The geology of an area usually determines the source and extent of hardness. Limestone that has been dissolved by slightly acidic waters is the usual source. Granite is not easily dissolved by acidic waters. Thus, areas rich in limestone generally have very hard water and areas that are mostly granite do not.

Acidic water is formed when water absorbs carbon dioxide. Carbon dioxide (CO₂) makes up approximately 0.03 percent of air and is a waste product of plant and animal respiration. Decomposition processes in soil and water also contribute to the carbon dioxide in the atmosphere.

When carbon dioxide combines with water, it forms a weak solution of carbonic acid, which is found in most carbonated beverages. This acid reacts with limestone to produce calcium carbonate (CaCO₃), which is a white compound that usually leaves a scaly deposit in pans and teakettles.

There is an important relationship between the amount of carbonates in water and the amount of bicarbonates, which are similar compounds. Bicarbonates work like Alka-Seltzer to buffer water against acids. If a water source has a high carbonate content, it also has a good buffering ability and is much less likely to be affected by acid rain and acid wastewater.

There are two different methods for testing for water hardness. The total hardness test is performed frequently in the water industry. It measures the amount of calcium and magnesium in water and the results are expressed as milligrams per liter of calcium bicarbonate. The second method determines calcium content only. Try a science experiment to determine water hardness.

Southern Nevada's water supply, which comes from the mineral-laden Colorado River, has a water hardness of 284 milligrams per liter or 16.6 grains per gallon.