As it leaves the skies in the form of rain, water is soft, meaning it is clear of impurities. But it picks up carbon dioxide and other impurities in the air. It also dissolves calcium and magnesium salts as it soaks through the earth and runs off into rivers or lakes and collects in reservoirs or wells.
Conventional water softeners have two separate chambers, the resin tank and the brine tank, and use an ion-exchange process to remove the hardness ions calcium and magnesium. Softeners are often able to also remove iron, manganese and radium.