Longitudinal and vertical variations of waterborne emerging contaminants in the St. Lawrence Estuary and Gulf during winter conditions

Abstract The occurrence of Contaminants of Emerging Concern (CECs) warrants further research in relation to land-sea transport and distribution in the water column. With its large physical scale and depths of up to 400 m, the estuarine system formed at the outlet of the St. Lawrence River offers a case-study to evaluate the longitudinal and vertical distribution of waterborne pollutants. This study reports for the first time the concentrations of 90 CECs in the St. Lawrence Estuary and Gulf (SLEG), including pesticides, pharmaceuticals and anthropogenic markers, steroid hormones, and perfluoroalkyl substances (PFASs). A total of 59 water samples were collected along a 1000-km reach of the SLEG during two winter expeditions (2019 and 2020) on board the CCGS Amundsen icebreaker. Micropollutants detected in >90% of surface water samples included, for instance, caffeine (1.4-231 ng L-1), sucralose (0.6-304 ng L-1), atrazine (0.5-34 ng L-1), hydroxyatrazine (0.2-12 ng L-1), metolachlor-ESA (0.5-80 ng L-1), perfluorooctanoate (PFOA; 0.1-1.7 ng L-1), and perfluorooctane sulfonate (PFOS; 0.03-1.9 ng L-1). The pollutant fraction attached to suspended solids was low, except in the turbidity maximum. Salinity and suspended solids influenced the KD distribution coefficients of contaminants. Surface concentrations decreased following a conservative behavior per the estuarine mixing salinity plots from Ile d’Orleans to the Cabot Strait. Samples collected at variable depths (30-100-200-300 m) showed much lower pollutant concentrations (on average, by 10 times) than corresponding surface concentrations, confirming that most of the contamination originates from upstream riverine sources and that vertical transfer to the deep water layer is limited.