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In this article we will discuss about:- 1. Meaning of Aquifer 2. Types of Aquifer 3. Aquifer Functions 4. Flow in Aquifer 5. Artesian Aquifer 6. Different Rocks as Aquifers.
Meaning of Aquifer:
It is defined as a rock mass, layer or formation which is saturated with groundwater and which by virtue of its properties is capable of yielding the contained water at economical costs when tapped.
The quality of an aquifer will, therefore, depend both on how much quantity of water a rock formation can hold per unit volume and at what rate it can yield water when tapped for supplies. It is a storage reservoir and a transmission conduit at the same time.
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Gravels, limestones and sandstones generally form good aquifers when occurring in suitable geological conditions and geographic situations.
Types of Aquifer:
Two basic types of aquifers are distinguished on the basis of physical conditions under which water can exist in them:
(a) The unconfined aquifer and
(b) The confined aquifer.
(a) Unconfined Aquifer:
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It is also called a water-table aquifer, and is the most common type encountered in the field. In this type, the upper surface of water or the water-table is under atmospheric pressure which may be acting through the interstices in the overlying rocks. Water occurring in this type of aquifer is called Free Groundwater. When tapped through a test well, the free water will rise to a level in the well equivalent to the water table of the area.
Water-Table:
The depth to upper surface of zone of saturation in free ground-water is called ‘water-table’. It fluctuates depending on many factors and may be as low as one meter or so or as deep as 100 meters or more.
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Perched water table is the term used for isolated water table in an aquifer held by a small extension of impervious rock within a large pervious tract. In such cases, the main water table is located much below. Supplies from such isolated reservoirs with perched water-table are often unreliable.
(b) Confined Aquifer:
It is a rock formation saturated with water and capable of yielding water tapped but unlike unconfined aquifer, has an overlying confining layer (an impermeable rock mass) that separates it from the influence of atmospheric pressure. Naturally, water held in this type of aquifer is not under atmospheric pressure but under a greater pressure due to the confining medium. The upper surface of water in a confined aquifer (the confined water) is called piezometric surface.
For establishing a piezometric surface, level of water in a number of test wells has to be made.
Artesian Aquifer is, in fact, confined aquifer of such a geometry developed in suitable geological situations so that the piezometric surface is always above the ground level at many places when projected in elevation. When water is tapped from such a confined aquifer, it rushes upto and even above the surface and may rise to the heights theoretically equivalent to the projected piezometric surface. Such wells are called Artesian Wells, or flowing wells and the type of groundwater obtained from them, which often needs no pumping, as Artesian Water.
Aquifer Functions:
Aquifers serve as underground reservoirs and distribution systems or conduits at the same time.
The Storage Capacity of a reservoir depends on the porosity of the rock on the one hand and the nature and inter-connections of the pores:
(a) Porosity:
It is defined as volume of voids in a rock mass expressed in percentage terms of the total volume of the rock. The openings responsible for porosity in rocks may have diverse origin. Pore spaces may be left between individual grains and particles during the process of deposition in sedimentary rocks. Opening may be also caused in the rocks due to development of cracks, crevices, joints, fissures, solution channels and gas holes in different categories of rocks.
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Sedimentary rocks are relatively highly porous because there is a great variation in degree of packing on the one hand and in size, shape and arrangement of the grains on the other hand. Limestones and dolomites are susceptible to the development of solution channels and caverns whereas igneous rocks may become porous due to escape of gases (gas holes). All types of rocks when subjected to tectonic forces often develop joints and cracks of varied geometry that make them porous to some extent.
Theoretically, porosity, n, is expressed by the relationship:
Porosity is a broad measure of the Storage capacity of an aquifer. Hence its range is of practical importance. As evident from table (18.1), porosity of common unconsolidated sediments and consolidated rocks varies from less than one percent to as high as 55 percent.
(b) Specific Yield:
Porosity is a property of primary importance in determining the water bearing qualities of a rock. Only porous rocks can be aquifers but high porosity itself is not sufficient to ensure water-yielding capacity of a rock. Thus, clays which are generally characterized with a high porosity, 50-60 percent, may be practically useless as aquifers. It is because the pores or voids in between clay particles are so minute that water is held up within these pores very firmly.
Our main interest is always to determine how much water can be actually obtained per unit area of the aquifer. This water is released by the aquifer under the force of gravity and depends both on the quantity and quality of the pores and other openings present in the rock. It is called Specific Yield and may be defined as “the quantity of water that a unit volume of aquifer drains by gravity”.
The term effective porosity is sometimes used to express the volume of pores which is effective in the release of water from an aquifer. It is also called kinematic porosity and is smaller than the total porosity.
Factors that are commonly responsible to adversely affect the porosity of rock are:
(i) Adhesion to Grains:
Some groundwater is attached to the surface of grains of rocks due to molecular attraction and is not easily available for circulation.
(ii) Unconnected Pores:
Though present in good abundance and even containing some water, such pores are separated from each other. Hence the liquid in them cannot circulate. Broadly, similar conditions may exist in crystalline rocks where all fractures are not thoroughly inter-connected.
Specific yield is, therefore, broadly a function of grain size and shape and also continuity of the interstices. Specific retention is the term used to express the amount of water retained by a unit volume of aquifer after allowing gravity drainage through it. Thus, specific yield and specific retention together sum up to the total porosity of an aquifer.
Specific yields (expressed in decimal fractions or percentage terms) range within wide limits in the same rock, as seen in Table (18.2).
As a general observation, coarse-grained rocks have higher specific yields compared to fine grained rocks, with porosity being of the same order.
Flow in Aquifer:
Movement of water through the aquifer is, in general, a function of three forms of energy contained in groundwater – pressure, velocity and elevation head.
According to Bernoulli’s equation, the sum of energy potentials of these forms, H, is:
where p is pressure; r is the specific weight of water; V is the velocity; g is the acceleration due to gravity; Z is the elevation head above a datum or depth of the water column.
The velocity factor in great many situations in groundwater is almost negligible.
Again, since the pressure extended due to atmosphere is almost a constant over an aquifer, it is only the depth of water column that will be responsible for driving water over a certain distance.
Permeability, hydraulic conductivity and transmissivity are important terms related to various aspects of movement of water in an aquifer:
(a) Permeability:
When used in a general sense, permeability is the capacity of a rock (or any other solid medium) to transmit fluids through it. It is often expressed as Intrinsic Permeability, of which darcy, d, is the unit which is the capacity of a rock of 1 cm length and 1 cm2 cross sectional area to allow a flow of 1 cm3/sec at a difference of 1 atmosphere.
Permeability is essentially related to the quality of pores and other interstices in a rock. Accordingly, rocks which are made up of well assorted grains and which are profusely traversed by interconnected joints, crevices and cavities allow the water to pass through them easily. On the other hand, rocks with closed or dead end pores do not allow water transmission. There is observed a great variation in permeability of rocks. In general, rocks are sometimes classified into three groups on the basis of coefficient of permeability. K(m/d).
(b) Hydraulic Conductivity:
In groundwater geology or hydrogeology, the quantitative measurement of flow or water is generally expressed by the term Hydraulic Conductivity rather than permeability.
The hydraulic conductivity K, may be defined as the flow velocity per unit hydraulic gradient. It is expressed as meters/day or meters/second.
Darcy’s Law:
Henri Darcy, a French engineer, pronounced in 1856 a relationship between rate of flow of water through a porous medium to other parameters such as hydraulic gradient and length of the column of the medium. His experimental observations have been established as Darcy’s Law, and form a fundamental equation in the flow of groundwater.
According to this law, the rate of flow of groundwater through a column of saturated sand is:
(i) Directly proportional to the difference in hydraulic head at the ends of the column, and
(ii) Inversely proportional to the length of the column.
Mathematically,
where V = Flux velocity or specific discharge; K = Hydraulic conductivity; (h1 – h2) = Difference in hydraulic head; L = Length of the column.
Hydraulic Gradient:
The difference in hydraulic head at two points divided by the length is often called as hydraulic gradient. l, so that the above equation may be rewritten as-
V = Kl
It is known that in a pipe flow condition which is analogous to flow in a porous medium, the discharge Q is given by relationship:
Q = V.A
where A is the cross sectional area, and V is the velocity or specific discharge.
From the above equation, the relationship of discharge to hydraulic conductivity is easily established:
Q = K l A
This relationship is of fundamental importance in groundwater studies. Here, Q represents discharge and is expressed as discharge per unit time such as cubic meters per day or gallons per minute. K, as usual, is the hydraulic conductivity and indicates the quantity of water that will flow through a unit cross-sectional area per unit time under a unit hydraulic gradient, at a specified temperature. The value of K ranges from 0.5 m/day to 200 m/day or even more.
(c) Transmissivity:
This term was introduced by Theiss in 1935 to express the capability of an entire aquifer to transmit water.
The coefficient of transmissivity T, (transmissibility) was defined by Theiss as:
“Rate of flow in gallons/minute through a vertical section of an aquifer 1 foot wide extending the full saturated length of the aquifer under a unit hydraulic head.”
(d) Types of Flow:
In general, groundwater flow through the interstices is of a laminar type although the turbulent flow also occurs in certain situations. In the laminar flow, the water droplets and drops move very slowly in a continuous, steady, ribbon-like fashion through the pores and other openings. In such a case, water moving laterally through the thickness of an aquifer flows in layers, there being no intermixing of water from overlying layers.
The turbulent flow is characteristic of subsurface streams and involves complete intermixing of water molecules, which move in an irregular fashion.
Artesian Aquifer:
An artesian aquifer has already been defined as a confined type of aquifer in which the contained water is under such a hydrostatic head that when tapped, the water will rush to the surface and may rise above the ground to a certain height. In fact the term Artesian Water is derived from the abundance of such wells in the ARTOIS region of central Europe. In some cases, where hydrostatic pressure is comparatively less, water may approach the surface in a well without actually overflowing. Such type of wells is called sub-artesian wells.
Artesian water is a good source of water supply in many regions of the world.
There are at least three geological considerations that need careful observation during planning water supply programmes from artesian water-resources:
(a) Existence of a Suitable Type of Aquifer:
An artesian aquifer must necessarily be enclosed at the top and base by impermeable layers. The aquifer must be suitably inclined to create sufficient hydraulic head. The piezometric surface must lie well above the ground surface. Monoclinial strata and broadly synclinal, basin type geological structures are considered ideal as artesian basins when these are overlain and underlain by impervious layers.
(b) Recharge Conditions:
Artesian aquifers being overlain by impermeable strata must have natural exposure at the surface somewhere in the catchment. In other words, their rims must be exposed to get replenishments from precipitation. It is obvious that such an exposure should be in areas of sufficient rainfall so that enough quantity of water is available for recharge.
In some cases, artesian aquifers may get their recharge from other type of subsurface (free) water where the free water aquifer is hydraulically connected to such a confined aquifer.
(c) Absence of Leakage:
In an artesian aquifer, there is always a possibility of leakage points due to localized development of fractured zones, reduced impermeability of the confining layer or connections with other subsurface water bodies. Such points will effectively reduce the total hydraulic head available in ideal conditions.
Artesian conditions are created in rocks by a number of ways: these may be due to original deposition or subsequent deformation such as folding and faulting. Further, both fresh water and mineralized water may be obtained from artesian well. In thicker artesian aquifers, the lower zones may contain even salt water.
Artesian wells, also called Flowing Wells are chiefly used for water supply and irrigation. In some cases they form important sources for the chemical industry. Hence, study of artesian sources aimed at rational planning for water supply programmes forms an integral part of water resources management.
A successful planning programme involves collection and coordination of all data on geology, hydrology, hydrogeology, climate and proposed extraction methods. There are very diverse types of artesian basins varying in area, type of rock, number of water-bearing formations, geological structure and depth from the surface.
It is, therefore, always necessary to first know the nature, geological structure and penological character as also salient hydrogeological features of the artesian belt with the help of test boreholes spread systematically over entire region under investigations. Similarly, location of natural drainage area for artesian waters is also important since it has a significant bearing upon the yield of the wells in the region. Such drainage areas include artesian springs and other means of ground seepage.
In volcanic terrain, conditions for artesian water aquifer are sometimes available where sloping lava layers are overlain and underlain by impermeable layers of clay and other materials.
Different Rocks as Aquifers:
Rocks show great variation in their water-bearing properties. Although generalizations do not hold good, it has been found that some rocks offer better chances of proving productive aquifers than others.
Following discussion is based on such observations:
(a) Sedimentary Rocks:
i. Gravels:
These are classed among the best kind of formations to yield groundwater. But these too show a great variation in their water bearing properties depending on degree of assortment, packing and cementing of the grains. Generally, the ideal gravel is a coarse, clean and well-sorted material, which has a very high effective porosity and permeability. Hydraulic conductivity K, of pebble gravels free from interstitial packing may range anywhere between 100-1000 m/day. Their porosity may range between 25-40 percent and specific yield between 15-30 percent.
ii. Sands:
Sand beds or loosely packed sandstones are rated next to gravels as water yielding materials. The sand layers are characterized with a porosity of 25-40 percent and specific yield of 15-25 percent. Their hydraulic conductivity is also quite high ranging between 10-100 m/day.
In many cases they prove even better than gravels because of their extensive continuity. The beach sands are example of best water-bearing formations in many regions. Compacted and cemented sands make sandstones which form a different class in themselves as aquifers.
iii. Sandstones:
These sedimentary rocks show great variation in their water yielding capacity, which is chiefly controlled by their texture and nature of the cementing material. Thus, whereas coarse-grained sandstones with rather imperfect cement are classed among the excellent aquifers, the fine-grained varieties which are thoroughly compacted or cemented may prove useless as water yielding rocks.
In permeability, sandstones vary from semi-permeable to highly permeable, with porosity ranging from 5-30 percent, specific yield from 5-15 percent and hydraulic conductivity between 1-100 m/day. Sandstone formations are among the most important groundwater reservoirs the world over.
iv. Limestones:
Some of the best known productive aquifers of the world are of limestone rock. At the same time, these vary rocks have proved totally unproductive in many other regions. This great variation in behaviour of limestones as water-bearing rocks is explained by the fact that in limestones and dolomites and similar calcium-rich rocks, the availability of water is controlled by development of secondary fracturing, solution channels, cavities and crevices and similar openings rather than by primary porosity.
It is, therefore, not surprising that the porosity of limestones varies from 1 to 20 percent. Hydraulic conductivity in highly karsted (cavernous) limestones may be as high as 100 m/day or even more, to be classed among the extremely permeable rocks.
v. Glacial Deposits:
Outwash plains deposited by glacial meltwaters during the last ice age cover a good part of the globe in northern hemisphere. When these accumulations are made up of sediments of uniform size and rounded nature, they present with best quality aquifers. When sufficiently thick and extensive, these may be depended as reliable groundwater reservoirs. These have been observed giving yields between 5-10 thousand m3/day.
(b) Igneous Rocks:
Igneous rocks are either intrusive or extrusive in nature. The intrusive igneous rocks are dense in texture with all the component minerals very closely crystallised so that there are available practically no openings or pores. Such rocks, therefore, are non-porous, impermeable and without any scope for holding any groundwater supplies.
The intrusive rocks are, however, sometimes traversed by extensive cracks and joint systems and fissures due to folding, faulting and other forces. When presence of such fissure-and-joint systems is established beyond doubt, the rock in this zone may prove to be holding groundwater supplies. Since the extent of the fractured zone is invariably limited both in depth and laterally, the groundwater will behave more as vein water rather than free groundwater.
The extrusive (volcanic) rocks exhibit great variation in their water bearing properties. The composition of these rocks and their mode of formation from lava as well as the nature of original topography are factors, which define the water bearing capacity of these rocks.
It has been observed that:
(i) Acid Volcanic Rocks (Rhyolites etc.) may or may not be holding water because acidic lavas are comparatively viscous and generally fragmentary at the time of eruption. Though interstices and fracture system will normally be common in such lavas, their being partially or completely filled subsequently with ash and other materials is a matter of common observation.
(ii) Basic Volcanic Rocks (Basalts etc.) are characterized by a greater gas content and high mobility at the time of eruption. They flow to great distances and form repeated and thick layers of flows. The congealing mass is often rich in cavities (due to escape of gases during cooling) and cracks and joints (due to contraction on cooling). These cavities and joint-systems bestow considerable porosity and permeability to this group of igneous rocks.
Similarly, the flow may conceal beneath them pre-existing valleys or ridges of sedimentary strata. These concealed valleys may prove good source of groundwater. Another situation may also occur where porous and permeable sedimentary strata form intervening layers in a volcanic flow. These inter-trappean formations also hold good prospects for groundwater supplies.
In all igneous rocks, the surface zone (100 m or so) when highly weathered and disintegrated, may become quite porous and permeable and hence water-bearing.
(c) Metamorphic Rocks:
Generally speaking, the crystalline and compact metamorphic rocks like quartzites, marbles and gneisses are non-porous and impermeable holding no prospects for groundwater reserves. The only exception is when they are profusely fractured and fissured, in which case their behaviour is similar to igneous rocks of plutonic nature. Groundwater will be found in the fractured zones without lateral and vertical continuity.
Metamorphic rocks which are inherently fractured and foliated such as schists, slates, phyllites and often gneisses, may prove exceptionally good aquifers, in a broader sense. Again, metamorphic rocks forming covering layers may have better chances as aquifers than those buried deeply. This is because in almost all cases, the fracture systems get closed and disappear with depth. Moreover, top layers are invariably weathered to a lesser or greater extent which adds to their original permeability.