Among the many environmental problems that offend and concern us, perhaps none is as powerful and dramatic as water pollution. Ugly, scummy water full of debris, sludge, and dark foam is surely one of the strongest and most easily recognized symbols of our misuse of the environment.

What is pollution? The verb "pollute" is derived from the Latin polluere: to foul or corrupt. Our most common meaning is to make something unfit or harmful to living things, especially by the addition of waste matter or sewage. A broader definition might include any physical, biological, or chemical change in water quality that adversely affects living organisms or makes water unsuitable for desired uses.

Paradoxically, however, a change that adversely affects one organism may be advantageous to another. Nutrients that stimulate oxygen consumption by bacteria and other decomposers in a river or lake, for instance, may be lethal...