 |  Environmental Science: A Global Concern, 7/e William P. Cunningham,
University of Minnesota
Mary Ann Cunningham,
Vassar College
Barbara Woodworth Saigo,
St. Cloud State University
Water Pollution
Be Alert BoxesBE ALERT FOR: Dissolved Oxygen
An important indicator of water quality is the concentration of oxygen dissolved within a water body. This substance, molecular oxygen, or O2, is as necessary for aquatic life as it is for us.
Do not confuse this oxygen with the oxygen atoms that make up water molecules. O2 is a totally different substance from the O in H2O. When dissolved in water, O2 molecules are simply dispersed among the much larger numbers of H2O molecules.
Secondly, unlike O2 in the air, the amount of O2 dissolved in water can change significantly over relatively short periods of time. As a result of respiration by aquatic life, particularly by decomposers, the O2 concentration can be driven essentially to zero.
BE ALERT FOR: Nonpoint Source Pollution
Be sure to distinguish the difference between point and nonpoint sources of pollution. When most people think of water pollution, what typically comes to mind are sewage treatment plants, landfill seepage, and chemical wastes released into the environment by specific industries. These are referred to as point sources of pollution. They are readily identified, and much attention has been paid to cleaning them up.
Unfortunately, point sources are only the tip of the proverbial iceberg. Nonpoint sources of water pollution are much more insidious. They result as the cumulative effect of thousands of commonplace activities occurring in thousands of locations. Contaminated runoff from myriad suburban lawns and farm fields is nonpoint source pollution. Parking lot runoff carrying oil and other chemicals that drips to the ground from automobiles is nonpoint pollution.
The problem with this type of pollution is that the individual often does not recognize the cumulative seriousness of small polluting actions. If done by millions of people, little things become big things. Solving point source pollution problems requires changing the behavior of a few. Solving nonpoint problems requires changing the behavior of all of us.
BE ALERT FOR: BOD
A common characteristic of polluted water is a high biological oxygen demand, or BOD. Technically, BOD is a measure of the amount of O2 used by the microbes in a water sample over a period of time. Microbes use oxygen in their respiration reactions to extract energy from organic matter. The greater the amount of organic matter available as microbial food, the more O2 these organisms remove from the water (i.e., the greater the BOD).
The problem for fish, clams, crustaceans, and other life forms is that the microbes are so efficient at using O2 that there may not be enough left to sustain these other members of the ecosystem. So pollutants that increase the amount of organic matter in an aquatic system pose direct threats to oxygen supplies.
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2003 McGraw-Hill Higher Education