What Is Selenium?

Selenium is a naturally occurring trace element with chemical symbol Se. It is often discussed with heavy metals in drinking water because it can persist in groundwater, form toxic dissolved species, and cause health effects when consumed above safe levels over time. Strictly speaking, selenium is commonly classified as a metalloid or trace element rather than a conventional heavy metal such as lead or cadmium, but its drinking water behavior and public health relevance place it in the same practical monitoring category.

Selenium is nutritionally essential in small amounts. It is incorporated into selenoproteins that support antioxidant function, thyroid hormone metabolism, and immune processes. The public health challenge is that the margin between adequate intake and excessive intake is relatively narrow compared with many other nutrients. Drinking water is usually not the only selenium source; diet, supplements, and occupational exposure can contribute. However, in areas with selenium-rich geology or mining activity, water can become a meaningful part of total daily intake.

In water, selenium does not behave as a single simple substance. It changes form depending on oxidation-reduction conditions, pH, microbial activity, and the presence of competing ions. The most important dissolved drinking water forms are selenate, generally associated with oxygenated and alkaline waters, and selenite, often found under less oxidizing conditions. These forms differ in mobility, treatability, and tendency to attach to mineral surfaces.

Scientific Identity

Selenium has atomic number 34 and occurs in several oxidation states, most importantly Se(-II), Se(0), Se(IV), and Se(VI). In drinking water assessment, speciation matters because selenate, Se(VI), and selenite, Se(IV), are the dominant inorganic dissolved species. Selenate commonly occurs as the oxyanion SeO₄^2-, while selenite occurs primarily as HSeO₃^– or SeO₃^2- depending on pH. Both are soluble enough to move through aquifers, but selenate is usually the more mobile form because it sorbs less strongly to iron and aluminum oxides.

Selenium is not microbial or radiological, but microbes strongly influence its environmental fate. Certain bacteria can reduce selenate or selenite to elemental selenium, Se(0), which is far less soluble and may precipitate as red or gray particles. Other biological and geochemical processes can produce organic selenium compounds, especially in wetlands, soils, and biologically active sediments. These transformations are important in aquifers and treatment systems because the measured total selenium concentration may represent multiple chemical species with different removal behavior.

Laboratory drinking water reports usually list selenium as total recoverable or dissolved selenium, reported in milligrams per liter or micrograms per liter. A result labeled simply “selenium” typically does not identify whether the selenium is selenate or selenite. When treatment is being designed for a difficult source water, selenium speciation testing can be valuable because reverse osmosis, ion exchange, adsorption, and biological treatment do not perform identically for all selenium forms.

How Selenium Enters Drinking Water

The most common source of selenium in private wells and groundwater systems is natural geology. Selenium is enriched in some marine shales, sedimentary rocks, coal-bearing formations, phosphate deposits, volcanic materials, and mineralized zones. As groundwater moves through these materials, selenium can dissolve into the water, especially where oxygenated water contacts selenium-bearing minerals and produces soluble selenate.

Mining and mineral processing can mobilize selenium by exposing sulfide minerals, waste rock, tailings, coal seams, and overburden to air and water. Coal mining, uranium mining, phosphate mining, metal mining, and oil shale development may all produce selenium-bearing drainage under certain conditions. Selenium can also occur in water associated with coal ash disposal, refinery operations, glass manufacturing, metal smelting, and some electronics or pigment-related industrial activities.

Agricultural irrigation can create selenium problems in arid and semi-arid regions. Irrigation water can dissolve selenium from naturally enriched soils and then concentrate it through evaporation. Drainage water may carry selenium into shallow aquifers, canals, wetlands, or reservoirs. This pathway is especially important in landscapes with seleniferous soils, high evaporation, poor drainage, and repeated irrigation cycles.

Corrosion is not typically the dominant selenium source in household plumbing, unlike copper, lead, or zinc, but industrial alloys, specialty materials, and some plumbing-related metal mixtures can contribute trace selenium in unusual cases. More often, selenium enters drinking water before it reaches the building, through aquifer contact, source-water contamination, or treatment plant intake from an affected surface water.

Occurrence and Exposure

Selenium in drinking water is geographically uneven. Many water supplies have very low selenium, while wells in selenium-rich basins can show elevated concentrations. Groundwater is often more vulnerable than surface water because it can remain in long contact with selenium-bearing formations. Private wells are a particular concern because they are not routinely monitored under municipal water regulations and may be located in small geochemical “hot spots” not reflected by regional averages.

Higher-risk settings include wells completed in marine shale, coal-bearing sedimentary basins, mining districts, arid irrigated valleys, and areas influenced by mine drainage or industrial wastewater. Selenium can also be present in reservoirs or rivers receiving agricultural drainage, coal combustion residual leachate, or mining-affected tributaries. In surface water, selenium may accumulate in aquatic food webs, which is why environmental agencies often monitor it for ecological risk as well as drinking water relevance.

Human exposure occurs primarily by ingestion. Bathing and showering are not considered major selenium exposure routes because inorganic selenium is not efficiently absorbed through intact skin and is not volatile like radon or some solvents. For households with elevated selenium in water, the greatest concern is water used for drinking, cooking, infant formula preparation, coffee, tea, and beverages made from tap water. Boiling does not destroy selenium and can slightly concentrate it as water evaporates.

Total exposure should be evaluated in context. People taking selenium supplements, consuming high-selenium foods, or living in regions where staple crops accumulate selenium may have less margin for additional water exposure. Conversely, in many locations, food remains the dominant selenium source and water contributes little. The drinking water question becomes important when laboratory results show selenium near or above health-based guideline values.

Health Effects and Risk

Selenium has a dual role in human health: essential at low intake and toxic at excessive intake. The main drinking water concern is chronic ingestion over months to years, not brief skin contact or occasional short-term exposure. Excess selenium intake can cause selenosis, a condition associated with hair and nail brittleness or loss, gastrointestinal upset, fatigue, irritability, garlic-like breath odor, skin rash, and neurological symptoms such as tingling or peripheral nerve effects in more severe cases.

Long-term high intake may affect the liver, endocrine function, and nervous system, though risk depends on dose, duration, chemical form, nutritional status, and selenium intake from food and supplements. Sensitive groups include infants, pregnant people, individuals using selenium-containing supplements, people with kidney disease affecting excretion, and residents relying on a single untreated private well with elevated selenium.

Selenium can bioaccumulate in aquatic food webs, particularly in fish, birds, and invertebrates, but human drinking water guidance is mainly based on direct ingestion and total dietary exposure. Ecological selenium problems can occur at water concentrations that are important for wildlife reproduction, even when drinking water risks are assessed separately. For household decisions, the key step is comparing a certified laboratory result with the applicable health-based drinking water standard in the jurisdiction.

Testing and Monitoring

Selenium cannot be reliably identified by taste, odor, color, or staining. A clear, pleasant-tasting well can contain elevated selenium. Testing requires laboratory metal analysis using methods such as inductively coupled plasma mass spectrometry, ICP-MS, inductively coupled plasma optical emission spectrometry, ICP-OES, or atomic absorption methods. The sample bottle, preservative, holding time, and filtration requirements should follow the laboratory’s instructions because metals data can be affected by collection and handling.

For drinking water compliance and household screening, total selenium is usually the first test. If the result is elevated, repeat sampling can confirm whether the finding is persistent or related to sampling error, well disturbance, or plumbing conditions. For private wells, testing both raw water and treated water is useful when evaluating treatment performance. If sediment is present, the laboratory may distinguish dissolved selenium from total recoverable selenium, which includes selenium associated with fine particles.

Speciation testing for selenate and selenite is not always included in routine residential panels, but it can be important for treatment design. Selenate is generally harder to remove by simple adsorption than selenite and may require reverse osmosis, strong-base anion exchange, or specialized treatment. Water chemistry parameters such as pH, alkalinity, sulfate, nitrate, chloride, total dissolved solids, iron, manganese, and arsenic should also be measured because they influence treatment selection and media life.

Private well owners in selenium-prone regions should test at least once and retest if nearby mining, drilling, irrigation drainage changes, drought, flooding, or well construction changes occur. Municipal water customers can review annual consumer confidence reports or local water quality summaries, but these reports may not describe conditions in individual private wells outside the regulated system.

Treatment Methods

Reverse osmosis is generally the best household treatment for selenium in drinking water when the system is properly sized, maintained, and verified by testing. RO uses a semi-permeable membrane to reject dissolved ions, including selenate and selenite. It is most commonly installed as a point-of-use unit under the kitchen sink, supplying treated water for drinking and cooking. This is often appropriate because ingestion is the critical exposure route, while whole-house treatment is usually unnecessary unless selenium concentrations are very high or treated water is needed at many taps.

RO can fail or underperform if the membrane is old, fouled, damaged, improperly installed, exposed to excessive pressure variation, or operated outside its design range. High total dissolved solids, hardness scaling, iron fouling, manganese, silica, organic matter, chlorine attack on non-resistant membranes, and poor prefiltration can reduce performance. RO systems also produce a waste concentrate stream containing the rejected selenium. After installation, the treated water should be laboratory-tested for selenium rather than assuming removal from a product label.

Point-of-entry selenium treatment may be considered for small public systems, high-use households, or situations where multiple drinking taps must be treated. However, whole-house RO is expensive, produces more concentrate, and may require corrosion stabilization after treatment because RO water can be low in minerals. For most homes, a certified point-of-use RO unit with periodic laboratory verification is the practical first choice.

Regulations and Guidelines

Selenium is regulated or guided by many national drinking water programs because of its chronic toxicity at elevated intake. In the United States, the EPA has established a federal maximum contaminant level for selenium in public drinking water of 0.05 mg/L, equivalent to 50 micrograms per liter. Public water systems subject to the Safe Drinking Water Act must monitor and manage selenium according to federal and state requirements.

The World Health Organization has published a health-based drinking water guideline value for selenium; recent WHO guidance has used a value in the tens of micrograms per liter range, commonly cited as 40 micrograms per liter. Other countries and regions may set different values. For example, national limits may differ across the European Union, Canada, Australia, and local jurisdictions, and some authorities may apply more conservative values for vulnerable populations or specific water supply contexts.

Regulatory values should always be checked against the current rule in the relevant location because standards can change as toxicology, dietary exposure assumptions, and analytical capabilities are updated. Private wells are often not covered by public water system regulations, even where a national selenium standard exists. For private well users, the regulatory value is best treated as a health benchmark for deciding whether treatment, an alternate source, or additional monitoring is needed.

Related Contaminants

Frequently Asked Questions

Is selenium in drinking water always harmful?

No. Selenium is an essential nutrient at low intake, and many water supplies contain only trace amounts. The concern begins when laboratory results show concentrations approaching or exceeding health-based drinking water limits, especially when a person also receives selenium from supplements or high-selenium foods.

Can I remove selenium by boiling my water?

No. Boiling does not break down or remove selenium. Because selenium is a dissolved inorganic element, boiling can slightly increase its concentration as water evaporates. If selenium is elevated, use an effective treatment such as reverse osmosis or an alternate low-selenium water source.

Is reverse osmosis enough for selenium?

Usually, yes for household drinking water, if the RO system is appropriate for the water chemistry and is maintained correctly. The only way to confirm performance is to test the treated water for selenium after installation and periodically afterward. Prefilters, membrane replacement, pressure, and fouling control matter.

Do refrigerator filters remove selenium?

Most refrigerator filters use activated carbon, which is not a reliable treatment for dissolved selenium oxyanions. Unless the filter is specifically certified and tested for selenium reduction, it should not be relied upon for elevated selenium.

Should a private well be tested for selenium?

Yes, especially in areas with marine shale, coal-bearing formations, mining, irrigation drainage, oil and gas activity, or known regional selenium issues. Selenium has no dependable taste, odor, or visual warning sign, so laboratory testing is the only practical way to know whether a well is affected.