An Earth cache is a special type of Virtual Cache that is meant to be educational. Therefore to log a find you must demonstrate that you have learnt something from the site and experience.
Send your answers to us in an email via our profile page.
Any logs not accompanied by an email will be deleted.
Logging Tasks:
1) Take a picture of yourself (optional) with your GPSr at the cache location where one can see you with the windmill and the reservoirs in the background and upload it to the web site.
2) In South Africa what rock formation would one find the highest water bearing aquifers?
3) Explain what hydrogeology is.
4) In your opinion describe how groundwater can become contaminated.
5) After observing the geological formation of the area estimate the yield of water in litres per second from this aquifer.
AN AQUIFER
An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt or clay) from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. The surface of saturated material in an aquifer is known as the water table.
Aquifers can occur at various depths.Those closer to the surface are not only more likely to be exploited for water supply and irrigation, but are also more likely to be topped up by the local rainfall. Over exploitation can lead to the exceeding of the practical sustained yield, i.e. more water is taken out than can be replenished, which can cause the lowering of water table and the subsequent contamination of the groundwater with saltwater from the sea.
Aquifers are typically saturated regions of the subsurface which produce an economically feasible quantity of water to a well or spring (e.g. sand and gravel or fractured bedrock often make good aquifer materials). Most land areas on Earth have some form of aquifer underlying them, sometimes at significant depths.
Aquifers near the coast have a lens of freshwater near the surface and denser seawater under freshwater. Seawater penetrates the aquifer diffusing in from the ocean and is denser than freshwater. For porous (i.e. sandy) aquifers near the coast, the thickness of freshwater atop saltwater is about 12 m for every 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 coast, salt-water may intrude into freshwater aquifers causing contamination of potable freshwater supplies.
TAPPING INTO GROUNDWATER
Looking at the place names around our country it is obvious that our pioneering forebears relied heavily on natural spring water. Just about every second farm and many town names end in the suffix fontein. Unfortunately, springs are no sure supply of water. Nor is South Africa blessed with a high annual rainfall, and frequent droughts have caused fountains, streams, rivers and dams to dry up. In many communities man, animals and plants can only survive on stored rainwater in man-made reservoir-like tanks and dams, or in aquifers that Nature has provided in the depths of the earth.
Unlike most other parts of the world, the South African geological formations consist mainly of old and firm, sedimentary, igneous and metamorphic rock. Such formations are all water bearing, or aquifers, but their water storage capacity is generally quite limited.
The Department of Water Affairs have calculated that South Africa doesn’t come near the global average of 28 metres of the world’s crust. Tests in the western Gauteng area have shown that dolomite, our best water-bearing formations, contains only 10 metres of water. On the Cape Flats this level drops to 2,5 metres, and where granite forms the sub-base the reading can be as low as 0,5 metres. Our formations also let far less water through, for very few South African boreholes yield more that six litres per second.
Yet, the total volume of South Africa’s groundwater reserves remains greater than the combined capacity of all our dams. Water will always be our most precious resource.
FINDING THE WATER
Before pointing out a suitable drill site, such an expert consultant will first seek clues from geological formations and existing boreholes in an area. He can also gain extra scientific information from aerial photos, satellite scenes or geological maps. He may also use detailed readings from magnetometers that detect the faintest variations in the earth’s magnetic field.
He may even go further with a resistivity test that measures the passage of electrical current through the ground, and which is usually conducted by water in the pore space of the rocks. Similar tests can be done with EM, the electromagnetic induction method. Other tools at the scientist’s disposal are the gravimeter that shows changes in the gravitational field, or even the seismic method that uses shock waves to pinpoint any density changes in the ground formation.
After all this, no diviner, expert hydrologist, or drilling contractor can even guarantee that a drilling operation will strike water.
TESTING THE SUPPLY
Test results will be given as a safe yield per 24 hours. If the figure given by the expert is 45 percent, it will mean that a specific amount of water can be continuously drawn from the borehole for just over 10 hours. Then it will take at least 13 hours for the aquifer to stabilize again before the next pumping session may commence.
Samples taken during the borehole testing must be sent to a laboratory for analysis to determine any safety standards. These days the contamination of borehole water is a very real threat, and can be a great health risk. That is why regular sampling and testing must be done on both existing and new boreholes, certainly at least every six months in an urban area and annually on a farm or smallholding.
Groundwater quality is one of the main factors restricting the development of available groundwater resources. Quality problems include high concentrations of total dissolved solids, nitrates and fluoride, which can be difficult and expensive to remedy.
When taking into account limitations such as groundwater quality, the potable groundwater exploitation of aquifers in South Africa is estimated at 14,8 billion m3/a, which declines to 12,6 billion m3/a during a drought. Nationally this represents almost a 30% reduction in the annual volumes of available groundwater for domestic supply due to water quality constraints.
ACKNOWLEDGEMENTS:
Borehole Water Association of Southern Africa on their book Groundwater – Guidelines for Boreholes with input from CSIR (Ematech),
Dept of Agriculture, Dept of Water Affairs and the Geological Society of South Africa.
Wikipedia,
Water Wheel and SA Country Life.
Congratulations to iNokia on the FTF!