Geoscience Reference
In-Depth Information
Figure 25.14a
, low rainfall is subject to loss by high rates
of evaporation and evapotranspiration. The remainder
runs off the surface, evaporates at the surface or infiltrates
locally. There is some groundwater flow, and in coastal
areas fresh ground water is in equilibrium pressure with
saline ground water.
Groundwater quality and quantity
With precipitation so seasonal, the value of groundwater
supplies cannot be overestimated. The volume of water
stored in rocks depends on the percentage of empty
spaces, i.e. their
porosity
. Not all ground water is available,
however, and the proportion that can drain under gravity
is called the
specific yield
. Ground water is not stationary
but flows through the rock. The rate of flow depends upon
the porosity and the degree to which the parts are
interconnected, i.e. the permeability. Rocks which are
both porous and permeable, i.e. can both store water and
allow water to flow through them, are called aquifers.
Alluvial aquifers consisting of geologically young alluvial,
terrace or fan deposits are important aquifers and follow
all river valleys and deltas. Sedimentary rocks such as
sandstone and limestone tend to have smaller pores, but
fracturing and fissuring can contribute greatly to specific
yield. Permeable basalt can be an important aquifer
locally, but other igneous and metamorphic rocks such as
granites, schists and gneisses may have no primary
porosity but depend upon weathering to enlarge joints to
provide some secondary porosity and permeability.
Ta b l e
25.3
summarizes the porosity and permeability values of
typical Mediterranean aquifers.
In cool temperate regions, groundwater and surface
water sources both contribute to society's demand for
water. Although the precise balance will vary, on average
each is contributing about 50 per cent to supplies for
drinking, for industry and for agriculture. Ground water
contributes also to the surface flow of streams, because
they are partially fed by drainage from aquifers. Rivers also
increase in discharge from source to mouth as they receive
more surface run-off downstream.
In sub-humid Mediterranean environments, however,
the hydrological cycle is less continuous. Higher rates of
surface evaporation mean that smaller amounts of water
are available for underground storage. As illustrated in
Salt water intrusion
Pumping of the groundwater resource has drastic
consequences (
Figure 25.14b
). The groundwater level is
lowered as surface run-off is reduced, giving reduced
infiltration and recharge. Under natural conditions the
freshwater-saline water interface is at an equilibrium
position; sea water extends under the land not at sea
level but at a depth below sea level equal to about forty
times the height of the freshwater table above sea level.
Extracting ground water from coastal aquifers therefore
not only lowers water tables but also causes an intrusion
of sea water inland at depth. This extension of saline
ground water further into the aquifer creates a serious
problem, and causes salt water to appear in wells,
degrading crops and soil over large areas. Furthermore the
recycling of irrigation water through salty soils continually
increases the amount of dissolved solids in the ground
water.
There are several important aquifers around the
Mediterranean, and many regions are dependent upon
them for their supplies of drinking water, and supplies for
industry and irrigation.
Table 25.4
shows the importance
of groundwater as part of the total water resources of the
Mediterranean part of four countries. Spain stands out as
a country where the annual consumption of water is
higher than the total groundwater resource. Increased
pumping from an aquifer will cause a lowering of the
groundwater level around the point of abstraction, to
cause a 'cone of dejection' in the water table. Upconing of
saline water occurs below boreholes, owing to pumping.
In the worst cases of massive overpumping from a large
number of wells a regional reversal of hydraulic gradients
takes place. The process is slow but virtually irreversible,
and once an aquifer has been invaded by saline water it is
extremely difficult to restore the quality of fresh water.
A salt-water aquifer has no value for irrigation or
potable supplies. Serious intrusion has been reported
from all along the Spanish Mediterranean coastline
(especially the Campo de Dalias in Almeríia), from Italy
(Sardinia), from Greece (Argolides), from France
(Roussillon and Var), from Lebanon and from Libya
(Tripolitania). Control of seawater intrusion can be
achieved by moving wells inland and/or reducing
Porosity
Permeability
(m day
-1
)
Type
(%)
Shallow alluvium
30-40
10-1,000
Sandstone
10-30
0·1-10
Limestone
5-30
0·1-50
Karstic limestone
5-25
100-10,000
Basalt
2-15
0·1-1,000
Fresh igneous/metamorphic
2
0·000001
Weathered igneous/metamorphic
10-20
0·1-2
