Do you have information about water hardness in the United States?

View a national map of hardness in surface water on the USGS water hardness website. Hardness data (reflecting mostly calcium, plus a little magnesium) for individual drinking-water suppliers is at the EPA Office of Ground Water and Drinking Water website.

It is important to note that the U.S. Environmental Protection Agency (EPA) 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.

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A drinking glass showing etching and cloudiness due to water hardness.
March 5, 2018

Hard water can etch and cloud glass.

Overly hard water (high calcium content) can cause film on glassware.

Photo of a young girl drinking water, which likely originated from groundwater sources. 
January 17, 2017

A young girl drinks water, which likely originated from groundwater

A young girl drinks water, which likely originated from groundwater sources. 

The USGS is near the midpoint of a complex undertaking to survey the quality of the nation’s largest drinking-water resource. From 2012 – 2023, the USGS is assessing groundwater throughout the country through extensive sampling. The latest results from five regional aquifers are now

Drinking water from tap
December 31, 2016

Drinking water from tap

The quality of the water we drink can potentially impact our health. The USGS has several programs and cooperative projects that characterize the quality of selected rivers and aquifers used as sources of drinking water to community water systems in the United States.

Attribution: Water Resources
Map of water hardness in the United States, 1975

Map of water hardness in the United States

Mean hardness as calcium carbonate at NASQAN water-monitoring sites during the 1975 water year. Colors represent streamflow from the hydrologic-unit area. Map edited by USEPA, 2005. Modified from Briggs, J.C., and Ficke, J.F., 1977, Quality of Rivers of the United States, 1975 Water Year -- Based on the National Stream Quality

Attribution: Water Resources
Copper pipes filled with solidified calcium and magnesium

Hard water precipitates in copper pipes

Calcium and magnesium ions tend to precipitate as mineral solids on the surfaces of pipes and especially on the hot heat exchanger
surfaces of boilers. The resulting buildup of scale can impede water flow in pipes and reduce the efficiency of heating elements.

From USGS Circular 1352: Water Quality in the Glacial

Hard water film on glass and spoon

Hard water film

One of the most common causes of cloudy dishes and glassware is hard water. As water moves through soil and rock, it dissolves small amounts of naturally occurring minerals. These minerals make your water hard, which means it doesn't rinse as well as soft water, and can result in a film left on the dishes. The minerals in hard water can also dry onto the surface of