GEO Brasil - UNEP

consolidated The dominant aquifers
in these areas are confined, and are
currently drawn through deep local
tubular wells (30, 50, 100, 150, 250 up
to 3,000 m, mainly) and occasionally
artesian wells The waters of these
aquifers are relatively protected
against domestic, industrial and
agricultural pollutants, with intensive
use of modern chemical fertilizers, and
good quality for domestic, industrial
consumption and irrigation In these
basins one may find the largest
underground water potentials in Brazil
in terms of reservoir or recharge,
resulting in the perennial situation of
rivers which drain these areas
amount of rain in the week equals the average for a whole month, or the
amount of rain in one month equals the average for a 6-7 month period
However, the normal average evaporation rate varies between more than
1,000 to 3,000 mm/year Therefore, the correct, in hydrological average terms,
is not to say that there is little rain in the semi-arid north-eastern Brazil, but
that it evaporates a lot
As a result, the gradual efficient use of the water available in that area has
become a practice of fundamental importance in order to reach sustainable
development conditions
It is important to emphasize that under humid tropical climate conditions –
pluviometer average from 800 mm/year to more than 3000 mm/year, chemical
processes of rock alterations are predominant and intense As a consequence, a
rock mantle is altered with thickness of up to 150 meters (50 m average), which
covers almost four (4) million km2 of the Brazilian territory In this scenery, porosity
values and permeability of the interstitial type of altered rocky mantle increase
depth gradually , being Sy = 5-15 percent and K = 10-4 and 10-5 m/s when in
contact with the healthy rock (Rebouças, 1988)
the state of the environment in Brazil
In the extensive rocky domain of
geological base and Pre-Cambrian age,
there are two hydro- geological
different contests:
a) In the 600,000 km2 of crystalline
lands or similar Pre-Cambrian
age, the dominant porosity /
permeability characteristics
present fissure type The most
promising possibilities of
underground water accumulation
are restricted to areas of fractured
rocks and to alluvial patches,
which are formed along the main
rivers which drain the areas where
these rocks appear
b) In the semi-arid North-eastern
Brazil, the experience indicates
that the most important “water
entrances” are crossed, in
general, up to 60 meter depths
(Rebouças, 2001) In addition, in
that area the normal average of
rain varies between 400 to 800
mm/year However, rain is
irregular, and sometimes the
In general, the characteristics of dominant porosities / permeability of the
interstitial type in the alteration mantle in this area develop to a fissure, in depths
of up to 250 meters
The annual recharge rates of accumulated underground waters in the alteration
mantle, are mainly sufficiently important to supply their rivers‘ basic drainage
during droughts
Finally, there is the karstic province which corresponds to the geological domains
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