Pollution is defined as a physical, chemical or biological alteration of water related to humans and resulting in adverse effects on human health, safety, well-being and the use of water for any purpose. either, or for the conservation and protection of the environment.
Pollution is defined according to international law as a physical, chemical or biological alteration of water related to humans and causing adverse effects on human health, safety, well-being and the use of water. waters for any purpose or for the conservation and protection of the environment.
Four types of pollution exist: physical pollution, nutrient pollution, organic pollution and toxic pollution. Accidental pollution and chronic pollution are also distinguished. Accidental pollution is a point source pollution, such as an oil spill, which is often large. While a chronic pollution corresponds to the permanent rejection of low doses of pollutants in the ecosystem (urban emissions, agricultural activities, …).
Physical pollution mainly comes from thermal and nuclear plants and factories using water as coolant. The water taken from the natural environment will be rejected by these structures at a higher temperature. This will cause an abnormal rise in stream temperature and therefore: (1) mortality in species that do not support the new temperature and (2) proliferation of other species favored by the new temperature of the stream. water. Aquatic species are generally more sensitive to temperature changes than terrestrial species.
For example, a 1000 MW plant uses and discharges several tens of cubic meters of water per second with a temperature of 7 to 8 ° C.
The increase of the temperature modifies the density and the viscosity, increases the evaporation, decreases the solubility of the gases, modifies the speeds of the chemical reactions, increases the respiration of the organisms (and therefore decreases the concentration of dissolved oxygen), increases the sensitivity of organisms to toxic substances and increases the toxic effect of chemicals.
Another physical impact on aquatic environments is represented by dams. Their impact depends on the depth, the size of the reservoir and the management of the water body. The main impacts encountered being:
- A break in the longitudinal gradient: there is a sudden change in the biotope downstream of these dams. The latter acting as an obstacle for biocenoses. For example, the construction of dams will prevent the passage of migratory fish.
- A modification of the hydrological regimes: the impact of the dam will depend on the management mode of the structure.
o By sluice: A low-capacity structure that passes water according to demand. The lotic environment is very unpredictable, with a significant drop in diversity.
o Case of large reservoirs: These reservoirs are likely to impose seasonal flow patterns that are extremely different from natural flow patterns.
o Diversion works: The sections of river beds are short-circuited by a canal which is equipped with a hydroelectric plant. In this type of system, the lotic sections will be powered by a low rate called reserved flow.
Organic pollution refers to polluting substances containing carbon such as PCBs, organochlorines such as DDT, etc … Emissions can be direct by pipes and rivers that then carry pesticides, fertilizers, organic load of cities, oil, oils. Naval traffic also generates organic releases from shipwrecks, accidents at sea, discarded waste and ballast water.
The impact of organic pollution varies according to the importance of the discharges. When they are small, the river will be able to recover its initial state some time after the pollution. When the discharges are too large, the pollution appears and is followed by effects on the biocenoses. The bacterial activity will consequently increase and lead to an increase in the use of dissolved oxygen. The proliferation of algae on the surface reduces the transparency of the water and photosynthesis is only possible in the surface layer.
This pollution comes from the overabundance of nutrients in aquatic ecosystems, such as phosphorus and nitrogen. Wastewater and agricultural and domestic fertilizers are the main sources. This form of pollution leads to the acceleration of the eutrophication of aquatic environments.
These nutrient pollutants will allow an increase in primary production at first. Then, when the intake is too high, there is a strong activity of degradation of organic matter and a drop in dissolved oxygen. The development of algae is limited by the absence of light, so there is accumulation of nitrogen and phosphorus that can no longer be degraded هنا.
The bodies of water are full of cyanobacteria, which make the water unsafe and prohibit even leisure activities.
The main toxicants found in the environment during chronic or acute pollution are generally heavy metals (lead, mercury, cadmium, zinc, …), halogens (chlorine, bromine, fluorine, iodine), complex organic synthetic origin (pesticides, …) or natural origin (hydrocarbons).
The toxic pollution of water has several origins: it can come from point releases (industries, urban wastewater treatment plants, …), scattered point releases (DTQD: toxic waste in dispersed quantities: many SME discharges, SMEs , artisans …) and / or diffuse releases less well known (pesticide application in agriculture, fallout of micropollutants emitted into the atmosphere, leaching of roads and highways, …).
Toxic substances discharged into the aquatic environment, have harmful effects for humans, wildlife and? ore. They contribute to the depletion of aquatic ecosystems. Some of them accumulate in living beings (bioconcentration), and move from one link in the food chain to another (biomagnification). They cause significant damage to the biological balance.