Almost all natural sources of water used for water supply can be attributed to three main groups:
- surface sources;
- underground sources;
- artificial sources.
Surface water sources include:
- seas or their separate parts (bays, straits),
- water currents (rivers, streams, canals),
- reservoirs (lakes, ponds, reservoirs, flooded quarries),
- swamps,
- natural underground water outflows (geysers, springs),
- glaciers and snowfields.
The characteristic qualities of river water are relatively high turbidity (especially during floods), a high content of organic substances, bacteria, and significant color. Along with this, river water is usually characterized by a relatively low content of mineral salts and, in particular, by relatively low hardness.
The water of the lakes is usually characterized by a very low content of suspended solids (i.e., low turbidity or, otherwise, high transparency), except for the coastal zone, where the turbidity of the water increases as a result of waves. The degree of mineralization of lake water is very different.
Surface sources are characterized by significant fluctuations in water quality and the amount of pollution in certain periods of the year. The water quality of rivers and lakes to a large extent depends on the intensity of precipitation, snowmelt, as well as on its pollution by surface runoff and wastewater of cities and industrial enterprises.
Seasonal fluctuations in river water quality are often very sharp. During the flood period, turbidity and bacterial contamination of water greatly increase, but its hardness usually decreases.
Natural waters are classified according to a number of signs. The simplest of them is the salt content of water: Fresh water – soluble salts up to 1 g / dm3 (such water is considered as drinking water); Brackish water – soluble salts are 1 – 10 g / dm3 (as a rule, such waters are mineral); Saline water – soluble salts are more than 10 g / dm3 (and this is so-called brines). In river water and groundwater, the salinity varies from 50 – 200 to 1500 – 2000 mg / dm3. The largest amount of dissolved impurities contains the waters of the oceans and seas.
Water is also classified by the predominant anion. There are hydrocarbonate, chloride or sulfate waters. Fresh water usually belongs to the hydrocarbonate class, since the content of calcium and magnesium bicarbonates in them comes to 60 – 70%.
According to the chemical composition, the impurities of natural waters can be divided into two types: mineral and organic.
Mineral impurities of water include N2, O2, CO2 gases dissolved in the atmosphere, which are formed as a result of oxidative and biochemical processes NH3, CH4, H2S, as well as gases from wastewater; various salts, acids, bases, which are largely in dissociated form, i.e. in the form of cations and anions forming them.
The organic impurities of natural waters include humic substances leached from soils and peat bogs, as well as various types of organic substances that enter the water together with agricultural runoff and other types of insufficiently treated runoff.
Due to the complexity of the individual classification of each type of organic impurities, the general ability of organic impurities to oxidize under the influence of specific oxidizing agents is used in the practice of analysis. This criterion does not give an accurate idea of the real concentration of organic substances in water, but it allows to approximately characterize and compare different types of water.
