Improving the assessment of anthropogenic impact on trace elements in fluvial sediments by applying Positive Matrix Factorization

Chennai, a major industrial hub in southeastern India, faces significant environmental challenges, including trace metal pollution in fluvial sediments. Previous studies on sedimentary trace metal dynamics in Chennai have predominantly relied on enrichment relative to global means, as no local reference level is available. However, this approach may lead to misjudgment because (a) the underlying enrichment processes are not considered, and (b) local background concentrations may be lower than global averages, masking true enrichment signals. To address this limitation, Positive Matrix Factorization (PMF) was applied to identify the processes controlling elemental concentrations. The results were then compared to enrichment factors (EF) obtained by relying on global background values.

A total of 130 fluvial sediment samples were analyzed for their elemental composition using XRF and for grain size distribution using laser diffraction. Sediments were mostly dominated by sand, with significant shares of clayey material being limited to the upper reaches of the Adyar River. PMF differentiated several natural factors driving the trace element composition in the fluvial sediments, namely marine influence and the source lithology of the sediments, which was differentiated into Precambrian lithologies, and the fine and coarse fractions of the Phanerozoic lithologies, respectively. Cu, Zn, Pb, and Cr were confirmed as anthropogenically enriched elements by PMF analysis. However, high monsoonal discharges flush fine sediments from the river catchments, leaving them mainly sand-dominated, which limits storing capacities for trace metal emissions. Furthermore, PMF successfully differentiated distinct source lithologies, despite the complex sedimentary system of the region. In contrast, EF analysis identified SiO₂, Cl, TiO₂, Cr, Cu, Zn, Br, Y, Zr, Hf, and Pb as having elevated concentrations (median EF > 1). However, most of these elements are enriched compared to crustal backgrounds due to natural processes, as shown by the PMF results.