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 sedimentbound 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.