Hard water refers to the amount of dissolved calcium and magnesium in the water (primarily calcium). The more calcium and magnesium is in the water, the harder it is! Hardness is measured in
Grains per Gallon (gpg), where 1 grain is 64.8 milligrams of calcium carbonate dissolved in 1 US gallon of water.
Drinking and utility water can come from a variety of sources, with Ground Water being a prolific source across the United States. Ground water tends to be high in water hardness as water is a solvent, and absorbs minerals such as calcium and magnesium as it passes through rocks.
In Tucson and the surrounding region - we see an average of about 10 Grains per Gallon... The water here is hard!