Discussion Ideas for Water Contamination Demonstration
Dorothy Merritts, Franklin & Marshall College, Lancaster, PA dorothy.merritts@fandm.edu
Most water on Earth contains substances other than hydrogen and oxygen. If rainwater falls through the atmosphere above a large urban and industrial area, for example, it would likely contain far more impurities than if it fell to Earth above the Arctic pole. This fact is increasingly significant since industrialization, and even remote locales now have substances in the atmosphere related to human activities. Rainwater in the Yukon Territory has been found to contain an insecticide (toxaphene), apparently blown in from the former Soviet Union.
One question to pose to students during the demonstration is whether or not they know the source of their water supply. Students often do not. Tap water is likely to be ground water, which supplies the majority of water to urban areas, or water from a nearby stream or river. If its source is a stream or river, it probably has passed through numerous settlement, coagulation, and filtration basins and then been disinfected at a municipal water treatment plant. Explain to students that if they were to look at the water in most rivers and streams near urban areas, they probably would not want to take a drink before the water underwent all of those procedures. During fieldtrips, we often point out the nature of water in streams passing through urban areas, and stop to examine the intake of water at our local water treatment plant. Students can see the accumulation of tires, plastic, and other garbage that is trapped by the first of a series of filters. If you teach near a water treatment plant, we recommend that you at least point out the facility during a class trip, or show students its whereabouts on a map.
Some impurities are pollutants at very small levels, while others can be ingested in much larger amounts before becoming hazards. For these reasons, chemists call substances such as the contaminants in drinking water "misplaced matter". Gasoline or motor oil in a car is not unwanted, but that in drinking water is. Chemists like to remind us that there is a little bit of everything in anything. In other words, few substances are pure. In terms of human health, however, we must decide: How much is too much, and how is that amount determined?
As a result of the Safe Drinking Water Act, passed in 1974 and amended in 1986, the Environmental Protection Agency has established drinking water standards for all public water supplies in the United States. To date, maximum contaminant levels and maximum contaminant level goals are identified for several dozen substances. A maximum contaminant level (MCL) is an enforceable standard set at a certain amount, or concentration, for which a given substance is considered safe to human health if a person is exposed to that level over a lifetime. For example, the MCL for lead, which can cause neurological disorders, is 0.05 mg/liter, and for trichloroethene, which is a possible carcinogen, the MCL is 0.005 mg/liter.
A maximum contaminant level goal (MCLG) is a non-enforceable standard that is even more stringent than MCLs, but is considered unlikely to be attainable because of technological or economic limitations. For example, in an ideal world a possible carcinogen such as trichloroethene would not be present in any measurable amount in our drinking water. Its MCLG, therefore, is 0 mg/liter. Because trichloroethene is found in many water supplies, however, and because it would be very costly to remove all traces of it from water, the MCL is set at a level that is economically viable to achieve.
