Between the main pollutants of rivers and streams, are the effluent drains generated by industrial activities, urban sewage, and substances used in agricultural activities and in mining activities.
Human economic activities have generated a worrying degree of contamination in fresh surface waters, rivers and streams on the planet, with water being the most important liquid for living organisms..
Water is the main component of our planet and represents approximately 75% of its total surface. All known life forms require water for their existence; the planet's water moderates the climate, produces a large part of the shaping and the terrestrial topography, carries away polluting wastes, mobilizes them, dilutes them and fulfills a very important biogeochemical cycle.
Additionally, water covers fundamental human needs such as food, hygiene and personal hygiene, housing and cities. Huge amounts of water are required for food crops, to sustain livestock, industrial and electrical energy production or for water transport.
Of the total water on the planet, only approximately 0.02% is fresh water, usable for human needs with previous purification treatments. Despite its vital importance, it is one of the worst managed natural resources.
There is a dilemma between its use by humans and its preservation as an indispensable resource. Nature has its own system for collecting, purifying, recycling, redistributing and reserving water, powered by solar energy, called the hydrological cycle..
By overloading aquatic systems with non-degradable polluting wastes and depleting reserve water from the subsoil, human activity is undermining the assimilation capacity and resilience of this system..
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Water pollution is understood to be any physical, chemical or biological change that alters its quality, with a negative effect on living organisms, or that makes it impossible to use in general..
Water pollution originates from point sources, unique, traceable, or non-point, dispersed and imprecise sources.
Point sources are easily located, as they produce discharges of pollutants in specific places, such as industrial effluent drainage pipes, black waters that flow into surface water bodies (rivers and lakes), oil spills, among others..
Point sources can be located, monitored and regulated, since their location is known.
Non-point, scattered sources cannot be associated with any particular discharge location. As an example we have the depositions from the atmosphere (acids, particulate matter), runoff of agrochemicals from farmland, animal farms, mines, emissions from land, water and air transport, among others..
The main non-point sources of pollution, which affect the water of rivers and streams, are agricultural activities, industrial activities and mining, both artisanal and mega-mining of traditional non-biological methods..
Intensive agriculture that uses powerful chemicals called agrochemicals to increase crop production causes intense environmental damage, as well as soil and water pollution..
Among agrochemicals, highly toxic biocides are used to eliminate so-called “weeds” (herbicides) and insect and small mammal pests (pesticides).
These substances reach streams and rivers through runoff from rain or already contaminated irrigation waters, and cause serious problems in aquatic life. They are a common cause of contamination.
Other widely used agrochemicals are inorganic fertilizers that are used as nutrients for the growth of plants in crops..
These fertilizers are salts of nitrates, nitrites, phosphates, sulfates, among others, which are soluble in water and are carried by irrigation water, rainwater and runoff to rivers and streams..
Once incorporated into surface water bodies, fertilizers make an excessive supply of nutrients to the waters, causing excessive growth of algae and other species that can deplete the dissolved oxygen available to other members of the ecosystem..
The remains of pruning and plant material from crops, if they are dumped into rivers, cause the depletion of dissolved oxygen in the water - essential for aquatic organisms - due to their aerobic decomposition..
Livestock activities also generate excess nutrients in aquatic ecosystems, with subsequent excessive growth of algae and depletion of dissolved oxygen in the water. This occurs due to the discharge of livestock feces into surface water currents..
The sediments of soils eroded by the elimination of the vegetal layer (product of agricultural activities and urban planning), are soils with very little adhesion, whose particles are easily carried away by runoff towards surface water flows..
The excess of sediment in the waters contributes turbidity, which hinders the passage of sunlight and reduces the rate of photosynthesis of the organisms that produce aquatic ecosystems. This negatively affects the food webs that support life in rivers and streams..
Industrial effluents provide a wide variety of toxic chemicals, which can be classified into organic and inorganic substances. Temperature variations are also considered pollutants if they affect the organisms in water bodies..
Organic substances contained in industrial effluents include oil, diesel, gasoline, lubricants, solvents, and plastics (which are very toxic to aquatic life)..
Salts, acids, metallic compounds, among other inorganic chemical compounds that can incorporate industrial effluents into surface waters, also act as powerful poisons in aquatic ecosystems..
Electricity generating plants and industrial activity in general, also generate thermal pollution of surface waters, which alters the optimum temperature for growth and development of aquatic life forms, and produces alterations of the immune system, among other conditions..
High temperatures also cause the loss of dissolved oxygen in the waters, which, as we have already mentioned, negatively affects the entire aquatic ecosystem and particularly causes respiratory difficulties to the death of fish..
Municipal wastewater or sewage contains, in addition to excess nutrients, infectious agents -bacteria, viruses, parasites- that contaminate surface waters causing diseases in animals, plants and humans..
Additionally, sewage is carriers of soaps, detergents, insoluble calcium and magnesium salts, oils, fats, acids and bases, which negatively affect organisms..
The effluents from mining activities are highly polluting of surface waters. These effluents contain heavy metals, arsenic, cyanides, acid drains, mercury, among other pollutants, which are discharged into rivers..
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