Chapter 4

Natural water quality in streams and rivers

4.1 Introduction

Quality of water in streams and rivers is a combination of their ionic composition, their mineralization, the dissolved organic matter content, and the total and permanent hardness. The composition and characteristics of water in streams and rivers is determinative to a certain degree for its utilisation as a resource for variable economic activities; it is also crucial for the structure and function condition of aquatic ecosystems and the hydroecological safety of the river basins.

The physical, chemical and biological characteristics meet the needs of the users (drinking and industrial water supply, irrigation) and the needs of the aquatic animals to a different extent; for that reason the quality of water is measured according to the different needs .The greatest requirements for the quality of water (taste, odour, colour, limpidity and some other organoleptic characteristics) are when it's used as drinking water. Different types of production demand water with a typical quality (low mineralization, low hardness, certain temperature etc.). The nutrients and dissolved gases determine the typical variety of the water biocoenosis.

The quality of surface waters is a result of the interaction between abiotic and biotic factors. Among the first ones are the climate, the conditions in the watershed for transformation of rainfall into a flow (geomorphological - indentation of the relief, slope of the basin; geological - structure of the rocks; soil - type of soils; vegetation - type, age and density of the forest vegetation, afforestation), the morphometric index of the river (width, depth, curve) and its hydrodynamic characteristics (speed of the river flow), temperature of water in rivers.

The abiotic factors directly and indirectly affect on the ionic composition, mineralization, and the function of the water ecosystems, etc. Among the second group of factors are the diversity of species and the structure of the water biocenoses, which to a great extent affect on the interrelations between the organisms, and their relations with the surrounding water environment. The hydrobionts control to a great extent the hydrobiological and hydrochemical processes.

The formation of the natural composition of water starts in the atmosphere and continues with the sheet flow. The chemical composition of rainfalls, the melted snow-cover, and the (melted) glacial water changes according to the rock structure, the soil, and the vegetation. The ionic composition and the mineralization of waters differ according to the proportion of the different types of water supplies; they change during periods of high and low waters i.e. it depends on the river regime. The hydrophysical, hydrobiological and hydrochemical processes are of great importance for the formation of the quality of water in streams and rivers.

The ionic composition and mineralization of waters are geographically connected to a certain area, and that defines their changes from the equator to the poles so as their specific characteristics in a certain region.

The natural hydrochemical and hydrobiological composition of water is preserved in river basins and areas that are not affected by any anthropogenic effect. In some cases the utilisation of water of natural quality needs an additional treatment (reduction of its hardness, mineralization and so on).

The composition and the quality of waters are defined according to the data on the ionic concentration and are studied by different indicators. Hydrochemical maps of a specific area indicate changes in time and among the river length of the water composition and quality.

4.2 Water composition in streams and rivers

The composition of water in rivers includes macroelements, microelements, dissolved gases, organic matter, and admixtures different in type, size, and concentration. The natural quality is defined to a great extend by the ionic composition and the mineralization. Usually among the anions contained in the river water the ones with the highest concentration are HCO3- (+ CO32-), SO42-, Cl-, and among the cations Ca2+, Mg2+, Na+ (K+). Most often the proportion between them is HCO3 > SO42- > Cl > Ca2+ > Mg2+ > Na+; That concentration is relatively constant or is characterised with a constant proportion to the concentration of some other substances, therefore they are called conservative.

In respect of the anions the river water is usually hydrocarbonated and sulphated, and in respect of the cations it is usually Calcium that is important. This characteristic is typical for waters with low mineralization. The higher the mineralization, the more the sulphate and chlorine ions so with the increase in mineralization the number of the Sodium ions predominates over the Calcium ones. One of the macrocomponents in freshwater is silicon acid and its concentration is often comparable to the major ions. The total calcium and magnesium ions content defines one of the major property of river water - its hardness. The proportion between the major ions reflects the environment and it changes according to the seasons of the year as well as along the river. The anions and cations contained in water determine its total mineralization.

River waters also contain some non-conservative substances - Fe, Cu, Zn, Pb etc. Their concentration is about 1·10-10 mol/dm3 and it strongly depends on the temperature, the biological activity etc. Most of them are part of disperse systems and the aquatic organisms (fish, crabs, etc). The ability of the conservative and the non-conservative substances to take part in a chemical interaction and their biological activity depend on their rate of oxidation. Phosphorus is of great importance for life in water biocenoses and its concentration in rivers is quite low: about 60 ppb (ppb = 1mg/kg); The presence or the lack of phosphorus as well as nitrogen limits the development of photosynthesising organisms.

Depending on the conditions in which river water is formed, it contains different concentration of dissolved gases. The solubility of the gases depends on their chemical properties, the temperature, the pressure, and the mineralization. Oxygen is of great importance concerning the quality of waters. The dissolved oxygen determines the mobility of the chemical elements. It defines the existence of the aquatic organisms and ecosystems. Oxygen goes into river flows by the air and during the photosynthesis. In deeper rivers the concentration of oxygen in depth depends on its income and expenditure during the biological oxidation-reduction processes.

CO2 dissolved in the river water is predominantly available in the form of a molecule as hydrated CO2. It gets into water flow by the carbon circle. HCO3-, CO2 & CO32- contained in water depend on the alkalinity (pH). About 80% of the CO2 contained in river waters is found in the form of HCO3-. If pH is between 4,4 and 8,3 HCO3 and free CO2 could be found.

Besides the inorganic components stream and river water contain large amounts of dissolved organic matter and its concentration varies from 0,5 to 10 mg/dm3. The basic components of the organic matter are carbon, oxygen and hydrogen. The content of nitrogen, phosphorus, sulphur, chlorine and calcium in organic compounds is less. The organic compounds dissolved in natural water influence the molecular-kinetic properties of waters and the ion hydration. Biogenic substances are not very important for mineralisation but are extremely important for the life of the aquatic organisms. There is a high concentration of nitrate ions (NO3), nitrite ions (NO2), ammonium ions (NH4) and ions of phosphorus acid (H2PO4). There are different types of organic matter. Some substances can be hardly dissociated biologically and others can be easily decomposed biologically. Among the natural ingredients the most difficult to decompound are the humus substances, which are products formed by dead plants.

Great amount of allochtonic organic matter with humus origin can be found in swampy rivers or swampy river sections. Stream and river water also contain humic acids and so on.

According to the so-called oxidation of water, several categories of stream and river waters are formed:

An important component of river and stream waters is the number of diverse simple and higher aquatic organisms, which to a great extent effect on the natural quality of waters.

The composition of water in streams and rivers determines the colour, odour, taste and density, which are crucial for their utilisation in different economic activities.

4.3 Conditions that form a natural water quality in streams and rivers

The natural quality of water in streams and rivers is formed by several groups of factors that work together and at the same time:

The natural factors determining the interaction between water in streams and rivers and the environment can be defined as direct and indirect. Direct are these that determine the input of minerals and organic matter (soil, rock composition, organisms etc.). Indirect are these that determine the conditions for the interaction of the chemical substances with the water (climate, relief, stream-flow regime). Concerning small rivers the most important factor is the dissolution of minerals and organic matter from the soil and the rocks in the catchment basin. The natural quality of water in big rivers also depends on the chemical composition of its feeders.

4.3.1 Climatic condition

Among the climatic factors the most important are precipitation and heat. The precipitation influence the river flow depending on its composition, amount, type, its distribution during the year, the temperature of the air (by heat conditions in river basins), and the temperature of water in river.

The effect of the climatic conditions on the ionic composition and the mineralization of water give them areal specificity. Precipitation

The precipitation fallen directly into the river basins and the melted snow-cover have a specific chemical composition; they form sheet flow. In the composition of the precipitation in natural conditions the hydrocarbonic ions predominate and the interrelation between the basic ions is: HCO3 > SO4 > NO3 > Cl > NH4 > Na > Ca > Mg. When precipitations fall through wood plants, that changes their chemical composition and pH, and then directly affects river water.

On the other hand a great amount of precipitation form high waters, which reduce the total number of the contained hydrochemical elements.

The air temperature is the most important factor that affects the temperature and the temperature regime of river waters. The air temperature is critical for the processes of evaporation in river basins; it slows or quickens the formation of weathering products in the catchment area and respectively the fall of some solid particles into the water flow. The temperature of river water directly affects on the abiotic conditions in water (Oxygen, pH, etc.), the diversity of species, and the function of the water biocenoses. The temperature of stream and river waters is a reflector of the changes in the environment and changes along the river and according to the climatic factors. The temperature of water in big rivers depends on the temperature of the water that comes from its feeders. The temperature of water changes a lot: from 0 to 20-40 °C. The decrease in temperature of water during winter in regions with temperate climate is accompanied with formation of ice, which changes the hydroecological conditions and respectively the quality of waters.

4.3.2 Conditions in the river basin Rock structure

Rocks are one of the major factors that determine the composition of the stream and river waters. The substances extracted from the rocks depend on their mineral structure. The major structural minerals are silicates (about 95%) and their structural units are the silicon-oxygen compounds (SiO4). Most of the 5% left are also Oxygen-consisting minerals: carbonates (CO32-), sulphates (SO42-), phosphates (PO43-). Oxygen-consisting anions are stable and they are preserved even after the weathering of the rocks.

Sedimentary rocks are the most important concerning the composition of river waters. For example carbonic rocks provide HCO3, sodium and magnesium ions. Igneous rocks (granite, diorite) are difficult to dissolve and river waters that form their composition in a catchment area with that type of rocks are characterised by low mineralization. The importance of metamorphic rocks depends on their type. The rock structure of the river basins directly affects the composition and the quality of stream and river waters and indirectly, by the composition of the subsurface waters. Soils

Soils are of great importance for the formation of the chemical composition and the quality of waters. Factors affecting soils are their composition, physical structure, and filtration ability. The solid part of soils contains minerals and organic matter. The liquid component contains soil solution and the organic part - micro-organisms (bacteria, fungi, etc.). Among the mineral compounds in the soil nitrogen ones are the only ones that are formed in the atmosphere. The rest of the mineral compounds are a part of the rock structure in such a form that are inaccessible for any plants. The natural process of weathering of rocks gradually destroys these mineral compounds and the biogenic elements become part of solutions by the drainage and infiltration waters. Forest vegetation

Forest vegetation is one of the most important factors for the formation of the natural quality of waters. The extent of its effect depends on its type (deciduous, coniferous), its age, its density, and also the combination of soil and tilt of the slope.

The geochemical control of the vegetation starts at the moment the atmospheric precipitation touch the overground organs. It depends on the vegetation type: if the vegetation is coniferous the acidity increases; if the plants are deciduous the acidity decreases. Older forests provide river waters with higher quality and in greater amount. Forest vegetation is a highly efficient mean for preserving freshwaters from bacteriological pollution.

Vegetation detains part of the fallen precipitation and the intensity of their infiltration, increases the evaporation, increases the infiltration ability of the soil, and in that way lowers the superficial water flow.

4.3.3 Hydrological processes

The composition and the mineralization of waters depends directly on the processes of formation of the river flow and flow regime. Source of supply

The genesis of the stream and river waters is a result of the incoming superficial (snow, rain, glacial, lake) and subsurface waters. The proportion between the sources of supply depends on climatic and hydrogeological conditions and it is of great importance for the formation of natural quality of stream and river waters. If superficial feeding predominates, stream and river waters are characterized by low mineralization and little organic matter content. The amount of substances incoming with rain and/or snow depends on the condition of the catchment area before the formation of the superficial flow. In dry weather the evaporation from the soils and the weathering process quicken, and for that reason the amount of salts on their surface increases. In case there is an intensive melting and a great amount of precipitation, then the superficial flow drains away in a short period of time and the amount of chemical substances going into the water flow is low. If subsurface feeding is more than 50% of the annual flow volume, river waters increase their mineralization. Subsurface karsts water increase the hardness of river waters. The proportion between the sources of supply changes during the year, which determines the changes in the chemical composition and mineralization of the river waters. Flow regime

In many cases ionic composition and mineralization depend on the height of the river flow and this dependence closely corresponds to the conditions in the river basins and their type of supply. If high waters are formed by precipitation and melted snow-cover that are characterised by low mineralization, the mineralization of water in streams and rivers will also be very low. In periods of low water, when river flow is formed predominantly by subsurface water, mineralization increases. In many cases this inverse proportional dependence between the number of ions and the amount of water changes into a direct or hyperbolic proportion.

4.4 Processes forming the natural quality of water in streams and rivers

4.4.1 Hydrophysical processes

Among the hydrophysical processes that affect the quality of stream and river water are the transportation of solid particles, heating and cooling, evaporation, freezing (formation of ice), absorption, diffusion, and dispersion of the light in water flows.

Important conditions for the formation of the quality of water are the processes of convection and diffusion. It is carried out by the movement of the dissolved natural compounds in the form of ions, associations and complexes and so on. Transportation of substances is described by the following formula:

I = Ic + Id

  Ic transportation of substances through processes of convection
  Id transportation of substances through processes of diffusion

Transportation of floating and bottom alluviums inflicts on the turbidness of water, the transportation of nutrients, and therefore the structure of the biocenoses. The number of solid particles and their concentration change the conditions for the light to go through the water and to a great extend determine the conditions for hydrobionts to exist and therefore determine the quality of water.

Heating and cooling are processes that are connected with a change in temperature and respectively in biochemical and hydrochemical processes. When an ice cover is present the intensity and activity of the processes of decomposition of complex organic compounds is reduced.

4.4.2 Hydrochemical processes

Water in streams and rivers is a polydisperse system in which the size of the particles of the disperse phase varies from 0.001 μm to a few mm. On the border of solid particles and solution there are constant processes of exchange interaction, adsorption of atoms and ions, isomorphic exchange of different types of ions and some other physical-chemical processes that lead to the extraction of dissolved substances from water. Important processes for the formation of the water composition are hydrolysis, dissolving of substances, sedimentation, sorption, ionic exchange and so on.

Oxidation-reduction processes. A great deal of the chemical processes in water are based on the interaction between the oxidants (Ox) and reductants (Red) or the so-called oxidation-reduction interactions, which can be generally described with the following equation:

Ox + ne- Red

If more than one oxidant and one reductant is available in the solution, the first to react are the strongest oxidants and strongest reductants. Oxidation is done even inside living organisms by enzyme interaction. At the same time some processes of reduction of substances are going on as the major reduction process is photosynthesis.

Oxidation-reduction processes go off in a different way according to the rate of oxygen content. Three zones are formed in water basins: anaerobic, borderline and aerobic. In aerobic conditions, which are typical for rivers, elements are in their high rates of oxidation (for example oxygen is in the form of nitrate NO3 and the ammonia is oxidised in nitrate). In anaerobic conditions, due to the exception of oxygen, reduction processes are predominant. Oxidation processes produce hydrogen ions H+ i.e. they affect on the pH of waters. Oxidation-reduction reactions are closely connected and are interrelated with the biochemical processes. A typical example is the oxidation of melted iron:

4Fe(HCO3)2 + 2H2O + O2 = Fe(OH)3 + 8CO2

and the desulphating in the next reaction

Na2SO4 + 2Corg + 2H2O = 2NaHCO3 + H2S

The second chemical interaction, which leads to the formation of hydrocarbonic-sodium waters, goes off exceptionally if microorganisms are present. For organisms that do not use sunlight to photosynthesise the reduction processes are the major source of energy.

Hydrolysis is a process of interaction between the ions of water (H and OH) and the ions of solution or the dissolved substances. This process is accompanied with the formation of compounds that can be weakly dissociated and dissolved, and with a change of the pH.

If water contains anions of weak acids and cations of strong bases, a hydrolysis of anions with connecting of protons is done.

HCO3- + H2O = HCO3- + H+ + OH- = H2CO3 + OH-

HS- + H2O = HS- + H+ + OH- = H2S + OH-


As a result pH increases.

Hydrolysis of cations is done if water contains anions of strong acids and cations of weak bases. Fe3+, Al3+:

Fe3+ + 3H2O = Fe3+ + 3H+ + 3OH- = Fe (OH)3 + 3H+

Al3+ + 3H2O = Al3+ + 3H+ + 3OH- = Al (OH)3 + 3H+


As a result pH increases.

Besides ions, O2 and CO2 are also involved in this process.

Dissolution is a process of chemical interaction between water and rocks or soils, in which substances from rocks and soils go into the water. Solubility of most natural minerals in water is connected with the anomaly specialities of water, conditioned by the polar nature of its molecules. In some cases dissolving is partial - only few ions (molecules) go from rocks and soils into river flows and these are usually the most soluble components.

Sorption is a process that goes on on the borderline of the liquid and solid phase, and which includes adsorption (increase of the concentration of some ingredients on the surface of the solid phase) and absorption (penetration of ingredients into the water). Absorption of oxygen from air is of great importance for river waters. Solubility of gases increases with the increase of temperature and decreases with saltiness of water.

Ionic exchange is a process in which solid particles can deprive ions from river waters and give the same amount of ions. Cation exchange is done more often than anion exchange. Ionic exchange increases when the concentration of ions in water increases.

As a result of nuclear decomposition radioactive products are excluded and/or supplied into water. Actinium (Ac), thorium (Th), uranium (U) and neptunium (Np) are decomposed.

4.4.3 Hydrobiological processes

An important factor for the water quality are the organisms that live there. Directly or by external metabolites they effect on each other and the environment. In regard to the hydrobionts their connection with water is stronger than the one between the ecosystems on the land. Hydrobiological processes that effect on the formation of quality of superficial waters depend on the quality, type and structure of the aquatic ecosystems. Aquatic ecosystems perform numerous valuable environmental functions. They recycle nutrients, purify water, attenuate floods, augment and maintain stream flow and provide habitat for wildlife and recreation for people.

A typical example that illustrates the effect of the aquatic organisms on the quality of waters is their importance for the organic compounds. It is usually considered that the organic compounds in waters dissociate to mineral form nitrogen and phosphorus, carbonic acid, and water. This rule is valid only for the unstable organic compounds with such a concentration that they can be quickly assimilated by heterotrophic organisms. When aquatic organisms die, two fractions of organic matter are formed: a slightly oxidised one, used by some bacteria and the zooplankton, and another one that is difficult to oxidise, which is used by some special bacteria. An important part of the process of transformation of the organic matter is the microbial synthesis. If the concentration of organic matter is high, the proportion between destruction and the biomass of the organisms using these substances is disturbed. As a result, photochemical and catalytic interactions, proceeding under the abiotic factors of the environment, form humus and resinous substances, bitomuids, etc.

The dissolved non organic phosphorus compounds, in the dissociated ionic form, are used by bacteria and plants as a source of phosphorus. Phosphates can be fixed in the cells of aquatic organisms as polyphosphates. They are accumulated in cells as an energy source. Negative accumulation of polyphosphate ions is equilibrated by the calcium, magnesium and potassium ions, which are of great importance for transportation of phosphates through the cell membrane. Together with the biological elimination of phosphorus, chemical sedimentation of the phosphorus compounds is going off. If iron or calcium is present orthophosphates settle as salts. Phosphorus compounds are used by aquatic biocenoses. The most active is the bacterioplankton, while phytoplankton by phosphatase assimilate the organic connected phosphorus. Zooplankton takes part in the circle of phosphorus in river waters.

If a molecule of substrate interacts with microorganisms the following processes proceed (Hartmann, 1993): transportation of the substrate to the bacterial cell as a result of turbulence or diffusion; sorption on the surface of the bacterial cell; enzyme interactions (processes that go on inside the cell). The last ones determine the speed of the biochemical oxidation in the whole biological system according to the principle of the minimum and the limiting phase.

Biochemical processes are closely dependent to the biophysical processes: formation, transmission and reception of information between cells, bioenergetics of water ecosystems and so on.