Seasonal changes in the bioavailable forms of cadmium (Cd), copper (Cu) and lead (Pb) in waste dump soils of a
rapidly expanding city were assessed by sequential chemical extractions. Three major waste dumps at Apir
Mechanic Village (AMV), Benue Industrial Layout (BIL) and Integrated Waste Dump (IWD) all located in
Makurdi, north-central Nigeria, were selected for the study. Composite soil samples were collected from each
waste dump during dry (January) and wet (July) seasons in 2011; characterized and subjected to the modified
Tessier et al’s sequential extraction to separate the metals into operationally defined pools with varying
bioavailability – water-soluble (F1), exchangeable (F2), carbonate-bound (F3), Fe, Mn and Al oxide-bound (F4),
organic matter-bound (F5) and residual (F6) in soil. The soils were essentially mineral soils with sandy clay loam
texture, moderately contaminated with Cd (1.36 – 2.98 mg kg-1), Cu (2.70 – 10.86 mg kg-1) and Pb (32.26 – 74.56
mg kg-1). Metal forms and their partitioning in the soils varied markedly among the waste dumps possibly due to
different anthropogenic sources of these metals coupled with seasonal effects. The metals were more bioavailable
in the wet than dry season. Across the waste dumps metal bioavailability followed the order: BIL > IWD > AMV.
Depending on the metal, mobility factors, Mf were high enough and appeared to vary in the order: Cd (20 = Mf
(%) = 65) > Pb (37 = Mf (%) = 50) > Cu (7 = Mf (%) = 37) suggesting the potential long-term risk of these
elements (especially Cd) in the soils.