(Cairns... Far North Queensland)
GROUND WATER RECHARGE
Groundwater recharge is the replenishment of an aquifer with water from the land surface. It is usually expressed as an average rate of millimetres of water per year, similar to precipitation. Thus, the volume of recharge is the rate times the land area under consideration, and is typically expressed in millions of cubic meters per year.
The quantity of recharge to an aquifer has been considered equivalent to the "safe yield" or quantity of water that could be removed from an aquifer on a sustainable basis. It is believed that the "sustainable yield" of an aquifer is almost always appreciably less than recharge. This is because sustainable yield must also allow for adequate provision of water to sustain groundwater-dependent ecosystems.
Nevertheless, a sustainable yield figure is derived from a recharge determination, and any sustainable yield study will usually involve the determination of recharge as a necessary first step. However, recharge is not bore understood, so it is difficult to estimate aquifer sustainability unless the recharge-related processes are carefully studied.
What is an Aquifer?
An aquifer is a body of saturated rock through which water can easily move. Aquifers must be both permeable and porous and 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 bore to be productive, it must be drilled into an aquifer. 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 bore 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 bore faster than it is replenished, the water table is lowered and the bore may go dry. When water is pumped from a bore, the water table is generally lowered into a cone of depression at the bore. Groundwater normally flows down the slope of the water table towards the bore.
Is an Aquifer an Underground River?
No. 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 (the porosity of such aquifers makes them good filters for natural purification). Because it takes effort to force water through tiny pores, ground water loses energy as it flows, leading to a decrease in hydraulic head in the direction of flow. 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 with the spread of contaminants being difficult or impossible to prevent.
What does an aquifer look like?
Every aquifer is unique, although some are more generic than others. The boundaries of an aquifer are usually gradational into other aquifers, so that an aquifer can be part of an aquifer system. The top of an unconfined aquifer is the water table. A confined aquifer has at least one aquitard at its top and, if it is stacked with others, an aquitard at its base.
Figure 1 shows an example of an aquifer system
How Does an Aquifer Work?
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. Ground water may flow through an aquifer at a rate of 50 feet per year or 50 inches per century, depending on the permeability. But no matter how fast or slow, water will eventually discharge or leave an aquifer and must be replaced by new water to replenish or recharge the aquifer. Thus, every aquifer has a recharge zone or zones and a discharge zone or zones.
Figure 2 is a simple cartoon showing three different types of aquifers: confined, unconfined, and perched.
Recharge zones are typically at higher altitudes but can occur wherever water enters an aquifer, such as from rain, snowmelt, river and reservoir leakage, or from irrigation. Discharge zones can occur anywhere; in the diagram, discharge occurs not only in springs near the stream and in wetlands at low altitude, and also from bores and high-altitude springs.
The amount of water in storage in an aquifer is reflected in the elevation of its water table. If the rate of recharge is less than the natural discharge rate plus bore production, the water table will decline and the aquifer's storage will decrease. A perched aquifer's water table is usually highly sensitive to the amount of seasonal recharge so a perched aquifer typically can go dry in summers or during drought years.
Why is Groundwater So Clean?
Aquifers are natural filters that trap sediment and other particles (like bacteria) and provide natural purification of the ground water flowing through them.
Like a coffee filter, the pore spaces in an aquifer's rock or sediment purify ground water of particulate matter (the 'coffee grounds') but not of dissolved substances (the 'coffee'). Also, like any filter, if the pore sizes are too large, particles like bacteria can get through. This can be a problem in aquifers in fractured rock.
Clay particles and other mineral surfaces in an aquifer also can trap dissolved substances or at least slow them down so they don't move as fast as water percolating through the aquifer.
Natural filtration in soils is very important in recharge areas and in irrigated areas above unconfined aquifers, where water applied at the surface can percolate through the soil to the water table. For example, in Figure 1, a protective layer of silt in the southern valley provides natural protection to the aquifer from septic systems, pesticide application, and accidental chemical spills.
Despite natural purification, concentrations of some elements in ground water can be high in instances where the rocks and minerals of an aquifer contribute high concentrations of certain elements. In some cases, such as iron staining, health impacts due to high concentrations of dissolved iron are not a problem as much as the aesthetic quality of the drinking water supply. In other cases, where elements such as fluoride, uranium, or arsenic occur naturally in high concentrations, human health may be affected.
How is an Aquifer Contaminated?
As shown in Figure 3, an aquifer can be contaminated by many things we do at and near the surface of the earth. Contaminants reach the water table by any natural or man-made pathway along which water can flow from the surface to the aquifer.
In general, any activity, which creates a pathway that speeds the rate at which water can move from the surface to the water table, has an impact.
In Figure 3, wastewater leaking down the casing of a poorly constructed bore bypasses the natural purification afforded by soil. Excessive addition of fertilizer, and agrichemicals, coupled with the enhanced recharge from crops, golf courses and other irrigated land and along road ditches, are common reasons for contamination arising from non-point sources. Removal of soil in excavations and mining reduces the purification potential and also enhances recharge; in some cases, the water table is exposed and becomes directly vulnerable to the entry of contaminants.
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Written and Authorised by Selwyn Johnston, Cairns FNQ 4870