Increasing salinity has a whole ser

Increasing salinity has a whole series of consequences
that include a reduction in the availability of potable
water (for humans and/or their stock), deterioration
in soil structure, reduction in crop yields, and decay
of engineering structures and cultural treasures. It is,
therefore, a major environmental issue.
Many semi-arid and arid areas are, however, natur-
ally salty. By definition they are areas of substantial
water deficit where evapotranspiration exceeds pre-
cipitation. Thus, whereas in humid areas there is suf-
ficient water to percolate through the soil and to leach
soluble materials from the soil and the rocks into the
rivers and hence into the sea, in deserts this is not the
case. Salts therefore tend to accumulate. This tendency
is exacerbated by the fact that many desert areas are
characterized by closed drainage basins, which act
as terminal evaporative sumps for rivers.
The amount of natural salinity varies according to
numerous factors, one of which is the source of salts.
Some of the salts are brought into the deserts by rivers.
A second source of salts is the atmosphere – a source
that in the past has often been accorded insufficient
importance. Rainfall, coastal fogs, and dust storms all
transport significant quantities of soluble salts. Fur-
ther soluble salts may be derived from the weathering
and solution of bedrock. In the Middle East, for exam-
ple, there are extensive salt domes and evaporite beds
within the bedrock, which create locally high ground-
water and surface-water salinity levels. In other areas,
such as the Rift Valley of East Africa, volcanic rocks
may provide a large source of sodium carbonate to
groundwater, while elsewhere the rocks in which
groundwater occurs may contain salt because they are
themselves ancient desert sediments. Even in the ab-
sence of such localized sources of highly saline ground-
water it needs to be remembered that over a period
of time most rocks will provide soluble products to
groundwater, and in a closed hydrological system such
salts will eventually accumulate to significant levels.
A further source of salinity may be marine trans-
gressions. At times of higher sea levels, it has some-
times been proposed (see, e.g., Godbole, 1972) that
salts would have been laid down by the sea. Likewise
in coastal areas, salts in groundwater aquifers may be
contaminated by contact with seawater.