What Is Selenium?
Selenium is a naturally occurring trace element that sits in an unusual regulatory category: it is both an essential nutrient the human body requires to function and a toxic contaminant the EPA limits in drinking water. In small amounts, selenium is incorporated into selenoproteins — including glutathione peroxidase, an antioxidant enzyme, and enzymes that activate thyroid hormone. Selenium deficiency causes real disease, most notably Keshan disease, a form of cardiomyopathy first documented in selenium-poor regions of China. But the same element that prevents deficiency at low doses causes selenosis — chronic selenium poisoning — at sustained higher intakes.
The adult Recommended Dietary Allowance for selenium is 55 micrograms a day. The Tolerable Upper Intake Level — the point where chronic toxicity risk begins — is 400 micrograms a day, according to the National Institutes of Health. That's roughly a sevenfold window between "not enough" and "too much," far tighter than the margin for most drinking water contaminants, which have no beneficial dose at all.
In drinking water, selenium is dissolved as an inorganic oxyanion — primarily selenate (Se(VI)) or selenite (Se(IV)), depending on the source and local chemistry. Like arsenic, its two common oxidation states behave differently in water treatment, which matters directly for how it’s removed. Selenium is odorless and colorless at the concentrations found in tap water; you cannot detect it by taste or smell, and confirming its presence requires laboratory testing.
Unlike carcinogens such as arsenic, where the EPA sets the health goal at zero because there’s no known safe threshold, selenium’s health goal is set above zero — a direct consequence of it being a required nutrient. That single fact shapes almost everything else about how selenium is regulated, monitored, and discussed by toxicologists.
How Selenium Gets Into Drinking Water
Natural Geologic Erosion
Selenium occurs naturally in sedimentary rock, shale, and coal deposits across the western United States. As groundwater and surface runoff move through seleniferous formations — most notably the Cretaceous-era Mancos Shale that underlies large parts of Colorado, Utah, and Wyoming — selenium dissolves and enters the water supply. This is the same basic mechanism that puts arsenic and uranium into western groundwater, and the three often turn up in the same wells.
Agricultural Irrigation Drainage
The most consequential source of selenium contamination in US water history is agricultural. When arid-region farmland with seleniferous soils is irrigated, the water mobilizes selenium that would otherwise stay locked in the rock. That water then collects as subsurface drainage and is typically channeled into evaporation ponds, wetlands, or waterways — concentrating selenium far above natural background levels. California’s San Joaquin Valley, Colorado’s Grand Valley, and parts of the Great Plains all have documented irrigation-driven selenium mobilization.
Industrial and Mining Discharge
Coal and uranium mining generate selenium-laden waste rock and leachate. Coal-fired power plants produce selenium-bearing fly ash and scrubber wastewater. Petroleum refineries discharge both selenate and selenite in process water. All three pathways have contributed to documented selenium contamination in surface water and groundwater near industrial sites, distinct from — but often overlapping geographically with — the agricultural drainage problem in the arid West.
The Kesterson Reservoir Warning
The starkest illustration of what concentrated agricultural selenium drainage can do occurred at Kesterson National Wildlife Refuge in California’s San Joaquin Valley. Starting in 1978, subsurface agricultural drainage water — carrying selenium mobilized from irrigated, naturally seleniferous soils — was channeled into the refuge’s ponds, and by 1982 it had become effectively the sole water source feeding them. Selenium bioaccumulated up the food chain, and by the 1983 nesting season, waterfowl at Kesterson were experiencing catastrophic reproductive failure: as many as 64% of eared grebe and American coot nests failed, and surviving chicks showed severe deformities — missing or malformed eyes, legs, beaks, and wings, along with brain and abdominal defects. Researchers measured selenium in the food plants and aquatic insects birds were eating at up to 64 times levels considered hazardous. The reservoir was partially drained and filled between 1987 and 1989 to stop the exposure. Kesterson remains the reference case toxicologists and regulators point to when explaining why selenium drainage from irrigated seleniferous land is treated as a serious contamination pathway rather than a theoretical risk.
Geographic Hotspots
| Region | Primary Driver | Notes |
|---|---|---|
| California (San Joaquin Valley) | Irrigation drainage over seleniferous soils | Site of the Kesterson case; ongoing drainage management programs |
| Colorado (Grand Valley, Mancos Shale belt) | Natural shale erosion + irrigation | Selenium co-occurs with elevated uranium in some wells |
| Nebraska, Wyoming, North & South Dakota | Natural erosion of seleniferous shale and glacial till | Livestock selenosis from seleniferous forage documented since the 1930s–40s |
Health Effects
The health story for selenium runs in two directions, and both matter. At the low end, inadequate selenium intake impairs thyroid hormone metabolism and antioxidant defense and, in severe cases, causes Keshan disease. At the high end — the direction relevant to contaminated drinking water — chronic overexposure causes selenosis.
Selenosis (Chronic Toxicity)
Selenosis develops from sustained intake above the Tolerable Upper Intake Level of 400 micrograms a day. Its hallmark signs are hair loss and brittleness, and nail loss, discoloration, or brittleness — often the first and most visible symptoms. Other documented effects include gastrointestinal symptoms (nausea, diarrhea, abdominal pain), a garlic-like odor on the breath and a metallic taste, skin rashes and dermatitis, fatigue, irritability, and in more severe cases neurological symptoms including numbness, tingling, and, at very high exposures, paralysis. A well-documented 1983-84 case in the United States involved 13 people who took a misformulated dietary supplement containing 27,300 micrograms of selenium per tablet — roughly 182 times the intended dose — over several months; more than half developed nail discoloration and brittleness that persisted for over 90 days after exposure stopped.
Cardiovascular and Metabolic Effects
Epidemiological studies have also associated chronically elevated selenium exposure with increased risk of type 2 diabetes and, in some populations, adverse cardiovascular effects, though the dose-response relationship is less firmly established than for the classic signs of selenosis (hair, nail, and skin changes).
Children
Children have a lower body weight relative to their water intake, so a given selenium concentration in drinking water delivers a proportionally larger dose than it would to an adult. Because the same nutrient supports normal growth and thyroid function at appropriate levels, the practical concern for children is less about any single tap-water reading near the MCL and more about total intake — water plus diet plus any supplements — pushing cumulative exposure toward the Tolerable Upper Intake Level, which is lower for children than for adults.
Pregnant Women
Adequate selenium intake supports normal thyroid hormone activity, which is important for fetal neurodevelopment, so both deficiency and excess carry theoretical risk during pregnancy. Selenium crosses the placenta, and clinicians generally advise pregnant women to meet — but not substantially exceed — the RDA (60 micrograms/day during pregnancy, per NIH guidance) through diet and prenatal vitamins rather than high-dose standalone supplements, particularly if their water supply already shows detectable selenium.
EPA Regulation and Limits
The EPA set both the Maximum Contaminant Level (MCL) and the Maximum Contaminant Level Goal (MCLG) for selenium at 0.05 mg/L — 50 parts per billion (ppb) — a standard that took effect in 1992 and has not been revised since. What makes selenium regulatory-history notable is that the MCL and MCLG are identical. For most EPA-regulated carcinogens, like arsenic or trichloroethylene, the MCLG is set at zero because there’s no known safe threshold. Selenium’s MCLG is set at the same level as its enforceable limit because it is an essential nutrient with a genuine toxicological threshold — the EPA’s assessment is that 50 ppb represents the point below which there is no appreciable risk from chronic exposure, not merely the lowest level treatment can practically achieve.
| Standard | Value | Notes |
|---|---|---|
| EPA MCL (enforceable limit) | 50 ppb (0.05 mg/L) | In effect since 1992; unchanged since |
| EPA MCLG (health goal) | 50 ppb (0.05 mg/L) | Set equal to the MCL — selenium has a genuine safe threshold, unlike zero-MCLG carcinogens |
| WHO guideline | 40 µg/L | Provisional; derived from allocating 20% of the tolerable daily intake to a 2 L/day water allowance |
| Adult RDA (dietary, all sources) | 55 µg/day | National Institutes of Health, Office of Dietary Supplements |
| Tolerable Upper Intake Level | 400 µg/day | The threshold above which selenosis risk begins |
Public water systems must monitor for selenium on the same standard schedule as other inorganic contaminants — typically every three years for systems with a history of compliance, more frequently for systems with prior detections. There is no pending EPA rulemaking to revise the selenium standard as of this writing; it has not been part of the recent regulatory activity surrounding arsenic, PFAS, or lead.
How Widespread Is Selenium?
Selenium exceedances of the 50 ppb MCL in public water systems are uncommon nationally — most of the country sits on geology with low background selenium, and utilities serving affected areas have decades of experience managing it through source blending or treatment. The exposure picture is different for private wells in the specific regions where seleniferous geology and irrigation drainage overlap.
USGS and state agricultural-extension studies have repeatedly documented elevated groundwater selenium in California’s San Joaquin Valley, Colorado’s Grand Valley and other areas underlain by the Mancos Shale, and parts of Nebraska, Wyoming, and the Dakotas, where seleniferous shale and glacial till caused livestock selenosis outbreaks as far back as the 1930s. In these regions, well owners — who are responsible for their own testing, unlike customers of a monitored public system — are the population most likely to encounter selenium at or near the MCL. Selenium in western groundwater also frequently co-occurs with nitrate from the same irrigated agricultural land, and with arsenic from the same underlying shale and volcanic geology, meaning a well that tests high for one is a reasonable candidate for testing the others.
How WaterVerge Tracks Selenium
WaterVerge pulls public water system selenium results from the EPA’s Safe Drinking Water Information System (SDWIS), the federal database of record for compliance monitoring. City and water-system pages display the most recent reported selenium levels alongside the 50 ppb MCL, and flag any system with a confirmed violation along with the corrective action on record.
SDWIS data reflects treated water leaving the utility or at designated compliance points — it is a system-wide measure, not a reading from any individual household tap, and it says nothing about private wells, which fall entirely outside federal monitoring. If you’re on a private well in an arid western state, or in any of the regions with documented seleniferous geology, WaterVerge’s data can tell you what nearby public systems report, but it cannot substitute for direct testing of your own well — see our well water testing guide for how to get that done.
How to Remove Selenium
Standard activated carbon — the media in most pitcher, faucet, and basic under-sink filters — does not meaningfully remove selenium. Selenium exists in water as a dissolved inorganic oxyanion, not an organic compound or particulate, so it passes through carbon media largely unaffected. Boiling water does nothing either; it concentrates dissolved selenium as water evaporates rather than removing it. Effective removal requires reverse osmosis, ion exchange, or activated alumina — and which one performs best depends on which form of selenium is in the water.
| Method | Removal Rate | Certification | Best For |
|---|---|---|---|
| Reverse osmosis | 90–99%, effective on both selenate and selenite | NSF/ANSI 58 | Under-sink systems; the most reliable single method regardless of speciation |
| Anion exchange | Strong for selenate (Se(VI)); performance degrades with high competing sulfate | Check for NSF/ANSI 53 selenium-specific certification | Utility-scale and whole-house treatment where sulfate is low |
| Activated alumina | Over 98% for selenite (Se(IV)); poor for selenate (Se(VI)) | Check for NSF/ANSI 53 selenium-specific certification | Point-of-use where selenite is the dominant species |
| Standard activated carbon (GAC/carbon block) | Not effective | N/A | Not recommended for selenium removal alone |
The speciation issue is the practical takeaway: selenate and selenite respond differently to ion exchange and activated alumina, the same way arsenite and arsenate respond differently to arsenic treatment. A lab test that reports only “total selenium” won’t tell you which species dominates your water; utilities addressing selenium at scale sometimes pre-oxidize selenite to selenate (or vice versa) to route it into whichever treatment train performs best. For a homeowner, that complexity is exactly why reverse osmosis is the more forgiving choice — it removes both forms without needing to know the split. Our guide to the best reverse osmosis systems covers current NSF/ANSI 58-certified options and what to check on a performance data sheet before buying. Whatever method you choose, confirm the specific model has been independently tested and certified for selenium reduction — not every unit that removes other contaminants has been validated for selenium specifically.
Frequently Asked Questions
Is selenium in my drinking water dangerous?
Usually not at the levels typically found in US public water systems, most of which report selenium well below the 50 ppb MCL or no detection at all. Selenium becomes a health concern with chronic exposure above the Tolerable Upper Intake Level of 400 micrograms a day from all sources combined — water, food, and supplements. If you’re on a private well in a seleniferous region (parts of California, Colorado, Nebraska, Wyoming, or the Dakotas), testing is worthwhile since wells aren’t federally monitored.
Why does the EPA set selenium’s health goal (MCLG) equal to its legal limit (MCL), when most contaminants have a gap between the two?
Because selenium is an essential nutrient with a genuine safe threshold, not a substance like arsenic or benzene where any exposure carries some cancer risk. The EPA’s MCLG represents the level below which it sees no appreciable health risk, and for selenium that level is the same 50 ppb it requires utilities to meet.
Can I get too much selenium from a multivitamin plus normal tap water?
It’s possible if you’re taking a high-dose selenium supplement on top of a selenium-rich diet (Brazil nuts are an extreme dietary source) and drinking water with detectable selenium, though most standard multivitamins contain far less than the 400 microgram Upper Intake Level on their own. If your water utility or well test shows selenium near the MCL, it’s worth mentioning to a physician before adding supplemental selenium.
Does boiling water remove selenium?
No. Boiling evaporates water and leaves dissolved selenium behind at a higher concentration — it does not remove the contaminant. Only reverse osmosis, ion exchange, or activated alumina treatment reduces selenium levels.
What is the Kesterson Reservoir case, and is it relevant to my drinking water?
Kesterson was a California wildlife refuge that received concentrated agricultural drainage water in the 1980s, causing severe selenium bioaccumulation and waterfowl deformities. It’s an ecological case, not a drinking water incident, but it’s the reason regulators and researchers treat irrigation drainage over seleniferous soils as a genuine contamination pathway — the same geology and irrigation practices that caused Kesterson also elevate selenium in nearby groundwater used for drinking.
Check Your City
Selenium levels vary enormously by geography, driven by local geology and agricultural history far more than by anything a utility does wrong. Search your city on WaterVerge to see your public water system’s most recent selenium results against the 50 ppb MCL, along with data on related contaminants like arsenic and nitrate that tend to travel with it in western groundwater. If you’re on a private well — especially in California, Colorado, Nebraska, Wyoming, or the Dakotas — SDWIS data won’t cover you, so use our well water testing guide to get your own water tested directly.