The oceans contain 97% of the earth’s water and of the remaining 3%, which is freshwater, 2% is locked up in glaciers and the polar ice caps, leaving 1% in rivers, lakes, streams, wetlands and groundwater. This 1% is the amount of water available to a global population, which is growing at a rate that puts increasing pressure on the earth’s freshwater resources.
Jordaan Fouché, a consulting geologist from Cape Town firm GeoWaterEx and who specialises in groundwater exploration, says the impact of climate change on the weather patterns has profound consequences for the planet’s people.
“The recent extreme water restrictions in Cape Town, imposed during the Western Cape’s worst drought in living memory, is a good example. Apart from the 50 litres per person per day allocated to human use, the water restrictions had wide-ranging consequences for the agricultural sector,” says Jordaan.
Drilling for water
One consequence of the water restrictions was large-scale borehole drilling by farmers trying to keep their production going.
“As a geologist, I am dumbfounded at the hit-and-miss way some of these borehole drilling sites are selected. I really think applying geological knowledge and using scientific equipment is a more effective way of locating underground water than some of these random methods,” Jordaan says.
Although there are people who believe that groundwater can be located using apparatus such as sticks, bent wires and bottles, Jordaan focuses on scientific methods.
“I look at the geological and geophysical data, using satellite photography and gravitational and magnetometric maps to identify areas favourable for groundwater production,” he adds.
Underground water cycles
Jordaan explains that groundwater is the water that saturates pores and fractures in rock and soil in sub-surface aquifers. The aquifers are replenished by water from precipitation (rain, snow, etc.) that lands on the surface and infiltrates into the groundwater system, restoring the water table.
An aquifer is an underground layer of water-bearing permeable rock, rock fractures or unconsolidated material such as gravel, sand or silt.
“It can take a long time to restore water to an underground system once it has been extracted from the aquifer. The judicious use of an aquifer requires prudent management to ensure that it stays productive in the long term.”
South Africa’s groundwater resources currently make up 15% of the total volume of water used nationally, of which about 64% is used for agricultural purposes and 8% by mining and households.
Threats to groundwater supply
Jordaan feels there is no immediate cause for concern that groundwater will run out, but there are some key factors to bear in mind.
“Pollution of underground water is a far greater threat right now than the threat of the aquifers drying up. A good example of this type of pollution is to be found in the underground water of the Cape Flats; this water is so contaminated by seepage from pit toilets, among other things, that it has become completely unusable.
“The Malmesbury shale of the Swartland is another example. The shale, high in salts and subject to decades of land cultivation and fertilisation, among other things, is liberally marked by salt pans. These salt pans are literally brine – as salty as the Dead Sea.”
Protecting the resource
This phenomenon is not specific to South Africa but occurs on a global scale as human practices render groundwater sources completely unusable. “But it’s also true that the current threat is simply due to a lack of rain. If there’s not enough rain, the groundwater is not sufficiently replenished.
“If climate change causes a decline in rainfall, it becomes imperative that we adapt our practices to prevent the disappearance of our groundwater resources. The less run-off there is, and the more water penetrates the soil like a sponge, the better. For this to happen, good plant cover, especially the cover of natural vegetation, is essential.”
Groundwater site location
Jordaan says a range of geophysical methods are used to identify areas where there is a high probability of finding good groundwater. Geologists study geological maps and aerial photographs to identify changes in the earth’s crust and rock formations that favour groundwater storage.
“We are looking for porous and broken rocks. We also look at drainage patterns and, based on this, try to identify low points into which water will flow, driven by gravity. In the case of dolerite, we typically look for a rocky structure that is not porous and acts as an underground water barrage.”
Afterwards, they visit the site and use geophysical methods for a more detailed investigation.
Jordaan explains that the earth has various features that help the geologist. These features include the earth’s ability to transmit sound, the electric and magnetic fields and gravity.
“At school we learned that the earth’s magnetic field and its gravity are uniform. Well, that’s not true. Certain factors can affect both these fields. These deviations are called anomalies. For example, if you take a magnetic recording over a basalt formation or a dolerite dyke, you will find a magnetic anomaly.
“Similarly, one finds gravitational anomalies in places where large bodies of water accumulate, such as the Okavango and the Niger Delta. In these areas, the gravitational force is stronger than in areas where there are no water bodies. Gravitational fields are also stronger in the rainy season than in the dry season.
“These earthly phenomena make it possible for us to use gravitational meters and magnetometers to detect the presence of water.
“Resistivity indicates the earth’s ability to transmit electricity. We know that water has high conductivity. Using resistivity meters gives us a very accurate indication of the presence of water.”
Apart from the earth’s ability to transmit sound, it can also transmit electromagnetic signals. Among other things, these signals can be passively generated by thunderstorms, sunspots or earthquakes. These events cause electromagnetic explosions duringwhich high-frequency signals are eliminated, while low-frequency signals are more easily transmitted.
“This is how one detects geological structures, such as underground channels, dolerite dykes, barrages, changes in porosity and fractures below the surface. Once this data has been interpreted, you can determine where to drill.”
The various methods, says Jordaan, complement one another and help geologists build up reliable information from which they can draw sound conclusions.
“I think gravity provides the best indication of where to look, while electromagnetics tells us how deep to drill. But really, there is no single infallible method because selection is ultimately based on the interpretation of the data.” – Izak Hofmeyr, Stockfarm