4 Types of aquifers: Aquiclude, Aquitard and Aquifuge and location of aquifers
Definition– An aquifer is an underground layer of rocks which can hold water. They act like sponges hence; they are also sometimes called sponge rocks. The water from this underground layer of the Earth’s crust can be used by drilling a well and pulling the water out with the help of machines (water pumps) or manually (with the help of the old traditional rope and bucket). The study of the water layers of water flow in the aquifers and the types and classification of aquifers is known as Hydrogeology. An Aquitard is an underground layer within the surface of the earth that prevents the flow of groundwater from one aquifer to another. An Aquitard which does not allow groundwater to pass through at all is called an Aquiclude or Aquifuge. Aquitards are made up of layers of either clay or non-porous rocks which do not allow the movement of fluids easily.
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Typical aquifer cross-section
Depth
The presence of Aquifers can occur in different layers across several depths. The aquifers which are closer to the surface are the easiest to be used for the supply of water and irrigation purposes. These aquifers are also mostly replenished by the local rainfall. Many desert areas have limestone hills or mountains within them or close to them that can be exploited as groundwater resources. There are several places in the world where aquifers exist. Some of these places have shallow aquifers that are exploited easily and much digging or drilling is not required for obtaining their water. Some examples of such places are parts of the Atlas Mountains in North Africa, parts of the Lebanon ranges which fall between between Syria and Lebanon, the Jebel Akhdar in Oman and parts of Sierra Nevada in the United States. Over exploitation of these aquifers can lead to the exhaustion of water in them as more water is taken out than can be replenished because of the changing scenario of climate change which has resulted in the paucity of good rainfall in many parts of the world. Along the coastlines of certain countries, such as Libya and Israel, increased water usage associated with population growth has caused a lowering of the water table and the subsequent contamination of the groundwater with saltwater from the sea.
A beach provides a model to help visualize an aquifer. If a hole is dug into the sand, very wet or saturated sand will be located at a shallow depth. This hole is a crude well, the wet sand represents an aquifer, and the level to which the water rises in this hole represents the water table.
In the year 2013 many large freshwater aquifers were discovered under the continental shelves off Australia, in China, parts of North America and parts of South Africa. They contain an estimated half a million cubic kilometers of “low salinity” water that could be economically processed into potable water.
http://www.whymap.org
Classification
Saturated versus unsaturated
Groundwater can be found at nearly every point on the shallow sub-surface of the Earth. Although there are incidents many a time when the water contained in the aquifers is not fresh water. The Earth’s crust can be divided into two regions: the saturated zone or phreatic zone (e.g., aquifers, Aquitards, etc.), where all available spaces are filled with water, and the unsaturated zone (also called the vadose zone), where there are still pockets of air that contain some water, but can be filled with more water.
Saturated means the pressure head of the water is greater than atmospheric pressure (it has a gauge pressure > 0). The definition of the water table is the surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0).
Unsaturated conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water that incompletely fills the pores of the aquifer material is under suction. The water content in the unsaturated zone is held in place by surface adhesive forces and it rises above the water table by capillary action to saturate a small zone above the phreatic surface (the capillary fringe) at less than atmospheric pressure. This is termed tension saturation and is not the same as saturation on a water-content basis. Water content in a capillary fringe decreases with increasing distance from the phreatic surface. The capillary head depends on soil pore size. In sandy soils with larger pores, the head will be less than in clay soils with very small pores. The normal capillary rise in a clayey soil is less than 1.80 m (six feet) but can range between 0.3 and 10 m (one and 30 ft).
The capillary rise of water in a small-diameter tube involves the same physical process. The water table is the level to which water will rise in a large-diameter pipe (e.g., a well) that goes down into the aquifer and is open to the atmosphere.
Aquifers versus Aquitards
Aquifers are those regions of the sub-surface that contain and produce a good enough quantity of water to a well or spring which is economically very easy to obtain for human consumption. The sand and gravel or broken or cracked bedrocks often make good and useful material for aquifers.
An Aquitard is a zone within the earth that restricts the flow of groundwater from one aquifer to another. A completely impermeable Aquitard is called an Aquiclude or Aquifuge. Aquitards comprise layers of either clay or non-porous rock with low hydraulic conductivity.
In mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated alluvium, composed of mostly horizontal layers of materials deposited by water processes (rivers and streams), which in cross-section (looking at a two-dimensional slice of the aquifer) appear to be layers of alternating coarse and fine materials. Coarse materials, because of the high energy needed to move them, tend to be found nearer the source (mountain fronts or rivers), whereas the fine-grained material will make it farther from the source (to the flatter parts of the basin or overbank areas—sometimes called the pressure area). Since there are less fine-grained deposits near the source, this is a place where aquifers are often unconfined (sometimes called the forebay area), or in hydraulic communication with the land surface.
Confined versus Unconfined
There are two members in the types of aquifers; confined and unconfined. Unconfined aquifers are also referred to as the water table which in scientific terminology is known as phreatic aquifers. The water table or phreatic surface being the upper boundary hence the name is given. Strikingly but not as a rule the shallowest aquifer at any given location is unconfined, meaning it does not have a layer which confines it (an Aquitard or Aquiclude) between the layer and the surface. The term “perched” refers to ground water accumulating above a low-permeability unit or strata, such as a layer of clay. This term is generally used to refer to a small local area of ground water that occurs at an elevation higher than a regionally extensive aquifer. The difference between perched and unconfined aquifers is their size (perched is smaller). Confined aquifers are aquifers that are overlain by a confining layer, often made up of clay. The confining layer might offer some protection from surface contamination.
Isotropic versus anisotropic
In isotropic aquifers or aquifer layers the hydraulic conductivity (K) is equal for flow in all directions, while in anisotropic conditions it differs, notably in horizontal (Kh) and vertical (Kv) sense.
Semi-confined aquifers with one or more Aquitards work as an anisotropic system, even when the separate layers are isotropic, because the compound Kh and Kv values are different (see hydraulic transmissivity and hydraulic resistance).
When calculating flow to drains or flow to wells in an aquifer, the anisotropy is to be taken into account lest the resulting design of the drainage system may be faulty.
Human dependence on groundwater
Most land areas on the Earth have some forms of aquifers hidden beneath like a layer beneath the surface. Sometimes these layers are found at significant depths. In some countries including India, these aquifers are rapidly being depleted by the excessive human intervention and exploitation to cater to the water needs of an ever increasing human population.
Aquifers that contain and supply fresh-water, especially those which have limited or almost no recharge by snow or rain can be over-exploited and depending on the local hydrogeology, may draw in non-potable water or saltwater intrusion from hydraulically connected aquifers or surface water bodies. This can be a serious problem, especially in coastal areas and other areas where aquifer pumping is excessive. In some areas, the ground water can become contaminated by arsenic and other mineral poisons.
Aquifers are important for the human habitation and also most important for agriculture. Deep aquifers in arid or semi-arid areas have for many years been the main water sources for irrigation. Many villages and even large cities draw their water supplies from the wells drilled into the aquifers.
Water supply to the cities, for the purpose of irrigation and water supplies for the use of manufacturing industries is provided through large wells drilled into the aquifers. Multiple wells for one water supply source are termed “well fields”, which withdraw water from the confined or the unconfined aquifers. Using ground water from the deep confined aquifers provides more protection to the surface water from being contaminated. Some wells, termed “collector wells,” are specifically designed to induce infiltration of surface (usually river) water.
Aquifers that provide fresh water for the purpose of irrigation and also for the domestic consumption are the ones which are found near the surface and drilling the surface to obtain water from these is fairly easy (within a couple of hundred meters) and some of these have recharge by fresh water. This recharge is typically from rivers or meteoric water (precipitation) that percolates into the aquifer through overlying unsaturated materials.
Occasionally, sedimentary or “fossil” aquifers are used to provide irrigation and drinking water to urban areas. In Libya, for example, Muammar Gaddafi’s Great Manmade River project has pumped large amounts of groundwater from aquifers beneath the Sahara to the densely populated areas near the coast. Though this has saved Libya money over the alternative, desalination, the aquifers are likely to run dry in 60 to 100 years. Aquifer depletion has been identified as one of the main causes for the rise in the prices of food in the year 2011.
Subsidence
In unconsolidated aquifers, groundwater is being continuously produced from the pores and spaces through slits, cracks or between the particles of gravel, sand, and silt. If the aquifer is confined by low-permeability layers, the reduced water pressure in the sand and gravel causes slow drainage of water from the adjoining confining layers. If these confining layers are composed of compressible silt or clay, the loss of water to the aquifer reduces the water pressure in the confining layer, causing it to compress from the weight of overlying geologic materials.
Saltwater intrusion
Aquifers near the coast have a lens of freshwater near the surface and denser sea water under the fresh water. Sea water penetrates the aquifer diffusing in from the ocean and is denser than the freshwater which is already present in the aquifer. For porous (i.e., sandy) aquifers near the coast, the thickness of fresh water atop saltwater is about 40 feet (12 m) for every 1 ft (0.30 m) of freshwater head above sea level. This relationship is called the Ghyben-Herzberg equation. If too much ground water is pumped near the coasts, the salt-water may intrude into the space meant for the fresh water inside the layers of the aquifers causing contamination of potable freshwater supplies. The water would then be salty in tase and also unclean and not fit for human consumption. Many coastal aquifers, such as the Biscayne Aquifer near Miami and the New Jersey Coastal Plain aquifer, have problems with saltwater intrusion as a result of over pumping and sea level rise. People living in these areas have to buy drinking water as a result.
Salination
Aquifers found in the surface irrigated areas in semi-arid zones with reuse of the unavoidable irrigation water losses percolating down into the underground by supplemental irrigation from wells run the risk of Salination.
Surface irrigation water normally contains salts in the order of 0.5 g/l or more and the annual irrigation requirement is in the order of 10000 m³/ha or more so the annual import of salt is in the order of 5000 kg/ha or more.
Due to the processes of excessive continuous evaporation, the concentration of salt in the water of the aquifer may increase continuously and eventually resulting into causing a great irreversible environmental hazard which would further cause disasters for the earth over a prolonged period.
For the control of salinity in such a case, there should be on an annual basis an amount of drainage water which is to be discharged from the aquifer with the help of a sub-surface drainage system and disposed of through a safe outlet. The drainage system may be horizontal (i.e. using pipes, tile drains or ditches) or vertical (drainage by wells). To estimate the requirement of the drainage of water, the use of a groundwater model with an agro-hydro-salinity component may be considered.
Examples
The Great Artesian Basin situated in Australia is certainly the largest groundwater aquifer in the world (spreading over a distance of 1.7 million km²). It is instrumental in catering to a large part of water supplies for the city of Queensland and also many areas of the remote parts of South Australia.
The Guarani which is an Aquifer and is located beneath the surface of Argentina, Brazil, Paraguay, and Uruguay, is another one of the world’s largest aquifer systems and is an important source of fresh water for the people living in these countries. It is named after the Guarani people; it covers an area of 1,200,000 km² and with a volume of about 40,000 km³, the thickness of this aquifer is between 50 m and 800 m and the maximum depth being of about 1,800 meters.
Aquifer depletion is a serious problem in some parts of the world, and it is especially a critical situation in parts northern Africa, for example the Great Man made River project of Libya. However, new and scientific methods for the management of groundwater such as the process of artificial recharge and the injection of surface waters during the seasonal periods of rainfall has extended the life of many freshwater aquifers, especially in the United States.
One of the most important aquifers, the Ogallala Aquifer located in the central part of the United States is one of the great aquifers in the world, however in several parts it is being rapidly being depleted by the growing municipal use, and continuous use for the purpose of the agricultural activities. This huge aquifer, which underlies portions of eight states of the United States, contains primarily fossil water from the time of the last glaciations.
Annual recharge, in the more arid parts of the aquifer, is estimated to total only about 10 percent of annual withdrawals. According to a 2013 report by research hydrologist Leonard F. Konikow at the United States Geological Survey (USGS), the depletion between 2001–2008, inclusive, is about 32 percent of the cumulative depletion during the entire 20th century (Konikow 2013:22).”
In the United States, the maximum uses of the water from the aquifers include the practices of irrigation for the purpose of agriculture and the other economic activities like the oil and coal extraction from the mines. The Cumulative total groundwater depletion of the United States shot up to a dangerously high level in the time period of the late 1940s and continued rapidly at an almost alarmingly steady linear rate through the end of the century. In addition to widely recognized consequences of the known fact that environmental, groundwater depletion also adversely impacts the long-term sustainability of groundwater supplies to help meet water needs of the continuously growing population of the nation.
Aquifers in India
The Central Ground Water Board of India (CGWB) has released an informative Atlas on the Aquifer Systems of India with the detailed data on the 14 principal and 42 major aquifers of the country. This atlas brings together valuable archival information from the Board on the aquifers in these states and presents it in graphical format through tables and maps. It is available at http://cgwb.gov.in/AQM/. The atlas also provides detailed data on six states: Chhattisgarh, Himachal Pradesh, Karnataka, Kerala, Meghalaya and Tamil Nadu.
The atlas presents the Board’s plans to manage different aquifers at the national level as well as in the above mentioned six states. It also exhibits the areas which have been identified for the purpose of water conservation and harvesting, groundwater development and artificial recharge. The data presented in the atlas shows information both at the national level and within these states.
It also highlights the fact that —groundwater levels across India are falling sharply and rapidly and that the quality of ground water is also declining considerably. The decline in the groundwater is particularly and specifically sharp around Delhi,
Western Uttar Pradesh, Haryana and Rajasthan. In the opinion of the ex- chairperson of CGWB S C Dhiman. “Construction around the Aravalli hills has severely disturbed the ground water recharge in the entire region.” The atlas also shows that certain areas of peninsular India are also severely affected by over-extraction of groundwater.
Dhiman, said that the recharge process was faster in the rocky parts of the country, and a few years of good rainfall could restore the health of the aquifers. Maps in the atlas indicate priority areas for water conservation and ground water recharge, and areas where water quality is affected by arsenic, fluoride and nitrates.
The baseline data of CGWB has been depicted using a GIS platform for initiating a National Aquifer Mapping Programme (NAMP) and demarcation, said water resources and parliamentary affairs minister, Pawan Kumar Bansal.
Maps in the atlas show various thematic layers such as climate, topographic settings on the occurrence of aquifers, movement, and chemical quality of ground water. The compilation has been done on a scale of 1:250,000 by integrating the geological and hydro-geological data of CGWB, the Geological Survey of India, and other agencies. Tables provide information on the characteristics of aquifers as they occur in different parts of the country. The atlas took two years to prepare and the purpose was to provide a broad overview of the groundwater situation in the country.
Long-term sustainability plan
In the opinion of the Groundwater expert and head of the Advanced Centre for Water Resources Development and Management (ACWADAM), Himanshu Kulkarni, the atlas will help to identify the vulnerable areas and can be used as a the basis for further research work in this field. His opinion was However that, “it is not a replacement for the aquifer mapping exercise to be conducted during the 12th Five Year Plan.”
