Geoscience Reference
In-Depth Information
Asia and Africa brought reliable rainfall to the seasonally wet margins of the tropical
deserts. Aquifers were replenished and groundwater levels rose, sometimes feeding
springs and small lakes. Formerly dry lakes refilled. The mobile late Pleistocene
dunes became vegetated and stable. Savanna woodland and grassland reoccupied
what are today the semi-arid regions of the world. Rivers flowed again in many parts
of the Sahara and Arabia. Countless small freshwater lakes supported communities
of Upper Palaeolithic/Late Stone Age hunter-fisher-gatherer people who left behind
their middens and hearths as enduring testimony to these brief episodes when the
deserts were green once again.
Some groundwater recharge also took place during previous interglacial periods.
However, because each interglacial episode was of relatively short duration, or roughly
one-tenth of each glacial-interglacial cycle during the past million or so years, the
duration of these recharge intervals was quite short. The consequence of all this is that
the groundwater in major aquifers, like the Great Artesian Basin in central Australia
or the Nubian Sandstone aquifers in North Africa, may be up to a million years old.
Current use of the fossil groundwater in Australia, Arabia and in some of the North
American deserts is well in excess of current recharge rates. In essence, we are now
mining a non-renewable resource.
26.7 Importance of avoiding pollution of water, soil and air in drylands
In the case of groundwater, it is not just the quantity used that matters. Equally
important is the actual quality of the water. Groundwater in the drier parts of northern
India and northern China is often severely polluted by industrial effluents and by
leakage from polluted rivers of toxic chemicals and untreated sewage. The same is
true of many other dryland regions in the world where environmental controls and
monitoring are inadequate or non-existent. The result is many hundreds of millions
of deaths from poisoning and from gastro-intestinal illnesses.
The abundant use of detergents and of fertilisers containing phosphates and nitrates
can lead to river pollution from cyanobacterial or algal blooms, particularly in turbid
desert rivers, where the seasonal river discharge can be sluggish. Algal blooms are
not confined to rivers but can be a hazard in coastal regions where the rivers carry
dissolved loads of nitrates and phosphates.
Perhaps the most widespread form of water pollution in arid areas is from salt. We
are only now starting to come to grips with the full complexity of how salt enters the
groundwater system and how it is transported from groundwater to rivers and soils
(Lawrie and Williams,
2004
). As long as evaporation greatly exceeds precipitation,
leaching of salt from the land surface will be difficult. It does not make sense to use
groundwater to cultivate crops on a large scale in fully arid areas where the potential
evaporation rates may amount to 5,000 mm/year or more, as at Kufra Oasis in south-
east Libya, given that all water contains some dissolved salts, and these salts will
