Thermal Pollution (College Board AP® Environmental Science): Study Guide

What is thermal pollution?

• Thermal pollution is an overlooked occurrence but a very real threat

• Most rivers, lakes, and oceans steadily gain and lose heat during summer heating and winter cooling

• But the sudden addition of hot or cold water causes thermal stratification, overwhelms the natural balance, changes the water's chemistry, stresses plants and animals and causes disease and death

Causes of thermal pollution

• Thermal pollution is usually caused by human activity raising the temperature of a body of water suddenly

• Natural causes include volcanic activity and geothermal processes

  • These release heated water and gases into water, raising the temperature and disrupting ecosystems

• The most common human cause is using water as a coolant for power plants and industrial manufacturing

  • Machinery is cooled with water, and the heated water is released into a river

  • This modifies the water temperature and produces thermal pollution

• Other industries that generate a lot of waste heat include

  • oil refining

  • pulp and paper mills

  • steel mills

  • chemical plants

  • desalination plants

• Agricultural irrigation in large-scale operations

  • Irrigation systems use large volumes of water to cultivate crops and is often taken from rivers, lakes and aquifers

• Deforestation exposes water to direct sunshine by reducing the shade that trees provide and exposing previously sheltered organisms to increased temperatures that they are not adapted to

• Road and concrete runoff

  • These surfaces absorb heat, which is then transferred during rainfall

• Riverbank erosion or modification can cause the river bed to widen and become shallower, allowing the water to heat up as more sunlight hits it

• Recreational activities such as boating, swimming and water skiing raise water temperatures, especially when large numbers of people are involved in small areas

  • Boats, jet skis and wave runners release warm exhaust from their engines into the water

Effects of thermal pollution

• Variations in water temperature affect the concentration of dissolved oxygen

  • The ability of water to hold dissolved oxygen decreases as temperatures rise

  • This makes lowers oxygen solubility in warmer months or areas of high thermal discharge

• Reduced oxygen levels in the aquatic environment stress species' aerobic respiration, compromising their growth, reproduction, and survival

  • Low oxygen levels can also enhance toxic algal blooms and other low-oxygen species

• Higher water temperatures increase metabolic rates and oxygen needs for respiration in aquatic species

  • To meet energy needs, organisms must respire faster at higher temperatures, consuming more oxygen

• Thermal pollution disrupts species and ecological balance because many are sensitive to any changes in temperature

  • Fish populations suffer reproductive cycle disruption

  • Higher temperatures delay fish spawning, and reduce egg fertilization, larvae survival, and fish numbers

• Thermal pollution can reduce aquatic biodiversity since many species have temperature ranges where they can survive and reproduce

  • Water temperature changes exceeding species' tolerance limitations can cause their decline or local extinction

  • For example, coral polyps need a temperature range of 73° to 84° F (23°- 29° C); beyond this optimal range, the polyps become stressed, bleach and die

• The severity of thermal pollution is dependent on the duration, speed, and amount of heat released into the water