In nature, water always contains dissolved minerals and nutrients. Water “hardness” is determined by the concentrations of minerals present in a particular water source—specifically calcium and magnesium. Simply put, hard water is mineral water that comes from your tap.  Water containing high amounts of calcium and magnesium leaves behind solid calcium carbonate deposits when it evaporates. … Minerals in soil and rock in a particular region will naturally be present in that region’s groundwater. All four soft-to-hard classifications are typical in California groundwater, depending on the source region.
—Written by Suzanne DeLorenzo, director of water quality at San Jose Water