Seasonal variations in heavy metal concentrations in soil and some selected crops at a landfill in Nigeria

Abstract

In this study, the seasonal variations in concentrations of the heavy metals - As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn - in soil and crops from a farm near the refuse dump site of Obafemi Awolowo University, Ile-Ife, Nigeria were investigated during the two major seasons of Nigeria. This was done to assess the pollution status of the farm and hence the safety levels of the crops produced. Soil and crop samples collected during the 2005/2006 rainy and dry seasons were treated and digested using standard wet digestion methods. Heavy metals were determined with Atomic Absorption Spectrophotometer (AAS). Analytical results of soil from landfill indicated that in the wet and dry seasons, values for Cd, Cu, Fe, Ni, Cr, Zn, Co and Pb were higher than normal levels of a typical agricultural soil, but As (3.20 and 4.13 mg/kg) was found to be within the acceptable range while Mn values of 597.00 - 828.37 mgkg^-1 were slightly above the usual background levels. The study showed highest concentrations of As (8.31 mg/kg), Cr (9.00 mg/kg) and Ni (40.00 mg/kg) in Manihot esculenta leaves; Cu (25.0 0 mg/kg) and Fe (176.00 mg/kg) in Xantosoma mafaffa tuber; Cd (14.50 mg/kg), Co (22.50 mg/kg), Mn (189.50 mg/kg), Pb (680.00 mg/kg) and Zn (440.59 mg/kg) in Talinum triangulare. In general, the levels of heavy metals in soil and crops were higher in the dry season than in the wet season, but this difference is not statistically significant. Particularly, the levels of As, Cd, Cr, Ni, and Pb were above the critical toxic level in plant leaves in both dry and wet seasons while Zn and Cu occurred at toxic levels only in the dry season.

Key words: Contamination, heavy metals, landfill, refuse dump, transfer factor.