A multi-tracer approach to disentangle anthropogenic emissions from natural processes in the St. Lawrence River and Estuary

The ability to differentiate anthropogenic signatures from natural processes in complex hydrological systems is critical for environmental regulation perspectives, especially to curb pollution and implement effective water management strategies. Here, we report variations in the concentrations of 57 chemical variables, including nutrients, major, trace and ultra-trace elements, as well as the concentrations of Escherichia coli in different water masses along the St. Lawrence River-Estuary continuum. The constant ratios among major elements indicate consistent carbonate and silicate weathering processes in the drainage basins. We also suggest applying Ce anomalies to trace waters of low alkalinity and low complexing capacity as the dominance of Ce3+ free ion could promote Ce oxidation, and thus negative Ce anomalies. Furthermore, the positive Eu anomalies and elevated Tl concentrations could highlight the cation exchange processes on clay particles. In the fluvial and estuarine sections of the St. Lawrence System, we demonstrate significant contributions of wastewater discharge and discuss the suitability of several wastewater tracers, e.g., excess of B, Na, K, as well as Rb/Sr and Gd anomalies. We also highlight the inputs of several minor and trace elements (e.g., Mn, Fe, Cu, Co, Ni) from south-shore tributaries to the St. Lawrence System. However, the complex anthropogenic activities in the watersheds did not allow clear source partitioning. Finally, increased mixing of different river water masses upstream of Quebec City, together with the estuarine salt front and suspended sediments, are also responsible for releasing these minor and trace elements into the aquatic media. The results presented here help support further environmental actions to curb the emission of contaminants in the St. Lawrence System and provide more robust tracers of natural and anthropogenic processes in aquatic environments.

Automated summary

Differentiating anthropogenic signatures from natural processes is crucial for environmental regulation. This study investigates the variations in chemical concentrations and trace elements in different water masses along the St. Lawrence River-Estuary continuum, and analyzes the impact of wastewater discharge and the sources of minor elements.