The sum of the cations that have a charge greater than one and are present in the water.
Water acquires ions when it passes through soil and rock that contain rich mineral deposits.
Sandstones, limestones, and siltstones are rich in calcium, while dolomites are rich in magnesium. Magnesium is required for chlorophyll in plants Hard water prevents fish from absorbing other toxic cations like lead, arsenic and cadmium. Hard water usually has high alkalinity, which helps maintain the pH levels suitable for aquatic life. Another method is the lime-soda process to remove ions. Water treatment plants regulate the water hardness allowing no more than 200 ppm calcium hardness and no more than 150 ppm magnesium.