Source apportionment of potentially toxic elements in soils from an urbanising region: Insights from multivariate analysis in Singida, Tanzania

Abstract

• The study uses advanced non-linear pattern recognition for source identification. • Cu, Pb and Zn exceed contaminant thresholds in some samples. • The Co, Cr, Ba and V concentrations show wide variance in some samples. • PMF identified anthropogenic activities as the primary source of elevated PTEs. • PTEs distribution suggests influence from both geogenic and anthropogenic sources. This study evaluates the spatial distribution and geochemical characteristics of potentially toxic elements (PTEs) in soil samples across the Singida area, Central Tanzania, highlighting the environmental implications of rapid urbanisation and contributing to a deeper understanding of soil pollution in urbanising landscapes. A total of 1884 soil samples were analysed with an Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The results of the study show that the background concentrations of the PTEs exceeded their corresponding Upper Continental Crustal (UCC) values in this order; Pb (86.25 %) > Ba (65.23 %) > As (45.65 %) > Cr (15.92 %) > Zn (15.18 %) > V (8.60 %) > Co (7.86 %) > Cu (5.68 %). However, only Cu (17 samples), Pb (2 samples), and Zn (1 sample) reached contaminant thresholds of 200 mg/kg, 200 mg/kg and 150 mg/kg, respectively in some samples. Agricultural practices and soil conditions are possible explanations for the high Cu values, which may be combined with other factors. This study also found that the Co, Cr, Ba and V concentrations vary greatly and even in some samples exceed the recommended levels. The principal component analysis, hierarchical cluster analysis, self-organising maps and positive matrix factorisation analysis revealed two main clusters: Ba, Zn and Pb (Factor 1) and Co, Cu, As, Cr and V (Factor 2). Cluster 1 is more prominent across most of the area, particularly the south. Cluster 2 is shown to be more prominent in the Northern part of the area such as Sekenke, Shelui, Lambi, Mtinko and New Kiomboi. Due to the growing rate of urbanisation, these areas have become relatively populous and have a high level of anthropogenic activities, such as gold mining, sunflower oil milling and agricultural activities which have been shown in the study to influence the spatial patterns of PTEs in the area. The level of anthropogenic influence on the PTEs calls for remediation and educative measures to be implemented.