Toxic metals and metalloids, particularly selenium, present in wastewater discharged from coal-fired power stations into lakes, rivers and other waterways can be dangerous to wildlife and people. While selenium is an essential nutrient, excess levels of the metal are harmful.

At coal-fired power stations, selenium and metals reach lakes and other water bodies from two main sources: scrubbers, which clean contaminants out of the air, and ash ponds, which store waste from coal combustion. Selenium occurs naturally in coal, and is especially prevalent in bituminous coal.

While scrubber wastewater is sometimes stored, reused or evaporated to a disposable sludge, it can also be treated in a settling pond and then released into a nearby waterway. Water from ash ponds can overflow in a storm or be siphoned off to a water body to prevent the pond from losing its structural integrity. Ash pond wastewater can also leach into surrounding soil and threaten groundwater.

Online monitors or offline analyzers can be used at coal-fired power stations to measure trace metals, such as arsenic, chromium, selenium and more, to ensure effluent levels meet regulatory guidelines.