One in ten private wells across the United States contains at least one contaminant at a level exceeding a primary drinking water standard set by the Environmental Protection Agency (EPA). For arsenic, a naturally occurring metalloid with no taste, color, or odor, that number is even more concerning. A 2017 study published in Environmental Science & Technology found that roughly 2.1 million Americans are likely consuming water from private wells with arsenic concentrations above the EPA's maximum contaminant level (MCL) of 10 parts per billion (ppb).
This isn't just an abstract problem; it’s a direct hit to the health of families in communities nationwide. Unlike public water systems, private wells are not regulated by the EPA, leaving homeowners solely responsible for testing and treating their water. This article will break down where arsenic contamination is most common, what it does to your body, and what you can do about it.
Arsenic is a naturally occurring element widely distributed in the Earth's crust. It can enter groundwater through the dissolution of arsenic-containing minerals and ores. Human activities, such as mining and agricultural practices, can also contribute to arsenic contamination, though naturally occurring sources are generally more prevalent in well water scenarios. The inorganic forms of arsenic are the most toxic, and these are the forms typically found in drinking water.
Short-term exposure to high levels of inorganic arsenic can cause nausea, vomiting, diarrhea, severe gastrointestinal issues, and even death. However, it's the long-term, low-level exposure that poses a more insidious threat. The World Health Organization (WHO) identifies long-term arsenic exposure as a cause of various health problems, including skin lesions, cardiovascular disease, neurological problems, and an increased risk of several cancers, including bladder, lung, and skin cancer.
While the EPA doesn't regulate private wells, it does set drinking water standards for public water systems. The arsenic MCL of 10 ppb was established in 2001, replacing a previous standard of 50 ppb. This change reflected a growing understanding of arsenic's long-term health risks, particularly its carcinogenic properties. It’s important to understand that this MCL is a legally enforceable standard for public water systems, and it's based on protecting public health with an adequate margin of safety.
Many environmental health organizations and scientists argue that even the 10 ppb standard may not be protective enough, especially for vulnerable populations like infants and pregnant women. Some states and organizations have even adopted lower, more stringent guidelines. For example, New Jersey has an MCL of 5 ppb for arsenic, demonstrating a more cautious approach given the scientific evidence of health risks at lower concentrations.
Arsenic contamination in private wells is not evenly distributed across the U.S. Geological Survey (USGS) data and numerous studies have identified several "hot spots" where naturally occurring geological conditions lead to higher concentrations of arsenic in groundwater. The primary geological factor is the presence of arsenic-rich minerals in bedrock and sediments. When groundwater flows through these formations, it dissolves the arsenic, carrying it into wells.
The Western U.S. is particularly susceptible to arsenic contamination. States like California, Nevada, Arizona, and Oregon consistently show higher rates of elevated arsenic in private wells. The geology of this region, characterized by extensive volcanic activity and arid conditions, creates ideal environments for arsenic mobilization into groundwater. A 2012 study in Environmental Health Perspectives by Ayotte et al. found that approximately 1 in 5 private wells in the western U.S. had arsenic levels above 10 ppb. California, for instance, has a large number of areas with naturally elevated arsenic in groundwater, particularly in the Central Valley and desert regions. If you live in a location like San Bernardino, CA, you might face higher risks related to arsenic contamination.
While less widespread than in the West, parts of the Northeastern U.S. also face significant arsenic challenges. Maine, New Hampshire, and parts of Massachusetts and Connecticut demonstrate elevated arsenic levels, often linked to metamorphic bedrock formations containing arsenic-bearing sulfides. Studies from the New England Water Science Center, a branch of the USGS, have consistently highlighted the prevalence of arsenic in groundwater across this region. Vermont, in particular, has seen significant attention to arsenic in private wells, with state health departments actively encouraging well testing due to widespread occurrences. A specific study published in Science of The Total Environment in 2021 by Korte et al. examined arsenic in groundwater across New England and found that many areas, particularly those with granitic bedrock, exhibited concentrations exceeding the EPA MCL.
Moving to the Midwest and South, states like Michigan, Wisconsin, Ohio, and parts of Texas and Oklahoma also report elevated arsenic levels in certain areas. In these regions, arsenic can be associated with glacial deposits, coal-bearing sediments, and specific types of sedimentary bedrock. While not as uniformly high as in the West, localized hot spots are a concern. For instance, in Michigan, research by the Michigan Department of Health and Human Services has identified certain counties with elevated arsenic in private wells, often linked to specific geological formations.
It isn't just whether arsenic-bearing rock is present, but the water chemistry around it that decides how much dissolves into your well. Two conditions matter most. In low-oxygen ("reducing") aquifers, iron minerals that hold arsenic break down and release it into the water. And in alkaline, high-pH groundwater common in the arid West, arsenic detaches from mineral surfaces more readily. That's why arsenic is so unpredictable at the neighborhood level: two wells on the same road, drawing from slightly different depths or aquifers, can test far apart. Depth matters too, since deeper wells sometimes tap older, more arsenic-rich water. The practical lesson is that you can't infer your well's arsenic from a regional map. You have to test your own water.
Because arsenic is invisible and private wells aren't regulated, testing is the only way to know your risk.
1. Test your well water through a state-certified laboratory, specifically for arsenic. A basic arsenic test is inexpensive, and if your area is a known hot spot, retest every few years and after any major change in your well or the surrounding land. 2. If arsenic exceeds 10 ppb, treat it. Choose a method proven for arsenic, because ordinary carbon filters and water softeners do not remove it, and boiling actually concentrates it. Effective options include: - Reverse osmosis at the kitchen tap, the most common point-of-use fix. - Adsorptive media such as activated alumina or iron-based media. - Anion exchange, best suited to arsenic in its more common oxidized form. 3. Confirm the fix works. Retest treated water to verify the system is actually pulling arsenic below the limit, and follow the maker's schedule for media replacement.
Arsenic isn't the only thing worth checking in a private well. See our guide to what to test your well for and how often for the full list.
You can't see, taste, or smell it, so the only way to know is a laboratory test. Use a state-certified lab and ask specifically for an arsenic analysis, especially if you live in a known hot-spot region like the Western U.S. or parts of New England.
No. Standard activated carbon filters and salt-based water softeners are not designed to remove arsenic. You need reverse osmosis, activated alumina, an iron-based adsorptive media, or anion exchange.
The EPA's enforceable limit for public water is 10 parts per billion, and some states, like New Jersey, use a stricter 5 ppb. Many health experts consider lower still to be safer, since arsenic is a known carcinogen with no clear "no-risk" threshold, so treat any result at or above 10 ppb as a call to act.
No. Boiling does not remove arsenic and slightly concentrates it as water evaporates. Use a certified treatment method instead.
Check your water quality by zip code to see if this contaminant has been detected near you. Find water filters that remove PFAS that are certified to reduce common contaminants. For information on PFAS specifically, see our PFAS contamination guide.
Check your water now. Enter your zip code at KnowYourExposure.com to see what contaminants have been detected in your local water supply – including PFAS, lead, and other regulated compounds.