The natural waters containing up to 0,1 % of dissolved substances, are named stale, from 0,1 up to 2,5 % - mineralised, from 2,5 up to 5 % - as waters with sea saltiness, more than 5 % - brine (see Physics of solutions). The majority of salts of acids and bases can also be dissolved in water. The solutions of these substances are electrolits.
The list of the main dissolved mineral components of natural waters include the ions Na+, K+, Ca2+, Mg2+, H+, Cl-, HCO3-, CO32-, SO42- and gases O2, N2, CO2 and H2S. In small amounts such ions contain: Fe2+, Fe3+, Mn2+, Br-, I-, F-, BO2-, HPO42-, SO32-, HSO4-, S2O32-, HS-, HSiO3-, HSO3- and gases CH4, Ar, He, Rn. Other substances are in water in the smaller amounts. Under the contents of the weighed substances and painted huminous substances we distinguish highly coloured and highly turbid water. Except for the painted organic impurities there are also colourless organic substances at natural waters - the products of life process of microorganisms and combinations acting with waste water. in natural waters contain hydrocarbons, chlorids and sulfits of alkaline metals In the greatest amounts; their nitrates, nitrits and salts of other acids - in smaller ones.
Water always contains ions H+ and OH- in small amounts. The active concentration of hydrogen ions in natural and waste water is usually characterized by pH. The result of measurement of pH of water is very important for the characteristic of balances in a solution and biological factors of environment. In distilled water deprived of dissolved salts as a result of dissolution of CO2from air the level of pH is reduced up to 5,7; the lowest level (about 2,8) is observed if only CO2would be above water. On occassion pH of natural mineralised (especially marshy) waters is below 4 owing to presence in them of organic acids basically of huminous origin; the level of ph is also low in ferruterous waters. pH of the majority of natural waters is within the limits of 6,5-8,5, that is explained by presence at them of ions HCO3-. Usually pH of superficial waters, than pH of underground waters owing to the smaller contents in them of CO2 is higher. pH of waters of mountain lakes is lower than 6, as the amount of ions HCO3- in them is insignificant. During summer intensive photosynthesis in water of open reservoirs the amount of CO2, decreases, and pH thus grows up to 9 and above; in the winter period at easing of photosynthesis and accumulation of CO2 pH of water is lowered. Except for seasonal fluctuations of pH of water the essential changes of its level are observed within day in connection with the discontinuance of photosynthesis in night time; these fluctuations are insignificant because of buffer action of ions HCO3-. Hydrocarbonic ions occur in natural waters owing to dissolution the limestone by the carbonic acid. These anions are in all waters, except for soft waters with pH lower than 4. In fresh sources they dominate. The level of pH of the majority of natural waters is caused just by the presence of these ions in them. The contents of hydrocarbonic ions in superficial waters usually does not exceed 250 mg/l, and in waters of northern reservoirs and number of the mountain rivers their concentration is about 50 mg/l. Such phenomenon is connected with the stability of ions HCO3-in water the presence of the appropriate amounts of neutral CO2, is necessary, the contents of which is limited by the pressure of carbonic acid in air. Carbonic ions are formed in natural waters from the ions HCO3-together with the loss of a part of CO2or at strengthening of the alkaline reaction of environment. The contents of them in fresh waters at the presence of ions Ca2+, as a rule, is insignificant owing to small solubility CaCO3. Usually significant part of natural waters is in a condition of saturation them with CaCO 3
The concentration of ions SO42- in natural waters containing the ions Ca2+, does not exceed 1500 mg/l owing to small solubility of CaSO4, however in well mineralised waters as a result of reduction of activity of ions the concentration of ions SO42- is frequently higher. So, in sea water with the contents of salts of 35 g/kg and Ca2+ - 0,0104 mole/kg the concentration of SO42- achieves 2,7 g/kg. Galogenic ions are submitted in natural waters mainly by chlorid-ion. The contents of chlorid-ions in water of natural reservoirs varies over a wide range. In river and lake waters, is especial in northern areas of our country, the concentration of them is insignificant. However with increase of the mineralisation water absolute and relative quantity of Cl- increases; it is the main anion in the seas and greater part of hydrochloric lakes; in sea water the chlorid-ions make 87 % of weights of all anions. The wide dispersion of ions of chlorine is promoted by industrial and physiological activity of the man: the ion Cl- is not acquired by living beings.
The concentration of fluorine in superficial waters is usually insignificant - below sanitary norm (0,7-1,5 mg/l); in underground waters the contents of fluorine sometimes considerably exceeds these norms.
The ions of alkaline metals - magnesium and calcium, determine general rigidity of water. The rigidity of water is accepted for expressing in milligrammes-equivalents on litre. Carbonic and uncarbonic rigidity are determined, and their sum formes general rigidity of water. The first one is formed by carbonates and hydrocarbonates dissolved in waterCa2+ and Mg2+, the second one - by other salts of these cathions.In other countries rigidity of water is expressed in conditional degrees: German degree corresponds to 10 g CaO in 1 l of water (1 mg MgO is equivalent to 1,4 mg of CaO); French degree - 10 mg of CaCO3 in 1 l of water; American degree corresponds to 1 mg of CaCO3 in 1 l of water; English degree - 10 mg of CaCO3 in 0,7 l of water. The ion of magnesium is very much widespread in natural waters; under the general contents in water it occupies) the second place among cations. However there are a little waters, where dominant cation is Mg2+. Ion of calcium is the main cation in well mineralised waters. However in connection with small solubility of CaCO3 and CaSO4 the concentration of Ca2+ in natural waters very rarely exceeds 1 g/l. Usually it is much lower. The huge amounts of are taken out by the rivers in the seas and oceans, where as a result of partial loss of carbonic acid the solutions of CaCO3are formed. CaCO3 is spent for construction of skeletons of sea animal.
From ions of alkaline metals in natural, especially in sea, waters there are in plenties ions of Na, in smaller - K, and also Rb (about 0,2 mg/l) and Li (about 0,1 mg/l). On prevalence in natural waters Na+ occupies the first place, making more half of all the cations contained in them. The amount of K+ usually makes 4-10 % of number of the Na+ (ions, present in water. In fresh waters the concentration of Na+ + K+ usually does not exceed several milligrammes on litre. The reason of occurrence of connections of nitrogen in water except for biological pollution is also direct fixing of nitrogen from an atmosphere by bacteria. The concentration of NO3- in water is significant more, in earth waters of the top horizons the amount of it achieves sometimes several tens of milligrammes on litre. The connections of nitrogen are nutritious substances for vegetative organisms, and in the period of vegetation their amount in superficial waters is sharply reduced. The reason of occurrence of connections of nitrogen in water except for biological pollution is also direct fixing of nitrogen from an atmosphere by bacteria. The concentration of NO3- in water is significant more, in earth waters of the top horizons the amount of it achieves sometimes several tens of milligrammes on litre. The connections of nitrogen are nutritious substances for vegetative organisms, and in the period of vegetation their amount in superficial waters is sharply reduced.
The iron in natural waters meets as two and valent ions, and also as organic and inorganic combinations which are taking place in colloid condition. In superficial waters the iron, is usual in oxid form, contains mainly in organic complexes, and also as colloid suspensions. The contents of iron in superficial waters reaches sometimes up to 1 mg/l; in underground - up to 10 mg/l and more. Waters, the amount of iron in which is great, usually have sour reaction. The manganese contains in natural waters, as a rule, in smaller concentration, than the iron, and meets less often. In the underground waters which are not containing dissolved oxygen, it is as two valent ions, in superficial waters - as organic combinations, including in colloid condition.
Other metals meet in natural water in very insignificant concentration. The more detailed items of information are available for sea water. In waters of the rivers and fresh lakes on the average contains, mg/l: Co - 0,0048, Ni - 0,001, Zn - 0,02. Cu - from 0,0001 up to 0,01. The large concentration of metals in water is the result of industrial pollution.
The radioactive elements are extremely rarely met in all waters, that is caused by their distribution in mountain breeds. In water of the rivers about 2*10-8 g/l of uranium, about 10-9 mg/l of radium.