Dr. Deepak Kumar
Thermal Pollution
Thermal pollution describes the introduction of waste heat into water bodies such as rivers, estuaries and coastal waters. Although other industries abstract water and then discharge it, the coastal power plants produce the greatest amount of water and thus waste heat. Coastal power stations of up to 1000MW capacity discharge up to 50cumecs (m3 s -1) at elevated temperatures. The effects of that discharge depend on the time and place of discharge and the characteristics of the receiving area. In particular, the effects depend on the ability to disperse and absorb the additional energy.
Power stations with a ready supply of cold water, such as those on the coast and in estuaries, use this to condense the steam used to turn the turbines. This flow of direct cooling, as opposed to the use of cooling towers, returns water to an adjacent area up to 10 degrees centigrade above ambient. The fossil and nuclear fuel power plants require this cooling. The high cost-effective value of direct cooling explains the large number of directly cooled estuarine and coastal power plants worldwide.
The Severity of the Pollutant and its Effects
The severity of thermal pollution is indicated by a plume of warm water from the outflow pipe of the power station. The impact on the receiving water is limited to the plume and its surrounding area, although the direction and distribution of the plume is dependent upon the physical conditions of the receiving area e.g. tidal currents, weather conditions and the ecological features exposed to the elevated temperatures.
Thermal pollution has a greater impact in temperate environments due to the greater difference between the temperature of the cooling water and the ambient water temperatures. This is especially so during the winter period when energy demands are at their highest and thus the power production and amount of cooling water are greatest. In tropical regions, there is little fluctuation experienced in power demand or the local sea temperature.
Thermally polluted water often undergoes biological and chemical changes that render it less valuable for drinking, recreational, habitat and industrial purposes. The main impacts of thermal pollution include:
The inhibition of plant growth leading to a change in character of the plant communities within the vicinity of the warm-water outfalls (see the following case study on Turkey Point).
A change in the behaviour of migratory fish if temperature is used as a trigger for migration.
Reduced O2 levels in the water column as the oxygen carrying capacity of seawater decreases with increasing temperature.
Interference with enzyme function and physiology of marine organisms for example increased temperatures increasing metabolic rate in aquatic organisms. Increased respiration at higher temperatures can alter an organisms energy budget if food is limiting.
The introduction of non-native species which are suited to living in warmer water temperatures e.g. southern species. These species may then out-compete the native species for the local resources such as food, breeding areas etc.
Interference with the life-cycles and reproductive physiology of marine organisms that live within the vicinity of the plume (see examples below).
Example
Most marine organisms rely on temperature thresholds as a trigger for spawning. Angulus Tenuis (the “thin tellin”), a marine bivalve, is a summer spawner and therefore spawns when the water temperature reaches a certain level. An increased water temperature around the out-fall pipes of a power station encourage A. tenuis to spawn earlier and for longer in the year. This can cause problems to their populations if the preferred food for their larvae is not available.
Example
In contrast is Arenicola marina, the lugworm. This species is a winter spawner therefore spawns when the water temperature falls below a certain level. If this species is present in areas where the receiving waters are thermally polluted then they spawn later in the year as they require cooler water temperatures and they may even not spawn if sufficiently low temperatures are not reached. In addition to the additional heat stress, direct cooling adds other pollutants such as antifouling compounds added to prevent the settlement of marine organisms within the power plant.
Turkey Point Nuclear Power Generating Station, South Florida - Case Study
Turkey Point nuclear power station is situated on the Atlantic coast of Florida, and serves more than 7 million people in the Florida area. This power station removes water from Biscayne Bay then passes it through the condensers. The output water with an increased temperature of up to 15°C above ambient is then pumped back into the receiving water. An area of nearly 40 hectares is affected by cooling water discharged from this power station.
The cooling water at Turkey Point is discharged into a shallow soft-bottomed area which is dominated by turtle grass, Thalassia.
Turtle grass is the largest and most robust seagrass in Florida and the Caribbean. Its leaves are about 1.2cm wide, which can grow up to 35cm long. Turtle grass will grow in water up to 25m deep, however it prefers shallow water of less than 10m deep. Turtle grass forms complex shallow water communities. The water temperature around the outfall pipe at Turkey Point is 30-35°C and a further temperature increase of 5°C would be detrimental to the turtle grass community. It is estimated that about 9.3 hectares of Thalassia has been destroyed around this outfall, with a further 30 hectares showing reduced growth rates.
Turtle grass is slowly being replaced by blue-green algae (cyanobacteria), since most other species cannot cope with the hot temperatures. Cyanobacteria are aquatic and photosynthetic, therefore produce their own food in the water by utilising energy from the sun. They are small bacteria which are usually unicellular although they often grow in colonies large enough to be visible to the naked eye. This change in vegetation cover will result in a localised change in community structure around the outfall pipe, as a result of thermal pollution. It has also been shown that marine animals cannot cope with a temperature rise of more than 2-3°C, with most sponges, molluscs (e.g. mussels) and crustaceans (e.g. crabs) being unable to tolerate temperatures above 37°C.