What is Selenium* and Why Does It Matter?

Selenium is a naturally occurring mineral found in soil and rock. It enters drinking water mainly through erosion — rain and groundwater slowly dissolve selenium-rich rock formations and carry it into rivers, lakes, and underground aquifers. Industrial activity also plays a role. Coal mining, oil refining, and certain agricultural practices can release selenium into nearby water sources. Unlike some contaminants that come entirely from human pollution, selenium has always existed in the environment. The problem is concentration — too much of it in your water is where things get complicated.

At low levels, selenium is actually essential to human health. Your body needs small amounts to support thyroid function, immune response, and antioxidant protection. The issue is the narrow gap between "enough" and "too much." Long-term exposure to elevated selenium levels can cause a condition called selenosis, which includes symptoms like hair loss, nail brittleness, fatigue, and nerve damage. The EPA and World Health Organization both recognize that chronic overexposure — not a single glass of water — is where the real risk lies. Children and pregnant women are generally more sensitive to mineral imbalances, so keeping exposure in check matters more for those households.

The EPA's current legal limit for selenium in drinking water sits at 50 parts per billion (ppb). The average level detected across 1,318 water systems in the U.S. is just 1.94 ppb — well below that threshold. However, one system has tested as high as 43.1 ppb, which is still within the legal limit but close enough to raise eyebrows. The EWG (Environmental Working Group) applies stricter health-based guidelines than the EPA's enforceable standard, and even a single system testing above those guidelines is worth noting. Legal doesn't always mean risk-free, especially for families with young children or people with specific health conditions.

Geographically, selenium detections cluster heavily in the Great Plains and Midwest. Kansas leads the country with 203 affected systems, followed by Texas (173), Illinois (150), South Dakota (103), and Iowa (93). This pattern isn't random. These states sit on selenium-rich geological formations — particularly the Pierre Shale and other sedimentary rock layers that span the central U.S. Agricultural irrigation in these regions also concentrates selenium in the soil over time, which then leaches into groundwater. If you live in any of these states and rely on well water or a smaller municipal system, it's worth getting your water tested. Even large municipal systems in these areas can show detectable selenium levels depending on their source water.

The good news is that selenium is removable. Reverse osmosis (RO) filtration is one of the most effective methods available — it can remove up to 97% of selenium from drinking water. Distillation and activated alumina filters also perform well against selenium. Standard carbon filters, like the kind in a basic pitcher or faucet attachment, are not designed to remove selenium effectively, so they're not the right tool here. If your water test shows elevated selenium, an under-sink reverse osmosis system is your most reliable option for protecting your family's drinking water. Echo Water's RO systems are built specifically for this kind of whole-home or point-of-use protection — filtering out selenium along with dozens of other contaminants before the water ever reaches your glass.

If you're unsure what's in your water, start with a test. Your local utility is required to publish an annual water quality report, called a Consumer Confidence Report (CCR), which lists detected contaminants and their levels. For well owners, independent lab testing is the only way to know what you're actually drinking. Knowledge is the first step — and once you know what you're dealing with, filtering it out is straightforward.

Regulatory Standards for Selenium*

Cities With the Highest Selenium* Levels

States Most Affected by Selenium*

Frequently Asked Questions

Related Contaminant Guides