Water mixing in a St. Lawrence river embayment to outline potential sources of pollution

Water mass balances on an isolated embayment (Hoople Bay) in the St. Lawrence River revealed a small stream (Hoople Creek), local groundwater and the St. Lawrence Main Channel as the 3 principal water sources. The latter had an average evaporative isotope signal that was inherited from the Great Lakes (δ18OH2O=−7.0‰) and an average Cl− content of 0.55 mmol/l. Hoople Creek and Hoople Bay waters were more variable in their isotopic composition and Cl− contents, while local groundwater was assumed to have a homogeneous composition year around. These parameters constituted an equation system that was solved with matrix operations to yield monthly contributions of the 3 endmembers. Influx of groundwater and Hoople Creek dominated the embayment only after higher snowmelt discharges, while the Main Channel contributed more than 50% during the remainder of the year. Preliminary results suggest that potential pollution in the Main Channel would strongly affect Hoople Bay and similar ecosystems along the river. Nevertheless, more detailed data are needed for a better water balance over longer time periods. The 3-component mixing technique serves as a good tool to evaluate seasonal water fluxes and may also become useful in other mass balances.