Characterizing trace elements in soils and rocks in the agricultural areas of Labunwa, Southwestern Nigeria

Abstract

Trace element (TE) concentrations in subsoils and underlying bedrocks of Labunwa, Odele area, were determined to ascertain the enrichment level and distribution of the TEs. Twenty-one subsoils (at 30 – 100 cm) and 13 rock samples were collected, pulverised and digested using aqua regia and total digestion, respectively. The samples were analysed using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The mean TE concentrations in ppm for the subsoil showed Cu (40.0), Pb (24.2), Zn (56.3), As (0.9) and U (2.7), among other elements, as against the mean concentrations of Granite gneiss (GGN) which showed Cu (22.9), Pb (61.4), Zn (64.6), As (1.0), U (2.6) and Pegmatite (PG), Cu (128.4), Pb (17.0), Zn (108.8), As (1.1), U (1.3) among other TEs. The relatively low concentrations of the TEs in the subsoil compared to the underlying parent rocks suggests that TEs in the subsoil may have been influenced by geogenic factors, such as weathering of the underlying rocks. TEs source apportionments in the subsoil using Bivariant plots, correlation coefficient, Bi-polar and dendogram analyses showed that these TEs are essentially from the underlying GGN and PG in the area. Pollution status indices, I-geo, Contamination factor and PI_N showed that the study area is practically unpolluted. These values suggest that locations with relatively higher concentrations of some TEs are probably due to local enrichment, and because most subsoil TEs are significantly lower in concentrations compared to the underlying bedrock, the area is safe for agricultural activities.