Palynology, and heavy metals in the sediments and water of the Ondiri swamp, Kikuyu, Kenya: past influences, present practices, future needs.

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.