WATER POLLUTION USUALLY describes the introduction or presence of harmful or objectionable substance in water in magnitude sufficient to alter the quality indices of natural water. It also connotes the presence of polluting substances in rivers, lakes, bays, seas, streams, underground water, or oceans in levels capable of resulting in measurable degradation of the water quality or usefulness. For example, if water contains too much contamination as a result of certain harmful chemical compounds or microorganisms, it could be rendered unsafe in its existing state for an intended purpose. This could be described as water pollution. In most cases, water pollution may arise from the use to which the water has been put. Although some kind of water pollution can occur through natural processes, it is mostly caused by human activities.
Water pollution has many sources and characteristics. These sources can be categorized into point and nonpoint sources. Point sources of water pollution are direct discharges to a single point, or simply stationary location discharges. Examples include discharges from sewage treatment plants, power plants, factories, ships, injection wells, and some manufacturing or industrial sources. Nonpoint sources of water pollution are more diffused across a broad area and their contamination is traceable to a single discharge point. Examples of nonpoint sources include mining activities and agricultural and urban runoffs. Water pollution arising from nonpoint sources accounts for the majority of contaminants in streams, rivers, bays, underground water, and seas.
A water pollutant is any biological, chemical, or physical substance if when present in water at excessive concentrations has the capability of altering the chemical, physical, biological, and radiological integrity of water, thereby reducing its usefulness to living organisms, including man. Although there are many sources of pollutants in our waters, the primary sources of water-polluting substances come from sewage, agricultural runoffs, oil spills, industrial wastewaters, land drainage, and domestic wastes. The major categories of common water pollutants include heavy metals, pathogens, nutrients, acids, organic chemicals, and radioactivity. Many of these substances are toxic and are capable of interacting additively or synergistically or antagonistically to give varying responses in aquatic ecosystems and in humans. However, the influence of a pollutant in natural waters varies according to the polluting substance, the local environmental conditions, and the organism involved.
MAJOR TYPES OF WATER POLLUTANTS
Heavy metals are toxic and include many metal pollutants that could have potentially harmful effects on human health and aquatic ecosystems. Common examples include cadmium, nickel, arsenic, lead, vanadium, mercury, and selenium. Typical sources of metal pollutants include wastes from domestic, industry, agriculture, urban, and mining drains. Acids are inorganic water pollutants caused by industrial discharges, especially sulfur dioxide from industrial power plants, drainage from mines, wastes from industry, and aerial acid deposition. Acids have the potential of causing harm to aquatic ecosystems via the mobilization of toxic heavy metal pollutants.
Organic chemicals such as insecticides, herbicides, petroleum hydrocarbons, detergents, and a range of volatile organic compounds such as solvents discharged into aquatic ecosystems have the potential of altering the integrity of natural waters. This variety of chemicals regarded as water pollutants arises from agricultural use of pesticides, especially insecticides and herbicides, industrial wastes, marine oil spillage, and domestic wastes. They are potentially harmful to human health and aquatic organisms. Nutrients arising from sewage and agricultural use of fertilizers may cause eutrophication in aquatic ecosystems.
Although nutrients are elements essential for the growth of living organisms, human-caused contaminations can greatly enhance the presence of nutrients (especially nitrogen and phosphorus compounds), leading to anthropogenic or cultural eutrophication. Continuous nutrient loading to aquatic systems could ultimately increase the phytoplankton population, resulting in algal bloom, by providing more food for the algae than is normally available. Nutrients may affect human health. Excessive algal population in water has the potential of unbalancing the food chain, discoloration of water, and reduction in the quantity of light radiation that is available to aquatic life. However, when the algae dies, the rotting algae could produce a strong, unpleasant smell and the remains could be toxic to aquatic fauna and flora. This process could also result in depletion of dissolved oxygen in water.
There are several sources of water pollutants, and these are domestic and industrial wastewaters, agricultural runoff water, and other nonpoint sources. Domestic wastes commonly carry organic matter, microbiological contaminants, and sometimes physical and chemical pollutants. Industrial wastes contain mostly chemical and radioactive pollutants, while agricultural run-off water may carry mainly nutrients, pesticides, and heavy metals. Moreover, water pollution can be broadly classified into different types and these include microbiological, chemical, physical, and thermal water pollution.
Biological hazards associated with water pollution include disease-causing (pathogenic) microorganisms, like parasites, bacteria, and viruses. People exposed to biologically contaminated waters can become sick from drinking, washing, or swimming. Disease-causing pathogens commonly associated with fecal contamination of water include Shigella dysenteriae, Salmonella typhi, Salmonella paratyphi, Vibrio cholerae, Entamoeba histolytica and poliomyelitis virus responsible for causing bacterial dysentery, typhoid fever, paratyphoid fever, cholera, amoebic dysentery, and infantile paralysis, respectively. Also, the consumption of microbe-contaminated seafood, especially shellfish, could lead to outbreaks of food poisoning.
CHEMICAL WATER POLLUTION
Chemical form of water pollution includes the presence of a wide range of chemicals from industry, such as lead, arsenic, nitrates, radioactive substances, metals and solvents, and even chemicals which are formed from the breakdown of natural wastes (ammonia, for instance). Effluents from chemical industries and oil pollution from accidental crude spillage are categorized under chemical form of water pollution. In aquatic systems, these chemicals are poisonous to fish and other aquatic life. Chemical pollutants can be generally categorized into persistent (degrade slowly) and nonpersistent (degradable) substances.
Nonpersistent pollutants include domestic wastes, fertilizers, and some classes of industrial wastes. These polluting substances can be broken down into simple nonpolluting molecules or compounds such as carbon dioxide, and nitrogen by chemical or biological processes. Persistent water pollution is the most rapidly growing type of pollution, and includes polluting substances that degrade or do not grade or cannot be broken down at all. These pollutants tend to remain in aquatic environments for a long period of time. Common persistent chemical pollutants include some pesticides (such as dieldrin, heptachlor, and DDT), petroleum products, polychlorinated biphenyls (PCBs), chlorophenols, dioxins, polycyclic aromatic hydrocarbons (PAHs), radionuclides, and heavy metals. Toxic metals discharged in effluent can be accumulated in seafood, especially fish and shellfish such as prawns, cockles, mussels, and oysters, to levels in excess of public health limiting levels, therefore posing serious health concerns to people who eat them.
Pesticides used in agriculture and around the home, especially those used for controlling insects (insecticides) and weeds (herbicides), are another type of toxic chemical. These chemicals are used to kill unwanted animals and plants, and may be collected by rainwater runoff and carried into streams, lakes, bays, rivers, and seas, especially if these substances are applied in excessive quantities. Some of these chemicals are biodegradable and may quickly decay into harmless or less harmful forms, while others are nonbiodegradable and can persist in the environment for a long time. When animals consume plants that have been treated with certain nonbiodegradable toxicants (NBTs), such as dichlorodiphenyltrichloroethane (DDT) and chlor-dane, these chemicals are absorbed into the tissues or organs of the animals and can accumulate over time. When other animals feed on these contaminated animals, the chemicals are passed up the food chain. Some of these can accumulate in fish and shellfish and poison people, animals, and birds that eat them. Materials like detergents and oils float and spoil the appearance of a water body, as well as being toxic; and many chemical pollutants have unpleasant odors.
PHYSICAL WATER POLLUTION
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.
CONCLUSION
The pollution of water resources can have serious and wide-ranging effects on the environment and human health. The immediate effects of water pollution can be seen in water bodies and the animal and plant life that inhabits them. Pollution poisons and deforms fish and other animals, unbalances ecosystems, and causes a reduction in biodiversity. Ultimately, these effects take their toll on human life. Drinking-water sources become contaminated, causing sickness and disease. Pollutants accumulate in food, making it dangerous or inedible. The presence of these toxic substances in food and water can also lead to reproductive problems and neurological disorders. The effects of water pollution are varied. They include poisonous drinking water, poisonous food animals (due to these organisms having accumulated toxins from the environment over their life spans), unbalanced river and lake ecosystems that can no longer support full biological diversity, deforestation from acid rain, and many other effects. These effects are, of course, specific to the various contaminants.
