Occurrence and risk assessment of heavy metals and organochlorine pesticides in surface soils, Central Kenya.

Abstract

The present study investigated pollution in surface soils of five dominant land use types in central Kenya. The mean concentration of heavy metals (Zn, Hg, Cd, Cu, Cr, As and Ni) in (mg/kg) and two organochlorine pesticides (DDTs and HCHs) (μg/kg) were determined. Heavy metal contamination and potential ecological risk using the Nemerow pollution index and Hakanson ecological risk index respectively were used in examining pollution level. With the help of multivariate analysis sources of heavy metal pollution were identified which were mainly from anthropogenic activities. Notably, heavy metal concentration in our study was compared to other regions within the country, results showed regional variation. Total levels of DDTs, HCHs pesticide and their constituent isomers in the surface soil were determined by a gas chromatography (GC-μECD). Pollution level in all land use areas according to Nemerow pollution index indicated low pollution status. Notably, in all land use areas the pollution level decreased in the following order Industrial land>Peri-urban>Agriculture land>Forest>River. In addition, heavy metals had low risks values according to Hakanson ecological risk index ranging from 0.01 to 0.58, with Hg having the highest mean value of 0.58. As expected, organochlorine pesticide were higher in agricultural land use, DDTs levels were comparatively higher than HCHs levels. Results on DDTs ratio (p, p'-DDT/p, p'-DDD + p, p'-DDE) were < 1 in all land use types which showed that their residues originated from historical sources. Lower α/ γ HCH ratio in forest and peri-urban land uses however indicated current input of lindane. Correlation analysis showed significant relationship between TOC and HCHs only. When compared to recent study done in agricultural soil (Nairobi surroundings), present study of OCPs (DDTs and HCHs) concentration in central Kenya was relatively high. The quality of soil in Central Kenya was classified as considerably polluted by OCPs but low polluted by heavy metals.