What is an aquifer?
The word “aquifer”, literally means “water bearer” and it is an underground layer of permeable rock, containing water, that can be accessed, by using a water well. The precipitation during rains, run into lakes, rivers, oceans and into the underground storage areas, called aquifers. In simple words, when a water bearing rock readily transmits water to wells and springs, it is called an aquifer.
Contextually, a well is a hole to extract water from the ground and it is drilled into the aquifer. If water is pumped to the surface from the well, at a faster rate, than the rate at which the aquifer is replenished, the water table is lowered.
Difference between an aquifer and an underground river:
Aquifer cannot be considered as an underground river, because of the pace of water flow and the life, aquifers support. The water in aquifers move slowly, at a snail’s pace, through pore spaces in an aquifer’s rock or sediment and the only life formed would be bacteria or viruses, which can penetrate through the pores of the rocks. But true underground rivers are found only in cavernous rock formations, where the rock surrounding cracks get dissolved to leave open channels, for the water to flow, rapidly like a river.
Movement of water in aquifers:
In aquifer, the water is stored between the grains of rock called pores .Pores, can range from microscopic spaces to caves. The aquifer property, that lets water through connected pores is known as permeability. Rocks such as granite and schist are poor aquifers, for their property of very low porosity. The rocks must be highly fractured to make a good aquifer.
Ground water squeezes through the spaces of the rocks and sediments, to move through an aquifer and hence they can be considered as good filters for natural purification. As the water flows through the tiny pores, ground water looses its energy, leading to the decrease in hydraulic head in the direction of flow. For faster flow of water in the aquifers, larger pore spaces, with higher permeability are required and in addition, produce less energy losses.
The amount of water in aquifer may vary from season to season and from year to year. Ground water may flow through an aquifer at an average rate of 50 feet per year or 50 inches per century, depending on the permeability.
Classification of aquifers:
There can be number of types and combinations in aquifers and in general, aquifers are broadly classified into three categories.
- Unconfined aquifers are covered by permeable geological formations (like solid rock) and the upper surface of saturation is called the water table. These are generally known as water table aquifers. They get recharged from the rainwater and surface water.
- Confined aquifers are covered by a semi permeable layer of rock and are not directly recharged from rainfalls, like in the case of unconfined aquifers. They need to be connected to an unconfined area to get recharged. Generally, in this layer some transfer of ground water between the confined aquifer and the other confining layers takes place.
- Perched aquifers occur, where ground water is perched above unsaturated rock formations as a result of discontinuous impermeable layers. The water tables of perched aquifers are highly sensitive to the amount of seasonal recharge and hence can go dry in summers or during drought years.
Other Principle aquifers by rock type:
- Sandstone and Carbonate.
- Carbonate rock.
- Other rocks.
Terms related to aquifers:
- Confining layer: is a geological unit, having little or no intrinsic permeability.
- Aquitard: A part of geological formation of lower permeability, than an aquifer and it will not transmit water at a sufficient rate to feed a spring.
- Aquifuge: A body of earth material, impervious to water and unabsorbtive.
- Aquiclude: An impermeable layer of rock, that does not allow water to move through it (like some shales).
- Aquifer yield: The maximum rate of withdrawal, that can be sustained by an aquifer.
- Surficial aquifers: They are shallow aquifers, typically less than 50 feet and thickness of about 60 feet.
- Hydrogeology: It is the study of water flow in aquifers and their characterization.
- Salt water Intrusion: It is the movement of saline water, into fresh water aquifers and hence can contaminate drinking water sources.
Increase in population and the rise in demand for water supply results in need for deeper wells and hence as a result, aquifers get depleted. More ever, water below the surface of the earth, is more likely to get contaminated with naturally occurring minerals like radium.
Irregular seasons, resulted from global warming may also result in the drying out of wells and hence aquifers. The best examples of such depletion can be witnessed by examining the case studies of Waukesha, Wisconsin, USA and Sana’a Yemen.
Sources of aquifer contamination:
The contamination of aquifers and ground water table are linked and hence contamination of one causes the other.
Some of the sources for such contamination are:
- On-site septic systems.
- Leaky tanks or petrol pipelines.
- Underground injection wells (Industrial wastage).
- Municipal landfills and Livestock wastes.
- Fly ash from coal fired power plants.
- Graveyards and road salt storage areas.
- Asphalt production and equipment cleaning areas.
- Wells for disposal of liquid wastes.
- Mill tailings in mining areas.
- Leaky sewer lines.
Non- point sources:
- Fertilizers on agricultural land.
- Contaminants in rain, snow, and dry atmospheric fallout.
- Pesticides on agricultural land and forests.
How to recharge aquifers artificially?
There are two ways to recharge the aquifers artificially- Passive and Active ways.
Passive recharge: In this method, artificial catchment areas are created to preserve heavy rain water, that may run in vain. Hence the stagnant captured water has the opportunity to run off into the aquifer, raising the level of the ground water table. This method is mostly employed in China and South west United States.
Active recharge: In this method, water is actively pumped into the aquifer, or at least to the porous rock, situated above the aquifer. This method can be used when recharging the aquifers using passive methods are impractical. In addition, this method can be used where the population density is large and the water scarcity is severe.
Preserving aquifers: safe guarding ground water supplies
Ground water is an essential source for human existence and saving aquifers are considered to be precious. Cleaning a contaminated aquifer is an expensive process and hence prevention of contamination of aquifers is supported. Some of such measures include:
- Replace leaking underground storage tanks.
- Landfills can be sited in locations, where leachates will not contaminate underlying ground water.
- By preventing hazardous material to enter into aquifers.
- By preventing use of excessive ground water resources.
- By preventing air, water and soil pollution.
- Controlling the effects of global warming and green house gas amounts.
The best possible solution to preserve ground water for an individual can be:
ACT: Acknowledge, Consider and Take action
- Acknowledge the causes of ground water contamination.
- Consider which apply to the particular individual.
- Take an action to preserve our own resources of aquifers.