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In this article we will discuss about:- 1. Meaning of Aquifers 2. Types of Aquifers 3. Ingression of Seawater.
Meaning of Aquifers:
An aquifer is a rock formation that has the capacity to host large quantities of groundwater. Well-defined aquifers consist of unconsolidated sedimentary rocks such as gravel and sand, which constitute beds of considerable thickness. These aquifers may occur along the watercourses, as stream channel fill sediments, in abandoned and buried valleys/stream channels, in plain terrains and in intermontane valleys.
The watercourses consist of alluvium that forms and underlies stream channels, as well as they form the nearby flood plain of the streams. Wells located in highly permeable strata bordering streams produce large quantities of water, as infiltration from the streams augment groundwater supplies.
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Abandoned or buried valleys/stream channels are those locations where the streams/rivers flowed in the geological past, and are no longer currently occupied by streams/rivers that carved them in the past. Although such valleys may resemble watercourses in terms of permeability and quantity of groundwater storage, the scope for recharge and capabilities for perennial yield are less in them, unless they are frequently recharged by rain-fed process.
Extensive areas under erosion regime by a network of streams and tributaries, characterised by plain country covered by unconsolidated and unsorted sediments are present in almost all continents. Geomorphologically, such terrains are known as pediplain.
These plains flank highlands or other geomorphic features that serve as the source for the sediments. In some places, gravel and sand beds form aquifers under these plains. In these plains, the groundwater reservoirs are recharged chiefly in areas accessible to downward percolation of water from precipitation and from occasional streams.
Intermontane valleys are stream-cut valleys developed in between two linear chains of hills or ridges. These valleys are underlain by huge volumes of unconsolidated rock materials interspersed with unconsolidated sediments like sand, gravels and alluvial deposits, both derived by erosion of the bordering hills/ridges.
Most of these intermontane valleys constitute large groundwater basins. The sedimentary deposits in these valleys are good quality aquifers, which are periodically replenished with surface runoff from the hill slopes and seepage from the streams that flow along these valleys.
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Several aquifers and groundwater basins/reservoirs have been delineated in India by remote-sensing. We need to continually attempt to recharge the aquifers and enhance the water resources in the groundwater basins. A massive exercise combining the remote-sensing application in identifying the above-mentioned geomorphologic features, with groundwater recharge in watersheds shall yield results.
Geological Formations as Aquifers:
Porous and permeable sediments deposited by river system, such as the alluvial deposits comprising unsorted sand and gravel, interspersed with boulders, cobbles and pebbles are the geological formation best suited for a good aquifer. Soil derived by mechanical weathering combined with chemical weathering of a variety of rocks such as granite, gneiss, quartzite and sandstone can form aquifers.
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Major parts of Karnataka, Telangana, Andhra Pradesh, Tamil Nadu, Madhya Pradesh and Orissa have produced sandy soil in this way. Further, the granite and gneiss bear fractures and joints serving directly as voids for groundwater or filled with the sandy soil derived in situ. This sandy soil cover on, or fracture fill in, granite and gneiss too serve well as a good aquifer.
Sandstone and conglomerate are the sand and gravel that have undergone induration with their grains held together with the aid of cement, but the cement material reduces the porosity and permeability. In order that they become permeable to percolating water, they need to develop fractures and joints by deformation. Crystalline and metamorphic rocks are relatively impermeable and are poor aquifers.
Where such rocks are fractured and are in decayed condition due to weathering, they can give rise to aquifers. Clay, mudstone, shale and siltstone are generally porous to varying measure, but their pores are so small that they become practically impermeable, unless they are jointed and fractured.
Volcanic rocks, such as the basalt flows of Deccan Trap occurring in the Deccan plateau of India, can form somewhat good aquifers, as they are permeable. Their permeability is attributed to the flow breccias, porous zones between the lava beds, lava tubes, lava shrinkage cracks and joints. The laterite derived by in situ chemical weathering of basalt, is a porous and permeable formation that can be a fairly good aquifer.
The density, porosity and permeability of limestone and other carbonate-rich sediments vary widely, depending upon the degree of consolidation and development of permeable zones after deposition and induration. The solution activity of water on limestone and carbonate rocks creates voids of varying dimensions from microscopic size to huge solution cavities and caverns.
Frequently, subterranean channels sufficiently large to carry the entire flow of stream are formed in limestone terrain. Large springs are frequently found in limestone areas. Under a well-developed groundwater system, the limestone terrain eventually forms a karst region or topography, where subterranean drainage through the limestone creates large ground water reservoirs.
The single unique feature of the Indian terrains is the diversity of the rock formation occurring in this terrain. Added to the variety of rock types, the diverse weathers such as arid to semi-arid, and the hot and humid conditions prevailing over different parts of India have created a variety of soil types, which are mostly in situ, but are frequently transported to the present locations.
These soil formations occur in an array of geomorphic units such as channel fills, intermontane valleys and as covers over vast pediplain. These rock and soil formations are good aquifers under a rain- fed regime, provided sustainable practices for groundwater reservoir development are adopted.
Types of Aquifers:
Aquifers are generally of large area and they can be visualised as large underground storage reservoirs. The groundwater flows out to the surface by the action of gravity or is drawn by the people for various human activities such as personal hygiene, agriculture and a host other purposes. The water percolates into the groundwater reservoir system and replenishes the aquifers by natural recharge fed by rainwater or as artificial recharge through human activity.
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The aquifers are of two types:
(1) Unconfined aquifer, and
(2) Confined aquifer.
(1) Unconfined Aquifer:
An unconfined aquifer is one in which a water table serves as the upper surface of the zone of saturation. It is also known as a free, phreatic or non-artesian aquifer. The water table of the unconfined aquifers is in undulatory form and slope, depending upon the areas of recharge and discharge, pumpage from the wells, and permeability of the formations. Rise and fall in the water table correspond to changes in the volume of water in storage within an unconfined aquifer.
Contour maps and profiles of the water table can be prepared for conceptual illustration of the groundwater reservoir in an unconfined aquifer environment, from the elevations of water in wells tapping the aquifer, to determine the quantities of water available in the aquifer, its distribution and movement. Isopach maps showing zones of groundwater saturation having equal or almost equal thickness are also prepared based on the conceptual projection of the three-dimensional form of the aquifers on a map or plan.
With the availability of appropriate computer-based satellite image processing software, Geographic Information System (GIS) software and the remote-sensing tools in current times, the high resolution Digital Elevation Models (DEM), which give us three-dimensional picture on the zones of groundwater saturation having equal thickness and isopach maps, as also on the groundwater reservoir, are prepared for the aquifers and reservoirs.
(2) Confined Aquifer:
Confined aquifers, also known as Artesian or Pressure Aquifers, occur where groundwater is confined under pressure greater than the atmospheric pressure by the overlying, relatively impermeable strata. In a well penetrating confined aquifer, the water level rises above the bottom of the confining bed due to the pressure.
Water enters a confined aquifer in an area where the confining bed rises to the surface or ends underground and the aquifer becomes unconfined. The region supplying water to a confined aquifer is known as a recharge area. Rises and falls of water in wells penetrating confined aquifers result primarily from changes in pressure rather than changes in storage volumes.
Confined aquifers have only small changes in storage and serve mainly as conduits for conveying water from recharge areas to locations natural or artificial discharge. The imaginary surface coinciding with the hydrostatic pressure level of the water in the aquifer is called as the piezometric surface of the confined aquifer.
The water level in a well penetrating a confined aquifer defines the elevation of the piezometric surface at that point. Flowing well results if the piezometric surface lies above the ground. A confined aquifer becomes an unconfined aquifer when the piezometric surface falls below the bottom of the upper confining bed. Contour maps, profiles of the aquifer, isopach maps and digital elevation models can be prepared for the confined aquifers, just the same way as in the case of unconfined aquifers.
A special case of an unconfined aquifer is the Perched Aquifer. This occurs wherever a groundwater body is separated from the main groundwater by impermeable formation or stratum of small areal extent and by the zone of aeration above the main body of groundwater. Clay layers or beds in sedimentary deposits often have shallow perched aquifers overlying them. The wells tapping perched aquifers yield temporary small quantities of water for short intermittent periods.
Water recharged to, or discharged from, an aquifer represents a change in the storage volume within the aquifer. For unconfined aquifers, this is expressed by the product of the volume of aquifer lying between the water table at the beginning and at the end of a period of time, and the average specific yield of the formation. In confined aquifers, assuming the aquifer remains saturated, the changes in pressure produce only small changes in storage volume.
Thus, the hydrostatic pressure (the pressure exerted by the water column) within an aquifer partially supports the weight of the overburden while the solid structure of the aquifer provides the remaining support. When the hydrostatic pressure is reduced the aquifer load increases. A compression of the aquifer results which forces out some water from it. Lowering of the pressure causes a small expansion and subsequent release of water.
A physiographic unit containing one large aquifer or several connected and interrelated aquifers is defined as a Groundwater Basin. The approach to the development of a groundwater basin has intimate links to the management of water resources by watershed development. The efforts towards the approaches for both may overlap.
The water-yielding capacity of a confined aquifer can be expressed in terms of its storage coefficient, which is defined as the volume of water that an aquifer releases from, or takes into, storage per unit surface area of aquifer per unit change in the component of head normal to that surface. The storage coefficient for an unconfined aquifer is the same as its specific yield. Storage coefficients can be determined from pumping tests of wells.
Ingression of Seawater in the Aquifers:
The groundwater basins or reservoirs or aquifers come in contact with sea along the coastline. The density of seawater is considerably higher than the density of the groundwater because of the total dissolved salts and minerals in the seawater. Owing to the difference in the densities of the seawater and the groundwater, under normal circumstances, the groundwater from the aquifers gradually flows into the sea.
The overdraft of groundwater in the coastal areas results in the fall of the groundwater table levels in those areas. It results in the reduction of the seaward flow of groundwater, or even total reversal causing seawater to enter into the aquifers.
Further, due to the lowering of water table in unconfined aquifers or the piezometric surface in confined aquifers, the natural gradient sloping downward toward the sea is reduced or reversed. Due to the difference in the densities of the two waters, an interface or boundary surface forms between the groundwater front and the seawater front.
The balance in the dynamics between the seawater and fresh groundwater governs the shape of this interface. When the interface moves landward due to the drop in the groundwater dynamics, the ingression of seawater into the aquifer takes place. The ingression of seawater into the groundwater system in coastal areas leads to contamination of groundwater with the salts contained in the seawater.
Seawater ingression wrecks the groundwater basin or reservoir or aquifers situated near the coastal areas, and it takes several years despite copious recharge of fresh water into the groundwater system, to reverse the situation for better. Almost all countries including U.S.A, U.K., Japan, the Netherlands and Germany besides India had to face the seawater ingression into the coastal aquifers due to water-stressed conditions prevailing in the coastal urban centres. The condition of groundwater in Chennai, Nellore, Ongole and Visakhapatnam in the east coast, all along the Gujarat coast and some places in the west coast, especially in the northern Kerala is affected this way.
The easiest way of prevention of seawater ingression into the aquifer is by gross reduction in the draft of water from a coastal aquifer. This helps in the rise of groundwater levels above sea level, thereby maintaining a seaward gradient of the groundwater table. Such a measure would entail proclamation of an unambiguous water policy by the government, followed by its strict implementation.
Alternatively, in order to prevent the seawater ingression or nullify the effect of the ingression that has already taken place, large amounts of freshwater recharge into the affected aquifer has to be made. Finding alternative sources of water to support the human activity and for augmentation of water resources is the only way to mitigate the stress on the groundwater source, especially on the coastal aquifers, appears to be the answer.