Heavy metal pollution in suburban topsoil of Nyeri, Kapsabet, Voi, Ngong and Juja towns, in Kenya

The increasing industrialization and agricultural developments in suburban areas of Kenya, has increased the threat of heavy metal contamination in soil. Until now, there are limited studies evaluating metal pollution in suburban towns. This study, therefore, investigated the concentration, distribution and environmental risk of lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), zinc (Zn), mercury (Hg), arsenic (As) and cadmium (Cd) in five townships. Heavy metal source apportionment was examined using principal component analysis and validated using cluster analysis. The relationship between heavy metals was analyzed using Pearson's correlation test and the ecological risk index calculated to determine the degree of contamination. The concentrations of heavy metals were 1.65-99.16, 0.2-12.50, n.d. (not detected)-2.28, 0.59-17.22, 0.18-4.93, 0.17-1.55, 0.01-0.23 and n.d.-0.03 mg kg(-1) for Zn, Pb, As, Cu, Cr, Ni, Cd, and Hg, respectively. These values were within the toxicity limit of Tanzania soil guidelines. The results further demonstrated that natural and anthropogenic activities influenced the distribution of heavy metals. Correlation coefficients highlighted an association among heavy metals suggesting a similar origin. Pollution indices revealed that pollution decreased in the order Juja > Ngong > Kapsabet > Nyeri > Voi with the overall ecological index indicating a low level of pollution. The concentration of As and Pb were found to be high in all the study sites posing a potential hazard to the environment while Juja and Ngong regions were at a higher risk of threat. The inappropriate disposal of industrial and municipal effluents, agricultural practices, and burning of fossil fuels, was identified as the key causes of metal pollution.