Impact of water pollution on Nairobi river, Kenya
Kinyanjui, Anne Wanjiru
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Nairobi River, like many other rivers in sub-Saharan Africa, is facing serious water pollution problems due to increased discharge of industrial, commercial and domestic effluents into the river system. Extensive literature review has shown that the effects of water pollution on microbial diversity of the river remain largely unknown. Samples were collected from different points along Nairobi River from the months of October 2005 to March 2006 with two aspects in mind: to assess physico-chemical and bacteriological characteristics, and macro invertebrates and algal species found within the river. The study was aimed at evaluating the impact of water pollution and bacteria on macro invertebrates and algae in the river. Samples collected were analyzed in the field and in the laboratory. Data obtained was subjected to SPSS statistical analysis. The highest turbidity levels of 11.00, 17.72 and 35.81 NTU and algal counts of 680, 690 and 947 organisms within the river were recorded at Ruai, Mwiki and Kamukunji sampling sites. The water here was dark coloured and murky. Nitrate levels recorded in all stations ranged between 0.031-0.068 mg/1 and did not exceed the specified USEPA, KBS and WHO limits. Naivasha Road and Museum Hill stations marked the beginning of pollution and as the river moved downstream, its status deteriorated even further. Total coliforms investigated along the river exceeded the KBS guidelines with Kamukunji, Mwiki and Ruai stations registering values of 30, 34 and 38 colonies respectively in stagnant waters. The presence of enteric bacteria, such as the coliforms was an indication of serious contamination of Nairobi River. Diversity of benthic organisms in Naivasha, Kamukunji, Mwiki and Ruai sites was low and was mostly dominated by pollution tolerant species from the orders Diptera, Oligochaeta and Hirudinea. Ruai station had the highest number of algae and those collected were 947 in total while the relatively clean water at Ondiri Swamp had the least number of 100 organisms. The increased presence of algae in the river was also an indicator of severe pollution. During the wet season, bacterial colonies in stagnant water increased due to total dissolved solids while those in flowing water were greatly influenced by turbidity. Algal diversity was determined by turbidity, air temperature, total phosphates, nitrates and biochemical oxygen demand. During the dry season, bacteria in stagnant water varied due to biochemical oxygen demand. Algal variation was due to biochemical oxygen demand and nitrate concentrations. It is concluded that field investigations coupled with laboratory analysis of samples obtained from the six sampling stations confirmed gross pollution of Nairobi River. Sustainable use and conservation of freshwater resources, water pollution prevention, institutional capacity building and awareness creation need to be promoted. Data derived from the study as a whole will be useful as it provides important baseline information required for prudent decision making on best development alternatives that incorporate environmental considerations. This data will also be useful to the Ministry of Environment and Mineral Resources, Ministry of Nairobi Metropolitan Development, National Environment Management Authority and the United Nations Environment Programme among other organizations in the country involved in the work of environmental protection and conservation.