Selenium in Drinking Water: What You Should Know

If you take a look at the periodic table, you will find selenium sitting at element number 34. It is a naturally occurring element that resides in the earth’s crust, soils, and rock formations, right alongside elements like sulfur.

Unlike many heavy metals or chemical contaminants that serve no biological purpose, selenium comes with a fascinating dual nature: it is actually an essential trace nutrient that your body needs in tiny amounts to stay healthy. However, as with many things in water chemistry, balance is everything. As someone who helps homeowners make sense of what comes out of their taps every day, here is a clear, honest breakdown of what selenium is, how it behaves in water, and how to manage it in your home.

1. What Selenium Is and Where It Comes From

Selenium is a naturally occurring mineral and metalloid found throughout the Earth’s environment. In nature, it is tightly bound within sedimentary rocks, shale, soils, and metal ores.

In human biology, selenium plays a vital role as a dietary micronutrient. It acts as an antioxidant, supports thyroid hormone metabolism, and aids immune system function. We typically absorb small amounts of selenium through our daily diet from foods like Brazil nuts, seafood, eggs, whole grains, and leafy vegetables grown in selenium-rich soil.

However, because it occurs naturally in the ground, it can easily find its way into regional water supplies when geological conditions or human activity disturb those mineral deposits.

2. How It Gets Into Drinking Water

Selenium typically makes its way into drinking water supplies through four primary avenues:

• Natural Leaching: Rainwater and subterranean groundwater naturally dissolve selenium-bearing rocks and soil as water moves through aquifers. Certain geographic areas—particularly in parts of the western United States—have naturally high selenium concentrations in their regional geology.
• Agricultural Drainage: Irrigation water flowing over soils high in selenium causes the mineral to dissolve and concentrate in agricultural runoff, which then drains into nearby rivers, lakes, or shallow wells.
• Coal Ash and Mining Operations: Coal combustion leaves behind ash containing concentrated trace metals like selenium. Rainwater washing over coal ash ponds, as well as runoff from metal mining, can transport selenium into surface water and aquifers.
• Oil Refining: Petroleum processing and refining operations produce wastewater that can contain elevated levels of selenium if not properly captured and treated.

3. What It Can Do to a Person’s Health: The Two-Sided Story

Selenium presents an honest health paradox: too little causes deficiency, but too much leads to toxicity.

• The Deficiency Side: A lack of selenium in the diet can lead to health complications, such as weakened immune function, thyroid imbalance, and specific muscular or cardiovascular conditions.
• The Overexposure Side (Selenosis): Consuming high levels of selenium in drinking water over extended periods—months or years—can cause a chronic toxic condition known as selenosis.

Key symptoms of long-term selenium toxicity include:

• Loss of hair and fingernails
• Skin lesions and persistent rashes
• A distinct garlic-like odor on the breath
• Neurological symptoms, including numbness, tingling, or weakness in the extremities

Because you already absorb selenium through food, drinking water with elevated selenium levels can push your overall daily intake past safe biological thresholds.

4. The Federal Picture: The EPA Limit

To prevent chronic health issues like selenosis, the U.S. Environmental Protection Agency (EPA) regulates selenium under the Safe Drinking Water Act.

The standard: The Maximum Contaminant Level (MCL) for selenium in drinking water is 0.05 mg/L (which is equal to 50 parts per billion).

Because public water utilities are legally required to keep selenium below 0.05 mg/L, municipal water users are generally well-protected. However, if you rely on a private well in a region with selenium-bearing rock or heavy agricultural drainage, monitoring falls entirely on you.

5. How a Homeowner Can Test Their Water

You cannot see, smell, or taste selenium at typical drinking water concentrations. The only way to know if it is present in your tap water—and at what exact concentration—is through lab analysis.

• Skip the DIY Home Test Strips: Simple dip strips sold at home centers are designed for basic pH, hardness, or free chlorine. They lack the sensitivity needed to detect trace metals down to parts-per-billion levels like 0.05 mg/L.
• Use a Certified Laboratory: Send a water sample to a state-certified drinking water laboratory. They will use advanced analytical equipment, such as ICP-MS (Inductively Coupled Plasma Mass Spectrometry), to deliver an accurate measurement of trace minerals including selenium, arsenic, lead, and cadmium.

6. How to Remove Selenium from Drinking Water

If a lab report shows selenium levels approaching or exceeding 0.05 mg/L, choosing the right treatment technology is critical.

What Actually Works

• Reverse Osmosis (RO): A point-of-use Reverse Osmosis system under the kitchen sink is the most dependable, practical home method for removing selenium. RO forces water under pressure through a semi-permeable membrane. The microscopic membrane pores allow pure water molecules through while rejecting dissolved mineral ions like selenium.
• Anion Exchange: In oxygenated drinking water, selenium is present almost entirely as negatively charged oxyanions — selenate (SeO₄²⁻) and selenite (mainly HSeO₃⁻/SeO₃²⁻ at drinking-water pH) — not as a positive ion, which is why strong-base anion exchange and reverse osmosis can remove it, while a cation-exchange water softener, which swaps positive calcium and magnesium ions, cannot. Strong base anion exchange resins can capture negatively charged selenium ions. However, competing ions in water (like sulfates and nitrates) can reduce its efficiency.
• Adsorption Media: Specialized media filters—such as activated alumina or iron-based adsorption media—can selectively trap selenium under specific pH conditions and controlled flow rates.

What Does NOT Work Well

• Standard Pitcher Filters & Basic Carbon Block: Standard activated carbon filters excel at removing chlorine and organic tastes, but they do not reliably remove dissolved inorganic minerals like selenium.
• Standard Water Softeners: Standard softeners use cation exchange resins designed to remove calcium and magnesium. They will not trap negatively charged selenium species.

Pre-Filtration and Micron Ratings

While physical sediment filters do not catch dissolved selenium ions, they play a vital pre-filtration role by keeping silt and rust from clogging your RO membrane or ion exchange media.

When choosing pre-filters, pay attention to micron ratings:

• Micron Rating: A micron measures pore size. For protecting an under-sink RO system, a 5-micron or 1-micron sediment pre-filter is standard industry practice.
• Nominal vs. Absolute Rating:
  • Nominal Rating: Traps approximately 60% to 85% of particles at that micron size under basic flow.
  • Absolute Rating: Traps 99.9% or more of particles at the rated micron size under strict lab test conditions.

Using an absolute-rated pre-filter ensures consistent physical protection for your main selenium removal equipment.

The Bottom Line

Selenium is a perfect example of why context matters in water chemistry—it is an essential element your body needs, but one you don’t want too much of in your drinking water.

If you rely on a private well in an area with high regional mineral content or nearby farming and mining, get your water tested by a certified lab. If tests show selenium near or above the 0.05 mg/L limit, a properly installed under-sink Reverse Osmosis system is your most dependable, straightforward fix for clean, safe drinking water.