Pollution occurs when, following the introduction of specific substances - solid, liquid, gaseous, radioactive - waters suffer significant physical, chemical or biological, capable of rendering it unfit or dangerous to public health, aquatic life, the fishing industry, industry and tourism.
Water pollution due to biological agents (microorganisms and fermentable organic matter) leads to a strong contamination, bacteriological water which results in the spread of diseases such as hepatitis colibacilozele or vital typhoid. In this type of pollution, along with urban wastewater can participate largely food industries, paper industry. Consider, for example, a medium size paper mill equivalent, in terms of pollution, a city of 500,000 inhabitants. No less dangerous are wastewater from animal husbandry major agro-industrial complex, characterized by a high concentration of animals in confined spaces, very small.
Chemical pollution resulting from discharges into waters of various compounds such as nitrates, phosphates and other substances used in agriculture, the residues and waste from industry or activities that contain lead, copper, zinc, chromium, nickel, mercury or cadmium. In fact, water pollution by nitrates and phosphates became more worrisome lately, especially in countries with developed and industrialized agriculture. Excess nitrogen fertilizer in the soil or other sources can make some of nitrates and nitrites to move into groundwater in large quantities. Drinking water with high concentrations of nitrates can cause "blue disease" of children - methemoglobinemia. A leading cause of water pollution is the hydrocarbon - present in all rivers of the world - as one of the effects of modern civilization.
Water pollution with organic synthesis is mainly due, detergents and pesticides. In the U.S., for example, was estimated at 13.1% the proportion of dermatitis (skin disease) caused by detergents. So great is the degree of water pollution with PCBs (polychlorinated biphenyls), which is used heavily in the plastics industry or are due to the utilization of the orezarii octoclordifenilului. Besides these substances, participate countless other synthetic organic chemicals such as phenols in inland waters.
Water pollution due to physical agents is due to release into water of solid materials, minerals, insoluble, as for example in the discharge of waste streams from quarrying or mining. This category includes water and thermal pollution. Thermal pollution is caused by discharges of cooling water from industry and from some thermal and nuclear. However, raising the water temperature as a result of these discharges, can lead to changes intolerable for most species of animals and plants in those areas. Are also accelerated bacterial decomposition phenomena, aquatic animals suffer because higher temperatures increase the intensity of metabolism. All this determines the so-called 'thermal pollution'.
A special problem is the radioactive pollution of water that can occur after falls from the atmosphere or radioactive materials, especially due to incorrect releases radioactive liquid or solid waste from industries that use atomic energy or nuclear research .