Geochemical Thresholds for a Suite of Elements in Soil from Tsofon Birnin- Gwari area, Nigeria: Implication for Exploration and Environmental Studies.

Abstract

The geochemical threshold values for a suite of elements in residual soil of the Tsofon Birnin-Gwari area, NW Nigeria were computed using two statistical methods. This area is within the Nigerian Schist Belts and is famous for artisanal and small-scale gold mining. The concentration of 17 trace elements in 32 samples was determined using wavelength dispersive X-ray fluorescence spectrometry (WD-XRF). The data was subsequently used to compute threshold concentrations for each of the elements using (i) mean+2SD and (ii) median+2MAD methods. Before the treatments, the data was smoothened by using ½DL for samples with element concentration below the detection limit. The results show that for all the elements, the mean+SD method provided higher threshold values compared to the median+2MAD approach. This may be attributed to the fact that the mean is likely to be influenced by a few positively skewed data points compared with the median. The standard deviation was generally high for all the elements, indicating wide dispersion around the mean. Accordingly, the threshold values from the first method show that nearly all element concentrations are at the background level, without anomalous values. However, based on the second method, some of the elements were found to have concentrations significantly above the upper limit of background fluctuation. For practical applications in exploration and environmental studies, it is concluded that the median+2MAD approach is more robust at estimating threshold values. However, other methods such as graphical and fractal analysis may be considered as potential alternatives for this purpose