Assessment of potentially toxic metals and their mineral species in soils of arable farmlands in the southeastern Niger Delta basin, Nigeria

Soils and sediments have the capacity to accumulate metals, and this is usually controlled by their certain properties. The concentration levels of these metals may pose a threat to the ecosystem through the food chain. This research documents concentration levels of selected potentially toxic metals and their available chemical species in arable soils within parts of the Niger Delta basin in Nigeria. The metals of interest included Cu, Pb, Zn, Ni, Co, As, Cr, Cd, Mn and V. The metal concentrations assessed were those contained in clay sized fractions of the soils which constituted 13% of total soil fractions. Cation exchange capacity (CEC) mean value indicated most metals to be bound to sesquioxides rather than soil humus. Pearson's correlation analysis indicated that Cr and Cu concentrations are significantly controlled by organic contents and pH of the soils. The metals were within average crustal concentration levels, except Pb and Cd which revealed mean value concentrations above average soil and continental rocks. Metallic pollution assessments of the soils by calculations of enrichment factors and geo-accumulation indices indicated the metals to be generally of low enrichment, with slightly-moderately polluted classifications respectively. Some metals exhibited high enrichment in some samples. The concentrations of metals were determined to be related to the underlying geology of soil formation within the Niger Delta basin. Anthropogenic influences on soil chemistry were considered minimal. Geochemical modelling of extractable metal concentrations indicated that most metals occur dominantly as free mobile ions, oxides, hydroxides and other aqueous species and these were readily bio-available in the environment.