naturally in rock formations across large parts of the American Southwest and Mountain West—and when groundwater moves through arsenic-bearing geology, it dissolves arsenic along the way before reaching wells and municipal water intakes.
That geological reality makes arsenic different from most other drinking water issues. There's no single event to trace it to and no specific industry to hold responsible. It's present in the aquifer, and utilities in affected markets treat to reduce it before distributing water. The question for businesses isn't whether it was ever introduced—it's what the EPA's limit means and whether compliance at that limit represents an acceptable exposure for employees drinking from the tap every day.
For the full picture on what contaminants are in tap water and what the regulatory framework requires, see the complete tap water guide for businesses. This piece covers arsenic specifically.
How Arsenic Gets Into Municipal Water
Arsenic is a naturally occurring metalloid found in trace amounts throughout the earth's crust. In regions where arsenic-bearing minerals—particularly arsenopyrite and other sulfide minerals—are abundant in bedrock, groundwater that sits in or flows through that rock picks up dissolved arsenic over time.
This is most pronounced in the western United States. Volcanic geology, hydrothermal activity, and specific mineral formations in Arizona, New Mexico, Nevada, Idaho, Montana, and parts of California create elevated arsenic concentrations in aquifers that feed municipal water systems. These states account for the majority of documented arsenic issues in public water supplies.
Industrial activity compounds the natural baseline in some markets. Mining operations—historically extensive throughout the Mountain West—leave behind waste rock and tailings that continue to leach arsenic into surrounding groundwater decades after operations end. Past use of arsenic-based pesticides in agricultural areas contributes additional localized contamination in some markets.
Utilities in high-arsenic areas treat incoming water to reduce arsenic before distributing it. The issue is that treatment brings arsenic down to the compliance threshold—not to zero.
What the EPA Limit Actually Means
The EPA set its arsenic limit at 10 parts per billion in 2001, reducing it from a prior ceiling of 50 parts per billion. That reduction reflected updated research on the cancer risks associated with long-term low-level exposure.
The EPA's non-enforceable health target is zero—no safe threshold for arsenic exposure has been established. The enforceable limit sits above that because treating to lower levels across thousands of water systems of varying size would be technically and economically impractical at scale.
Water testing at 9 parts per billion is legally compliant. The EPA's own health position says that level carries risk.
Arsenic is a confirmed human carcinogen. Long-term exposure through drinking water is associated with skin, bladder, and lung cancers—the cancers most consistently linked to arsenic in peer-reviewed literature. The International Agency for Research on Cancer classifies arsenic as a Group 1 carcinogen, its highest classification, based on human study evidence.
Which Markets Face the Highest Exposure
High-arsenic municipal water concentrates in specific geological settings. Several of Bottleless Nation's core markets fall directly within the affected zone.
Phoenix, Arizona and the broader Valley. Arizona's geology makes it one of the highest-arsenic states in the country. Multiple municipal systems in the Phoenix metro area have reported arsenic levels between 5 and 9 ppb in routine monitoring—legally compliant, but with little margin against a limit set for practical reasons rather than health ones. Smaller suburban and rural systems around the metro have faced closer compliance challenges. The Arizona Department of Environmental Quality publishes arsenic monitoring results by utility.
New Mexico. Parts of central and southern New Mexico have naturally elevated arsenic from volcanic geology. Smaller systems serving communities outside major metros have faced ongoing compliance challenges.
Mountain West and parts of the Midwest. Portions of Idaho, Montana, Nebraska, and South Dakota have arsenic in groundwater systems drawn from arsenic-bearing glacial deposits and bedrock.
For businesses in these markets, the annual Consumer Confidence Report will include the utility's arsenic monitoring results. A result consistently in the 5 to 9 ppb range is compliant—but it's operating close to a ceiling that was set for practical reasons, not health ones.
How Reverse Osmosis Addresses Arsenic
In treated municipal water, arsenic is predominantly present as arsenate (As-V)—utilities apply oxidation that converts the more difficult arsenite form to arsenate before distribution. Arsenate is an ionic compound with a negative charge, which makes it well-suited for removal by reverse osmosis.
Reverse osmosis membranes reject ionic compounds based on size and charge at the molecular level. Studies on reverse osmosis systems for arsenate removal document removal rates of 90 to 95 percent under standard operating conditions. That means a building drawing water at 8 parts per billion arsenic from the municipal supply receives water at the dispenser at roughly 0.4 to 0.8 parts per billion after point-of-use purification.
The reverse osmosis guide for businesses covers how each purification stage works and what it removes in plain language. Carbon filtration alone does not remove arsenic—the same limitation that applies to nitrates applies here. A reverse osmosis stage is what separates purification systems that address arsenic from those that don't.
A bottleless water purification system with a multi-stage reverse osmosis process removes arsenic at the point of use regardless of what the incoming municipal supply delivers. In high-arsenic markets like Phoenix, this closes the gap between what the utility is required to treat to and what employees are actually drinking.
Frequently Asked Questions
Is arsenic in tap water a problem everywhere in the US?
No. Arsenic contamination concentrates in areas with specific geological conditions—primarily the Southwest, Mountain West, and parts of the Midwest. Businesses in Phoenix, New Mexico, and similar markets face a higher baseline than businesses in geologically different regions. Your utility's annual Consumer Confidence Report includes arsenic monitoring data showing where your local system tests relative to the EPA limit.
What is the EPA limit for arsenic and how was it set?
The EPA's arsenic limit is 10 parts per billion, reduced from 50 in 2001. The health goal—non-enforceable—is zero, because no safe level of arsenic exposure has been established. That limit reflects what's technically and economically achievable across thousands of water systems, not a threshold below which health risk disappears.
What health effects are linked to arsenic in drinking water?
Long-term exposure to arsenic through drinking water is associated with skin, bladder, and lung cancers. The International Agency for Research on Cancer classifies arsenic as a Group 1 carcinogen—sufficient evidence of carcinogenicity in humans. Risk increases with exposure level and duration.
Does a standard carbon filter remove arsenic?
No. Carbon filters remove chlorine, chloramines, and organic compounds—they don't remove dissolved ionic compounds like arsenate. Reverse osmosis is the technology that addresses arsenic at the point of use, removing it at rates of 90 to 95 percent under normal operating conditions. A purification system without a reverse osmosis stage doesn't address arsenic.
How do I find out if my building's water has elevated arsenic?
Your utility's annual Consumer Confidence Report includes arsenic monitoring results. The EPA's drinking water dashboard shows historical monitoring data by utility. Comparing multiple years of data gives a more complete picture than any single annual result—in high-arsenic markets, trends matter as much as any individual reading.