In discussions of plant growth, aeration refers to an exchange that takes place in soil or another medium allowing oxygen to enter and carbon dioxide to escape into the atmosphere. Crop growth is often reduced when aeration is poor. In geology, particularly with reference to groundwater, aeration is the portion of the earth's crust where the pores are only partially filled with water. In relation to water treatment, aeration is the process of exposing water to air in order to remove such undesirable substances in drinking water as iron and manganese.