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.

Loads and elimination of trace elements in wastewater in the Great Lakes basin.

The growing use of trace elements in industrialized societies is driving an increase in the occurrence of trace elements in anthropogenic waste streams globally. Yet, the large-scale sources of many trace elements to wastewater and their elimination during treatment remain poorly understood and potential environmental impacts on freshwater systems therefore unclear. We screened 42 wastewater treatment facilities in the North American Great Lakes basin and deployed a black-box approach to calculate representative estimates for average per-capita trace element loads and basin-scale effluent discharge rates, as well as trace element removal efficiencies across different treatment technologies. Our results show different removal of specific groups of trace elements during wastewater treatment: average removal efficiencies were 25% for alkali metals, 50% for alkaline earth metals, 74% for transition metals, and 85% for rare earth elements. Higher elimination of the majority of trace elements was generally achieved by more advanced, tertiary treatment types. Elemental loads generally followed natural abundance patterns, but anomalous loading rates were observed for various trace elements across the sampled facilities. By examining geospatial attributes of the sampled sewersheds, trends in select trace element loads were qualitatively tied to possible point sources and diffuse sources. Overall, these results illustrate the potential of wastewater surveillance to inform environmental management of emerging trace element contaminants.