A river runs below it: Africa's hidden reserve

 

SCIENTISTS HAVE mapped the location of vast reservoirs of underground water across Africa that could be used to help counter the effects of climate change on the continent’s rural populations.

Over a two-year period, researchers from the British Geological Survey (BGS) and University College London gathered information from existing hydro-geological maps and 283 aquifer studies to produce a comprehensive chart of the continent’s forgotten underground reserve.

The study, funded by the UK’s department for international development, and released by the BGS last month, also attempts to quantify the amount of groundwater available across Africa for the first time.

The results show that the volume of groundwater is up to 100 times that found on Africa’s surface.

Groundwater is broadly defined as water that seeps beneath the soil into aquifers – bodies of rock and sand that contain sufficient saturated permeable material to store and transmit water – below the water table.

Under natural conditions, groundwater is moved by gravity through rocks and soil until it discharges into a streambed, lake, ocean, or a spring. It is replenished through a process called recharge, which is the addition of water to the saturated zone from the surface or adjacent aquifers.

Surprisingly, the greatest groundwater stores mapped by the study were beneath the Sahara desert in northern Africa, in the large sedimentary basins in Libya, Algeria and Chad. The amount stored in those basins is said to be equivalent to 75m thickness of water across that region.

But due to changes in climate that have turned the Sahara into a desert over hundreds of years, many of the aquifers have not been recharged with additional water for more than 5,000 years.

Dr Alan MacDonald of the BGS, lead author of the study, said that while some countries have been well mapped to date, others in Africa remain somewhat of a mystery when it comes to the exact amount of groundwater available.

“There was not a lot of data available across the middle of Africa. Places like the Democratic Republic of Congo and the Central African Republic are very under developed and have very difficult terrain, not to mention political instability.

“But the rest of the continent has had a lot of studies done on groundwater levels and we wanted to pool the information to try and get a quantitative idea of what is available,” he explained.

MacDonald maintained that, because groundwater is subterranean, it has rarely been part of the narrative when discussions take place about how ordinary people in Africa can tackle the negative effects of climate change. However, he believes that if aquifers are assessed individually by professional hydrologists, and accessed properly, they could act as a buffer to the droughts and floods associated with variable weather patterns brought on by climate change.

“Across much of Africa, you can find groundwater within 50m of the surface if you drill a bore hole. And what we have found is that bore holes that tap into this resource are still working during times of drought. The groundwater appears to be much more resilient to climate change than other water sources,” he said.

A UN report released late last year, Drinking Water Equity, Safety and Sustainability, said that access to improved drinking water sources in sub-Saharan Africa had significantly increased since 1990, reaching an additional 126 million people in urban and 111 million in rural areas.

But the gains have been negated by population growth to the extent that the number of people without access to drinking water was greater in 2008 than in 1990. It is estimated that more than 300 million people do not have access to safe drinking water.

MacDonald said that what the BGS study does not deal with are the quality and sustainability of the groundwater, and this cannot be ignored. While groundwater is often cleaner than surface water because its movement through the aquifer removes many impurities, there is not a huge amount of information in this regard for Africa, he says, so one needed to be cautious.

“There is the possibility that groundwater in Africa could be contaminated with arsenic and fluoride, so checks must be carried out. In terms of sustainability we have found that if people start taking a lot of groundwater for the irrigation of crops, that’s a game changer – commercial farming is unsustainable.

“High-yielding boreholes should not be developed without an understanding of the local groundwater conditions. But rural people in villages using low-yielding hand pumps to access the resource for their consumption will allow it time to replenish naturally.”

The fact that much of the data used by the BGS has come from existing studies raises the question of whether the more developed African countries have already been able to tap into the resource.

South Africa, like much of the continent, is water-stressed, and the BGS maps show it has low to moderate groundwater reserves in aquifers that would yield half to one litre of water per second.

Dr Shafick Adams, research manager at the Water Research Commission of South Africa, says that while groundwater may seem limited in South Africa overall, over the last couple of decades a number of local municipalities have begun to avail of it.

“In the past, groundwater was not perceived as an important resource and therefore been given limited attention in South Africa. However, in the last few decades, South Africa has done excellent work in the characterisation and assessment of its aquifers. A growing number of municipalities utilises groundwater as part of their bulk water supply on a regular basis and provide examples of successful management of this resource,” he said.

While Adams welcomed the BGS study, he warned it did not account for localised conditions, so could not be used for planning at a local or well-field scale.

“It is very important to contextualise the BGS study. Groundwater is not 100 times more all over and it does not mean that boreholes can be sited anywhere and that groundwater will be available. Finding sustainable groundwater resources requires a scientific approach with inputs from groundwater experts.”

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