Dissolved mineral constituents can be hazardous in large concentrations to animals and plants; for example, too much sodium in the water may be harmful to people who have heart trouble. Boron is a mineral that is good for plants in small amounts, but is toxic to some plants in only slightly larger concentrations.
Water that contains a lot of calcium and magnesium is said to be hard. The hardness of water is expressed in terms of the amount of calcium carbonate—the principal constituent of limestone—or equivalent minerals that would be formed if the water were evaporated. Water is considered soft if it contains 0 to 60 mg/L of hardness, moderately hard from 61 to 120 mg/L, hard between 121 and 180 mg/L, and very hard if more than 180 mg/L. Very hard water is not desirable for many domestic uses; it will leave a scaly deposit on the inside of pipes, boilers, and tanks. Hard water can be softened at a fairly reasonable cost, but it is not always desirable to remove all the minerals that make water hard. Extremely soft water is likely to corrode metals, although it is preferred for laundering, dishwashing, and bathing.
Groundwater, especially if the water is acidic, in many places contains excessive amounts of iron. Iron causes reddish stains on plumbing fixtures and clothing. Like hardness, excessive iron content can be reduced by treatment. A test of the acidity of water is pH, which is a measure of the hydrogen-ion concentration. The pH scale ranges from 0 to 14. A pH of 7 indicates neutral water; greater than 7, the water is basic; less than 7, it is acidic. A one unit change in pH represents a 10-fold difference in hydrogen-ion concentration. For example, water with a pH of 6 has 10 times more hydrogen-ions than water with a pH of 7. Water that is basic can form scale; acidic water can corrode. Water for domestic use generally should have a pH between 5.5 and 9.
In recent years, the growth of industry, technology, population, and water use has increased the stress upon both our land and water resources. Locally, the quality of groundwater may have been degraded. Municipal and industrial wastes and chemical fertilizers, herbicides, and pesticides not properly contained may have entered the soil, infiltrated some aquifers, and degraded the groundwater quality. Other pollution problems include sewer leakage, faulty septic tank operation, and landfill leachates. In some coastal areas, intensive pumping of fresh groundwater has caused salt water to intrude into freshwater aquifers, a situtation known as saltwater intrusion (Figure 3).