Application of Geoaccummulation Index on the Assessment of Heavy Metal Pollution of Soils in Selected Urban Farms in Nairobi County

Heavy metal toxicity has proven to be a major threat due to health risks associated with it. The heavy metals, do not have any biological role, but are usually accumulated in the tissues to harmful levels. This study was conducted to access the contamination levels of selected heavy metals in agricultural and nonagricultural soils from several urban areas in Nairobi County. The soil samples were collected in triplicate sand made into a composite sample from each site. The sites from which the samples were collected included: Uthiru, Kibera, Dandora and Molem. The soil samples were air dried and ground to pass through a 2 mm sieve and were later subjected to wet digestion. The digested samples were filtered through What man No 1 filter paper, washed with distilled water and the filtrates analyzed using Atomic Absorption Spectrophotometer. The metals analyzed were: Pb, Co, Cr, Zn, Cu and Cd. The results showed that the concentration levels of the heavy metals in the farming sites (agricultural soils) were between 0.075 to 85.5 mgkg-1whereas in the non-farming sites (non-agricultural soils) the range was 0.05 to 61.875 mgkg-1. Cadmium was not detected in any of the sites whereas lead was only detected in Kibera and Dandora (in both farming and non-farming sites). In order to determine the contamination levels of the heavy metals in the sites, geoaccummulation index (Igeo) was calculated for each of the heavy metal. Geoaccummulation index is a factor that is used to measure the degree of contamination of the soil by heavy metals.  Based on the standards for Igeo values, the contamination levels of Cu which had Igeo of 4-8, was heavily to extremely contaminated in all the sites, Zn (Igeo of 2-4) was moderately contaminated in all the sites and Cr (Igeo 7) was found to be extremely contaminated in the all the sites. Co was only detected in Uthiru farm (Igeo 6) indicating that its heavily contaminated.  Pb was detected in Kibera (Igeo -6) and Dandora (Igeo -5) indicating that these soils were uncontaminated with lead. The study showed that Dandora had the highest levels of contamination of Cu (Igeo 8), Zn (Igeo 4) and Cr (Igeo 8). The heavy metal pollution in the Dandora site could be attributed to the anthropogenic activities. The activities observed near the Dandora farming site included a car wash and a dumpsite.

 Key words: Heavy metals, urban agricultural soils, Nairobi County, geoaccummulation index

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