Contaminant Guide

Arsenic in Well Water

Arsenic is a naturally occurring element that dissolves from rock and sediment into groundwater, particularly in bedrock and glacial aquifers across the northern and western United States. It is colorless, odorless, and tasteless — you cannot detect it without a laboratory test. Long-term exposure above the EPA limit is linked to bladder cancer, skin cancer, and cardiovascular disease.

What is arsenic?

Arsenic is a naturally occurring element found in rock and soil. It dissolves into groundwater from minerals underground. You cannot taste it, smell it, or see it — the only way to know if it's in your water is to test it. And unlike bacteria, you cannot boil arsenic out of your water. Boiling actually makes it worse by concentrating it.

Private wells are not covered by federal drinking water rules. That means no one is checking your well for arsenic — unless you do it yourself. City water systems must test and report their arsenic levels every year. Private well owners get no such protection.

How common is arsenic in well water?

Arsenic is one of the most common natural contaminants in U.S. well water. It shows up more often in certain parts of the country:

  • Michigan and Wisconsin — Glacial deposits and bedrock in the Upper Midwest often contain sulfide minerals that release arsenic into groundwater.
  • Maine and New Hampshire — Crystalline bedrock wells throughout New England have a higher chance of arsenic contamination.
  • Nevada, Arizona, California, Montana, Idaho, and Wyoming — Geothermal activity and a history of mining have raised arsenic levels in many Western aquifers.

A 2017 study found that about 2.1 million people across the U.S. drink from private wells with arsenic above 10 micrograms per liter (µg/L, also called parts per billion). Newer research suggests even more people may be exposed at lower levels.

Here's something important to understand: arsenic in your well depends on your local geology, not on whether a factory or farm is nearby. Two neighbors with wells just feet apart can have very different arsenic levels depending on which underground layer their well draws from.

Health effects

Arsenic is linked to cancer in humans. The health problems it causes come from drinking contaminated water over months and years — not from a single exposure.

The EPA limit vs. the safe level: an important distinction

The EPA's safety limit for arsenic is 10 µg/L, but scientists say any amount carries some risk. The limit was lowered from 50 µg/L in 2001 after decades of research showed that level was causing harm. The current limit of 10 µg/L was chosen because it was the lowest level that water systems could realistically achieve — not because it's considered truly safe.

For well owners, this matters: even a result below 10 µg/L carries some risk. Many health professionals recommend treatment at any detectable level, especially in homes with young children or pregnant women.

Documented health effects

  • Bladder cancer — Arsenic is linked to cancer in humans, and studies show a clear pattern: the higher the arsenic level, the higher the risk. Some research finds elevated risk even below the EPA's limit.
  • Skin cancer — Skin lesions and a thickening of the skin called keratosis have been widely documented in Bangladesh, Taiwan, and Chile, where people were exposed to high arsenic levels over many years. Arsenic is linked to cancer in humans through this pathway as well.
  • Cardiovascular disease — Long-term arsenic exposure has been connected to heart disease and problems with blood flow in the legs and feet.

Who is most at risk?

  • Private well users, because there is no required monitoring
  • Infants and children, who take in more water relative to their body weight
  • People with low selenium intake — selenium may help reduce some of arsenic's harmful effects
  • Long-term residents in high-arsenic areas who have had decades of exposure

Testing your well

Arsenic must be tested by a certified laboratory using ICP-MS lab analysis (a highly accurate measurement method). Do not rely on field test strips — they are not precise enough at the levels that matter for your health and should never be used to make treatment decisions.

When you order a test, ask for total arsenic. A basic inorganic metals panel or a certified well water test package will usually include it. Results will be listed in µg/L or ppb — those two units mean the same thing.

The EPA recommends testing your private well at least once if you never have. Test again if you deepen your well, notice any change in your water, or if your area floods near the wellhead.

Find a certified lab and learn how to submit a water sample

Treatment options

The good news: arsenic can be removed from well water reliably. Several proven treatment options exist. A licensed water treatment professional should install and maintain whatever system you choose. Because effectiveness depends on your water's chemistry — things like pH and other minerals — always test your water before picking a treatment method.

  • Reverse osmosis (RO) — An under-sink system that filters water at the tap. Removes 90–99% of arsenic. Works on both forms of arsenic found in groundwater. The membrane needs to be replaced periodically.
  • Activated alumina adsorption — A filter media that attracts and holds arsenic as water passes through. Works best for one form of arsenic (arsenate, or As V). May need a pH adjustment to work well for the other form. Can filter water for the whole house.
  • Iron-based adsorption media — Filters made with greensand, iron oxide, or iron oxyhydroxide are effective at capturing arsenic, especially arsenate. Commonly used in whole-house systems.
  • Distillation — Boiling water and collecting the steam removes arsenic effectively, but it uses a lot of energy and works slowly. Best suited for small amounts of drinking water.

One important warning: standard activated carbon filters — including popular pitcher-style filters — do not reliably remove arsenic. Do not use them as your main protection against arsenic.

Compare arsenic treatment systems for private wells

In your area

Arsenic levels can vary a lot from one county to the next, and even from one aquifer to another. If you live in Michigan, Wisconsin, or New England, your bedrock well has a higher-than-average chance of containing arsenic. You can explore state and county-level data through our interactive reports.

View Michigan well water data and county-level arsenic patterns

Regulatory framework

MCL: 10 µg/L

The EPA's Maximum Contaminant Level (MCL) for arsenic in public water systems is 10 µg/L (micrograms per liter), enforceable under the Safe Drinking Water Act (SDWA) for community water systems serving 25 or more people. This standard applies only to public water systems; private wells are not federally regulated.

MCLG: 0 µg/L

The Maximum Contaminant Level Goal (MCLG) for arsenic is 0 µg/L. The MCLG represents the level at which no known or anticipated health effects occur, with an adequate margin of safety. For known or probable human carcinogens — arsenic is classified as both — EPA sets the MCLG at zero, acknowledging that no safe threshold has been established.

The gap between the MCLG (0) and the MCL (10 µg/L) reflects the limits of economically and technically feasible treatment at the time of rulemaking. This distinction is significant: compliance with the MCL does not imply the absence of health risk.

Regulatory history

The arsenic MCL in the United States was 50 µg/L from 1975 until 2001. The Arsenic Rule (66 Fed. Reg. 6976, January 22, 2001) lowered the MCL to 10 µg/L after a comprehensive review of epidemiological evidence linking 50 µg/L to elevated bladder and skin cancer rates. Full compliance was required by January 23, 2006. The 10 µg/L standard aligns with the World Health Organization (WHO) guideline value.

Detection methods

Regulatory-grade arsenic analysis requires ICP-MS (Inductively Coupled Plasma Mass Spectrometry) per EPA Method 200.8. This method achieves detection limits well below 1 µg/L, well suited to the 10 µg/L MCL.

  • Certified laboratory required — Results used for regulatory compliance must come from a state-certified laboratory.
  • Field test strips — Commercial arsenic test strips have detection limits typically around 10–50 µg/L and high false-negative rates near the MCL. They are not suitable for health-based decision-making.
  • Speciation — ICP-MS measures total arsenic. Speciation analysis (separating As III from As V) is available from some labs and informs treatment selection, since arsenite (As III) is harder to remove with some media.
  • Sample collection — Proper sampling protocol (first-draw vs. flushed sample) affects results. Follow the certified lab's instructions precisely.

Geochemistry and occurrence

Arsenic enters groundwater primarily through natural weathering and dissolution of arsenic-bearing minerals — particularly iron oxyhydroxides, sulfide minerals (arsenopyrite, pyrite), and volcanic rock. Under reducing (anoxic) groundwater conditions, iron oxyhydroxides dissolve and release adsorbed arsenate; this mechanism drives elevated arsenic in many glacial and alluvial aquifers. Oxidizing conditions can also mobilize arsenic from sulfide minerals.

Secondary sources include historic mining activity (acid mine drainage), legacy agricultural pesticide use (lead arsenate and calcium arsenate used through the mid-20th century), and geothermal inputs in the western United States.

Aquifer types with documented elevated arsenic in U.S. well water: bedrock, glacial outwash, and alluvial aquifers.

Treatment technology detail

Treatment efficacy depends on arsenic speciation and competing water chemistry parameters (pH, phosphate, silica, iron, sulfate). Key technical considerations:

  • Reverse osmosis — Membrane rejection of arsenic is high (90–99%) for both As III and As V. Rejection efficiency is relatively insensitive to pH. Requires pre-treatment for high iron/turbidity to prevent membrane fouling.
  • Activated alumina — Works via ion exchange; most effective for As V at pH 5.5–6.0. As III must be oxidized to As V (e.g., with chlorination or aeration) before activated alumina treatment. Competing phosphate and fluoride ions reduce capacity.
  • Iron-based media — Iron oxyhydroxide and iron oxide media (e.g., GFH, Bayoxide E33) adsorb As V and As III effectively across a wider pH range. Competitive inhibition from phosphate and silica. Media have finite capacity and must be regenerated or replaced.
  • Coagulation-filtration — Used at municipal scale; ferric coagulants co-precipitate arsenic. Not practical for point-of-entry residential applications.
  • Distillation — Effective but energy-intensive; useful for small volumes only.

Data access

Arsenic well-water occurrence data are available from the USGS National Water Information System, EPA's ECHO database, and state-level well records. Our site aggregates county-level patterns from Water Quality Portal (WQP) records.

Access our data API and methodology documentation

References

  1. Ayotte, J.D., Medalie, L., Qi, S.L., et al. (2017). Estimating the high-arsenic domestic-well population in the conterminous United States. Environmental Science & Technology, 51(2), 839–849. https://doi.org/10.1021/acs.est.7b02881
  2. Zheng, Y., & Flanagan, S.V. (2017). The case for universal screening of private well water quality in the US and testing requirements to achieve it: evidence from arsenic. Environmental Health Perspectives, 125(8), 085002. https://doi.org/10.1289/EHP629
  3. Lombard, M.A., Bryan, M.S., Jones, D.K., et al. (2021). Machine learning models of arsenic in private wells throughout the conterminous United States as a tool for exposure assessment in human health studies. Environmental Science & Technology, 55(8), 5012–5023. https://doi.org/10.1021/acs.est.0c05239
  4. U.S. Environmental Protection Agency. (2001). National Primary Drinking Water Regulations; Arsenic and Clarifications to Compliance and New Source Contaminants Monitoring. Federal Register, 66(14), 6976–7066.
  5. International Agency for Research on Cancer. (2012). Arsenic, metals, fibres, and dusts. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Vol. 100C. IARC Press.