- Kufra Oasis, Libya - NASA Earth Observatory
Aquifers are essential to our survival. They store ground water and enable life to flourish in regions that would be otherwise virtually uninhabitable. Humans have become very good at exploiting natural resources such as aquifers, but this very fact makes them vulnerable. Our ever-increasing water demands mean that some aquifers are being depleted more rapidly than they can be replenished, which means the water supply is coming under threat.
What Kind of Rock Makes an Aquifer?
An aquifer must be porous (contain void spaces) so that it can store water, and permeable (the gaps must be well connected) to allow the water to flow through the rock. This means that only certain rock types are suitable. The best aquifers are coarse-grained sedimentary rocks in which the grains are spherical and all about the same size.
When the grains in a rock are all spherical and around the same size they can only pack together loosely because they can’t squeeze into spaces once they are touching their neighbours. This leaves a large amount of void space (pores). Larger grains lead to larger pore spaces. The pores will also be quite well connected, so fluid can flow easily between them.
Conversely, a wide variety of grain sizes allows the grains to pack together more tightly because small grains can fill gaps between the larger ones, thereby decreasing the porosity. The gaps will also be less well connected and fluid will take longer to seep between pores. If grains are angular they also have more potential to squeeze into small gaps, so angular grains will tend to lead reduce porosity and permeability.
Mineral ‘cement’ between grains helps to hold the rock together but also fills up pores and connecting channels, reducing porosity and permeability. Therefore, a coarse, poorly cemented sandstone with spherical grains makes a good aquifer while a well-cemented rock containing only fine grains or one containing coarse, medium and fine grains makes a poor aquifer.
Where Do We Find Aquifers?
Aquifers are found across the world in the shallow regions of the Earth’s crust. At depths of around 10km rocks have been so compressed and altered that their void spaces have been closed and they can be assumed to be impermeable. These deep rocks effectively confine groundwater to the shallow crust.
Rain seeps down through the Earth’s crust, effectively ‘filling up’ the rocks overlying the impermeable deep layer. Rocks with poor permeability can reduce the downward percolation of water, so the replenishment of water in an aquifer may be inhibited by an overlying impermeable rock layer. In this case, if the aquifer reaches the surface at some point water can be replenished at that location and, from there, flow through the rest of the layer.
Extracting Water From an Aquifer
For exploitation of an aquifer to be sustainable it is necessary for its water content to be replenished at least as quickly as it is extracted. Water is normally drawn from aquifers using wells, although if an aquifer outcrops at the Earth’s surface then drilling a well may not be necessary as groundwater can seep from it naturally. Where this occurs in a desert it will normally result in the formation of an oasis.
Wells drilled into aquifers may either be ‘artesian’ (the water flows from the well under its own pressure) or pumped. If water is extracted from the aquifer more rapidly than it is replenished the water level (and therefore pressure) in the aquifer drops, and artesian wells may cease to flow.
Extraction of water from aquifers can be a large-scale operation. For example, the Ogallala Aquifer in the United States provides water for the crops fields of the mid-West. According to Jane Braxton Little in the March 2009 Scientific American article “The Ogallala Aquifer: Saving a Vital U.S. Water Source” (accessed August 15 2010), $20 billion a year depends on the aquifer. Unfortunately, this water resource is being over-exploited
Aquifers are also sensitive to pollution, such as is the case in Bangladesh where aquifers have been poisoned with arsenic. According to the article “Ground Water” by SOS-arsenic.net (accessed August 15 2010) it is estimated that the most significant health risk from drinking water from tubewells in Bangladesh is chronic arsenic poisoning.
Aquifers are a vital resource and good management is essential. Because rates of water replenishment and flow are so low within an aquifer, any issues such as over-exploitation or pollution may take thousands of years to remedy. Aquifers are also sensitive to changes in precipitation, so any changes in rainfall patterns have the potential to increase the vulnerability of these natural resources.
Sources
Price, Michael, 1996. Introducing groundwater, 2nd Edition, Stanley Thornes,UK.
Margaret Wallace - Hello, my name is Mags Wallace. I did an MESci in geophysics and Geology from Liverpool University, followed by a PhD in Physical ...
