Magazine article UNESCO Courier

Mining Fossil Water

Magazine article UNESCO Courier

Mining Fossil Water

Article excerpt

Mining fossil water

FAR below the surface of the earth, vast quantities of life-giving water have lain concealed for thousands of years in huge aquifers (layers of water-bearing porous rock) formed in the extensive sedimentary basins that exist in every continent.

Most of these aquifers were first discovered and exploited during the nine-teenth century, but, in some countries, it is only in the last few decades that intensive use has been made of them.

In arid areas such reservoirs have, of course, become of outstanding importance and often represent the sole permanent source of water. Fortunately reservoirs of this kind are to be found in the subsoil of several countries in arid areas, including, in particular, the huge desert area which stretches from the Atlantic Ocean to the Gulf, covering the north of the African continent (the Sahara) and the Arabian peninsula (see map).

Naturally enough, the primary concern in these countries was to improve prospection and exploitation techniques. More and more wells were drilled and water production was often substantially increased in each basin.

This increased exploitation had the predictable effect of reducing water levels and in some cases the quality of the water drawn deteriorated. It soon became evident that what was needed was to establish a strategy for the long-term exploitation of each basin as a whole.

Research carried out in a number of countries soon showed that the water confined in these deep aquifers represents a non-renewable resource. There was clearly a need for a special conception of water resources and special procedures for evaluating them. A model example of this type of approach is Unesco's Study Project on Water Resources in the Northern Sahara.

In absolute terms hardly any ground-water exists completely independently of the natural water cycle. However, water moves at very different speeds in different aquifer layers, and in addition the distances travelled may vary greatly. Where water has to cover hundreds or even thousands of kilometres at speeds of the order of several metres a year, it may stay in the subsoil for periods of up to tens of thousands of years.

This does not mean that this "fossil' water, as hydrologists call it, is stagnant or that there is no renewal of water in these very extensive deep aquifers. It is simply that the renewal is very very slow.

It is also, strictly, incorrect to call this water a non-renewable resource; it is only non-renewable in relation to its use on the human time-scale. On the larger scale of the natural water cycle, water drawn is never lost, but is merely transferred and modified.

However, the local structure that provides the resource may, through intensive exploitation, undergo irreversible changes such as the compaction of the soil or an intrusion of salt water. Thus harnessing the water resources of one of these aquifers amounts to drawing on the water reserve capital and, by analogy with the extraction of mineral ores, it can rightly be described as the "mining' of groundwater.

"Water mines', productive drilling fields exploting deep aquifers on the basis of withdrawal from storage, are in operation in various countries in the arid zone where they account for the bulk of water production. …

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