It is a natural component of the earth's crust and occurs in small amounts in bodies of water, underwater sediments, and soils. It is commonly found in combination with sulfur and iron in minerals such as pyrite. Arsenic is used mainly to preserve wood and to control insects and weeds. When electric utilities burn these fuels at their power plants, arsenic is released in very small amounts.
Calcium arsenate is likely to be the arsenic compound most frequently released.
Most of this arsenic is carried by particles of ash. It is mainly on the surface of the particles. Coal-burning power plants are equipped with devices to capture ash particles before they reach the air. Arsenic-carrying ash captured by these devices is usually sent to ash ponds or land disposal sites. Natural arsenic occurs commonly and comes mainly from the soil.