Journal of Environmental Geology

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Enrichment of trace elements in urban soils and sediments in a typical sedimentary environment; Onitsha metropolis southeastern Nigeria

Author(s): Timothy Iyobosa Asowata* and Akinade Shaddrach Olatunji

This study was carried out to evaluate the concentration, spatial distribution, and fate of potentially harmful elements in soils and sediments in Onitsha metropolis [N=162]. The samples were air-dried, sieved and digested with Aqua regia prior to elemental analysis using ICP-MS. Five [5] potentially harmful elements [Pb, Zn, As, Cd, and Mn] were selected for geoenvironmental assessment. A five-step sequential extraction was carried out to ascertain the fate of these selected elements in the soils and sediments. Elemental concentrations revealed that Pb ranged from 7.2 ppm-2026.0 ppm; Zn, 3.0 ppm-8395.0 ppm; As, 0.5 ppm -27.3 ppm, Cd, Undetected-10.3 ppm and Mn, 10.0 ppm -2127.0 ppm. Elevated concentrations of these elements were found in soils when compared to the sediments. The result also revealed that there was a marked variation between the metal concentrations in the topsoil compared to the sub-soil. The result of the sequential extraction showed that Zn and Cd were more in the carbonate bound fraction, while As were higher in the Organic bound fraction with Mn and Pb more in the hydroxide and residual fraction respectively. It was observed that the concentration of elements that were leached at the demineralized fractions was generally low in all the elements. The relative bioavailability of the metals can be summarized as Cd>Zn>As>Mn>Pb. Owing to the extremely toxic nature of the selected metals and the ease with which they could be leached to the environments where they could become extremely bio-available, it is important that a monitoring programme is instituted in order to avoid catastrophic consequences.

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