Next to a supply of air, nothing is so essential to life as a supply of high-quality water. We drink it, cook our food in it, use it as a source of energy, and lift a hundred pounds or so of it each time we stand up. Water carries nutrients in and removes waste materials from our bodies. Contaminated water also spreads numerous diseases.
We judge the quality of water by taste, smell, color, and lack of pathogenic organisms or harmful contaminants. Often bad taste, odor, or color indicates contamination. Most of the water we drink has been treated to remove harmful substances and has had chlorine, ozone, or chloramines added to kill bacteria. Ordinary water contains dissolved gases such as oxygen, nitrogen, carbon dioxide, and other atmospheric components, as well as harmless minerals.
Pollutants are usually present at very low concentrations, commonly measured and reported as parts per million (ppm) or parts per billion (ppb). A solution containing 2 grams (0.071 ounces) of lead in 1 million grams of water (1,000 liters, or 264.2 gallons) is a 2 ppm solution of lead in water. A 1 ppb solution of calcium in water contains 1 gram (0.036 ounces) of calcium in 1 billion grams (2,205,000 pounds) of water. A concentration of 1 ppm is the same as 1 milligram (3.6 × 10 −5 ounces) per liter.
It would be impractical and expensive to remove all impurities from water. The Safe Drinking Water Act of 1974 gives the Environmental Protection Agency (EPA) authority to set limits for dangerous contaminants. For each substance, the EPA sets Maximum Contaminant Level Goals (MCLGs), levels at which the substance could be consumed over a long period of time with no known adverse effects. The MCLG is the amount of contaminant that can safely be present in two liters of water drunk each day for seventy years by a person weighing 70 kilograms (154 pounds). In addition, the EPA sets Maximum Contaminant Levels (MCLs), the maximum permissible level of a contaminant in drinking water.
Removing all pollutants from water would be difficult and expensive, but concentrations below the MCL and MCLG are considered harmless. Lead damages kidneys, and chronic exposure to even tiny amounts may cause damage to the nervous system. The MCLG of lead is 0; the EPA maintains that no amount of lead should be consumed for an extended time. The MCL of lead is 0.015 ppm, but consumption of even low levels of lead in water is not recommended. Both the MCLG and MCL of mercury are set at 0.002 ppm.
Technicians use specialized analytical equipment to monitor pollutants. In the field, pH meters are used to measure acidity; very acidic or basic water may be corrosive. Turbidometers measure suspended solids, which may be harmless but often carry or hide pathogenic organisms. In laboratories, samples are subjected to gas chromatography to determine the presence of organic compounds such as vinyl chloride, high pressure liquid chromatographs measure pesticide traces, and absorption and emission spectroscopy are used to detect heavy metals . Such instruments are capable of detecting as little as one part per trillion of pollutants.
Biological tests are also commonly performed on drinking water. Biochemical oxidative demand (BOD) is a measure of the concentration of biodegradable organic matter. While coliform bacteria such as Eschericia coli are seldom dangerous themselves, they act as indicator bacteria. Water containing coliforms is likely to contain other, more dangerous pathogens.
The acceptable level of pollutants depends on the use intended. We need not flush toilets or water lawns with water pure enough to drink. River water commonly contains traces of animal wastes that are acceptable for irrigation but must be removed before human consumption. Ocean water too salty for consumption can be used for industrial cooling and may be purified by distillation or reverse osmosis to render it suitable for drinking.
Common pollutants include traces of human or animal waste; disease organisms; radioactive materials; toxic metals such as lead or mercury; agricultural chemicals such as pesticides, herbicides, or fertilizers; and high-temperature water discharged from industrial plants. Polluted water may be dangerous to drink, may harm crops, and may cause eutrophication.
Metals such as lead, cadmium, chromium, and mercury are toxic even at low concentrations (the MCL for cadmium is 0.005 ppm). Aquatic microorganisms often concentrate toxic materials from soil or water and may
convert inorganic substances such as mercury to organic forms such as methylmercury. These organisms may be consumed by fish, which in turn are eaten by animals higher on the food chain, and eventually the toxic materials can find their way into human diets. Organic mercury is sometimes absorbed by the central nervous system. Mercurial wastes discharged into the bay at Minimata, Japan, resulted in birth defects and neurological disorders among many children. Even small amounts of lead or mercury may be converted by aquatic microorganisms into toxic organic mercury compounds such as methyl- or dimethylmercury, which, acting as neurotoxins, may be passed up the food chain, eventually causing damage to the central nervous system of humans.
Lead and copper ions in water pose health risks and contribute to the corrosion of pipes and fittings, as does water that is at a high or low pH. Lead solder was banned from pipes in 1986, but much old plumbing still contains a mixture of 50 percent lead and 50 percent tin solder in joints. In plumbing systems having pipes and fittings of two different metals, corrosion may lead to the failure of joints.
Hot water discharged by industries, such as at power plants, and nitrates and phosphates from feedlot runoff cause algae to grow rapidly, rendering water unfit for consumption by humans or farm animals. High-BOD organic matter in sewage, feedlot runoff, or excess fertilizer from farm fields accumulates in ponds and lakes. Oxidative processes then consume so much oxygen that fish and aquatic plants die.
Organic contaminants such as vinyl chloride or hydrocarbons, hormones from animal feed, and pesticides and herbicides often find their way into streams or aquifers. From these sources, the contaminants may make their way into water supplies.
Hard water contains metallic ions, such as magnesium or calcium ions, that interact with soap to form insoluble films or scum. Hardness is not hazardous to health but may form scale in boilers and clog water pipes. Excess calcium and magnesium can be removed by ion exchange water softeners.
MacKenzie, Susan Hill (1996). Integrated Resource Planning and Management: The Ecosystem Approach in the Great Lakes Basin. Washington, DC: Island Press.
Stanitski, Conrad L.; Eubanks, Lucy P.; Middlecamp, Catherine H.; and Pienta, Norman J. (2003). Chemistry in Context: Applying Chemistry to Society, 4th edition. Boston: McGraw-Hill.