Pollutant source identification in a Platinum Belt headwater catchment: a comparative geospatial assessment of dissolved and sediment-bound metals

Abstract

Land-use stressors contribute to trace metal accumulation in aquatic ecosystems, yet South African river health assessments rely solely on dissolved metal monitoring, overlooking sediment analysis as a potential complement. The study objectives were to (i) monitor wet and dry seasonal variation in dissolved and sediment-bound Cr and Zn concentrations, (ii) identify at which spatial scale each sample type best predicts trace metal concentrations as a function of land-use and geology, and (iii) evaluate concordance between sample types across 14 sites in the Gwathle River catchment, South Africa. Using inductively coupled plasma mass spectrometry (ICP-MS) for water samples and energy dispersive X-ray fluorescence (EDXRF) for sediments, seasonal analyses revealed significantly higher dissolved Cr in the dry season, while no significant differences were observed for dissolved Zn. In addition, there were no significant seasonal differences in either sediment-bound Cr or Zn. Statistical modelling revealed that dissolved and sediment-bound Cr concentrations were best predicted by cumulative-scale mining impacts, while dissolved Zn correlated with sub-basin-scale lithology, and sediment Zn with combined urban and lithological factors. Spatial concordance analysis showed substantial agreement between water and sediment Cr signals along a land-use intensification gradient, indicating its conservative geochemistry enables effective multi-media source tracking. In contrast, Zn showed negligible phase concordance, suggesting dissimilar dissolved versus sediment-bound behaviour. The strong performance of Cr across both media demonstrates that paired sampling comprehensively traces pollution along intensification gradients. As a complementary tool for monitoring trace metals in South African aquatic ecosystems, sediment analysis may offer a pragmatic screening approach for assessing metal contamination across catchments, particularly where capacity constraints exist. This approach enables rapid and cost-effective delineation of persistent catchment contamination and profiling of metal distribution patterns across large areas, providing valuable insights for water resource management.