What are the main causes of fresh water pollution ?


Classification of water pollution depends on the criterion used for categorization of pollution. Based on the medium in which pollutants occur, types of water pollution may be distinguished as fresh water pollution and marine pollution. Fresh water pollution can be categorized into pollution of surface water and pollution of ground-water. When the pollutant enters a lake, pond, or river it is known as surface water pollution. If, however, the pollutant finds its way into aquifer, along with water of percolation, it deteriorates the quality of groundwater, and is called groundwater pollution.

You have noticed that groundwater bodies and surface water bodies together have been kept under the class fresh water bodies because their salt content is very low; always less than 5 ppt (parts per thousand). As against this, the water bodies containing salt concentration equal to or above that of sea water (i.e., 35 ppt or above) are called as marine water bodies. Estuaries and brackish waters have salt content somewhere in between 5 to 35 ppt. The pollution of oceans, seas, estuaries, salt marshes and other similar water bodies is known as marine pollution or ocean pollution. We will discuss this separately because the factors polluting oceans acquire different dimensions ensuing from the magnitude of water bodies involved.

Surface Water Pollution

Sometimes the natural causes of water pollution are so intricately mixed up with man-made causes, that the two become indistinguishable from each other. For example siltation (which means active transport of suspended particle by water or wind in a series of bounces) together with sedimentation (which means passive deposition of silt in a water body) is a common problem of most’ of the water bodies. Rivers bring silt from mountains as a result of rumbling of rocks- during their flow .towards plains. The natural deposition of silt in the form of sedirnents results from sharp fluctuations in the flow of water, ranging between zero flow to flash floods, within a short span of time. The man-made sedimentation of water bodies may also take place. Sewage, industrial effluents and discharge from agricultural farmlands sometimes bring tonnes of silt into river beds, turning them into swampy, marshy streches of foul smelling land. The natural and artificial or mail-made causes, in this case, are difficult to separate.


Similarly, fluoride—a strong pollutant, which causes knock knee disease, occurs naturally in Water bodies but it also result from industrial activities such as ceramic industries, phosphate fertilizer plants and aluminum factories.

We will now discuss some of the man-made sources of water pollution i.e., pollutants released into water bodies as a result of human activities.

Domestic sewage, industrial waste, agricultural residues, radioactive substances and heated waste waters are some of the important pollutants which result from human activities. Although the same water body may receive pollutants from more than ope source(s) simultaneously, for the purpose of simplification, sources of water pollution may be studied under the following sub-headings:

  1. Domestic waste water and sewage
  2. Industrial wastes
  3. Agricultural wastes
  4. Physical pollutants (radioactive and thermal)

Of these, the first three are discussed here. The physical pollutants include the radioactive and thermal pollutants.

(a) Domestic Waste Water and Sewage :


It includes water-borne wastes derived from household activities such as bathing, laundering, food processing and washing of utensils. Domestic waste contains garbage, soaps, detergents, waste food, paper, cloth, used cosmetics, toiletries and human excreta. This waste water which is known as sewage, is the largest primary source of water pollution.

A major ingredient of detergents is phosphate. When discharged into water, phosphate supports luxuriant growth of algae, called algal blooms. These produce offensive smell and choke the water bodies.

(b) Industrial Waste :

Most of the rivers and fresh water streams which pass near the major cities, townships or other human dwellings are polluted by industrial wastes or effluents. You may spend sometime studying Table-ti.1 which lists some of the major Indian rivers and the corresponding industry(ies) polluting them. You will notice that some of the common industries are paper industry, textile and sugar mills, distilleries and thermal power plants among others. The kinds of effluents generated by industries are also numerous. The paint and varnish industries produce aromatic long-chained hydrocarbons, textile industries put out various dyestuffs and metal salts which are used as mordants. The other industrial effluents contain a host .of pollutants such as oils, greases, plastics, metallic wastes, e.g., copper, zinc, arsenic, cadmium, lead, mercury, acids, alkalis, cyanides and chlorides, produced by various industries .

(c) Agricultural Waste :

It includes the following types of waste: manure, and other wastes from farm and poultry houses, slaughterhouse waste, fertilizer runoff from croplands, harvest wastes, pesticides, and salt and silt drained from irrigated or eroded land. These wastes enter waterways as .runoff from agricultural lands. You have read in Unit 8 that if a water body receives fertilizers (phosphates, nitrates) or manures, the water becomes rich in nutrients leading to eutrophication and oxygen depletion. Seepage of excessive nitrates into groundwater followed by its consumption by children produces a serious disease known as methaemoglobinaemia; Nitrate poisoning has been reported in various areas of Rajasthan.


Pesticides, especially DOT (Dichloro-diphenyl-trichloro-ethane) used in the control of mosquitoes and agricultural pests, have become the most serious pollutants of water. Being long-lasting under natural conditions, the pesticide goes on increasing in soil and water with successive applications. Serious cases of fish mortality have occurred following leaching of pesticides from agricultural fields to nearby rivers after rainfall. Most of the pesticides, being fat soluble, reach the adipose tissue of animals including man. On fat breakdown, the pesticides are released in the blood stream producing toxic effects.

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