INTRINSIC VULNERABILITY OF GROUND WATER POLLUTION OF
AQUIFERS IN NAIROBI AREA, KENYA
Nairobi city the capital of Kenya is located at approximately, 1° 9’S, 1° 28’S and
36° 4’E, 37° 10’E. The lithostratigraphy includes the metamorphic and igneous
rocks, as well as the inter-bedded lacustrine and alluvial sediments. Sub-surface
water has been encountered in aquifers of un-similar geological units in
succession. The aim of this project was to establish the distribution of surface and
sub-surface water, aquifers intrinsic vulnerability to pollution from anthropogenic
sources and come up with ground water vulnerability map. The acquired data was
analysed using internationally recognized software, i.e. GIS (Arc map 10.1), surfer
and Microsoft excel. The analysis was based on DRASTIC method of assessing
aquifer vulnerability to pollution.
The results show that the degree of ground water vulnerability to pollution is a
function of hydrological, geological and hydrogeological conditions, which show
that the effective protection provided by the natural environment may vary
drastically from place to place. The contamination level is determined by the
attenuation processes that occur within the zone between the contaminant source
and the aquifers, and the dilution effect of the aquifers themselves.
However, the class, mode of deposition, intensity and timespan of application of
the contaminants, as well as over-abstraction which leads to downward leakage of
ground water heads that has been contaminated, rendering the sub-surface water in
confined aquifers vulnerable. Results from the past analysis shows that some
boreholes within Nairobi indicate that contamination is evident for non- pathogenic
bacteria of faecal origin (coliform). This calls for spatial separation between
contaminants loading potential areas from the possible pollutant source areas.