Hard water contains high levels of dissolved calcium and magnesium ions. Calcium and magnesium occur naturally in soils. As groundwater or surface water comes into contact with these minerals, they may dissolve and enter the water supply. Calcium and magnesium help give water a pleasant taste and are necessary for our health. A small portion of our necessary dietary intake of calcium and magnesium comes from the water we drink.
The Environmental Protection Agency (EPA) has developed the National Primary Drinking Water Regulations with enforceable standards on the number of contaminants allowed in drinking water. These standards protect the public from contaminants that may pose a risk to human health.
All public water providers must meet the requirements of these standards when supplying water to their customers.
In addition, the EPA has a set of standards known as the National Secondary Drinking Water Regulations. These standards are recommended guidelines that water providers can follow, but the standards are not enforceable. Substances found on this list are not harmful to human health but may cause unpleasant taste, odor, and color changes.
This link is to a complete list of EPA’s Primary and Secondary Drinking Water Regulations: the EPA has not placed calcium or magnesium on either list. The reason is that neither substance is considered harmful to health or thought to cause concerns with taste, color, or odor.
https://www.epa.gov/sites/production/files/2016-06/documents/npwdr_complete_table.pdf
One parameter on the Secondary list related to water hardness is Total Dissolved Solids (TDS). TDS is a measure of all dissolved minerals in the water. The recommended maximum standard for TDS in drinking water is 500 milligrams per liter (mg/L). Above this level, the taste and color of the water may be affected, and it may leave deposits on appliances and dishes.
Water hardness is measured in the laboratory by the amount of calcium carbonate present. Although there is no set standard for the classification of water hardness, the following chart provides generally accepted classifications based on the concentration of calcium carbonate.
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Water Hardness Measured as Calcium Carbonate (CaCO3)
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Concentration in (mg/L)
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Classification
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0 to 60
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Soft
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61 to 120
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Moderately Hard
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120 to 180
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Hard
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More than 180
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Very Hard
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Source: United States Geological Survey (USGS); The USGS Water Science School, http://water.usgs.gov/edu/hardness.html
Water classified as hard will leave mineral deposits on faucets and dishes and may have a salty taste but will not negatively affect human health. Some parts of the country have very hard water with typical concentrations of calcium carbonate ranging from 200 to 300 mg/L in drinking water.