Distribution of Heavy Metals in Urban Soils; A Case Study of Calabar Area, South-Eastern Nigeria

Abstract

This work presents an attempt to establish the degree of anthropogenic influences of selected trace metals distribution in soils from Calabar area of south-eastern Nigeria. Fe, Mn, Pb, Cu, Zn, Cd, Co, Cr, As and Ni concentrations were determined for soils of different categories within the study area. Various geo-statistical and other techniques were used for assessment of the metal concentration in the Calabar urban soils. Geochemical data show the metal levels to be within background values and geo-accumulation index (I geo) confirms that these metals are unpolluted - moderately polluted, thereby posing no significant environmental impact. The slightly elevated concentration of Pb are attributable to emissions from automobile along motored roads, waste disposal from automobile repair shops and other forms of indiscriminate dumping of effluents within the commercial areas. Principal component analyses; correlation, factor and cluster analyses indicates dominant lithogenic control and subordinate anthropogenesis. Metal speciation reveals that the trace elements are unsaturated and bound in immobile phases with some occurring as free mobile ions. Geochemical mapping and techniques have in recent times found diverse applications in studies of urban environment using various sampling media. This has been geared by the need to identify contaminated land and subsequent health risk assessments. Soils are considered as a very important ecological crossroad in the landscape because it constitutes a recognizable repository for the transfer, retention and domiciliation of current or potentially toxic pollutants within the geo-system. The contents of these potential pollutants may be from natural sources but increases with time are related to anthropogenesis of urbanization. Rock weathering often mobilizes elements into the soil leading to distribution and dispersion along the geochemical cycle in the secondary environment. Contents of these metals in soils govern the composition of the elements in plants and animals with an attendant influence on human and animal health. Trace element distribution in soils depends on factors such as nature of the parent material, weathering processes, human activity and climatic conditions (Martinez et al., 2003). Trace metals such as As, Pd, Cd are characterized by long residual periods, high invisibility, little transfer, high toxicity and complexity of behaviours (Alloway, 1995). Trace metals, even the most potentially toxic ones can be essential