You can view a national map of hardness in surface water. Hardness data (reflecting mostly calcium, plus a little magnesium) for individual drinking-water suppliers is on the following pages:
It is important to note that the U.S. Environmental Protection Agency has not set a legal limit or standard for hardness in water. This is primarily because the constituents that contribute to hardness (generally calcium and magnesium ions) are not toxic; that is, they do not cause harmful health effects. Instead, there is a generally accepted division of water into categories of soft, moderately hard, hard, and very hard, as explained in the water hardness chart. Most water utilities try to provide water that is not in the very hard category because of the unpleasant effects such as scaling in equipment and the need for more soap and synthetic detergents. In addition, many homeowners in hard-water areas use water softeners to further reduce hardness by substituting sodium for calcium and magnesium.