Arsenic in Drinking Water: Health Risks & Removal (2026)

📅 Last Updated: July 16, 2026

Published January 2026 | Written by Filter Tested Editorial Team | Last updated: July 11, 2026 | Read our methodology

Published: January 2026 | Reading Time: 15 minutes | Technical Review: Toxicologist & Water Quality Engineer

Understanding Arsenic in Drinking Water

Arsenic is element 33 on the periodic table, a metalloid that occurs naturally in Earth's crust at an average concentration of 1.8 parts per million. It enters groundwater through dissolution of arsenic-bearing minerals including arsenopyrite (FeAsS), realgar (As4S4), orpiment (As2S3), and iron oxides that adsorb arsenic from surrounding rock. Unlike contaminants introduced by human activity, arsenic is almost always naturally occurring - though mining, pesticide use, and industrial discharge can amplify local concentrations.

The EPA estimates that approximately 2.1 million Americans drink water from private wells with arsenic above the 10 ppb MCL. An additional 500,000-700,000 people served by public water systems receive water that exceeds the standard during certain seasonal conditions. Globally, over 200 million people in 70 countries drink water with arsenic above 10 ppb, making it the most widespread natural drinking water contaminant worldwide.

How Arsenic Enters Groundwater

Arsenic release into groundwater depends on geology, hydrology, and geochemistry. The primary mechanisms are:

• Reductive dissolution of iron oxides: In oxygen-poor (reducing) aquifers, iron oxide minerals dissolve and release adsorbed arsenic. This process explains the high arsenic in alluvial aquifers of Bangladesh, the Mekong Delta, and parts of the US Great Plains.
• Oxidative weathering of sulfide minerals: Arsenopyrite and other arsenic-bearing sulfides oxidize when exposed to air or oxygenated water, releasing arsenic. This occurs in mining areas and where water tables fluctuate.
• Competitive desorption: Phosphate from agricultural fertilizers competes with arsenic for binding sites on iron oxides, potentially releasing arsenic into groundwater. Silicate and carbonate ions can have similar effects.
• Geothermal influence: Volcanic and geothermal areas (Yellowstone, the Pacific Northwest, parts of Chile and Argentina) produce arsenic-rich water through hydrothermal mineral dissolution.

Sources and Geographic Distribution

High-Risk Regions in the United States

Arsenic in US groundwater follows distinct geological patterns:

• Southwestern States (Arizona, Nevada, New Mexico, California, Utah, Colorado): Arsenic occurs naturally in volcanic rocks, alluvial basin fill, and geothermal systems. Arizona has over 20,000 private wells exceeding 10 ppb, particularly in Yavapai, Gila, and Cochise counties where concentrations reach 50-500 ppb. The Central Valley of California has extensive arsenic contamination in shallow alluvial aquifers.
• Upper Midwest (Michigan, Wisconsin, Minnesota, North Dakota, South Dakota): Glacial deposits and sandstone aquifers contain elevated arsenic. The Fox River Valley in Wisconsin has documented concentrations up to 12,000 ppb in private wells. Confined aquifers with reducing conditions are particularly susceptible.
• New England (Maine, New Hampshire, Vermont): Arsenic-bearing sulfide minerals in metamorphic bedrock create high concentrations in fractured rock aquifers. Maine has the highest percentage of private wells exceeding 10 ppb of any eastern state - approximately 20-25% of wells in coastal and central Maine. The famous "arsenic belt" runs from the Kennebec River valley through Augusta and Waterville.
• Texas and the Great Plains: The Ogallala Aquifer and southern High Plains aquifers show arsenic levels of 5-50 ppb in many areas, related to volcanic ash deposits in the geologic formation.
• Pacific Northwest (Washington, Oregon, Alaska): Volcanic and geothermal activity creates arsenic hotspots. The Yakima Valley in Washington and the Willamette Valley in Oregon both have documented contamination.

Private Wells vs. Public Water Systems

Private wells are not regulated by the Safe Drinking Water Act and have no legal requirement to meet the 10 ppb standard. The EPA estimates 13% of private wells nationwide exceed 10 ppb arsenic, compared to 1-2% of public water systems. Well owners bear full responsibility for testing and treatment. Public water systems exceeding the MCL must notify customers and implement treatment - typically activated alumina, modified lime softening, or reverse osmosis at the treatment plant.

As(III) vs As(V): The Two Forms of Arsenic

Arsenic in drinking water exists in two oxidation states, and treatment effectiveness depends critically on which form is present. Testing for "total arsenic" alone is insufficient - you need speciation analysis.

As(III) Arsenite: The Mobile, Toxic Form

In reducing (oxygen-poor) groundwater, arsenic predominantly exists as As(III), specifically as arsenous acid (H3AsO3) or its ionized forms. At neutral pH, As(III) is uncharged (H3AsO3), making it extremely difficult to remove by adsorption or ion exchange - most treatment media rely on electrostatic attraction to charged species. As(III) is also 25-60 times more toxic than As(V) because it mimics phosphate and disrupts cellular energy metabolism (ATP synthesis). It crosses cell membranes more readily through aquaglyceroporin channels. Most treatment technologies achieve only 30-60% removal of As(III) without pre-treatment.

As(V) Arsenate: The Treatable Form

In oxidizing (oxygen-rich) conditions, arsenic converts to As(V), existing as arsenate ions (H2AsO4- and HAsO4^2-). These are negatively charged at drinking water pH (6.5-8.5), making them readily adsorbed by activated alumina, iron oxide media, and ion exchange resins. As(V) is also less toxic because cells absorb it less efficiently. Most treatment technologies achieve 90-99% removal of As(V).

Pre-Oxidation: Converting As(III) to As(V)

If your water contains As(III) - common in deep wells and reducing aquifers - pre-oxidation is mandatory before adsorptive treatment. Methods include:

• Chlorine (sodium hypochlorite): 1-2 mg/L free chlorine contact for 10 minutes fully oxidizes As(III) to As(V). This is the most common approach for whole-house systems. The chlorine also provides disinfection.
• Potassium permanganate (KMnO4): Dose of 1-5 mg/L oxidizes As(III) at pH 6-9. Creates visible pink color at overdose, making it self-indicating. Commonly used in Greensand iron filter systems.
• Ozone (O3): Highly effective but requires ozone generator equipment ($2,000+). Used primarily in commercial applications.
• Chlorine dioxide: Alternative disinfectant that oxidizes As(III) without forming trihalomethanes. Requires on-site generation.
• Passive aeration: Simply exposing water to air oxidizes As(III) slowly over hours. Not practical for whole-house treatment but relevant for storage tank systems.

Health Effects of Arsenic Exposure

The IARC classifies inorganic arsenic in drinking water as carcinogenic to humans (Group 1) based on extensive epidemiological evidence, particularly from studies in Taiwan, Chile, Argentina, Bangladesh, and the United States.

Cancer Risks

• Skin cancer: Both squamous cell carcinoma and basal cell carcinoma show dose-response relationships with arsenic exposure. Non-cancerous skin changes (hyperpigmentation, keratosis) often appear first as early warning signs.
• Bladder cancer: A meta-analysis of 20 epidemiological studies found a relative risk of 1.44 for every 10 ug/L increase in drinking water arsenic above 10 ppb.
• Lung cancer: Even non-smokers in high-arsenic areas show elevated lung cancer rates. The risk combines multiplicatively with smoking.
• Kidney and prostate cancer: Moderate but consistent associations in cohort studies.

Non-Cancer Health Effects

• Cardiovascular disease: Chronic exposure accelerates atherosclerosis, increases risk of hypertension, and causes peripheral artery disease. A 2023 review in Environmental Health Perspectives linked arsenic above 10 ppb to 15-30% increased cardiovascular mortality.
• Diabetes: Epidemiological studies consistently associate arsenic exposure with type 2 diabetes risk. Mechanism involves disruption of insulin signaling and pancreatic beta-cell function.
• Neurotoxicity: Arsenic crosses the blood-brain barrier. Studies in Maine and Bangladesh link childhood arsenic exposure to reduced IQ (2-5 point decrement), memory deficits, and behavioral problems.
• Peripheral neuropathy: Numbness, tingling, and weakness in extremities, typically symmetric and "stocking-glove" in distribution.
• Respiratory effects: Chronic cough, bronchiectasis, and reduced lung function.

Latency Period

Arsenic-related health effects typically manifest after 5-20 years of chronic exposure. This long latency means that by the time symptoms appear, years of damage have accumulated. The implication for well owners: test your water before symptoms develop, not after.

EPA Standards and Regulations

The EPA established the arsenic MCL at 10 ppb (0.010 mg/L) in January 2001, replacing the previous 50 ppb standard that dated to 1942. Public water systems had until January 2006 to comply. The 10 ppb standard was a compromise - the EPA's maximum contaminant level goal (MCLG) is actually zero, but treatment technology limitations and cost considerations prevented a lower enforceable limit. The agency estimated that reducing the standard from 50 ppb to 10 ppb would prevent 30-50 cases of bladder and lung cancer annually.

For private well owners, no federal requirement exists to test or treat. However, the EPA and CDC recommend annual testing for arsenic and other contaminants, particularly in high-risk geological areas. Some states (New Jersey, New Hampshire, Maine, Vermont) have enacted requirements for arsenic disclosure during real estate transactions.

How to Test for Arsenic

Certified Laboratory Testing ($25-$50 for total arsenic)

Arsenic testing requires specialized analytical equipment - atomic absorption spectrophotometry with hydride generation (EPA Method 200.9) or inductively coupled plasma mass spectrometry (EPA Method 200.8). Sample containers must contain preservative (typically nitric acid to pH <2). Laboratories certified for drinking water arsenic analysis report results to 1 ppb precision.

Request speciation if total arsenic exceeds 5 ppb. Speciation analysis separates As(III) from As(V) and adds $30-75 to the cost but is essential for treatment selection. If As(III) exceeds 20% of total arsenic, plan for pre-oxidation.

Home Test Kits ($15-$30)

Colorimetric test kits (Industrial Test Systems Arsenic Quick, Hach Arsenic Test Kit) provide semi-quantitative results. TheITS kit uses a reaction that produces color from yellow to brown, detecting 0, 10, 25, 50, 100, 250, and 500 ppb. These kits are useful for screening - a positive result above 10 ppb should be confirmed with a certified laboratory. Accuracy: +/- 10 ppb at lower concentrations.

When and How Often to Test

• Test new wells before first use
• Test annually if your well is in a high-risk geological area
• Test after seismic activity, flooding, or changes in nearby land use
• Test if neighbors report arsenic problems
• Test after any well repair or pump replacement (deepening a well can tap a different arsenic-bearing zone)

Arsenic Removal Technologies

Reverse Osmosis (95-99% Removal)

Reverse osmosis forces water through a semipermeable membrane with 0.0001-micron pores, rejecting arsenic molecules along with dissolved salts, heavy metals, and other contaminants. RO is equally effective against As(III) and As(V) - the size exclusion mechanism does not depend on charge. NSF/ANSI 58 certification for arsenic reduction requires >95% removal. Point-of-use RO systems (under-sink) typically produce 50-100 gallons per day. Whole-house RO systems are available but expensive ($3,000-8,000) due to the need for large membranes, storage tanks, and booster pumps. Pre-treatment with sediment and carbon filters protects the membrane. Annual maintenance: $80-150 for filter replacements, $100-200 for membrane every 2-3 years.

Activated Alumina (90-95% for As(V))

Activated alumina (Al2O3) is a porous aluminum oxide specifically manufactured for water treatment. The positively charged aluminum surface adsorbs negatively charged As(V) ions through electrostatic attraction and ligand exchange. Optimal pH: 6.0-7.0 for maximum capacity. At pH above 8, capacity drops significantly. Activated alumina does NOT effectively remove As(III) - pre-oxidation is required. Media capacity: 3,000-8,000 bed volumes depending on arsenic concentration and pH. A 2.5 cubic foot system treats approximately 500,000-1,000,000 gallons before media replacement. Cost: $1,200-2,500 for whole-house system. Media replacement: $400-800 every 3-5 years.

Iron Oxide/Hydroxide Media (85-95%)

Iron oxide-based filter media (AdEdge AD33, Bayoxide E33, Metsorb) use amorphous iron hydroxide surfaces to adsorb both As(III) and As(V), though As(V) removal is more efficient. Some iron oxide media show 60-70% As(III) removal without pre-oxidation, higher than activated alumina. pH range: 6.0-8.5. These media are increasingly popular for whole-house arsenic treatment because they handle both arsenic species reasonably well. Capacity: 5,000-15,000 bed volumes. System cost: $1,500-3,000.

Distillation (100% Removal)

Distillation boils water and condenses the steam, leaving arsenic and other non-volatile contaminants behind. This is the only method that achieves complete arsenic removal regardless of speciation. Countertop distillers produce 0.5-1 gallon per cycle (4-6 hours). Energy cost: approximately $0.25-0.40 per gallon at $0.13/kWh electricity rates. Distilled water tastes flat due to lack of minerals; some units include a post-filter cartridge to add calcium and magnesium. Best for drinking and cooking water in homes where whole-house treatment is impractical.

What Does NOT Work

• Standard activated carbon: No measurable arsenic removal
• Brita and PUR pitcher filters: Not designed for arsenic
• UV sterilization: Kills bacteria but does not remove dissolved metals
• Water softeners: Ion exchange targets calcium and magnesium, not arsenic
• Boiling: Concentrates arsenic as water evaporates

Best Arsenic Water Filters

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