AQUIFERS, HYDROGEOLOGY, GROUND WATER

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Aquifers are defined as underground layers of rock that are saturated with water that can be brought to the surface of the earth through natural springs or by pumping using machines. It is a body of saturated rock through which water can easily move beneath the earth’s surface. An aquifer is as an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials (gravel, sand, or silt) from which groundwater can be extracted using several mechanical means. Aquifers are amongst the major sources of water for mankind, and it is critical that these natural sources of water are free from all possible biological or chemical contamination- thus the study of geomicrobiology. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Aquifers act as reservoirs for groundwater. And water from aquifers sometimes flows out in springs. Wells drilled into aquifers provide water for drinking and for agricultural purposes and other industrial and domestic purposes beneficial to mankind.

Aquifers must be both permeable and porous, and they usually include such rock types as sandstone, conglomerate, fractured limestone and unconsolidated sand and gravel. Fractured volcanic rocks such as columnar basalts also make good aquifers. The rubble zones between volcanic flows are generally both porous and permeable and make excellent aquifers. In order for a well to be productive, it must be drilled into an aquifer – which is the major source of the well water. Not all rock types are good sources of aquifers for water source. For example, rocks such as granite and schist are generally poor aquifers because they have a very low porosity. However, if these rocks are highly fractured, they make good aquifers. A well is a hole drilled into the ground to penetrate an aquifer. Normally such water must be pumped to the surface. If water is pumped from a well faster than it is replenished, the water table is lowered and the well may go dry because of the low level of its water table. When water is pumped from a well, the water table is generally lowered into a cone of depression at the well. Groundwater normally flows down the slope of the water table towards the well.

Aquifers are not necessarily underground rivers. Almost all aquifers are not rivers. Since water moves slowly through pore spaces in an aquifer’s rock or sediment, the only life-forms that could enjoy floating such a ‘river’ would be bacteria or viruses which are small enough to fit through the pore spaces. True underground rivers are found only in cavernous rock formations where the rock surrounding cracks or fractures has been dissolved away to leave open channels through which water can move very rapidly, like a river. Ground water has to squeeze through pore spaces of rock and sediment to move through an aquifer. And it is worthy of note that the porosity of such aquifers make them good filters for natural purification of the water that they provide. According to geological findings, it takes effort to force water through the tiny pores of the aquifers and/or rocks; thus the ground water loses energy as it flows, leading to a decrease in hydraulic head in the direction of flow. However, aquifers with larger pore spaces usually have higher permeability, produce less energy loss, and therefore allow water to move more rapidly. For this reason, ground water can move rapidly over large distances in aquifers whose pore spaces are large or where porosity arises from interconnected fractures.

Ground water moves very rapidly in fractured rock aquifers; and in such scenarios, the spread of contaminants can be difficult or impossible to prevent. An aquifer is filled with moving water and the amount of water in storage in the aquifer can vary from season to season and year to year. No matter how fast or slow the water in an aquifer may move, the water will eventually discharge or leave an aquifer and must be replaced by new water to replenish or recharge the aquifer for future use. Every aquifer has a recharge zone or zones and a discharge zone or zones from which the water in the aquifer can be replenished and released from respectively (Figure 1).

Figure 1. Anatomy of the different types of aquifers.

Recharge zones are typically at higher altitudes, and they indicate the points or zones from which water may probably enter the aquifer. Water can enter the recharge zone of an aquifer through or from rain, snowmelt, river and reservoir leakage, or from irrigational processes. Discharge zones are typically at low altitudes, and they usually occur in springs near the stream and in wetlands that are at low altitudes. They can also occur in high altitude springs and in wells too. The three different types of aquifers are: confined aquifers, unconfined aquifers, and perched aquifers (Figure 2). Perched aquifers are close to the recharge zones of aquifers. A perched aquifer’s water table is usually highly sensitive to the amount of seasonal recharge so a perched aquifer typically can go dry seasons of the year. Unconfined aquifers are those into which water seeps from the ground surface directly above the aquifer. Confined aquifers are those in which an impermeable dirt/rock layer exists that prevent water from seeping into the aquifer from the ground surface located directly above.

Figure 2. Common sources of ground water (aquifer) contamination.

Instead, water seeps into confined aquifers from farther away where the impermeable layer doesn’t exist. Confined aquifers are less contaminated than unconfined aquifers. The reason is because confined aquifers have impermeable dirt/rock layer that prevent water from seeping into the aquifer from the ground surface located directly above it. This is not obtainable in unconfined aquifers. Natural filtration in soils is very important in recharge zones of aquifers and even in irrigated areas above unconfined aquifers, where water applied at the surface can percolate through the soil to the water table. Though aquifers are natural filters that trap sediment and other particles such as microorganisms; and this is why they provide natural purification of the ground water flowing through them.

However, aquifers could still be contaminated through various natural and man-made activities. Despite the natural purification mechanisms of aquifers, the concentrations of some elements in ground water (e.g. lead, uranium, or arsenic) can be high in instances where the rocks and minerals of an aquifer contribute high concentrations of certain elements. Aquifers can be contaminated from various sources including mining activities, excavations, surface runoff of water, fertilizers, other agrochemicals, landfill leakages and leakages from oil wells or tanks and septic tanks (Figure 2). In summary, aquifer contaminants reach the water table by any natural or manmade pathway along which water can flow from the surface to the aquifer as aforementioned.

Further reading

Jee C and Shagufta (2007). Environmental Biotechnology. APH Publishing Corporation, Darya Ganj, New Delhi, India.

Latha C.D.S and Rao D.B (2007). Microbial Biotechnology. First edition. Discovery Publishing House (DPH), Darya Ganj, New Delhi, India.

Maier R.M, Pepper I.L. and Gerba C.P (2000). Environmental Microbiology. Academic Press, San Diego.

Mishra B.B, Nanda D.R and Dave S.R (2009). Environmental Microbiology. First edition. APH Publishing Corporation, Ansari Road, Darya Ganj, New Delhi, India.

Paul E.A (2007). Soil Microbiology, ecology and biochemistry. 3rd edition. Oxford: Elsevier Publications, New York.

Pelczar M.J., Chan E.C.S. and Krieg N.R. (2003). Microbiology of Soil.  Microbiology, 5th Edition. Tata McGraw-Hill Publishing Company Limited, New Delhi, India.

Pepper I.L and Gerba C.P (2005). Environmental Microbiology: A Laboratory Manual. Second Edition. Elsevier Academic Press, New York, USA. 

Roberto P. Anitori (2012). Extremophiles: Microbiology and Biotechnology. First edition. Caister Academic Press, Norfolk, England.

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