courtesy  :  wikipedia

water pollution

Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities, in such a manner that negatively affects its legitimate uses.^[1]: 6 Water pollution reduces the ability of the body of water to provide the ecosystem services that it would otherwise provide. Water bodies include for example lakes, rivers, oceans, aquifers, reservoirs and groundwater. Water pollution results when contaminants are introduced into these water bodies. In addition to damage to many species, water pollution can also lead to water-borne diseases for people.^[2] Water pollution traditionally is attributed to four sources, which provide the organization of this article:^[3]

• sewage
• industry
• agriculture
• urban runoff including stormwater.

Water pollution can also be classified as surface water pollution (for example lakes, streams, estuaries, and parts of the ocean in marine pollution) or groundwater pollution. Sources of water pollution are either point sources or non-point sources. Point sources have one identifiable cause, such as a storm drain or a wastewater treatment plant. Non-point sources are more diffuse, such as agricultural runoff.^[4] Pollution is the result of the cumulative effect over time. Supplying clean drinking water is an important ecosystem service provided by some freshwater systems, but approximately 785 million people in the world do not have access to clean drinking water because of pollution.^[5]

A practical definition of water pollution is: “Water pollution is the addition of substances or energy forms that directly or indirectly alter the nature of the water body in such a manner that negatively affects its legitimate uses”.^[1]: 6 Therefore, pollution is associated with concepts attributed to humans, namely the negative alterations and the uses of the water body. Water is typically referred to as polluted when it is impaired by anthropogenic contaminants. Due to these contaminants it either does not support a human use, such as drinking water, or undergoes a marked shift in its ability to support its biotic communities, such as fish.

Agriculture

Oxygen depletion, resulting from eutrophication is a common cause of fish kills.

Agriculture is a major contributor to water pollution. The use of fertilizers leads to Nutrient pollution, in which excess nutrients, usually caused by nitrogen– or phosphorus-containing compounds that are the main components.^[6] Sources of nutrient pollution include surface runoff from farm fields and pastures, discharges from septic tanks and feedlots (sewage – see below – is also high in nutrients). In addition to plant-focused agriculture, fish-farming is also a source of pollution.

Additionally, ag runoff often contains high levels of pesticides.^[3]

Industrial wastewater

Red mud stored in large ponds in (Stade, Germany). The waste is a byproduct of aluminum manufacturing, is dangerously alkaline and does not biodegrade. An estimated 175 million tonnes are produced annually.^[7]

Using the US as an example, the main industrial consumers of water (using over 60% of the total consumption) are power plants, petroleum refineries, iron and steel mills, pulp and paper mills, and food processing industries.^[3]

Some industries discharge chemical wastes, including solvents and heavy metals (which are toxic) and other harmful pollutants such as nutrients. Certain industries (e.g. food processing) discharge high concentrations of biochemical oxygen demand (BOD) and oil and grease.^{[8]: 180  [9]} Some industrial discharges include persistent organic pollutants such as per- and polyfluoroalkyl substances (PFAS).^[10][11]

Sewage

Sewage typically consists of 99.9% water and 0.1% solids.^[12] Globally, about 4.5 billion people do not have safely managed sanitation as of 2017, according to an estimate by the Joint Monitoring Programme for Water Supply and Sanitation.^[13] Lack of access to sanitation often leads to water pollution, e.g. via the practice of open defecation. Simple pit latrines may also get flooded during rain events. When sewers overflow during storm events this can lead to water pollution from untreated sewage. Such events are called sanitary sewer overflows or combined sewer overflows.