[Encyclios]

Water pollution

As long as industry was non-existent, or in an embryonic state, as long as chemical detergents and fertilizers did not exist and as long as urban agglomerations were far from their current size, the self-purification processes of water and soil were sufficient to avoid water pollution. The increase in population, industrial development and the widespread use of chemicals in all human activities have caused a significant increase in the amount of sewage and a radical change in its composition: it is no longer organic substances easily biodegradable, but a mixture of organic and inorganic substances, some of which are very harmful and extremely difficult to eliminate.

Inland waters are polluted by industrial, agricultural and urban discharges. The pollutants diffused in industrial effluents are mainly chemical compounds in solution or in the form of emulsion and foams: strong acids and bases, mineral salts (particularly of chromium, zinc, cadmium, copper, nickel, lead, ammonia salts and also chlorides, fluorides, sulfides, cyanides, sulfites and hydrosulfites), hydrocarbons, tar, vegetable oils and fats, phenols, starches and sugars, dyes, etc.. There are also solid materials of various sizes and of organic or mineral nature (colloids, residues of wood and paper processing, waste and residues of the food industry, sands, crushed stone, etc.) and radioisotopes.

In urban effluents, on the other hand, putrescible organic substances more or less contaminated by pathogenic and parasitic microbial forms, and foams from detergents and soaps prevail. Agricultural effluents mainly contribute nutrients and biocides by leaching fertilizers and pesticides from the fields.

Marine pollution results not only from polluted inland water runoff, but also from the direct discharge operated, without effective purification, by coastal industries and urban settlements and from the disposal of waste by all types of vessels. It should also be taken into account that most of the atmospheric pollutants end up sooner or later in the sea. The sea is seriously contaminated even at a considerable distance from the coasts; the most widespread pollutant is oil because of the criminal practice of discharging tankers into the sea, the repetition of accidents and shipwrecks involving oil tankers, the extraction of oil from continental platforms.

A form of water pollution destined to spread more and more is the thermal one, largely due to the increasing use of water in industrial cooling processes, especially in thermoelectric and nuclear power plants, but no less serious are the effects of thermal discharges from steel mills, sugar refineries, aluminum factories and, in general, of all the many industrial processes that require, at the end of processing, the disposal of residual heat.

The increase in water temperature has, as a first effect, the decrease in the solubility of oxygen, also accelerates all the processes of development of aquatic life, accentuating the phenomena of eutrophy. Along the course of the rivers, thermal dams can be formed, which prevent the ascent of fish. The most serious effects of thermal pollution are, however, due to sudden drops in temperature that cause, in animals now adapted to a warmer environment, the so-called “cold stress”, which can even be fatal; these phenomena are common downstream of power plants when production activity is suspended.

Pollution of natural waters, both marine and inland, has a fairly complex genesis. The presence of sufficient dissolved oxygen is essential for the life of aquatic biocoenosis; if organic substances are introduced into the water in large doses, they are broken down by aerobic bacteria and transformed into simpler substances through the use of a certain part of dissolved oxygen; If pollutants are released in higher concentrations, the aerobic bacteria will consume all the oxygen dissolved in the water and the oxygen absorbed from the external environment; then an anaerobic bacterial population will be established, indifferent to the absence of oxygen and able to break down organic compounds, transforming them into harmful substances, lethal for the life of aquatic plant and animal biocoenosis. Even the discharge of toxic compounds and industrial waste is fatal to aquatic organisms: 0.14 mg/l of copper sulfate are already sufficient to kill a trout for asphyxiation at the gill level, sulfides, cyanides and ammonia instead determine the asphyxiation in the bloodstream and cells in general.

Detergents, which often cover entire water surfaces with a thick foamy layer, due to their complex chemical structure with branched chains, are hardly attacked and degraded by bacteria into simpler and less harmful compounds; these substances therefore strongly alter the physical characteristics of water, modifying its surface tension and causing the disappearance, among other things, of aquatic flora, plankton and, with them, the components of the entire trophic pyramid. This causes, in addition to the extension of large surface layers of decaying matter, with its miasma and various colors, the spread of bacteria and viruses (typhoid, dysentery, cholera, viral hepatitis, etc.) in both fresh and marine waters and the absorption of these pathogenic microorganisms by shellfish for food (such as mussels, oysters and other edible lamellibranchs) and bred near outlets of drains with the consequent danger of serious epidemics.

The superficial layers of oil and other hydrocarbons constitute real impermeable barriers between air and water, preventing the dissolution of atmospheric oxygen in the liquid environment and causing the death by asphyxiation of the entire underwater ecosystem. These substances also damage the beaches, the coastal vegetation and the surface aquatic fauna: sea birds, for example, which, entangled in the dense oily layers, often die from insufficient body thermoregulation and from poisoning.

Another form of pollution of sea water is constituted by the formation of mucilage. More serious, however, is the pollution that occurs in lake basins in which the mixing between hypolimnion and epilimnion and the water exchange are rather slow. Although lakes are relatively more resistant to pollution than rivers, if they exceed a certain limit, the consequences last for decades after the complete cessation of polluting discharges.