Selenium in Drinking Water
Selenium is a naturally occurring mineral essential to human health in trace amounts but toxic at elevated levels — it has one of the narrowest margins between beneficial and harmful dose of any element. In drinking water, selenium exceeds the EPA's 50 ppb MCL primarily near coal mining operations, coal-fired power plant sites, and in naturally selenium-rich agricultural soils of the western United States. Chronic high exposure causes selenosis, characterized by hair loss, nail brittleness, and neurological effects.
Quick Answer
Selenium (Se) is a trace element that functions as a component of selenoproteins essential for thyroid hormone metabolism, antioxidant defense, and immune function. It occurs naturally in soil and rock at varying concentrations — the western High Plains, Colorado Plateau, and parts of the Pacific Northwest have selenium-rich soils that leach into groundwater. Industrial selenium contamination comes primarily from coal mining, coal combustion waste (fly ash and bottom ash), and copper smelting. The narrow therapeutic window — roughly 55 µg/day dietary intake is recommended while 800 µg/day causes toxicity — makes selenium unusual among essential nutrients.
Why Is Selenium in Drinking Water a Concern?
Selenium toxicity (selenosis) from drinking water is documented in communities near coal mining operations in Appalachia and coal ash disposal sites. The 2008 Kingston, Tennessee coal ash spill — the largest industrial spill in U.S. history — contaminated nearby waterways with selenium and other metals. Selenium mobilization from coal ash ponds has been an ongoing regulatory battle, with the EPA establishing regulations for coal combustion residuals that have faced repeated legal and political challenges.
Well owners near coal mines, coal ash disposal ponds, and copper smelters in Kentucky, West Virginia, Wyoming, and Colorado face the highest industrial selenium exposure. Agricultural regions in the western High Plains — Kansas, Nebraska, South Dakota, and parts of Montana and Wyoming — have naturally selenium-rich soils that can leach into shallow wells. Livestock in these regions have historically shown selenium toxicity ('blind staggers') from grazing selenium-rich vegetation.
Health Effects of Selenium in Drinking Water
Selenosis with chronic exposure above the MCL: hair loss (often the first symptom), brittle and pitted fingernails, garlic odor on breath, fatigue
Gastrointestinal distress, nausea, and vomiting at elevated exposures
Neurological effects — numbness, tingling, and paralysis at high chronic doses
Liver cirrhosis with very high long-term exposure
Essential nutrient at low levels — selenium deficiency (Keshan disease) causes heart problems; too little is as concerning as too much in selenium-poor regions
No established acute toxicity at concentrations near the EPA MCL
How Does Selenium Get Into Drinking Water?
Coal mining and coal combustion residuals (fly ash, bottom ash) — major source near power plants and mines
Naturally selenium-rich rock and soil in the western U.S. — particularly phosphate and shale formations
Copper and gold mining operations
Agricultural drainage from selenium-rich soils — particularly in the San Joaquin Valley, where irrigation concentrates selenium
Phosphate fertilizer production and application in selenium-rich soil regions
Regulatory Limit
EPA Maximum Contaminant Level (MCL)
50 ppb (0.05 mg/L)
The EPA MCL for selenium is 50 micrograms per liter (µg/L), or 50 ppb. This standard was set in 1992 and has not been revised since. The MCLG is also 50 µg/L, reflecting the EPA's view that selenium at the MCL does not pose an unacceptable health risk (unlike carcinogens where the MCLG is typically zero). The World Health Organization drinking water guideline is also 40 µg/L — similar to the U.S. standard.
How to Test for Selenium in Your Water
Selenium testing requires a certified laboratory and is not included in standard water quality panels. A selenium-specific test costs $15–$30 or is included in comprehensive metals panels ($100–$200). Well owners near coal operations (Kentucky, West Virginia, Wyoming) or in agricultural selenium-belt regions (Kansas, Nebraska, South Dakota) should include selenium in comprehensive testing. Public utilities test and report selenium in Consumer Confidence Reports.
How to Remove Selenium from Drinking Water
These treatment methods have demonstrated effectiveness for Selenium removal.
Frequently Asked Questions
Related Pages
Data Sources & Provenance
All data on this page is sourced from official U.S. government or public datasets.
Quick Reference
Category
Minerals
Risk Level
low
EPA Limit
50 ppb (0.05 mg/L)
Most at Risk
Well owners near coal mines, coal ash disposal ponds, and copper smelters in Kentucky, West Virginia, Wyoming, and Colorado face the highest industrial selenium exposure.
Well Water Relevant
Yes
Selenium by State
Treatment Options