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Groundwater is the water located in the fractures of rock formations of the Earths upper crust in liquid, solid, or gaseous state.

Renewable groundwater resources can be divided into two parts: those that are formed naturally in catchment areas and those that are formed through seepage in irrigated areas.

Origin of groundwater

There are different opinions concerning water origin on Earth. Most researchers believe that initially water was formed from magma in the process of cooling and crystallization of the latter; magma, in turn, formed in the result of heating and remelting of cold planetary matter due to the heat from radioactive processes.

Most part of precipitation, surface and ground waters of Earth formed through the so-called zone melting by A.P. Vinogradov along with fusion of its lighter rocky shell, lithosphere. Partly, water on Earth is formed from streams of cosmic particles. The protons contained in it catch electrons in upper parts of the atmosphere and turn into hydrogen which reacts with atmospheric oxygen thus forming water. According to data by L.S. Abramov, 1.5 km3 of water forms every year falling onto the Earths surface in the form of precipitation.

The water confined to the lithosphere and mainly circulating in the sedimentary rock is essential to the human practical activities; in the restricted sense this water is named groundwater. It has origin of two kinds; most part of groundwater is formed through seepage, infiltration of precipitation, and partly through condensation from the air when temperature of the air in rock pores drops to the dew point. Some part of groundwater is formed simultaneously with terrigenous material deposition at the bottom of a water body being buried in the pores of sedimentation masses; this water is called sedimentation water.

Water of infiltration and condensation take active part in the overall hydrological cycle on the Earth; sedimentation water that form part of ancient sedimentation masses may be involved in the overall cycle in the result of geological processes of compression into folds, breaking of overlying strata, formation of fissures, etc., and the cycle time is measured in terms of geological periods. The share of contribution of deep water, ascending from the mantle, to the overall hydrological cycle is still lower; its role is significant only in terms of geological time scale, providing global water storage and hydrological cycle.

In mineral deposit areas, groundwater is formed and replenished mainly due to seepage. This is evident from the increase of water inflow to mine openings in springtime as well as after rainfall, albeit with some delay, which is observed almost in every mine and pit.


Classification of groundwater

Groundwater is classified proceeding from the following various features: formation conditions; mode of occurrence; hydraulic characteristics; lithologic composition of water-bearing rocks/strata as well as their age; physical properties of groundwater; chemical properties of groundwater.

According to formation conditions, groundwater falls into different groups, of which infiltration and partly condensation waters are most significant.

According to the mode of occurrence and type of enclosing rocks, groundwater is divided into the following types:

  • Porous groundwater, occurring and circulating in pores of the rocks that form the top surface part of the Earth crust;
  • Stratal groundwater, occurring and circulating in pores or fissures of the rocks that water-resisting rocks overlap or underlie are in turn divided into pore-stratal and fissure-stratal ones;
  • Fractured-rock (fissure) groundwater, circulating in hard (magmatic, metamorphic, and sedimentary) rocks pierced through by uniform fissuring;
  • Karst groundwater, circulating in carbonate, gypsiferous and saliferous karsted rock masses;
  • Fissure-vein groundwater, circulating in particular tectonic fissures and tectonic fracture zones.

According to its hydraulic properties, groundwater is divided into non-artesian (or free-surface/ unconfined) type and head (confined) type when the water-bearing stratum is covered with water-resisting rock and the groundwater therein is under hydrostatic pressure conditioning head.

Depending on the age of water-bearing rocks, groundwater is given a suitable name: water of coal formations, Jurassic sediments, chalk (Cretaceous) deposits, Tertiary deposits, etc.

According to salinity or concentration of dissolved salts, groundwater is classified as follows:

  • Fresh groundwater: contains up to 1 g/l dissolved matters;
  • Brackish: contains from 1 to 10 g/l salts;
  • Saline: from 10 to 50 g/l;
  • Brine: over 50 g/l.

In terms of temperature, groundwater comes under four headings:

  • Cold, with a temperature below 20 ;
  • Warm: 20-37 ;
  • Hot: 37-42 ;
  • Very hot (thermae), with a temperature over 42

In practice, not only the total dissolved solids but also the composition of these salts is essential when characterizing and assessing groundwater. Depending on the prevalence of the salts dissolved in water, they distinguish between hydrocarbonate, sulfate, and chloride waters; and depending on the prevalence of cations, they break into calcium, magnesium, and sodium waters.

In addition to salts, groundwater contains various gases, i.e. carbon dioxide gas, nitrogen, hydrosulphuric, etc., which often are of great practical importance. Depending on the usefulness of the gas dissolved in the water, there are carbonic acid, hydrosulphuric, radon, and other types of groundwater. Generally, such waters are used for medicinal purposes. Groundwater with any therapeutic properties is called balneologic.

The groundwater containing any dissolved substance in concentration with which this substance can be extracted is called commercial groundwater: iodine, bromine & iodine, bromine, etc.


Factors, processes, conditions for formation of groundwater composition, its chemical composition and properties

Basic processes forming the chemical composition of groundwater

The chemical composition of groundwater is quite complex. This depends on many factors: complexity of water structure; isotopic composition of hydrogen and oxygen; composition of the rocks with which groundwater interacts; complexity of biological processes running therein; inconstancy of gas composition; different temperature and pressure at different depths; etc.

There are three isotopes in nature: protium (prevails); deuterium; and tritium. Although the content of deuterium and tritium in nature is negligibly small, their presence has an influence on the chemical composition of water (in particular, on its biological effect). There are also three oxygen isotopes in nature. Combination of hydrogen and oxygen isotopes yields forty two types of water, of which nine are most stable. There is no so-called light water 20 in nature. Ordinary water has polymeric structure in the form 2nn. The main processes that form the chemical composition of water are as follows: dissolution; hydrolytic decomposition; ion exchange; diffusion-driven desalination; and biological processes. In addition to these factors, mixing of waters, which occurs at the areas of water of different origin, and water concentration at evaporation are of importance as well; also superposition of complex processes taking place in groundwater; carbonation of waters in rock metamorphism areas; oxidation and reduction processes.

Dissolution is the process of passing of all the constituents the rock contains into solution. The Earth crust has always various easily soluble substances that are present in rocks in the form of admixtures (chlorides, sulfates, etc.) or in the form of salt deposits. When contacting with water, such salts pass into solution, and as a result groundwater enriches with chemical elements.

Hydrolytic decomposition, or desalination, is the process of selective dissolution and carryover of some rock constituents by groundwater. Hydrolytic decomposition progresses especially intensively at the upper part of the earth crust; this is caused by the vigorous weathering processes running here. Groundwater desalination property enhances due to dissolved oxygen and carbon dioxide gas present in it. In the result of desalination, some rock constituents pass into solution and travel with water. Due to desalination groundwater is enriched with sodium, potassium, calcium, magnesium salts, and other elements.

Ion exchange. Usually argillaceous terrigenous material in absorbing complex contains calcium cations, since salts of this metal prevail in territorial waters in the form of solution. Under diagenesis of the clay sediments deposited in seas, ion exchange processes run; calcium cations in the absorbing complex of clay particles change by sodium cations that prevail in the pore solution of clay particles, since the sea water saturating deposit pores contains chiefly sodium salts in dissolved form. In the consequence of ion exchange processes, the chemical composition of groundwater changes considerably. Diffusion-driven desalination happens: at long-standing contact of water with rocks; it differs from ion exchange processes in that the cations contained in the crystal lattices of the minerals, which are constituents of rocks, interact with solution cations.

Biologic, or to be more precise, microbiological processes are of paramount importance for transformation of the chemical composition of rocks. In V.I. Vernadskiys opinion, one of he most active substances in the earth crust is a living substance, viz. community of living organisms. Surface and ground waters are populated by living organisms. In oil-field areas, bacteria were found at a depth of over 2000 m. Organisms vital activity is an active factor of formation of the chemical composition of natural water. Organisms vital activity gives rise to the following reactions: oxidation processes; processes of reduction (deoxidization) of the oxidized substances contained in groundwater by means the anaerobic bacteria; reduction (deoxidization) of sulfates to hydrogen sulfide; reduction (deoxidization) of nitrates; concentration of scattered elements; formation of the gas composition of some types of groundwater; and so on.

Chemical composition of groundwater

To date, more than 80 elements of the Mendeleev periodic table have been discovered in dissolved form in natural waters. Therefore, groundwater is natural solution. The most commonly encountered elements in natural waters are CI, S, , Si, N, , , , Na, Mg, Ca, Fe, Al; other elements are found more rarely and usually in small amount.

Determining the chemical composition of groundwater in the course of hydrogeological investigations is essential. In accordance with the existing standards, practical evaluation of the groundwater (for water supply, construction, mining, irrigation, and other purposes) studied in this paragraph can be widely differing.

The groundwater property is determined by the quantity and ratio of the salts contained in it in dissolved form; these are present in water in the form of ions, cations and anions. Among the ions, the following have the most of practical importance: +, Na+, +, Mg2+, Ca2+, Fe2+ cations; Cl, SO, 3 anions. Out of undissociated compounds, SiO2, Fe2O3, l2O3 are of frequent occurrence; out of gases O2, , N2, 4, H2S and widely met, and sometimes , Rn, etc.

Water reaction. In order to accurately determine the chemical composition of groundwater one should know the concentration of hydrogen ions or so-called active reaction of water that is quantitatively expressed by pH value represented by the common logarithm of hydrogen ion concentration (or more exactly by their activity) taken positive: = lg (+). One must know this value to solve quite a number of theoretical and practical problems (assessment of groundwater aggressiveness, its corrosion capability, etc.). At a temperature of 22 , the concentration of hydrogen and hydroxyl ions in pure water comes (separately) to 10~7; consequently, for neutral water = 7; at > 7 water has alkaline reaction; and at <* 7 has acid reaction. According to the value, water is broken down into highly sour (acidic) ( << 5), sour (acidic) ( <= 57); neutral ( = 7); alkaline ( = 79); and highly alkaline ( > 9). For the most part groundwater has faintly alkaline reaction. The water of sulphide and especially pyrites and coal deposits is as a rule sour and often highly sour.

It is necessary to determine the concentration of hydrogen ions at the water sampling site; the most common way of determination is colorimetric method that is based on the property of indicators to change their color depending on the concentration of hydrogen ions. Sodium ion is widespread in natural waters, mainly in combination with 1" ion, more rarely with SOI" ion, and still more rarely with 3-. Almost all Na+ salts are freely soluble in water. In dry areas, capillary ascension of groundwater containing Na+ salts brings about formation of sodium alkali soils.

Much lesser potassium ion appears in groundwater than Na+. This is due to the fact that + enters into the composition of secondary non-water-soluble minerals as well as because + is well sorbed by soil from where it is extracted by land plants. In addition to stable isotope, radioactive isotope + is always present creating conditions for natural radioactivity of natural water.

Calcium ion is found in natural water in concentration of up to hundreds of milligram per litre and is its main component. 2+ percentage (with respect to other cations) decreases with increasing total salinity. 2+ salts are responsible for water hardness.

Magnesium ion in natural fresh water comes to about 25% of the 2+ concentration; it also bring about water hardness along with the latter.

Iron ion is present in groundwater mainly in the protoxidic form, Fe2+, in concentration from fractions to a few milligrams per litre. At ^ 7 value, under the influence of 2 it easily changes in the oxydic form, Fe3+, forming at that hardly soluble hydroxide Fe(OH)3, and Fe2O3-3H2O is formed, then depositing in the form of brown flocculi. Presence of iron compounds imparts an off-flavour taste to it. The water that contains iron compounds is harmful for many types of production.

Chloride ion can most often be found in groundwater in the form of Na+ ion compounds in concentration from several to hundreds and thousands milligrams per liter. ll~ ions can accumulate in groundwater not only because of dissolution of chlorides from rock formations, but also in the result of contamination with various sewage disposals. In this case, l" presence is indicative of the groundwater non-potability.

Sulfate ion concentration in groundwater is from fractions to several thousands milligrams per litre depending on the composition of water-bearing strata, nature of the processes sulphide compounds weathering and oxidation.

Three types of carbon dioxide are encountered in groundwater: free (gaseous), water-dissolved CO2; hydrocarbonate (bicarbonate) type in the form of 3- ion; carbonate in the form of 3- ion. Inclusion of free carbon dioxide in water is of great practical importance because it imparts corrosive nature to water. Nitrogenn compounds occur in groundwater in the form of nitrate ion NO5, nitrite NO2 and ammonium NH4+ ions. If they are of inorganic nature, nitrogen compounds are harmless. However, at the dissociation of organic substances, the nitrogen compounds are measures of water contamination with pathogenic bacteria; in this case the content of NOi and NH ions in drinking water is acceptable in the form of traces, and that of NO5 ion should be not more than 10 mg/l.

Organic impurities in natural water have diverse origins; they are found mainly in shallow water.

Organic substances of animal origin are almost always indicative of water contamination with pathogenic bacteria. The organic substance quantity in water is assessed according to its oxidability which implies the oxygen or potassium hypermanganate quantity that is used for oxidation of organic impurities. 1 mg of O2 or 4 mg of nO4 correspond to 21 mg of organic substance. In drinking water, oxidation shall not exceed 10 mg/l of nO4.

The bacteriologic properties of groundwater are determined by the presence of microorganisms, including probably pathogenic ones too, in it. From hundreds to millions of bacteria per 1 cm3 are discovered in water samples. Bacteriologic pollution of water can be assessed by coli-titer, water volume in cubic centimeters where there is one colon bacillus, and by coli test, number of colon bacilli per 1 liter of water.

Physical composition of groundwater

When groundwater (hydrogeological) survey is carried out, the following key physical properties of groundwater are to be determined: its temperature; color; transparency; taste; odor; and specific gravity.

Groundwater temperature varies over a wide range. In high-mountain areas and in permafrost zones, it is low; in some places highly saline water has even negative temperature (-5 and below). In young volcanic activity areas as well as in geyser outbreak places (Kamchatka, Iceland, etc.), water temperature at times exceeds 100 . In middle latitudes, the temperature of shallow groundwater usually ranges within 5-12 and is determined by local climatologic (predominantly) and hydrogeologic conditions.

When carrying out a hydrogeologic investigation, groundwater temperature is measured directly in its source, i.e. well or borehole. A bucket/lazy thermometer, the bulb of which is wrapped in heat-insulating material (cotton wool, woolen cloth, etc.), is used for such measurement. Electric thermometers or thermoelements are used for measuring water temperature in deep wells.

Groundwater color depends on the mechanical or organic impurities contained/dissolved therein. Organic impurities impart yellowish and brownish colors to water; ferrous compounds and hydrogen sulfide impart greenish blue coloration to water. For the most part, groundwater is colorless. Water color is defined and quantified by degrees by comparing with the reference colority (chromaticity). Water with a colority of 20 and below is considered tolerable.

Groundwater transparency depends on the mechanical impurities, colloids, and organic substances. The transparency is determined by using a transparency (cylinder) tube, 30-40 cm high, marked with a centimeter scale and special pattern drawn on the bottom: water is released through a tap from the tube until the pattern is clearly visible through the remaining water sheet. The height of the remaining water column is measured in centimeters and defines the water transparency degree. Water is deemed tolerable if its sheet is 30 cm and over.

The groundwater taste is imparted by dissolved mineral substances, gases, and impurities. Sodium chloride adds sweetish taste to water if its concentration is up to 500 mg/l and saline taste if its concentration is over 600 mg/l; magnesium sulfate imparts bitter taste; iron salts astringent flavor; organic substances sweetish flavor; calcium and magnesium hydrocarbonates as well as free carbon dioxide palatable and refreshing flavor. Brackish rainwater has off-flavor taste. The taste is determined by using water warmed up to 20-30 . It should be borne in mind that mouth-feel sensations differ from each other.

Groundwater is normally odorless. However, in some cases groundwater has the smell of rotten eggs (because of hydrogen sulfide presence), smell of marsh, putrefactive, musty odor, etc. Drinking water must not have any odor. To exactly determine the odor of water, it is warmed up to a temperature of 40-50 . Odor strength is graded on a five-point scale.

The specific gravity of water gives an indication of its salinity expressed in Baume degree: one Baume degree corresponds to one percent of the weight content of sodium chloride in water. Approximate specification of the specific gravity is carried out by means of a salimeter, namely by using a pycnometer.


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