Determination of the levels of some heavy metals in tree barks and dustfall among some streets of Nairobi city
Mungai, Njogu John
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This study determined the levels of some heavy metals (Cd, Pd, Zn, Mn and Cu) in dust dustfall and tree barks along some streets in Nairobi city centre and the industrial area. It further established the rates of total (wet + dry) deposition in the two areas. In total there were ninety sampling stations. Of these forty were for dust samples, forty for tree bark samples and ten for the dustfall. Half of the sampling stations were in city centre and the other half in the industrial area. All the samples were collected once a month from february to June 1996. The results were graphically represented based on the mean and monthly data across the stations. The standard deviations, pollution index and enrichment factors were also calculated. The finding of the study showed that Nairobi city and its industrial area are not highly polluted with respect to metals and the determined pollution levels are comparable to those from various countries in the world. The metal levels in all the samples were in the order Mn>Zn>Pb>Cu>Cd. The high levels of Mn were attributed to natural sources while the other metals were linked to an anthropogenic origin. The metals analyzed for in the study are related to traffic volume as was shown by the strong correlation coefficients among them. The mean rate of total (wet + dry) deposition was 81.8±86.0 gm-2 Mo-1. This is 9 times higher than the WHO limit of only 9gm-2 Mo-1. This suggests that the air in Nairobi is highly polluted with respect to dust, which pose a health risk. For the two major zones, industrial area and town center have mean values of 26.3±35.3 gm-2 Mo-1. Thus on average there is five times as much dust depositing per unit area in the city center compared to industrial area. For the tree barks the metal levels were in the range 18.3-2310.8, 17.3-1108.8, 8.7-124.1, 3.2-10.8 and 1119-4147.4 gg-1 for Zu, Pb, Cu and Mn. All these levels are higher than literature values for average land plants. Metal levels in dust are within the range levels reported in various countries. The mean values of Pb, Cd, Cu, Zn and Mn are 456.9±604.2, 7.7±1.5, 173.5±220.6, 691.7±692.2 and 3192.7±2596.8 ugg-1 respectively. There was positive correlation between the elements Mn, Zn, Cd, Cu and Pb. This suggests that these elements are either directly or indirectly from automobiles. This finding is supported by the high correlation values between these metals and traffic volume. The same metals are present in same order of Mn>Zn>Pb>Cu>Cd in tree barks. The three sample types (dust, tree barks, dustfall) showed a positive correlation of the metals analyzed. This supports that tree barks are effective biomonitors and hence a recommendation for their use in a full ecological evaluation of pollution.