A common form of physical water pollution is thermal pollution. This includes warm water from cooling towers, floating debris, foam, and garbage. In highly industrialized areas of the world, power plants are used in generating electricity, where warmer water generated in the process is generally released back to the environment. In nuclear plants, water is used in large quantity to cool reactors. The discharge of high-temperature water into a natural body of water can affect the downstream habitats, therefore altering the ecological balance. It can lead to cultural eutrophication, thereby promoting algal bloom. This development has the potential of threatening certain fish species, as well as disturbing the chemistry of the receiving water body.
Heat may also affect man's legitimate use of water for fishing. Another common and widespread type of thermal pollution is the unsafe removal of vegetations that should naturally keep streams and small lakes cool. Natural vegetations, mainly trees and other tall plants, are usually seen around streams and sizable water bodies and they block direct sunlight from heating and thereby increasing the surface temperatures of these waters. People often remove this shading vegetation in order to harvest wood from the trees, to make room for crops, or to construct buildings, roads, and other structures. When these vegetations are removed and the aquatic ecosystems are left uncovered, the water temperature could increase by as much as 18 degrees F (10 degrees C).
Many wastes are biodegradable, that is, they can be broken down and used as food by microorganisms like bacteria. Biodegradable wastes may be preferable to nonbiodegradable ones, because they will be broken down and not remain in the environment for a very long time. However, too much biodegradable material can cause the serious problem of oxygen depletion in receiving waters. Like fish, aerobic bacteria that live in water use oxygen gas, which is dissolved in the water when they feed. Invariably, the oxygen is not very soluble in water. Even when the water is saturated with dissolved oxygen, it contains only about 1/25 the concentration that is present in air. So if there are too many nutrients in the water, the bacteria that are consuming it can easily use up all of the dissolved oxygen, leaving none for the fish, which will die of suffocation. Once the oxygen is depleted, other bacteria that do not need dissolved oxygen take over. But while aerobic microorganisms convert the nitrogen, sulfur, and carbon compounds that are present in the wastewater into odorless, oxygenated forms like nitrates, sulfates, and carbonates, these anaerobic microorganisms produce toxic and smelly ammonia, amines, and sulfides, and flammable methane.
Nutrients are major chemical pollutants and they include nitrates and phosphates found in sewage, fertilizers, and detergents. Although phosphorus and nitrogen are essential elements necessary for plant growth, in excess levels nutrients overstimulate the growth of aquatic plants and algae. When discharged into rivers, streams, lakes, and estuaries, they cause nuisance growth of aquatic weeds, as well as blooms of algae, which are microscopic plants. Excessive growth of these organisms can clog navigable waters, deplete dissolved oxygen as they decompose, and block light from penetrating deeper waters. Weeds can make a lake unsuitable for swimming and boating. Algae and weeds die and become biodegradable material. If the water is used as a drinking-water source, algae can clog filters and impart unpleasant tastes and odors to the finished water. It can also impair respiration by fish and aquatic invertebrates, which could lead to a decrease in animal and plant diversity.
Suspended solids originate from eroded stream banks, construction, and logging sites. They are a form of physical water pollution. These pollutants are also referred to as particulate matter because they contain particles of much larger size which remain suspended in the water column. Although they may be kept in suspension by turbulence, once in the receiving water, they will eventually settle out and form silt or mud at the bottom. As these sediments enter the rivers, lakes, and streams, they tend to decrease the depth of the body of water. If there is a lot of biodegradable organic material in the sediment, it will become anaerobic and contribute to the formation of algal bloom. Toxic materials can also accumulate in the sediment and affect the organisms that live there, and can build up in fish that feed on them, and so be passed up the food chain, causing problems along the food web. Also, some of the particulate matter may be coated with grease, which is lighter than water, and float to the top, creating an aesthetic nuisance.
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