Imprints of wastewater discharge on trace element dynamics in the Grand River, Ontario

Discharge of treated wastewater effluent can be an important source of contaminants to downstream environments, but only a handful of specific effluent parameters are regulated and monitored in Canada. Consequently, the importance of effluent discharge for the surface water budgets of trace elements remains poorly understood. Here, we report concentrations of > 50 major and trace elements in > 30 riverine and effluent samples collected in the Grand River catchment, Ontario, in an attempt to assess imprints of effluent discharge on riverine trace element loads. We find that effluent-derived loads of major and trace elements generally outweigh those of tributaries when contrasted to their hydraulic contribution at the point of confluence. In particular, effluent-derived loads of conservative elements (> 30-fold the receiving riverine load), but also those of heavy metals and rare earth elements (> tenfold and twofold their receiving riverine loads, respectively) exerted important controls on trace element dynamics in the Grand River. Yet, multiple elemental tracers suggest that detectable imprints of these trace element inputs remain spatially restricted and limited to the catchment's upper reaches, urban areas, and confluences and effluent inputs with low mixing ratios. This study presents important baseline data for trace elements in this complex river system and highlights the need for expanded surface water quality monitoring to disentangle anthropogenic from natural factors affecting trace element budgets.