Interesting. But when there is no replenishment of the groundwater then it is not likely to be sustainable to tap into it, or?

See also this part in the article:

However, the scientists are cautious about the best way of accessing these hidden resources. They suggest that widespread drilling of large boreholes might not work.

Dr Alan MacDonald of the BGS, lead author of the study, told the BBC: "High-yielding boreholes should not be developed without a thorough understanding of the local groundwater conditions.

"Appropriately sited and developed boreholes for low yielding rural water supply and hand pumps are likely to be successful."

With many aquifers not being filled due to a lack of rain, the scientists are worried that large-scale borehole developments could rapidly deplete the resource.

BBC link
www.bbc.co.uk/news/science-environment-17775211

MacDonald et al 2012 Quantitative maps of groundwater resources in Africa_Environ Res Lett 7.pdf[/attachment]