The effects of land use types on the hydrology and water quality of the upper-athi river basin, Kenya
The Athi/Sabaki River basin harbours two of the major urban and industrial centres in Kenya, namely, Nairobi city on the upper catchment areas and Malindi in the south-eastern outlet ofthe basin. Nairobi city is the heart of the industrial production in Kenya. In the upper catchment areas (headwaters) land has been extensively used for urban settlement, transport and industrial activities while in the south-eastern parts agricultural production especially livestock keeping dominates. The study on land use changes focuses on the city of Nairobi and its environs drained mainly by Nairobi, Mathare and Ngong rivers. These streams drain areas of diverse land use activities. The land use changes in a spatial manner from the rich agricultural system through residential cum urban to industrial. The contribution of these land use activities to pollutants generation and hence water pollution and quality degradation is quite enormous. Water, river sediments and riverine vegetation from all the 10 sampling stations on the Ngong, Nairobi and Mathare Rivers were analysed to find the extent of water pollution and quality degradation downstream the rivers. The results of the study indicated a downstream increase in water pollutants and water quality degradation for the three rivers investigated. Water quality and hydrology of all the three rivers indicated a seasonal trend in variability. Water quality degradation was more physical during the rains (wet season) and chemical during the dry season. Nairobi River which drains most of the city centre and upper parts of the catchment had mean values of Total Suspended Sediments (TSS) of 158 mg r' (100lt year") at Muthangari and 256 mg r' (6317 t yr") at Outering Road Bridge after passing through the city and Gikomba areas. A TSS concentration value of33 mg r' (37 t year") was measured at Mutoine tributary (Ngong forest), 88 mg rl (188 t year") at Langata road bridge (downstream Nairobi dam) and 160 mg r' (1733 t year") after industrial areas at Embakasi in the Ngong River. In the Mathare River, TSS concentration of 71 mg r ' (1194 t year") was measured at Thika Road Bridge and 251 mg r' (2987 t year") at Outering Road after passing through the Mathare slums. Organic pollution detected was due to frequent sewer bursts and un¬sewered slum areas with a five day measured value of Biological Oxygen Demand (BOD5)and Chemical Oxygen Demand (COD) exhibiting an increasing trend in the three streams. A value of 7.8 mg r' and 123 mg r' in BOD5 were recorded in Nairobi River at Muthangari and Outering Road Bridge, respectively. Heavy metals were detected in water samples but most of them were found to be adsorbed in the river sediments. Concentration values of 1.0 mg r' and 0.1 mg r' for Zinc and Lead in water were measured but river sediments had the highest adsorption levels of 700 ppm (700 mg rl) and 51 ppm (51mg rl) for Zinc and Lead respectively. The dissolved metal ions in water appeared to surpass the recommended WHO and Kenya standards limits for drinking water. Heavy metal concentration in the tissue of riverine vegetation was found to be significant. Sphaeranthus napierae (spp 1), Commelina benghalensis (spp, 2) and Xanthium pungens (spp, 4) had the highest absorbed values of Zinc, Copper and Nickel and these varied with the part of the vegetation tissue. High Zinc concentration values were recorded in the root system with a value of 0.68 ppm recorded in sampling point Njiru 2 (10) for plant species 1 (Sphaeranthus napierae) and 0.66 ppm in species 2 (Commelina benghalensis) at Njiru 1 (9) and 0.55 ppm for species 4 (Xanthium pungens). In Mathare River, Zinc values of 0.85 ppm were recorded in the root system of species 1, 0.52 ppm in species 2 and 0.53 ppm in species 4. The stem and leaf had the least heavy metal concentration with some metal ions not being detected in the leaves. Generally, heavy metal concentration decreased upward the plant system (i.e.) from the roots to the leaf system. The study recommends the use of storm rain water (natural purification after heavy storm down fall) and Commelina benglensius, Sphaeranthus napierae and Xanthium pungens plant species in cleaning the heavily polluted river water and restore its aesthetic quality in the basins studied. Species 3 (Pennisetum purpureum-Napier grass) was commonly found along the river profiles where it is harvested as folder for cattle feeding in and around Nairobi. Generally, pollution and pollutant levels varied with season and distance away from the city in the three rivers. In addition, the streams were found to be less polluted chemically (less ionic concentration) away from the city due to dilution effect and self-purification ofthe river waters during the wet season. The study revealed that storm rainwater can be used to clean up the dirt, foul smelling and highly polluted waters of the rivers passing through the city. Riverine vegetation along the streams proved useful in adsorbing some of the pollutants especially heavy metals. In addition, strategic and well constructed ponds along the rivers would be useful in reducing the water quality degradation problem in the stream investigated.