Water existing in the voids of the geological stratum below the surface of the earth is called groundwater. Groundwater is found in pores and fissures of rocks. It is regulated by the quantum and speed of rains, extent of vaporization at the time of rain, temperature, slope of land, dryness of air, porosity and permeability of rocks, vegetative cover and water absorbing capacity of the soil.
Groundwater is 0.58 per cent of the total water resources available in nature and it is 22.21 per cent of fresh water part (2.6%) of total water reservoirs. It is located up to a depth of 4 kms of the earth’s surface (Table 4.1). It is also called sub-surface water since it is found below the surface of earth.
Description of development of groundwater is available since ancient times. The Old Testament contains many details about groundwater in the form of springs and wells. Rollman has given description of underground water channels of Egypt and Syria in 80 B.C., while Greek and Roman philosophers have also described the principles of formation of waterfalls. Homer, Thales and Plato have explained creation of waterfalls from the water of seas.
They said that sea water flows underground through water routes under the mountains. Seneca and Pilny also followed the Greek thinking. Shilpy Vitrviyas, while explaining infiltration, clearly told that in hilly areas it rains heavily due to which the water, after percolating into lower layers, appears in the form of waterfalls at lower levels of hills. French philosopher Bernard Pelisy (1510-1589) repeated the principle of infiltration in 1580.
Clear knowledge about the hydrological cycle became available in the 17th century when for the first time analysis was made on the basis of observations and quantitative figures. From this point of view, the contribution of Pire Perat (1608-1680), Edmen Meriote (1620-1689) and Edmund Hailey (1652-1742) remain praiseworthy.
Perat measured rain for three years and found out the flow of the upper part of the basin of river Siene. Meriotte, after measurements of river Siene near Paris, clarified the work of Perat. Meenjar has called Marriott as ‘Father of Hydrogeology’. English geographer and scientist Edmund Hailey, after measurement of the process of vaporization concluded that flow of all rivers and waterfalls depends on vaporization activity of seas.
During the first decade of the nineteenth century, the first artesian wells were constructed in France. Thus, at present hydrogeology has become an independent branch in which various geographical aspects of groundwater are studied.
Origin of Groundwater:
Total water existing on earth is 13, 84,12,0000 cubic kms, out of which 8,00,0042 cubic kms is groundwater. Apart from this, 61,234 cubic kms is in form of soil moisture. Groundwater and soil moisture together constitute the sub-surface quantity of water. Groundwater is stored in different layers of earth by infiltration through pores and fissures of permeable rocks.
Groundwater mainly comes from three sources. They are, First: ‘Meteoric Water’, which is the main source of groundwater and is received in the form of rain and snow. This water infiltrates from the surface through fissures, pores and joints of rocks till it is stored on non-permeable rocks in the form of groundwater; Second: ‘Connate Water’, which exists in pores and cavities of sedimentary rocks of seas and lakes. It is also called sedimentary water. Thirdly: ‘Magmatic Water’ which converts into water after condensation of vapour as a result of volcanic action at the time of entering hot rocks.
The main source of groundwater is rainfall. It infiltrates through seepage slowly into the earth and collects there. It is also called ‘plutonic water’. Groundwater is an important part of the water cycle, which also includes that part of surface and atmospheric water which goes underground through rainfall, rivers and lakes.
Sources of Groundwater:
Water received on the surface of the earth from different sources becomes groundwater when it goes underground after information through pores of permeable rocks.
It is found from following sources:
(i) Meteoric Water:
This is the main source of groundwater. This water is received in the form of rain and snow. Water from tanks, lakes, rivers and seas is again received by earth after vaporization. Water is received by melting of snow or rain, hence it is called ‘meteoric’ or ‘shooting star water’. From the surface of the earth, this water infiltrates down below through rock joints, pores and fissures of rocks and is stored at the level of impermeable rocks in the form of groundwater.
It originates in the atmosphere, falls as precipitation and percolates through the soil to become groundwater. You may have noticed the fluctuation of the water level in wells. During the rainy season the level goes up, while in the summers the level goes down. This is indicative of the fact that groundwater significantly depends upon water from the atmosphere.
Another way in which the groundwater may be derived directly from atmospheric moisture is condensation of water vapour from air circulating through the pores and interstices. This is also known as ‘condensational water’ and is the basic source of replenishment in the arid and semi-arid areas.
During summers, the land is warmer than the air in the soil. This results in a difference of pressure between the water vapour in the atmosphere and the soil. The water vapour from the atmosphere penetrates into the rocks as the temperature of the water vapour drops in the cooler soil. A certain amount of water may accumulate this way.
A third source is effluent seepage from lakes, rivers, oceans and also man-made channels, but the importance of this varies with the climate of the area concerned. In fact, in humid regions, the groundwater contributes to stream flow by means of effluent seepage, and the gradient of this saturated groundwater more often than not slopes towards the surface water bodies and the oceans.
(ii) Connate Water:
Water contained in pores and cavities of sedimentary rocks under seas and lakes is called connate water. It is also called ‘sediment water’. It is the second important source of groundwater. This is the water that is entrapped in the interstices of sedimentary and volanic rocks at the time of deposition. Connate water is highly mineralized and salty and does not mix readily with meteoric groundwater. Connate water is usually found deep down in the lower layers of the zone of saturation.
(iii) Magmatic Water:
Hot magma enters rocks due to volcanic action after which its vapour drops are condensed and converted into water. This is called magmatic water. Apart from it, other sources are those in which groundwater becomes again available on the surface of the earth.
They are mainly springs, wells, and geysers.Such water is considered to have been generated in the interior of the earth. It has consequently travelled to the upper layers of the earth’s surface for the first time; this is also known as magmatic water.
Rock Structure and Groundwater:
Below the surface of earth, availability of groundwater depends on composition of rocks. Water holding capacity and water yield depends on the composition of rocks and, on such basis is decided vertical or horizontal distribution of water. It is clear that geology occupies an important place in hydrogeology. Springs and streams are included in special groundwater in lands permanently covered by snow.
Below the surface of the earth, groundwater remains in permeable group of rocks. Such groups of rocks are called aquifers. From aquifers water can move towards springs or wells in sufficient quantity. Due to the composition of aquifers, sufficient water remains mobile in ordinary local conditions.
Groundwater reservoirs and water-filled group of rocks (rock bed, strata or deposit) are synonyms. As against it, aquifers are such a non-permeable group of rocks which neither holds water nor it is permeable. Its composition is like solid granite. Rocks without solid minerals or other such parts of soil can retain groundwater. Such spaces are known as voids, pores or fissures. Because such voids work as water pipes for groundwater, they are very important. Hence their shape, type, irregularity and distribution are specially studied in groundwater.
Basic spaces are formed by those very geological reactions which form rock groups and they are found in igneous and sedimentary rocks. Interstices are formed after rock formation. Rock joints, fracture, solution holes and voids created by vegetation come in this category.
According to shape or form, these spaces are classified as capillary, over-capillary, and sub-capillary. Capillary spaces are so small that water is held there by surface tension. Over-capillary spaces are those which are bigger than capillaries. Sub-capillary spaces are so small that water is held there by adhesive forces. Because of their linkage with other voids, they are called isolated ones.
Figure 4.1 shows different types of spaces and their relation with porosity. From the viewpoint of recycling of groundwater, granular sedimentary deposits are of special importance. Porosity of such deposits depends on different types of granules and their arrangement, distribution as per size, degree of cementation and compaction.
Solution of mineral substances emanating from consolidated groups of rocks and the condition of broken rocks is also important. Due to above mentioned reasons, range of porosity varies from 0 to 50. Table 4.1 mentions main details of sedimentary materials.
Vertical Distribution of Groundwater:
Underground availability of groundwater can be divided into ‘Saturated Zone’ and ‘Zone of Aeration’. In spaces of saturated zone, compressed liquid water exists. In spaces of aerated zone, partly water and partly air exists. In most of the parts of the earth, above every single saturated zone there exists a single aerated zone which extends up to the surface of the earth. This has been shown in Figure 4.2.
The upper part of the saturated zone remains either up to the point of saturation or is bounded by impervious levels. The lower part of saturated zone extends to the underlying impermeable rocks like bed rock or clay. In the absence of overlying impermeable rocks, upper level of saturated zone is called water table or groundwater level.
It has been termed as level of atmospheric pressure and after intrusion of level of the aquifer, it is found on the basis of height of water level in the well. In reality, saturation remains a little above the underground level due to capillary gravitation, even then water is available at lesser pressure than atmospheric pressure.
Normally, water existing in the saturation zone is considered as the form of underground water. In the aerated zone, suspended water or vadox water exists. This normal part can also be further sub-divided into soil water portion, middle portion and capillary portion (Figure 4.2). Expansion and distribution of water in each zone has been described in the coming paragraphs.
Inter-granular spaces vary widely in terms of size. Minute voids between the component particles of clay, shale and slate may feature on one end, while large spaces between the pebbles of well-sorted and unconsolidated valley gravel may feature on the other end of the spectrum. Massive spaces are those that occur between large blocks of rocks such as fractures, joints and bedding planes sometimes enlarged by the process of solution.
Capillary interstices or spaces are those that are small enough to hold surface tension forces (Fig. 4.3). They can be further classified into two types namely super capillary and sub-capillary. The former is large and may sometimes be as large as a limestone cave. The latter is very small and water is held in them mainly by molecular forces.
Here, let us briefly mention an important dimension of groundwater flow. The movement of groundwater is influenced by gravity just like surface water. Just as everything else has a low, a low was also formulated to express the relationship between capillary of laminar flow and the hydraulic gradient. This was first stated by Poiseuille and is referred to as Poiseuille’s law in physics. But it was Darcy who confirmed the application of this law to the movement of groundwater through natural materials.
Since then geologists know it as Darcy’s law, which is expressed in the form of an equation:
q = KH/L
q is the velocity of groundwater flow H is the difference in head between the two points separated by the distance L
K is the hydraulic conductivity.
The amount of groundwater flow can be determined with the help of the equation
Q = KIA
Q is the volume of water flowing from a porous medium with a cross sectional area A under a hydraulic gradient I and K is the hydraulic conductivity of the porous medium
It should be mentioned, that the original interstices were created at the time of origin of the rock, while secondary interstices are a result of the actions of subsequent geological, climatic or biotic factors of the original rock.
Soil Water Zone:
Water existing in soil water zone is lesser in comparison to saturated zone. Sometimes, due to rains or additional quantity of irrigation water this portion can also be temporarily saturated. This zone extends from the surface of the earth to the root zone. Its thickness depends on type of soil and type of vegetation. Since, soil water sends moisture to the roots hence, due to its importance for agriculture soil and agriculture scientists have made a deep study of movement, and distribution of water of this zone.
Briggs has divided soil water in three parts on the basis of water concentration. The first is Hygroscopic Water: This is the water drawn from air which forms thin films of moisture on level of granules. Due to excessive adhesive strength, this water does not become available to plants.
The second is Capillary Water, which exists on all sides of soil granules in continuous layers. It remains held up due to back pressure strength and starts moving by capillary action. This water becomes available for plants. Gravitational Water is the additional soil water which flows away from soil due to gravitational force.
Hygroscopic co-efficient is the quantity of maximum moisture which pre-dried soil draws from 50 per cent moisture containing atmosphere at 250 cms. Wilting point is the quantity of water drops in which plants fade away permanently. It has been proved by experimentation that it has no definite standard, but it depends on plants, environment, root system and place of tested soil.
Field capacity is the quantity of water in soil which exists even after the flowing away of excess water by gravitational force and infiltration of water underground. Moisture equivalent is the quantity of water which, after using 1,000 times centrifugal force of gravitation, remains in saturated soil after being decentralized.
Field capacity of sand is more than moisture balancing figure but it is equal for loamy sand. Because field capacity and wilting point indicate the maximum and minimum limits of water respectively for development of plants, hence water required for development of vegetation is equal to the difference of these two. Water necessary to saturate all voids of soil is the maximum water content in it. This is called maximum water capacity.
Study of soil moisture has developed many methods of measuring soil moisture according to change of place and time. The most correct method is ‘weight measurement method of soil samples, in which samples are weighted and again weighted after drying.
In this method, gravitational suction block is used, which is penetrated into and taken out of soil. These porous parts develop moisture equalization with soil, due to which correlation of weight can be established with moisture.
Richards and other persons told that for finding out tension measurement or capillary magnanimity, tension measurement zero (on saturation) to 0.85 remains limited to atmospheric pressure.
According to soil texture, more than half of the available range of water comes within this limit. Many instruments have been developed on the principle of electric resistance measurement from materials kept in soil, in which the relation between resistance and soil water content has been established.
To amalgamate electrodes, various sucking materials are used, out of which some are Plaster of Paris, nylon and fiber glass. Because soil moisture is received from heat management of soil, hence based on this principle, different units have been developed when heating elements are buried under soil.
To determine soil moisture, ‘Neutron Scattering’ is a useful method. It is known that fast neutrons become dormant as compared to other organs while colliding with hydrogen. Hydrogen exists in most of the soils almost completely in the form of water.
Using these facts, a fast neutron source is combined with a dormant neutron source and inserted into special type of joint hole in the soil. Counting of dormant neutrons due to hydrogen of the soil is measurement of soil moisture. After calibration, changes in soil moisture can be known easily according to time and depth in the well.
The middle part extends from lower portion of soil water up to border of the upper portion of the capillary part (Figure 4.4). Thickness of this portion can vary from zero to hundreds of feet. It is zero feet when the border part mixes up with a high water table on the surface of the ground. Its thickness is hundreds of feet in deep groundwater level.
The main function of this part is to connect parts near the surface of earth with groundwater levels and through which water should go down in vertical form. Non-moving water of middle part or pellicle water remains stationary on account of capillary tension and humidity and is equivalent in field capacity of soil water. The additional water is gravitational water which moves in downward direction due to gravitational force. The capillary part extends from groundwater level to capillary support.
Thickness of capillary part does not vary only according to composition of soil and rocks, but in a layer having innumerable voids, where size of voids is large, its border looks like inverted sides even with a microscope. Physically, with rising height, quantity of moisture reduces or capillary water remains existent in almost all voids above groundwater level. As height increases, water remains only in small voids and at still higher levels, only the smallest interconnected voids contain water, in which water above the groundwater level is present.
Since all spaces of the saturated zone are filled with groundwater, hence per unit porosity is directly measured with sphere of available water content. The total quantity of such water cannot be pumped out of the well or discharged from the land, because atomic and backpressure retains some part of water at the same place.
Hence, retained water is that water which remains retained in spite of gravitational force. Special retention capacity of any soil or rock is the percentage ratio between water retained against gravitational force after saturation, and total water retaining capacity.
Thus, special product is a part of porosity of the aquifer. Its measure depends on size and shape of granules, distribution of voids and layer tolerance. For uniform sand, special product can be up to 30 per cent, but in alluvial aquifers its measure is between 10 to 20 per cent.
To find out special product, Meinzer has suggested seven methods:
1. To saturate samples in laboratory and allow discharge of water from them.
2. Saturated water level and sufficient mass of material remain over the capillary portion and discharge natural water downwards.
3. Collect samples just above the capillary part after lowering down of groundwater level.
4. After pumping out water from the well in known quantity, determine the width of sediments throwing out water.
5. To determine width of sediments saturated by measured seepage of one or two streams of water.
6. To know special product indirectly after finding out moisture measure by centrifuging.
7. Determine special product or special hold after finding out porosity through mechanical analysis.
All these methods have limitations. Samples of laboratory can be disturbed or may have defective representation. In field experiments, control and measurement of variables is a difficult task and thus decided counting may lack truthfulness.
A large part of earth is permanently covered under snow. It includes most of the part of Tundra area and the whole of Antarctica, which remain permanently frozen. About 60 per cent part of Alaska also has land portion full of frost. Studies have been made in Russia and Alaska regarding water supply in permanently snow-covered lands and the resulting mechanical problems.
Based on availability of groundwater, lands permanently covered under frost have been categorized into supra-permafrost, intra-permafrost and sub-permafrost areas. Supra-permafrost groundwater being in the upper side of frost land creates a temporary supply of shallow water. Water located in supra-permafrost land is called artesian water or inter-frost groundwater. This water remains in open areas like near rivers, voids or fractures etc., of frost land.
Thus, water resources located under permanent snow can become optimistic sources, but there are difficulties like excessive expenditure, probable high salinity, and possible extension of frost land up to non-permeable rocks in its exploitation.