What dirty water?
The medium can isolate, undo, recycle or accumulate matter or energy by itself. But the overcoming of this capacity, both in quantity and in speed, can be considered as a contamination. Therefore, any medium that is incorporated into the medium can be "managed" by it, and it will not cause major problems if this increase is made to some extent (to the extent that it can be isolated, destroyed, recycled or accumulated by itself). The problem would be to overcome it. The size will vary depending on the contaminant.
When the concentration of toxic pollutants is very low it does not produce significant adverse effects, but if the “threshold level” is exceeded, they can affect the growth, reproduction or metabolism of living beings, being able to be lethal when the concentration is very high. These levels are not fixed, among other things because the tolerance of each being is different. Sometimes, a certain living being is able to remove a certain contaminant, which despite its high concentration in the medium, will hardly be affected. On other occasions, however, bioaccumulation occurs, since most living beings tend to accumulate certain pollutants (such as DDT in fats), so their concentration along the food networks increases considerably. This type of substance, despite its limited impact on plants and herbivores, can be deadly for eagles, otters, etc., which are found on the vertex of the trophic networks, as well as for the human being.
All contaminants disappear over time, but their duration can vary greatly: a few days for some organic pollutants and several thousand years for certain radioactive substances.
Water pollution
As for water, what has been said so far is also useful, but since water dissolves almost everything, there are often more contaminants in water than anywhere else.
In general, the anthropic pathways of water pollution (i.e., those caused by human action) are grouped into three groups (see figure 1). On the one hand, pollution by agrarian activities, on the other, industrial and, finally, urban pollution. At the local level, one or the other (or in some cases several) will have greater importance and, therefore, the characteristics of the water will be different.
Agriculture and agriculture Agriculture
The pollution caused by agricultural activities, in terms of fertilizers and pesticides, is diffuse, that is, it does not present specific points of discharge.
When the farmland is fertilized excessively, the fertilizer, especially nitrate and phosphate, which is not absorbed by the plants and is not retained by the land, is gradually carried by the water and finally reaches the channels, both superficial and underground. If the subscriber is made by air, they will arrive directly to the superficial channels. When nitrates and, above all, phosphates reach the water, water is also "abona". Therefore, algae grow more and, at least theoretically, feed on them (both invertebrates and vertebrates). But this, in the end, causes an increase in the consumption of oxygen and if the water has oxygen deficit, many of its animals will die and the diversity of species will decrease (only those most tolerant to the scarcity of oxygen will remain). The problem is greater if the waters are hot. This process is called eutrophication (in fact the waters are usually more eutrophicated at the end of the channels). In addition, the influence of pesticides, whose effects depend on their chemical characteristics, must be added to eutrophication. II. After the World War the use of synthetic organic pesticides was imposed (the previous ones were arsenic, mercury, copper and lead). Among them are chlorinated hydrocarbons, such as DDT, aldrin, dialdrin, heptachlor, lindane, endrin and toxaphene. All these products are hardly degradable, so they accumulate in living beings and the concentration increases as the trophic chain increases. They can be fatal when the concentration is very high, but before reaching it there are problems of growth, development and fecundity (malformations, sterility, fragile egg shells in birds...). In recent years, organophosphoric compounds such as malation and diazinon and carbamate pesticides such as carbofurane and oxamila have been replaced. The former degrade very quickly and theoretically should cause less damage; the latter are very soluble in water and little retained to the ground. However, if both arrive in the channels before degrading (for example, by heavy rains), they can cause significant damage, since they are very toxic.
However, in the case of agricultural activities carried out in certain nurseries and buildings, the point of discharge is usually precise and, in this sense, can be analyzed as industrial activities. Organic pollution (causing eutrophication) and chemicals used in treatments, cleanings, etc.
Industry industry industry
Unlike agriculture, the water pollution produced by the industry occurs at certain points of discharge, which facilitates the adoption of measures. However, the type of industrial activity is very varied and therefore the pollution that can generate or generate the different sectors is different (see ). The most persistent problems are those caused by substances that remain longer without degrading in the environment, such as heavy metals (which cause malformations, development problems...) and polychlorinated PCBs or bifelines (related to cancer and other development problems). Chemical products also cause the sudden death of fauna and flora. In addition, the industry can provoke eutrophication.
Finally, there is the so-called thermal pollution, that is, water heating, usually used in the cooling systems. This transforms communities of aquatic living beings and disappears the species that need more oxygen.
Cities and cities Cities
Pollution in the cities is also carried out from certain points of discharge, although it can be considered from two origins. On the one hand, it is the one that is generated in the kitchens and toilets of the homes, being its main components the organic matter, the microbes, the soaps and the oils. The first three have as main consequence the eutrophication. On the other hand, rainwater is a pollution accumulated on roofs, roads, parking lots, etc. that can cause a greater problem than the previous ones. The components are industrial oils, certain liquids poured from the vehicles, heavy metals detached from the exhaust tubes, etc.
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