What Is Arsenic?

Arsenic is a naturally occurring metalloid element found in rocks, sediments, soils, geothermal fluids, and some ore deposits. In drinking water, it is most often a groundwater problem rather than a surface-water problem, although surface sources can be affected by mine drainage, industrial discharges, volcanic geology, or sediment disturbance. Arsenic is classified here with heavy metals because it behaves as a toxic trace element in water and is regulated primarily through inorganic chemical contaminant standards.

The most important drinking water forms are inorganic arsenic species, especially arsenite, commonly written as As(III), and arsenate, As(V). These dissolved forms are colorless, odorless, and tasteless at concentrations of health concern, so household users cannot identify arsenic by appearance, smell, or taste. Clear well water can contain arsenic above a health-based limit, and boiling the water does not remove it.

Arsenic is a high-risk contaminant because harm is usually associated with long-term ingestion rather than a single obvious exposure event. People may drink contaminated well water for years before testing reveals a problem. Chronic exposure has been associated with cancers, skin changes, cardiovascular disease, diabetes-related effects, developmental concerns, and injury to multiple organ systems.

Scientific Identity

Arsenic has the chemical symbol As and CAS number 7440-38-2. Elemental arsenic itself is not usually the form measured directly in drinking water; laboratories typically report total arsenic after preserving the sample and analyzing the dissolved and particulate-associated fraction included by the test protocol. For health evaluation, total arsenic is commonly compared with drinking water standards, but treatment performance depends strongly on arsenic speciation.

In oxygen-rich water, arsenic is commonly present as arsenate, As(V), an oxyanion that can be removed efficiently by many adsorption and membrane technologies. In low-oxygen groundwater, arsenite, As(III), may dominate. Arsenite is generally more mobile, more difficult to adsorb, and less efficiently rejected by some treatment media unless it is first oxidized to arsenate. This is why two wells with the same total arsenic concentration can respond differently to the same filter.

Arsenic chemistry is closely tied to pH, oxidation-reduction conditions, iron and manganese minerals, sulfide, phosphate, silica, alkalinity, and organic matter. Arsenic can be released when iron oxides dissolve under reducing conditions, when pH rises and weakens adsorption, or when competing ions such as phosphate displace arsenate from mineral surfaces. These geochemical controls explain why arsenic can vary sharply between neighboring wells completed at different depths.

How Arsenic Enters Drinking Water

The most common pathway is natural leaching from arsenic-bearing minerals into groundwater. Aquifers containing volcanic ash, sulfide minerals, iron oxides, geothermal deposits, or certain sedimentary formations may release arsenic under specific geochemical conditions. Reducing aquifers, where oxygen is depleted, can dissolve iron oxide coatings that previously bound arsenic, releasing both iron and arsenic into the water.

Mining and ore processing can intensify arsenic contamination because arsenic commonly occurs with gold, copper, lead, zinc, and sulfide ores. Mine tailings, waste rock, acid mine drainage, and smelter emissions can move arsenic into streams, soils, and shallow groundwater. Industrial sources have included wood preservatives, pesticides, glass production, semiconductor manufacturing, coal combustion residues, and metal refining wastes. Many of these uses are now restricted in some countries, but contaminated soils and sediments can remain long-term sources.

Arsenic can also enter water through corrosion or dissolution of arsenic-containing materials, although this is usually less important than geologic release for drinking water wells. In distribution systems, arsenic may accumulate in iron-rich pipe scale and later be released if water chemistry changes, such as shifts in disinfectant type, pH, phosphate dosing, or redox conditions. For this reason, utility system changes sometimes require monitoring for metals mobilized from existing deposits.

Occurrence and Exposure

Arsenic occurs worldwide, with significant groundwater regions reported in parts of South Asia, Southeast Asia, Latin America, the western and midwestern United States, Canada, Europe, and many other areas with favorable geology. The Bengal Basin, parts of Bangladesh and India, is one of the best-known examples of widespread natural arsenic contamination, but the problem is not limited to any single region. In the United States, elevated arsenic has been found in parts of the Southwest, Great Basin, New England, Midwest, and areas influenced by geothermal or volcanic geology.

Private wells are a major exposure concern because they are typically not regulated or routinely tested by government agencies after installation. A household well may be safe for bacteria yet still contain arsenic, and a neighboring well may have a very different result because of depth, casing construction, aquifer zone, and local mineral chemistry. New wells, newly purchased homes served by wells, and wells in known arsenic-prone geology should be tested by a certified laboratory.

Exposure occurs primarily through drinking water used for drinking, cooking, beverages, and reconstituting infant formula. Arsenic is not effectively absorbed through intact skin during bathing, so ingestion is the dominant route for household water exposure. However, using contaminated water to cook rice, soups, or other absorbent foods can increase dietary intake because arsenic remains in the food as water is absorbed or evaporates.

Health Effects and Risk

Inorganic arsenic is toxic because it interferes with cellular energy metabolism, oxidative stress pathways, DNA repair, vascular function, and enzyme systems. The main public health concern in drinking water is chronic exposure over months to years. Arsenic does not need to create a visible illness immediately to be dangerous; risk accumulates with concentration, duration, total water intake, age, nutrition, and individual susceptibility.

Long-term ingestion of inorganic arsenic has been associated with skin changes such as hyperpigmentation and keratosis, peripheral neuropathy, cardiovascular disease, hypertension, diabetes-related outcomes, impaired immune response, and developmental effects in children. Arsenic exposure is also associated with increased risk of several cancers, especially skin, bladder, and lung cancer. The strength of evidence for inorganic arsenic as a carcinogen is one reason drinking water standards are set at very low microgram-per-liter levels.

Infants, children, pregnant people, and individuals with high daily water consumption may receive proportionally higher doses. People using private wells for all drinking and cooking needs can have higher cumulative exposure than people using treated municipal supplies. Nutritional factors, including folate status and overall methylation capacity, may influence how arsenic is metabolized and excreted, but adequate nutrition should not be considered a substitute for removing arsenic from drinking water.

Arsenic can bioaccumulate in some organisms and is important in food-chain studies, but drinking water risk focuses mainly on repeated ingestion of inorganic arsenic. Organic arsenic forms found in many seafoods are generally less toxic than inorganic arsenic, which is why a drinking water arsenic result should not be interpreted the same way as total arsenic in a seafood-based diet.

Testing and Monitoring

Arsenic cannot be reliably detected with taste, odor, color, or simple household observations. The appropriate method is laboratory metal analysis using a properly collected water sample. Common analytical techniques include inductively coupled plasma mass spectrometry, often abbreviated ICP-MS, inductively coupled plasma optical emission spectroscopy, and graphite furnace atomic absorption methods. Certified laboratories typically report results in micrograms per liter, also expressed as parts per billion for water.

For private wells, testing should include total arsenic at minimum. If treatment is being designed, speciation testing for As(III) and As(V) may be valuable, especially when considering adsorption media or when reducing groundwater is suspected. Field parameters such as pH, iron, manganese, sulfide, phosphate, silica, alkalinity, hardness, and total dissolved solids can help predict treatment performance and fouling risk.

Sampling should follow the laboratory’s instructions. Many arsenic samples require acid-preserved bottles, and some tests require filtered or unfiltered samples depending on the monitoring objective. A first-draw sample is generally more relevant for lead and copper corrosion studies, while arsenic assessment usually focuses on the water supply itself after appropriate flushing unless pipe-scale release is being investigated. If a well result is high, confirmation with a second laboratory sample is often prudent before selecting a permanent treatment system.

Monitoring frequency depends on risk. A new private well should be tested before use, and existing wells in arsenic-prone areas should be retested periodically because groundwater chemistry and pumping patterns can change. After installing treatment, both the treated water and untreated raw water should be tested to confirm removal and to establish a maintenance schedule.

Treatment Methods

Reverse osmosis is often the best point-of-use treatment for arsenic in household drinking water because it can remove a broad range of dissolved contaminants, including arsenate and many forms of arsenic when the system is properly selected, maintained, and verified by testing. A typical under-sink RO unit treats water used for drinking and cooking, which is usually the most practical strategy because ingestion is the primary exposure route. RO systems work by forcing water through a semi-permeable membrane that rejects many ions and dissolved species.

RO performance can decline when membranes are damaged, fouled, poorly maintained, incorrectly installed, or used outside their design range. High total dissolved solids, hardness scaling, iron, manganese, sediment, chlorine exposure to non-chlorine-tolerant membranes, or inadequate pressure can reduce effectiveness. Arsenite, As(III), may be less reliably removed than arsenate by some systems, so pre-oxidation to As(V) or a treatment train may be needed for reducing groundwater. Post-installation laboratory testing is essential; a filter label alone is not proof of arsenic removal in a specific well.

Point-of-use treatment is usually appropriate for arsenic because drinking and cooking water drive risk. Point-of-entry treatment may be considered when arsenic levels are very high, when multiple taps must supply drinking water, for small public systems, or where pipe-scale interactions and household uses justify whole-house treatment. However, whole-house arsenic treatment is more expensive, creates more waste media or concentrate, and requires careful operation to avoid breakthrough.

Regulations and Guidelines

Arsenic is regulated in many countries because of its well-established chronic toxicity and cancer risk. In the United States, the EPA maximum contaminant level for arsenic in public drinking water systems is 10 micrograms per liter, commonly written as 10 µg/L or 10 parts per billion. This federal standard applies to regulated public water systems, not to most private household wells.

The World Health Organization guideline value for arsenic in drinking water is also commonly cited as 10 µg/L, although WHO has described this value in relation to analytical feasibility, treatment practicality, and health risk management. Some countries, states, provinces, or local authorities may use different limits, interim values, or more stringent health advisory goals. Regulatory limits vary by jurisdiction, and local water suppliers must follow the rules that apply in their country or region.

For private well owners, the practical target is usually to reduce arsenic as low as reasonably achievable and at least below the applicable local drinking water limit. If a well exceeds a regulatory or health guideline value, water used for drinking, cooking, and infant formula should be treated with a verified technology or replaced with a safe alternative source until treatment is confirmed by laboratory analysis.

Related Contaminants

Frequently Asked Questions

Can I see, taste, or smell arsenic in water?

No. Arsenic at harmful drinking water concentrations is typically colorless, tasteless, and odorless. Clear, pleasant-tasting well water can still contain arsenic above health-based limits, so laboratory testing is the only reliable way to know.

Is arsenic mainly a private well problem?

Private wells are a major concern because they are not usually monitored under public water system regulations. Public systems must test and manage arsenic according to applicable laws, while private well owners are responsible for their own testing, treatment, and maintenance.

Does reverse osmosis remove arsenic?

Yes, properly certified and maintained reverse osmosis systems can substantially reduce arsenic, especially for point-of-use drinking water. Performance depends on arsenic form, membrane condition, pressure, pretreatment, and water chemistry. A post-installation laboratory test is necessary to confirm actual removal.

Why does arsenic speciation matter?

Arsenic can occur as As(III) or As(V). As(V), or arsenate, is generally easier to remove by adsorption, anion exchange, and many RO systems. As(III), or arsenite, is more difficult for some technologies and may require oxidation before final removal.

Is boiling arsenic-contaminated water safe?

No. Boiling does not remove arsenic. Because water evaporates while arsenic remains behind, boiling can slightly increase the arsenic concentration in the remaining water. Use a verified treatment system or an alternative safe water source instead.