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QCC: S.16.4.4

Background Information:

The raising of the air or water temperature by artificial means is called thermal pollution. Here we will primarily deal with thermal pollution of water. (Literature on thermal pollution is largely water associated, and this curriculum is developed based on the watershed. Thermal air pollution may be related to chloro-fluoro carbons (CFCs), carbon dioxide and global warming.)

Four major causes of thermal pollution of water may be identified, viz. use of water as a cooling agent; soil erosion; deforestation of shorelines; and run-off from hot paved surfaces. However, the foremost cause is the use of water as a cooling agent in power plants (natural gas, coal or nuclear) and factories and industrial facilities. It is estimated that almost half of all water withdrawn in the United States each year is used for cooling electric power plants. The easiest and cheapest method is to withdraw water from a nearby water body and return the heated water to the same water body. The discharge of hot water used to cool the plants into water bodies, like streams, rivers, lakes and oceans causes the thermal pollution. The contributions of soil erosion, deforestation of shoreline, and run-off from paved surfaces to the problem are less obvious. Soil erosion makes the water muddy, which in turn increases the absorption of light, thus increasing the water temperature. Deforestation of shorelines contributes to the problem in two ways. First, plant roots hold soils together, so deforestation enhances soil erosion. Secondly, vegetation provides shade. With the vegetation gone, the amount of light hitting the water surface is increased, thereby raising the water temperature. As anyone who has walked barefoot on an asphalt parking lot on a summer day knows, paved surfaces are hotter than natural ones. Run-off from paved surfaces is also hotter, raising the temperature of the water into which it flows.

The effects of thermal pollution are difficult to define with precision. However, two types of effects are discernible, namely, thermal shock and thermal enrichment. Change in water temperature above its normal level, due to hot wastewater, can harm fish and organisms adapted to a particular water temperature regime from thermal shock. For example, warmer water may interfere with fish growth, reproduction, and food supply. In some cases, fish may be killed due to the sudden and rapid rise in temperature. However, some argue that heated water may be used for beneficial purposes, calling it thermal enrichment. For example warm water from power plants may be used for irrigation to extend plant growing season in frost-prone areas, speed the growth of fish and shellfish for commercial production, melt snow on sidewalks, desalinate ocean water etc. However, the harmful effects of thermal pollution seem to outweigh the beneficial effects.

Increase in water temperature affects the ecosystem. Recently people have realized that only small changes in temperature (typically 1 or 2 degrees Centigrade) are needed to have considerable environmental impact. The amount of oxygen dissolved in the water is believed to be affected, most importantly, because oxygen is less soluble in warm water. Also, increase in temperature increases photosynthesis and aquatic plant growth, like algae, which may crowd out the zooplankton. Excess plant growth is harmful because of eutrophication. Increased plant growth means increased dead plants. These dead plants are decomposed by bacteria, which in turn use up oxygen thus depleting the water of oxygen for other aquatic animals like fish. Warmer temperature is also known to increase the metabolism of fish, thereby increasing their need for oxygen. Increase in the fish metabolism rate will increase the consumption of aquatic insects, resulting in an increase in fish population and depletion of aquatic insects - an imbalance in the food chain or food web and ecosystem. Higher temperatures can cause enzymes and microbes to speed up metabolism, and can eventually kill them. Also, changes in temperature cause fish to migrate to regions where the water is at the best temperature for them, and can kill any species which cannot move away. Thus thermal pollution can contribute to ecological imbalance in an otherwise balanced ecosystem in many ways.

Internet Resources:

Thermal Pollution: A Global Problem

Thermal pollution

Effects of Thermal Pollution on Aquatic Organisms

Thermal Pollution and Aquatic Life, By Joseph A. Mele, Ph.D. (All rights reserved)

Ways to reduce thermal pollution

Research Proposal: Thermal Pollution (Middles School)

Federal Laboratories with Expertise in Environmental pollution and control; Water pollution and control; and Thermal pollution

Glossary to Scientific and Environmental terms


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