GROUND AND SOIL WATER CHARACTERISTICS
Laboratory of Water Quality, Water Problems Institute, Russian Academy of Sciences. Moscow, Russia.
Keywords: physically bound water, free water, water structure, soil and ground water, saturation zone, heavy metals, geological barriers, migration capacity, microphlora, water pollution.
2. Water structure and its physical characteristics.
3. The process of groundwater chemical composition formation.
4. Groundwater biological characteristics.
5. Anthropogenic influence on ground water chemistry
Water bedded under the earth’s surface in the crust is called ground water. Geological structures holding groundwater are located in different physico-chemical conditions—this determines the different aggregative states of water: liquid, solid, and gaseous.
Liquid water is located in the upper part of the earth’s crust under relatively low temperature and pressure. Water deeper than 20-70 km is in gaseous state under high pressure. Water in the solid state is located in the zone of permafrost.
Liquid water in rocks is in different forms: in the crystal lattice of minerals, physically bound with rocks, and free. The maximum volume of physically bound water is found in clay—up to 40%.
Free water moves under gravity and possesses extraordinary ability to dissolve rock. All ground waters are solutions of different chemical composition—their characteristics depend in part on their location in the crust.
The chemical composition of soil and subsoil waters forms as a result of stage-by-stage transformation of the chemical composition of water that fell as precipitation. Other transformations are controlled by many factors: climate, relief, lithology, intensity of water exchange, the biological production of the landscape, and the geochemical situation. The complex combination of these factors creates the mixed character of soil and subsurface waters.
Chemical composition of artesian water is created under the influence of interactions between water and rocks. The scale of the interaction is determined by the duration of the contact between water and rocks as well as by the composition of the rocks. The duration depends on the intensity of water exchange in the hydrogeological structure.
In recent decades the increase in groundwater use and human economic activity has led to noticeable deterioration in groundwater quality. As it is extremely difficult to remove existing groundwater contamination, the best strategy is to eliminate sources of contamination.
The extraordinary mobility, high chemical activity, diversity of forms, and ability to change phase determine the unique role of water in all geochemical processes in the atmosphere, hydrosphere and lithosphere. All natural waters are linked to each other. It does not matter in which state they are. Changes of state take place steadily. Transfer of chemical elements happens with the help of water. Groundwater plays a key role in this eternal movement of water and soluble substances, circulating in global and many other smaller cycles.
Groundwater is usually understood as water found in the crust, in different aggregative states—liquid, gaseous and solid. It lies in soil, in sedimentary beds and in cracks in massive-crystal rocks. The lower border of liquid water may be as much as 70 km beneath the surface. In other words the zone of water distribution includes virtually all the crust. The total volume of the Earth’s groundwater is estimated to be 23.4 million km3, i.e. approximately 65% of all terrestrial water.
Soil and ground water are always aqueous solutions of various chemicals. The source of the water is precipitation. The chemical composition of soil and ground water is closely related to geographical conditions. A high flushing regime, low water mineralization and high sediment loads are associated with land with low annual temperature and rapid biomass accumulation. In arid zones the concentration of organic acids is very low, and water mineralization is high, as products of weathering and soil formation are not removed from the soil profile due to the low precipitation. The chemical composition of deep ground water is determined by the geological characteristics of the environment and is controlled by processes of interaction between water and rocks. All this gives rise to an extraordinary diversity of chemical composition in soil, subsurface and ground waters. In recent decades groundwater use has increased, as have changes in its physical and chemical characteristics, under the influence of anthropogenic activity. Anthropogenic pollution of ground water now occurs all over the world. The best strategy for maintaining good water quality is preventive measures, such as elimination of pollution sources.
2. Water structure and its physical characteristics.
Ground water in rocks is found in different states: water in the crystal lattice of minerals, water physically bound with rocks, free water, ice and vapor.
Water in the crystal lattice of minerals is chemically bound with its atoms and ions and may be removed from the lattice only by heating.
Physically bound water is located in rocks as a hydrated covering surrounding mineral particles and can be subdivided into hard-bound and low-bound. Hard-bound or hygroscopic water is found in clay material and is firmly held there by molecular and electric cohesion. It only moves when it goes into the gaseous state, and it can be removed by heating (up to 100 – 120 oC).
The maximum volume of hard-bound water in rocks is called maximum hygroscopic rock moisture capacity, and this does not exceed 1% for massive-crystal rocks, or 18-20% for clays. Plants cannot use this kind of water.
Low-bound or film water forms films on hard-bound water when the rock moisture exceeds rock hygroscopicity. The bond strength between rocks and this kind of water is significantly lower than the hygroscopic one. On the other hand, the power that keeps the water in rocks is 70 000 times greater than the power of gravitational acceleration, as water moves in rocks from a thicker to a thinner film. Maximum film water concentrations are: in clays – 25-40%, in loamy soils – 15-23%, in clay sands – 9-13%, in sands – up to 6%.
Physically bound water may be removed from rocks by heating (up to 110 oC). The characteristics of this kind of water differ strongly from those of free water. Its permittivity is significantly lower, and the freezing point is significantly lower than 0 oC, dropping to -100 oC when converting to hygroscopic water. The dissolving power of this water is also low. The maximum volume of physically bound water is called ‘maximum molecular rock moisture capacity’.
Free water moves under gravity and is located in rock cracks and pores. It is formed in rocks when the humidity within the rock is higher than the maximum molecular rock moisture capacity.
Water in the solid state—ice crystals—is formed under temperatures below zero in the layer of winter freezing. In the zone of permafrost, ice consolidates separate rock particles.
Water in the gaseous state is located in pores free of liquid water. It moves from layers with higher pressure and temperature to layers with lower pressure and temperature.
The diversity of water forms and states creates various interactions of water with the lithosphere, atmosphere and biosphere. During the process of such interactions the physical and chemical characteristics of the water undergo changes.
As a result of the development of human activity, the characteristics of water consumers have changed over time, and these changes pose hazard to the populations of many countries, particularly those that suffer from deficit of suitable water. The study of soil and ground water chemical composition formation is therefore a priority line of investigation dedicated to medical, biological and environmental problems.
The unique structure of the water molecule, caused by the presence of the hydrogen bond, determines its anomalous physical and chemical characteristics. For instance, according the calculations of D. Herd, the melting point of water, by analogy with the hydrides of other elements, should be minus 120 oC, and boiling point - minus 112 oC. Water melting is accompanied by compression, not expansion. The maximum density temperature is plus 4 oC. The specific heat of evaporation and surface tension are the highest among all other liquids. Most substances are perfectly soluble in water. This characteristic can be explained by the high permittivity of water that equals 80 at a temperature of 22 oC.
The key point about the hydrogen bond is that the hydrogen ion, having a small radius, is capable of electrostatically attracting other ions similar to electrons. In each water molecule four hydrogen bonds are formed: 2 at the expense of 2 hydrogen protons and 2 – 2 pairs of unshared electrons. Because of the presence of 2 plus and 2 minus poles, a water molecule is an electric dipole with dipole moment (1844 debye). Each water molecule is surrounded (as a tetrahedron) by 4 other molecules. Owing to the hydrogen bonds, these molecules have a high degree of orderliness. This fact gives the structure of water similarities to the structure of solid substances. However, the orderliness of a solid substances’ molecules is the same throughout the solid, but it is only the orderliness of the closest atoms in water that is similar to that of solid substances. O.Ya. Samoylov considered the orderliness of water to be an ice-like frame that is slightly fuzzy because of thermal motion. The frame’s cavities are filled with monomolecular water molecules. In ice, all water molecules are bound to each other but in water there are always some (10 to 50%, according to different publications) monomeric molecules.
When ice melts the water molecules change from being in a hexagonal arrangement to a closer tetrahedral modification, the volume of which is 20% less. Similarly expansion of the liquid state, as determined by the increase in thermal motion of water molecules, causes a 10% increase in volume. The overall result of contraction of water molecular volume at the moment of ice melting is 10%. Contraction of volume together with temperature increase prevails up to 4 oC. As a result of increase in thermal molecular motion, the number of monomeric molecules and the water volume are increasing. Complete destruction of hydrogen bonds and transfer of the molecules to a monomeric state occur at 250 to 370 oC, according to different sources.
The structure of physically bound water adsorbed onto the surface of rocks is close to the monomeric condition. There are no hydrogen bonds there. This leads to a change in permittivity value to 2.2, and, consequently, to acute decrease in the dissolving power of the water and lowering of the freezing point. This is why adsorbed and film water have few solutes as compared to other forms of water.
The structure of water changes under the process of dilution. If ion size exceeds the size of cavities in ice-like water frames it "tears" the hydrogen bonds. If the ion charge is not high, broken hydrogen bonds will not be effectively substituted by the interaction between the ion and water, and water molecules will have higher mobility than they have in pure water. The elements K, Rb, Cs, Br and I are among the ions that disorder the structure of water. The influence of ions on water structure is proportionally related to polarizing power; to put it more precisely its charge density equals the ratio of ion charge to radius. The ions of K, Rb, Cs, Br and I have a high charge density. Ions with this characteristic strengthen the ice-like water structure and lower water mobility.
In order to go out of the nearest ion entourage, a water molecule has to go through a so-called potential barrier the value of which increases under the restorative influence of ions with high charge density. Conversely, the value of the potential barrier decreases under the destructive effect of ions with low charge density.
The value of the potential barrier E is for pure water and equals E + δE in the case of ion presence, where δE is the value characterizing changes in potential barrier under the influence of ions.
If δE>0, ionsare effectively bound to the nearest solution molecules. This phenomenon is called positive ion hydration. If δE<0, water molecules located near ions become more active than in pure water. This phenomenon is called negative ion hydration.
Ions with low charge density, i.e. with negative hydration, are not sufficiently protected by water molecules and are easily sorbed in natural conditions. The low potassium concentrations in natural waters—despite the abundance of this element in the lithosphere and the high water solubility of its compounds—exemplifies this phenomenon. Its low charge density and large radius lead to destruction of water structure and fast ion capture by natural sorbents.
Ions with positive hydration have the same influence on water structure as increased pressure. According to the Le Shatelieux principle, increased pressure leads to the transition of ions with negative hydration to solution. Thus, in near-surface conditions, carbonaceous and sulfate minerals are easily dissolved in water and at the same time ions with positive hydration go into solution (Ca2+, Mg2+, SO42-, HCO32-).
At a certain depth these minerals become solution-resistant, and new formations of tufa, anhydrite, etc. are usual there. Furthermore ions with negative hydration (Pb2+, Rb2+) migrate in waters of artesian basins and hardly migrate in subsurface water.
When ions with positive hydration prevail, water polarity and dissolving power are higher.
2.1. Spatial forms of groundwater bedding.
Movement of groundwater and its physical, chemical and biological characteristics depend on the peculiarities of the ground hydrosphere. In a hydrogeological section of the earth’s crust (downward from the surface) one can distinguish a zone of suspended water, a cryolithozone, and asaturated zone.
Water located in the pores of the upper layer of the zone of suspended water confer fertility and is called soil water. In soils all kinds of water are found—from physically bound to vaporous. In periods of intense moistening of soils, free gravitational water prevails. Downward motion under gravity is the main type of motion.
Ice, vapor and hygroscopic water have no ability to dissolve substances and are beyond the reach of vegetation.
The soil water fraction in the whole volume of surface waters is not big (~0.06%), but its role in the life of the Earth’s biocenoses is extremely important. In this boundary part of the lithosphere all four components of biosphere meet: atmosphere, lithosphere, hydrosphere, and living substances. The transformation of incoming solar energy takes place here, too. In this connection the physical characteristics of soil water reflect three essential and interlinked aspects that determine its role in the Earth’s hydrosphere.
Firstly, soil water plays the most active part in surface water exchange. In the terrestrial hydrological cycle, the soil layer serves as the first "water divisor" causing partition of the precipitation into three parts: surface runoff, groundwater flow, and evapotranspiration.
Secondly, soil water is one of the elements in the Earth’s climate system which significantly influences the climate because of its specific location on the border of atmosphere and lithosphere.
Thirdly, soil water is an important factor in the existence and development of vegetation, i.e. it plays a vital part in the food chain of ground ecosystems. The root fraction of the vegetation is situated in soils, providing for the uptake of mineral elements to plant tissues as well as water to the stomata of leaves—their evaporative organs. Therefore, transpiration activity—a process playing an important role in the hydrological cycle and primary production of organic material—is determined not only by vegetation parameters but also soil water reserves. Soil holds the specific water body, on the terrestrial part of the Earth, that accomplishes temporal control on vegetation’s water supply.
It is worth dwelling on transpiration as an important part of the hydrological cycle. Total evaporation from the Earth’s land (of which transpiration is 80-90%) is approximately 2/3 of the land precipitation. At first glance, this is a very large volume of water, falling as precipitation on the land surface, that remains unused. Actually, no more than 1-3% of the water passing through plants during transpiration is required for the production of plant tissue during photosynthesis. The other 97-99% goes to the atmosphere with the help of transpiration. However, analysis of vegetation in the light of the theory of dissipative structures (all living organisms including plants are dissipative structures; a necessary condition for their existence is the export of permanent entropy) shows that during the evolutional of land plants there arose a unique mechanism for such export—water vapor transfer on the surface of mesophilic cells of leaves, i.e. transpiration. Thus, a key part of the hydrological cycle is closed by transpiration (the main component in the balance of soil water), promoting the circulation of bioelements, i.e. life on the Earth, through vegetation.
In humid areas a part of the soil water reaches the level of subsoil water, supplying the latter and to a large extent determining its chemical composition.
In the forest-steppe zone soil water penetrates deeper than the lower border of the rhizosphere but does not reach subsoil water. The chemistry of subsoil water in such landscape conditions is not directly related to soil formation. At the same time, the processes of dilution, hydrolysis, acidification and reduction in rocks, exert significant influences on soil formation.
In arid zones water reaches only to the root-inhabited layer; it moves together with solute compounds within the soil profile. A geochemically inert layer serves as the boundary between the upper part of the soil and a deeply located lower part. Water in a liquid state cannot move through this layer. Mobile elements accumulate above the subsoil water as a result of evaporation.
The cryolithozone is coincident with the areas of permafrost. It covers part of the zone of suspended water and the upper part of the saturated zone. Its thickness changes from several meters to 1500 meters and more, depending on climate and geological conditions. The great majority of the water in this zone is in a solid state or as physically bound water; its freezing occurs below 0 oC.
The zone of total saturation covers the upper part of the Earth’s crust to 8-20 km. The uppermost aquifer in the zone of total saturation is called the aquifer of subsoil water. The zone of supply of subsoil water coincides with the zone of its location.
All ground water in the zone of total saturation, excluding subsoil water, is called artesian water, i.e. it is held under pressure.
An artesian basin is most often a system of inter-bedded water-bearing and water-proof layers within geological structures. The name stems from the province Artois in France where for the first time in Europe, in the twelfth century, pressure water was discovered. Depending on bedding conditions, fracture and cavern water in volcanic metamorphic rocks and limestones may be either subsoil or artesian water.
The bedding conditions for ground water are determined by the local geological structure. Structures with similar conditions of bedding and chemical composition are called hydrodynamic. Three hydrodynamic zones are distinguished according to the intensity of water exchange among the hydrogeological structures: these are the upper, middle and lower zones.
The upper zone—or zone of intensive water exchange—is actively drained by the local drainage network. Its thickness depends on the relief and is greatest in mountain regions, –and least in flat terrain. Its thickness can vary from 10 to 1000 m. The turnover time for water exchange varies from decades in mountain regions to centuries and millennia in plateau areas.
The middle zone—or zone of slow water exchange—is situated in mountains and transition areas (from mountains to plateaus). Its lower border is the level of the ocean. The turnover time is tens and hundreds of thousands of years.
The lower zone is a zone of difficult water exchange. It is located below the ocean level. Groundwater flow is minimal, and the time of water exchange may be up to tens and hundreds of millions of years.
The groundwater regime, i.e. temporal and spatial changes of level, temperature and chemical composition, depends on the intensity of water exchange.
The waters of these hydrodynamic zones have different origins. Waters of the upper and middle zones arise from penetration of precipitation into the Earth’s crust. As regards the mechanism of penetration through the zone of suspended water, the zones can be divided into infiltration, in which water is in a liquid state, and condensation, in which it is in a vaporous state. The thickness of the zone of such waters on plateaus is 1 to 2 km; in mountain-fold areas it is up to 3 km.
Waters in what are called sediment reservoirs have a totally different origin. They are formed in the process of sedimentation. These are greatly changed waters; they are widely spread in the zone of difficult water exchange—in the lower aquifers of plateau-type artesian basins.
3. The processes of groundwater chemical composition formation
Many factors influence groundwater chemical composition. Ye. V. Pinneker divides them into:
physical geographical (climate, relief, water exchange, peculiarities of weathering);
geological (geological structure, petrographic composition of rocks, tectonic movements, gaseous factor);
physical chemical (solubility of compounds,, oxidation-reduction and acid-base conditions);
physical (temperature, pressure);
biological (influence of living substances,, soil processes);
artificial (anthropogenic activities).
All these factors run in certain combinations and their roles are not similar. All the factors are inter-related and collaterally subordinated. For instance, intensity of water exchange has a strong influence on the chemical composition of ground water, and this depends on relief, the ratio of precipitation to evaporation, and lithologic composition of rocks.
S.L. Shvartsev hierarchically arranged the main factors controlling groundwater composition in the zone of hypergenesis (see Figure 1). He showed that groundwater composition, climate, relief, and geology have indirect influence, and their influence is revealed in biological production, intensity of water exchange, physical chemical parameters of the environment etc. It is worth mentioning that different sets of factors impact soil/subsoil waters, from those that influence water formation in the zone of slow water exchange.
Figure 1. Hierarchy of main factors influencing the formation of groundwater chemical composition in the zone of hypergenesis. Source: S.L. Shvartsev.
Soil and subsoil water chemical composition has to be considered as a result of stage-by-stage transformation of precipitation in the system soil - zone of suspended water - subsoil water. The first stages of the process start in the atmosphere. One of the features of precipitation is its geographical zoning. The same holds true for soil and subsoil water as physical geographical factors have a strong influence on its chemical composition.
The mineralization of precipitation increases from north to south. In low-mineralized precipitation the following anions occur in the proportions: HCO3- > SO42- > Cl-. As mineralization increases, however, the ratio changes: SO42- > HCO3- > Cl-. In the steppes where precipitation mineralization may be more than 1 g/l, the following ratio is established: SO42- > Cl- > HCO3-. Salts migrate in the atmosphere as aerosols of marine and continental origin. Sulfates and carbonates show the highest mobility. The proportions of cations’ are more stable: Ca2+ > Mg2+ > Na+.
On the continents aerosols of continental origin prevail. Their composition is determined by soil composition, vegetation, climatic and meteorological conditions. In coastal regions aerosols are of continental-marine origin. The subacid character of precipitation here (pH = 5.6 – 6.8) and SO4-H acid precipitation of anthropogenic origin (pH = 4 – 5) has special significance. This character of precipitation may be explained by acidification caused by gas-cycle photochemical reactions of sour gases, i.e. gases containing a lot of sulfur.
Heavy metals in precipitation are of special interest. In the air they migrate in particles with a size < 0.4 mkm, or in a vapor-gaseous environment, or in precipitation (in solution). Atmospheric water is often the main source of heavy metals in soils and in surface waters in humid and arid zones. Atmospheric transport and deposition of metals on the land surface are global phenomena. In Table 1 gas spaces of heavy metals that reached the surface of the European territory of Russia with precipitation provides evidence of the large scale of the process described.
Table 1. Concentration and gas space of heavy metals that reached the surface of the European territory of Russia in precipitation.
Precipitation falling on the Earth's surface is taken up by vegetation. Transpiration adds to the air near the ground surface such elements as P, Cl, K, and Ca and increases mineralization. These elements forming part of the surface mixture are taken out of the air by precipitation and dew. In forest ecosystems leaching of elements occurs when precipitation makes contact with vegetation. Their concentrations increase in subsurface waters under forest. Mineralization—concentrations of K, P, solute organic compounds, Ca, Mg, Cl-, and SO42-—increase. Conifers acidify subsurface waters, and, conversely, deciduous trees make them more alkaline.
When subsurface waters under forests are filtered through the substrata, they once become again enriched with the same elements and indices. Water filtration through a humus layer is accompanied by decrease of all indices as a result of root absorption. The decrease in concentrations of solute organic compounds, K, P, Ca, and heavy metals is the most significant.
As water continues to filter through the soil profile, it loses solute compounds and becomes neutral. Thus, by filtering through each soil layer, and consequently changing its chemical composition, the water actively takes part in circulation of biological material in the landscape. Plants accumulate elements in the process of creation of organic compounds; this involves materials that had been carried by the air—including organics and heavy metals—and they subsequently return them to the soil as fallen leaves, dead branches, etc. Soil water carries the compounds and elements away into root-inhabited layers, where plants take them up again by root adsorption. This is one of the functions of soil waters in ecosystems. The significant transformation of the chemical composition of precipitation, in the confined space of the soil profile, is explained by processes described above.
Table 2 gives an example of the changing chemistry of soil waters in turf-lowpodzol soils developed on light-loamy rocks. The vegetation is mixed deciduous-pine forest.
Table 2. Changing chemistry of soil water in turf-lowpodzol soils in Middle Russia.
The well-known Russian scientist V.I. Vernadsky in his monograph "Biosphere" edited in 1926 wrote that the main sources of cations and anions for subsurface and surface water were not rocks but soil and vegetation. According to S.L. Shwartsev's calculations, the "biogenic part" of ground water chemical flow, i.e. that arising from the decomposition of vegetation in soil, is: for tundra – 62.5% of total flow; for mixed forest – 50.6%; for forest-steppe – 42.6%; for steppe – 16.2%, and for humid savanna – 88.6%.
In steppe and forest-steppe landscapes, the biomass of decaying vegetation plays a small part in the processes of dilution, but evaporation and crystallization of salts have a strong influence on soil and ground water chemical composition.
Geochemical barriers—transitional zones where one geochemical situation is substituted by another and where characteristics of element transport change—are located across the path of vertical transfer of soil water to the subsoil and from local catchments to local depressions.
Very important in these is the above-mentioned barrier of root absorption, which depends on the biological absorption capacity and on peculiarities of the vegetation—each holding a characteristic association of elements. For instance, the capacity for biological accumulation of elements in deciduous forest is twice as much as in coniferous forest, and 25 times as much as in meadow steppe.
The acid-base equilibrium changes from subacid to neutral and alkaline in the zone of suspended water, with organomineral compounds of Al, Fe and Ca with fulvic acids. Calcium compounds and bicarbonate-ion consequently transfer into a soluble state.
At an evaporating barrier calcium carbonate precipitates out of the soil mixture, as do more the soluble gypsum, epsomite, soda, and halite. The elements that become concentrated at this barrier are Ca, Mg, Sr, Cr, Mn, Fe, Co, Ni, Zn, Cu and Pb.
When soil mixtures interact, soil cation exchange occurs. Colloidal soil fractions have a strong ability for cation absorption. Humic substances and clay minerals are charged negatively and absorb cations from non-saturated solutions. They form sorption geochemical barriers. The following elements tend to concentrate at this barrier: Mg, P, S, K, Ca, Cr, V, Co, Ni, Cu, Zn, As, Hg and Pb.
According to V.M. Shvets's calculations, soil water contains significant concentrations of dissolved organic compounds: in the waters of humid climate zones - 35 mg/l of organic carbon; in the waters of steppes and deserts - 20 mg/l, and in deep pressure ground waters - up to 40 mg/l. Organic compounds are acidified in the huge areas of waterlogged tundra and taiga, where reducing conditions are formed due to the absence of oxygen. As a consequence, hydrosulfuric and gley waters are formed. Bacteria take oxygen from the mineral compounds, oxidize soluble organic compounds and produce humic acids. Bacteria make Fe and Mn switch into bivalent state. Gley process occurs where there is a lack of sulfates. When concentrations of sulfates are high, hydrogen sulfide is formed. In hydrosulfuric water, the migration capacity of metals decreases as a result of sulfide formation. The following elements become concentrated at this barrier: Fe, Co, Ni, Cu, Zn, Ag, Cd, Hg, Pb and As.
Decay of organic compounds enriches ground water with carbonic gas and organic acids, especially humic ones. Dissolved organic substances form organic-mineral compounds with cations. This contributes to their migratory capacity in the upper part of the Earth's crust.
The migratory capacity of chemical elements depends not only on the geochemical situation but also on the forms of their existence in natural waters. In soil and ground water they migrate in colloidal and soluble states.
The colloid fraction consists of associations of organic acids, Fe and Al hydroxides, silicic ortho- and meta-acids. In real soluble states elements migrate in the form of neutral molecules, simple ions and complex compounds which are formed as a result of interactions of simple ions with different charges. Ions with negative charge are called ligands. Most often an element migrates in several forms—as neutral molecules, ions and complex compounds. Table 3 shows in which forms prevailing elements migrate in subsurface water of different geographic zones. The macro-components (Na+, K+, Ca+, Mg+, Cl-, etc.) mostly migrate in the form of simple ions. Heavy metals migrate as ions and as complexes with ligands—both organic and mineral.
The chemical composition of soil and subsoil waters also obeys the law of geographic zoning. Each climatic zone is characterized by its own set of elements that readily migrate in water.
Table 3. Main forms of heavy metals in subsurface water in different geographic zones.
In the permafrost zone soil and subsurface water is mostly in the form of ice. In tundra water is fresh and ultra-fresh, acid and with high concentrations of Fe2+ and organic compounds (up to 220 and 20 mg/l, respectively). The main geochemical barriers are the reductive-oxidative and sorption ones.
Acid water also dominates in the taiga; carbonaceous rocks tend to have neutral water with low mineralization (less than 0.3 g/l) and high concentrations of Fe2+. Heavy metals are a bit higher than in tundra, and their migration capacity is high. The concentrations of organic compounds in soil and subsoil water may reach 25 mg/l, and the main compounds are humic acids and low molecular weight acids, amino acids, sugars, alcohols, ethers, carbonyl compounds, etc.
Geochemical barrier is a sorption one, and in areas of carbonaceous rocks, a carbonaceous barrier.
In forest-steppe and steppe, water is generally neutral and alkaline, well mineralized and with few organic compounds.
Table 4 shows the different migration capacity of elements in different geographic zones. Clearly, alterations in reductive-oxidative and soda-and-acid conditions, which are observed in marshes, changes the mobility of elements.
Table 4. Elements' mobility in soil and subsoil water of different types of landscapes. Source: M.A. Glazovskaya.
Longitudinal hydrochemical zoning of soil and subsurface water arises under the influence of landscape, climate, hydrodynamic and lithological factors. In a number of cases petrochemical composition of rocks may interrupt the zoning of subsurface water composition. From tundra to steppe and semi-desert, and from rain forest to dry savanna, water exchange drops; contact of water with rocks and the ratio rocks/contiguous water increase. Water mineralization increases with permanent changes from bicarbonate-silicon water rich in iron and silicon to bicarbonate-calcium and magnesia, and bicarbonate-calcium-sodium waters. Concentrations of organic compounds in water and the number of biogenic elements entering water during the process of vegetation decay decline in the same direction. The influence of physico-chemical factors such as weathering of rocks increases, and the influence of soil water chemical composition on subsurface water decreases.
The chemical composition of deeper water depends on mainly the interactions between water and rock. The strength of the interaction is mostly determined by the duration of contact between water and rock, and the duration depends on water exchange intensity of the given hydrogeological structure.
In a zone of active water exchange the hydrogeological structure is actively washed out; intensive leaching of chlorides and sulfates occurs, fresh bicarbonate water is formed. This is a zone of river network drainage. The zone typically consists of medium-sized intermountain basins—often artesian basins supply areas.
In the zone of slow water exchange the deeper water-bearing rocks have the worst water exchange characteristics and lowest water velocity. This zone is up to 500 m deep in artesian basins on plateaus and up to 1000 to 2000 m deep in folded structures. Bicarbonate sulfate-chloride water is a specific trait of this zone.
In the zone of difficult water exchange water velocity is less than 1 mm/year. Deep sedimentary complexes containing ancient buried highly-mineralized chlorine-sodium-calcium brines belong to this zone; in hydrogeological massifs bicarbonate sodium water is often formed.
In Table 5 the relationship between vertical hydrodynamic and hydrochemical zoning of hydrogeological structures is clearly traced.
Table 5. Changes in water exchange characteristics of geological structures.Source: S.L. Shwartsev.
The deeper the water is located, the slower is the water exchange, and the higher its mineralization. Gas and chemical compositions change significantly with depth. In artesian basins bicarbonate waters gives place to chlorine-sodium water, and concentrations of organic compounds and microelements increase. The main cause of these changes is the merging of infiltration water into sedimentation water, with depth. The chemical composition of the water takes on great significance. In artesian basins where the mantle is composed of salt-bearing sediments, brines can have very high mineralization—more than 270 g/l (in the plains of Siberia, Eastern European, North America, China and elsewhere). Most researchers believe that initial source of deep brines is sea water. As a result of long term metamorphosis and complex interaction between rocks and solutions, brines are formed with ion-salt compositions very different from sea water. The problem of the origin of different deep brines has not yet been solved.
4. Groundwater biological characteristics.
Soil is rich in microelements. In fertile soils the biomass of microorganisms may reach several tons per hectare. In soil and ground water concentrations of bacteria, fungi and protozoa may be up to 500 000 or more in 1 ml of water . The number of cells depends mainly on the presence of nutrients, especially organic compounds. Microflora is a main factor of decay of organic compounds entering soil, particularly from leaf fall and root excretions. At the same time soil microflora deposits nutrients in its own biomass, not letting them leach from the soil profile. Microflora development occurs in capillaries (with a diameter of more than 1 micrometer) filled with water, and microflora even exists in physically bound water. The ecological role of microflora is not confined, however, just to mineralization of organic substances and nurient deposition. It interacts with vegetation—fixation of atmospheric nitrogen by bacteria is a striking example. Interaction of plants with several kinds of fungi facilitates uptake of phosphorus compounds that would otherwise be difficult to access.
There are bacteria that concentrate sulfur in their cells (sulfur-bacteria)—also iron hydroxides (iron-bacteria), and other elements. A lot of iron, sulfur and manganese deposits were formed by the activity of microorganisms.
During the decomposition of organic compounds, water becomes enriched with carbonic gas, hydrocarbonate-ions, calcium, magnesium, phosphorus, organic acids, carbohydrates and other compounds. In waterlogged soils there is a decrease in reductive-oxidative potential, and growth of anaerobic bacteria that can use nitrates, sulfates, carbonic acid, and organic compounds as oxygen sources. Their activity produces gases such as nitrogen, nitrous oxide, hydrogen sulfide, and methane.
Thus, the activity of microorganisms has a powerful influence on the geochemical situation of ground water, thereby influencing the migration of chemical elements.
5. Anthropogenic influence on ground water chemistry.
Ground water is widely used as a source of household and drinking water supply. It has several advantages over surface water: it is less vulnerable to pollution; it has better composition and smaller seasonal change. The share of ground water in household and drinking water supply is approximately 60% in European countries, 75% in the USA, and 35% in Russia.
A lot of economic activities, such as fertilizer application, deforestation, construction, and mining, lead to deterioration of ground water quality. Mineralization increases, with elevated levels of sulfates, chlorides, biogenic elements, heavy metals, and pesticides. Contamination of ground water by oil and related products in areas of oil production, refining, storage and transportation represents a serious danger. Rehabilitation work in the USA has shown that, despite enormous financial expenditure, it is impossible to achieve complete remediation of past oil pollution.
Usually ground water contamination has a local character and only in a few cases does the polluted area reach dozens of square kilometers. According to data from the State water cadastre of Russia, in the year 2000, only in 24 cases of ground water pollution was the area more than 100 km2, out of 2776 cases. It should be noted that more than 70% of cases of contamination are concerned only with the uppermost aquifer. Vulnerability of aquifers to economic activity and, in particular, pollution, is not the same. Pressure aquifers with a clayey bed are better protected from the penetration of contaminants from the Earth's surface. Subsoil waters are more vulnerable.
Ground water vulnerability is a characteristic of natural systems where water quality is within established standards for a given period of time. Ground water vulnerability depends on many factors: climatic peculiarities of the territory, degree of interaction between soil and subsurface water, level and character of contamination, thickness of the zone of suspended water and its filtration characteristics, presence of geochemical barriers, lithology, duration of penetration of contaminating substances into the aquifer, and ground water migration capacity.
For pressure ground water the main factors determining its vulnerability to contamination are:
the ratio of the pressure aquifer to overlying head of free-flow aquifer;
waterproof characteristic of the superstratum;
water conductivity, and
infiltration supply and duration of water exchange of the aquifer.
Nowadays there are techniques allowing researchers to estimate ground water vulnerability to contamination and such estimates are used when planning action to ameliorate ecological situations and in water protection strategies.
The essence of such strategies is to support life on Earth and avoid hydrosphere pollution. Success depends on development of production technologies, reduction of water consumption, planned reduction of waste discharge (until its total elimination), and development of ecological understanding in the population.
all ground water, excluding subsoil water, bedded between waterproof rocks and having a water head.
Form of element
a compound where element retains its migration capacity. An element can migrate as a dredge, a colloid, or in a solution as an ion or a complex compound.
a transition zone where one geochemical situation meets another.
water situated in the Earth’s crust as a liquid, vapor or solid substance, bedded in soils, sedimentary and massive-crystal rocks.
a geological structure with similar hydrogeological characteristics determining the character of water distribution and peculiarities of chemical composition.
the capacity of a chemical element to be in a solute state under defined geochemical conditions and, therefore, retaining its mobility.
consisting of a single component.
the quality of a material that allows it to store a given amount of water.
water solution located in the porous zones in soils in different forms and phase states.
water of the first permanently existing aquifer, closest to the land surface. This is a saturation zone.
Water exchange zone
a zone of active, slow or difficult water exchange, in which a defined volume of water can pass through it in a defined period of time.
Zone of hypergenesis
a zone characterized by the formation of specific rocks and hypergene bodies.
Zone of suspended water (unsaturated zone)
zone of soil situated between the land surface and subsoil water. The porous zone contains liquid and gaseous water, and air.
Biogeochemical cycles in the biosphere (1976). Moscow, "Nauka". [This symposium volume depicts cycles of various substances in the biosphere]
Bloch A.M. (1965). Water structure and geological processes. Leningrad, "Nedra". [This book reveals the links between the physical structure and chemical peculiarities of water]
Glazovskaya M.A. (1988). Geochemistry of natural and anthropogenic landscapes in the USSR. Moscow, "Vyshaya shkola". [Fundamentals of landscape geochemistry are given. The peculiarities of elements’ migration in different landscapes are described]
Dobrovolskiy V.V. (1998). Biogeochemistry fundamentals. Moscow, "Vyshaya shkola". [This book presents fundamentals of biogeochemistry].
Zektser I.S. (2001). Ground water as an environmental component. Moscow, "Nauchniy mir". [This book is all about groundwater and its role in the environment].
Kirukhin V.A., Korotkov A.I., Shwatsev S.A. Hydrogeochemistry (1993). Moscow, "Nedra". [This book presents the basic processes of groundwater chemical composition].
Andrey G. Kocharyan graduated from the Geological Faculty, Institute of Chemistry and Oil, Baku, in 1960. He received his Ph. D. in geochemical techniques of deposits’ search in 1968 at Baku State University, Geological faculty. Since 1979 he has held a position of Head of the Laboratory of Water Quality, Institute of Water Problems, Russian Academy of Sciences, Moscow. Dr. Kocharyan is the author of more then 150 scientific publications. His research interests are in the fields of water quality and water resources protection. He is currently concerned with water quality formation in catchment areas.
Kocharyan A.G. Forms of heavy metal migration in waters, bottom sediments, and makrophits in Volga reservoirs. Actual problems of regional use of reservoirs’ biological resources. Rybinsk, 2005, pp. 151-161. (in Russian)
Kocharyan A.G. On the choice of water quality priority indexes for local standardization. Safety of energetic facilities. Vol. 12, NIIES, 2003, pp. 429-436. (in Russian)
Dolgonosov B.M., Kocharyan A.G. Methodology of ecological state amelioration in river basins. Watereconomy of Russia. Vol. 5, № 4, 2003, pp. 289-302. (in Russian)
Arkhipova N.A., Kocharyan A.G., Lebedeva I.P., Safronova K.I. The role of reservoirs in chemical composition transformationof river waters. Water economy of Russia. Vol. 4, № 4, 2002, pp. 297-307. (in Russian)