What Is Arsenic and Why Is It Dangerous?

Arsenic is a metalloid element that occurs naturally in the Earth’s crust and is widely distributed in the environment. In its inorganic form, arsenic is a potent carcinogen and toxicant. Chronic ingestion of arsenic-contaminated water has been linked to cancers of the skin, bladder, lung, kidney, and liver. The World Health Organization (WHO) classifies inorganic arsenic as a Group 1 carcinogen, meaning there is sufficient evidence of carcinogenicity in humans. Non-cancer health effects include skin lesions (hyperkeratosis and pigmentation changes), peripheral neuropathy, cardiovascular disease (including hypertension and ischemic heart disease), developmental effects in children, and diabetes. The severity depends on the concentration of arsenic, duration of exposure, and individual susceptibility. The U.S. Environmental Protection Agency (EPA) has set a maximum contaminant level (MCL) for arsenic in public drinking water at 10 parts per billion (ppb), a standard that also serves as a health-based guideline for private well owners.

How Arsenic Contaminates Well Water

Arsenic enters groundwater largely from natural deposits in rocks, sediments, and soils. Geologic formations rich in arsenic-bearing minerals—such as arsenopyrite, realgar, or orpiment—can release arsenic into the water through chemical weathering and dissolution, particularly under certain pH and redox conditions. In many regions, including the southwestern United States, parts of New England, the Midwest, and areas of Southeast Asia, elevated arsenic occurs naturally. Human activities also contribute: mining and smelting operations, agricultural use of arsenic-based pesticides and herbicides (now largely restricted but historical residues remain), wood preservation using chromated copper arsenate, and improper disposal of industrial waste. Well construction and depth matter; shallow wells are more vulnerable to nearby surface contamination, while deep wells may tap into naturally mineralized aquifers. Seasonal changes in water table levels can influence arsenic concentrations.

Arsenic Speciation: Inorganic vs Organic Forms

Understanding the chemical form of arsenic is critical for health risk assessment and treatment selection. Inorganic arsenic (arsenite [As(III)] and arsenate [As(V)]) is the more toxic variety. As(III) is typically found in more reducing (anoxic) groundwater and is more difficult to remove with conventional treatment because it is uncharged. As(V) is common in oxidizing conditions and is more readily removed. Organic arsenic forms (e.g., monomethylarsonic acid, dimethylarsinic acid) are generally less toxic and can arise from biological activity, but they are seldom a significant concern in well water. Laboratory test reports should specify the species or at least indicate total arsenic and whether it is inorganic. When considering treatment, it is often necessary to oxidize As(III) to As(V) for efficient removal.

Testing for Arsenic in Well Water

Testing is the only reliable way to determine arsenic levels in a private well. The EPA recommends that private well owners test annually for arsenic, as concentrations can vary over time due to changes in water chemistry and geology. At a minimum, test whenever a new well is drilled, when a household moves into a home with an existing well, or if there is a known local contamination problem. Two testing approaches exist:

  • Home test kits: Inexpensive and easy to use, but they typically provide only a semi-quantitative result (e.g., presence/absence or a broad concentration range). They have lower sensitivity and may not detect low levels near the 10 ppb MCL. They should be considered a screening tool, not a definitive analysis.
  • Certified laboratory analysis: The gold standard. A certified lab uses methods such as inductively coupled plasma mass spectrometry (ICP-MS) or graphite furnace atomic absorption (GFAA) to detect arsenic down to very low levels (sub-ppb). Laboratories provide accurate quantitative results that can be used for decision-making. Contact your local health department or the EPA Drinking Water Hotline for a list of certified labs in your state.

How to Collect a Proper Sample

  1. Use only sample containers provided by the laboratory; they are treated to avoid contamination.
  2. Select a cold water tap used for drinking (do not use a softened water tap).
  3. Remove any aerator or screen from the faucet.
  4. Let the water run for 2–3 minutes to flush the line.
  5. Fill the sample container to the indicated line, avoiding overflowing or touching the inside of the cap.
  6. Label the container, complete the chain-of-custody form, and ship or deliver promptly on ice if required.

Interpreting Test Results and Understanding Risk

If your laboratory report shows total arsenic below 10 ppb, your water meets the EPA MCL, but some health experts recommend even lower action levels (e.g., 5 ppb) for sensitive populations. If arsenic exceeds 10 ppb, you should take immediate steps to reduce exposure. Consider using bottled water for drinking and cooking until treatment is installed. For results between 10 and 50 ppb, point-of-use treatment can be effective. For levels above 50 ppb, a point-of-entry (whole-house) system may be warranted. Keep in mind that the risk is cumulative: even low levels over many years can be harmful. Pregnant women, infants, and individuals with chronic health conditions are at greater risk.

Treatment Options to Remove Arsenic from Well Water

Several technologies effectively reduce arsenic in well water. The choice depends on water chemistry, arsenic speciation, flow rate, budget, and maintenance requirements. Always test water after treatment and periodically thereafter to confirm performance. The CDC provides guidance on well water treatment systems.

Activated Alumina (AA) Filters

Activated alumina is a porous aluminum oxide that adsorbs both As(V) and, less effectively, As(III). Pre-oxidation of As(III) to As(V) (using chlorine or aeration) improves removal. AA media must be replaced periodically when exhausted. Regeneration with caustic soda can extend media life but adds complexity. Best suited for point-of-use applications with low to moderate arsenic levels.

Reverse Osmosis (RO) Systems

RO forces water through a semipermeable membrane, rejecting a high percentage of dissolved solids, including arsenic. Point-of-use RO units under the kitchen sink are common. They can reduce arsenic by more than 90%, especially As(V). As(III) removal is lower unless pre-oxidized. RO systems waste water (typically 3–4 gallons per gallon of product) and require membrane replacement every 2–3 years. Suitable for addressing combined contaminants (e.g., arsenic, nitrate, fluoride).

Anion Exchange Resins

Strong-base anion exchange resins can remove As(V) by exchanging chloride or hydroxide ions for arsenate. They are less effective for As(III). The presence of competing anions (sulfate, nitrate) reduces capacity. Resins require periodic regeneration with brine, and the waste brine must be disposed of properly. Best for water with low TDS and moderate arsenic levels.

Iron-Based Adsorptive Media

Granular ferric hydroxide (GFH) or iron oxide coated sand are highly effective for both As(V) and As(III). They work via adsorption and have high capacity. No regeneration is needed; media is replaced at exhaustion. These systems are robust and well-suited for point-of-entry treatment. They are less sensitive to pH changes and competing ions compared to activated alumina.

Distillation

Distillation removes arsenic by boiling water and condensing the steam. It effectively eliminates nearly all inorganic contaminants but is energy-intensive and slow. Usually limited to small-volume point-of-use applications. The distiller must be cleaned regularly to remove accumulated solids.

Maintenance and Monitoring

All treatment systems require ongoing maintenance—changing filters, media, membranes, or cleaning. Keep a log of maintenance actions and test water annually to ensure the system is still performing. Untreated water should be used for non-potable purposes only if the arsenic level is elevated, as bathing and washing do not generally pose significant ingestion risk (but avoid swallowing water).

Costs and Considerations for Treatment Systems

Costs vary widely. A home test kit may be $20–40; laboratory analysis typically ranges $50–150 per sample. Point-of-use RO systems start around $200 and can exceed $1,000 installed. Point-of-entry iron-based adsorptive systems range from $1,500 to $4,000 plus installation. Replacement media or membranes add ongoing costs. Some health departments offer low-cost testing events or subsidies for well owners. Check with your local health department or state environmental agency. The investment in treatment is far less than the long-term health costs of chronic arsenic exposure.

Regulatory Standards and Health Advisories

While private wells are not federally regulated under the Safe Drinking Water Act, the EPA has established a non-enforceable health advisory level for arsenic in drinking water. Many states have their own guidelines or require testing at the time of property transfer. The WHO recommends a guideline value of 10 ppb. In regions with known high arsenic, such as parts of Nevada, New Hampshire, and Maine, state agencies may recommend testing annually. If you are buying a home with a private well, request recent test results and have the water tested independently.

Protecting Your Family’s Health: Next Steps

  • Test now. Order a lab-certified test kit from a certified laboratory. Do not rely on taste, odor, or appearance—arsenic is undetectable by senses.
  • Interpret results with a professional. If arsenic is found, consult a water treatment specialist who understands local geology and water chemistry.
  • Treat effectively. Choose a treatment system certified by NSF/ANSI Standard 53 for arsenic reduction. Ensure the system is properly installed and sized for your household water demand.
  • Retest after treatment. Confirm that the system is removing arsenic to below your target level. Retest annually and whenever the system is serviced or media replaced.
  • Stay informed. Keep up with local water quality advisories and any new research on arsenic health risks. Consider testing for other common well water contaminants (bacteria, nitrate, lead) at the same time.

Conclusion

Arsenic in well water is a serious but manageable health threat. Understanding the sources, health effects, testing methods, and treatment options empowers homeowners to protect their families. Private well owners must take responsibility for water quality because no regulatory authority monitors their supply. By testing regularly and installing appropriate treatment, you can ensure that your well water remains safe for drinking and cooking. For further information, consult the EPA’s private well water page or your local health department.