Going underground for water

The total volume of water in aquifers is more than 100 times the amount found on the surface and 20 times the fresh water stored in African lakes.

The World Economic Forum in Africa has already flagged water security as one of the fastest-growing social, political and economic challenges today. (Image: Flickr)

Wilma Den Hartigh

Many African countries have always been considered water scarce, but new research findings suggest that the continent isn’t as dry as previously thought. Scientists have for the first time produced the most detailed maps yet which show that Africa has vast reserves of groundwater, hidden in aquifers under the surface.

This incredible discovery by researchers from the British Geological Survey (BGS) and University College London is good news for a continent that is considered one of the most vulnerable regions in the world to global warming and climate variability.

The new report, Quantitative maps of groundwater resources in Africa, published in the environmental research letters journal on IOP science online, comes at a time when water security in Africa has become a talking point.

According to the paper, the maps indicate that the total volume of water in aquifers is more than 100 times the amount found on the surface and 20 times the fresh water stored in African lakes.

This is a considerable water reserve – but what does it mean for people who live in water-scarce regions in Africa?

A greater focus on water security for Africa

The report helps us to understand more about nature’s way of storing water in aquifers – these are underground layers of porous rock that act like large sponges, allowing water to infiltrate the soil through pores and cracks. The water held in aquifers is known as groundwater.

The new maps offer an accurate assessment of groundwater reserves, which are an important buffer to climate variability and change on the continent.

Dr Anthony Turton, a South African scientist specialising in water resource management, agrees that there is an emerging trend in the global water sector to focus on groundwater resources.

“The aquifers are a remarkable discovery and it is good news for South Africa,” Turton says. “If we can get the water out, it could substantially increase water availability on the continent,” he says.

But Turton also has another idea – he thinks that the aquifers present great opportunities not only for water extraction, but also storage.

“It is a good water security solution,” he says. “Instead of only seeing aquifers as a resource to pump water out of, we should pump water into them,” he says.

Whether the problem is one of too little water over long periods of time, or too much water at once, the World Economic Forum on Africa has already flagged water security as one of the fastest-growing social, political and economic challenges today.

Analysts predict that the demand for water in all sectors will increase. It is up to decision makers and water experts to find solutions to deal with the 40% global shortfall between expected demand and available supply by 2030.

More than 300-million people in Africa don’t have access to safe drinking water and greater access to improved water supplies has become an international priority.

Increasing the number of hectares under irrigation is another pressing issue. According to the report, only 5% of arable land is irrigated. There is a need to increase this figure to help meet the rising demands for food production to combat growing food insecurity.

Where are the aquifers?

The origin of the maps is an interesting story involving earth- penetrating radar, satellite cameras and the search for oil reserves.

“Satellites have the capacity to map the earth to identify oil reserves,” Turton explains. “As the satellites detect oil in underground reservoirs, over the years they also pick up the presence of fresh water aquifers.”

Drawing up the new maps was a major feat as areas of North, Central and West Africa didn’t have good quality maps. Where available, the new aquifer maps are based on an extensive review of available hydro-geological maps, but scientists also had to consult many individual studies.

According to Helen Bonsor from the BGS, one of the authors of the paper, the greatest groundwater volumes are in large sedimentary aquifers in North African countries such as Libya, Algeria, Chad, Egypt and Sudan.

Turton says South Africa and the SADC region also have large hidden underground water reserves. “There are about 12 to 15 aquifer systems, of which three are considered very important for the future.”

“In South Africa a very substantial resource lies in a massive dolomite aquifer system east of Johannesburg in Gauteng,” he says.

This aquifer covers a vast area, extending from Springs and Brakpan east of Johannesburg; to Lenasia south of the city; Zuurbekom, Carltonville and Magaliesberg on the West Rand; Kuruman in the Northern Cape and even as far as parts of Botswana.

“To give you an idea of the size, the Witwatersrand mining basin’s aquifer storage capacity is about the size of Lake Kariba,” he says.

Other countries such as Mozambique, Zambia, Zimbabwe, Namibia, Botswana and Ghana also have hidden aquifers.

Getting the water out

Although there is a lot of water underground the next big question is how to get it out, and if it is economically viable to extract it.

The BGS paper points out that in certain areas, appropriately sited and developed boreholes for low-yielding rural water supply and hand pumps could be successful. However, the paper also warns that the potential for higher-yielding boreholes might not be possible.

Extracting the water could also be very costly. “In many cases the water will have to be pulled up from a fairly great depth,” Turton says.

Dr Alan MacDonald, lead author of the study, told BBC News that high-yielding boreholes should not be developed without a good understanding of local groundwater conditions.

Large parts of Africa don’t receive rain very often and this means that many aquifers are not filled regularly. Scientists are concerned that aquifer water resources might be depleted if large-scale development of boreholes goes ahead.

Turton points out that the logistics of mass extraction of water from aquifers is a big challenge. “So much cabling, piping and drilling will be needed, which will increase the cost of getting the water out.”

However, he says large-scale drilling becomes more viable if the rock is very porous and has good transmissivity, a measure of the quantity of water that an aquifer can transmit horizontally.

Transmissivity is also used to determine the water that an aquifer can deliver to a pumping well.

“Highly transmissive systems are very suitable for exploitation,” he says.

Aquifers for water storage

Turton believes that extracting the water for use in hand pumps might not be the best option. “The true value of aquifers lie in their use for future storage of water,” he says.

Underground storage is an efficient way to store water. The resource won’t be vulnerable to evaporation losses and it is fairly safe from contamination. Water found in aquifers is often cleaner than surface reserves because the permeable rock layers act as natural filters that purify water by removing impurities.

Turton says in Africa the conversion ratio of rainfall to runoff is very low and a large amount of water is lost to evaporation.

At continental level, the conversion rate is only 20% and in South Africa, the figure is even lower, between eight and 10%. In two of South Africa’s most important river basins, the Limpopo and Orange, the ratio is only 5%.

“This has to be improved and aquifers are an excellent opportunity for this,” he says.

The challenge from an engineering perspective is to find new ways to use ground water aquifers as underground storage dams.

“Instead of storing water in dams above ground, water can be treated and stored underground to prevent large-scale evaporation. Building dams isn’t the only way to go,” he says.

“It is possible to store water 200 to 300 metres underground for 25 years or even longer.”

The idea presents many engineering challenges, but it is not impossible.

A few years ago, the Department of Water Affairs in cooperation with the Water Research Commission, the Council for Scientific and Industrial Research, and Groundwater Africa, released a document about the possibility of developing a national artificial recharge strategy as part of the country’s water management planning.

Artificial recharge (AR) is the process whereby surface water is transferred underground via boreholes and infiltration basis to be stored in an aquifer.

When needed, the water can be pumped from the aquifers via boreholes to users. Locally, AR is an ideal way to promote water conservation.

Turton says that South Africa can learn from Botswana’s example. The ministry of water in that country built many smaller dams on a river system to capture water in peak periods. The dams are designed to leak, allowing the water to seep naturally into the ground.

“This type of development could attract investment into the country and we need more investment in the water sector to attract the technology we need to do this,” he explains.

Source: Department of Water and Sanitation

Would you like to use this article in your publication or on your website? See Using Brand South Africa material.