| dc.description.abstract | Ondiri swamp is located 200 metres south of Kikuyu town, 20km west of Nairobi City
center. Presently the area around the swamp has a high population density of 500 persons
per square kilometre. The swamp is the source of Nairobi River, and it is also a source of
water for irrigation for farmers around the swamp. 70% of the people around the swamp
are involved in agricultural activities as a means of earning a living.
The present study aimed at establishing the vegetation changes in the swamp catchment
and determining the level of heavy metals in the swamp water and sediments. This was
done to establish the swamps sustainability vis-a-vis past and present landuse practices
around the swamp. The objectives were achieved through raising of a lO'Scm-scdimcnt
core and determining its palynology. The changes in pollen taxa composition were
inferred to represent vegetation change through time. A total of three bottom sediment
samples and six water samples from the swamp were analyzed for heavy metals zinc,
copper, lead, and cadmium, with the use of the Atomic Absorption Spectrometry (A.A.S).
Interviews were also conducted with the locals to establish the value of the swamp to the
surrounding community and observed changes in the swamp.
The results indicate that at the bottom of the core the catchment area had already
undergone major vegetation changes. The lower section of the core (105-75cm) is
dominated by herbaceous' taxa Asteracea, Pliyllanthus, Amari Cheno, all indicative of
disturbance. Exotic taxa appear at 75cm depth and Eucalyptus and Cupressus dominate
them. A drought event is inferred at depths between 70-65cm. Between 65clll and the top
of the core no major vegetation changes are noted. The presence of Podocarpus ami
Olea, in the core is interpreted as a result of long distant transport to the swamp site. No
heavy metals were. detected in the water column, but the bottom sediments had
appreciable concentrations of zinc, copper, and lead, with average concentrations of
91 ppm, 6 ppm and 20 prm respectively. Cadmium remained below detection limits with
the equipment of analysis. The source of the heavy metals is hypothesized to be
geologically controlled. The soil, which has weathered from the local lithology, is the
'carrier' of the heavy metals. Anthropogenic contribution is limited to enhancing of
. erosion rates which leads to higher sedimentation rates in the swamp .
.The agricultural activities around the swamp lead to high rates of erosion. Use of
fertilizers and pesticides is common. These practices threaten the existence of the swamp,
which is an important resource to the community around it and downstream Nairobi
River. The locals have noted already changes on the swamp including more growth of
macrophytes, lowering of swamp water level, and drying up of small springs along the
swamp banks.
Conservation strategies suggested in the present study include erosion and runoff control, conservation education, organic farming, and creationof buffer zones. | en |
