Microbiological survey of Athi river and its upstream tributaries

Abstract

River water is open to many polluting agents especially those waters which gain direct entry of discharges from urban centers. Some of these polluting agents at certain levels may be lethal to both flora and fauna that depend on such waters. Therefore there is increasingly high demand of frequent monitoring of such waters in order to avoid disasters of public health importance occurring to the users downstream. This study aims at determining the pollution status of Athi River in terms of both chemical and microbial pollutants. Microbial and chemical survey was conducted in 10 and 8 purposively selected sites (potential polluting sites) respectively in Ngong, Nairobi and Athi rivers to determine the levels of contamination by fecal Escherichia coli and trace chemical elements; lead (Pb), arsenic (Ar), chromium (Cr), and zinc, Zn). Day variation and seasonality during sampling of the water samples was a key factor establish the pollution levels in different times of the day and seasons. The recommended WHO procedures of water sampling and analyses were followed. Energy dispersive X-ray flourescence (EDXRF) analytical techniques was used in the determination of lead (Pb), chromium (Cr) zinc (Zn) and arsenic (Ar) levels dissolved in the water samples. High faecal Escherichia coli counts as high as 7.6 x 107/100mls of water were observed in all the samples from Ngong and Nairobi Rivers and at sampling site No. 10 in the main Athi River. In terms of chemical pollution, higher levels for lead, arsenic chromium of 0.217mg/1 and 0.05mg/1 respectively which are above the WHO recommended guidelines in drinking water were observed at certain sampling sites. Lead was constantly high in most of the points with an average of 0.06mg/1 in all sites. The WHO recommended guidelines for lead (Pb), chromium (Cr), and arsenic (Ar) in potable water are 0.01mg/1 0.05 mg/1 and 0.01mg/1 respectively. This study showed that both microbial and chemical pollution particularly lead, arsenic and chromium pose a pollution risk to Athi River thus endangering the health status of the people downstream. As far as day variations and seasonality differences of pollution levels were concerned, there was no significant statistical difference between seasons and day variations on chemical concentrations levels (Pb) P - 0.9829, (Ar) P - 0.2850, (Zn) P - 0.1192 but statistical difference was observed between sites for Chromium P - 0.0046 and P - 0.0043. For day variation still there was no pollution level difference Pb = 0.67270, Ar = 0.2908, Cr = 0.9299 and Zn = 0.2173. Slight differences on microbial pollution levels between seasons were observed with post rain period having the highest pollution levels in relation to dry and wet period. However, when the data was statistically tested, there was no significant difference among the three seasons P = 0.0087. However, significant differences were established between the sites P = 0.0053. This means that microbial pollution to Athi River is continuous irrespective of the day variation.