Some beneficial elements, such as calcium, magnesium, and fluoride, either occur naturally in water at sufficiently high concentration to positively influence human health or can be added to water as supplements. In addition, some microbes result in remediation of waterborne contaminants (bioremediation).

Calcium (Ca) and magnesium (Mg) are two of the three most abundant cations (along with sodium) in natural waters. These cations result from dissolution of a variety of rock-forming minerals, including feldspars, carbonates, and sulfate evaporites such as gypsum. Water “hardness” is a measure of the combined calcium and magnesium concentrations in water (see Figure 4.2). Because high concentrations of Ca and Mg generate residues when used with soaps, and boiler scale when water is heated and evaporated, “softening” by ion exchange is recommended for

FIGURE 4.2 Water hardness across the United States, 1975 (milligrams per liter).

SOURCE: USGS web product; http://water.usgs.gov/owq/map1.jpeg.