A province-wide mapping of per- and polyfluoroalkyl substances (PFAS) in surface waters of the St. Lawrence River watershed, Québec, Canada
Termeh Teymoorian, Gabriel Munoz, Sung Vo Duy, Marc-Antoine Vaudreuil, Min Liu, Jinxia Liu, Sébastien Sauvé
Abstract
This study investigates the spatial distribution of per - and polyfluoroalkyl substances (PFAS) in surface waters of the St. Lawrence watershed, Québec, Canada. Water samples (232 sites, overall n = 447) were collected along a ~ 700-km longitudinal gradient of the St. Lawrence as well as in tributary rivers, streams, creeks, and lakes with different anthropogenic pressures. St. Lawrence samples had a mean and median Σ 77 PFAS both at ~12 ng/L (the maximum at 26 ng/L) and PFAS composition profiles dominated by perfluorooctane sulfonate (PFOS: 19 % of the summed PFAS), perfluorobutanoic acid (PFBA: 16 %), and perfluorooctanoic acid (PFOA: 14 %). Samples were also characterized by frequent detections of perfluorobutane sulfonamide and perfluoroethylcyclohexane sulfonate. PFAS concentrations slightly decreased from west to east along the St. Lawrence River and cross-sections revealed higher levels in the Great Lakes/Central water mass than in the Ottawa River/Northern water mass. Tributary rivers displayed a much wider span of PFAS levels and were influenced by local point sources, with common PFAS signatures revealed by a Kohonen mapping (artificial neural networks). Top hotspots (Σ 77 PFAS = ~100–3600 ng/L) included watersheds downstream three airports, where positive ion mode fluorotelomer betaines 6:2 FTAB and 5:1:2 FTB also occurred at the highest levels (max = 156 and 570 ng/L, respectively). A mass balance analysis indicated, however, that tributaries contributed marginally (~10 %) to the ΣPFAS flux transiting in the St. Lawrence River, and that the bulk of the contamination mostly originates from upstream sources in the Laurentian Great Lakes. While PFOS and PFOA concentrations observed in the St. Lawrence River remained within Canadian and USEPA guidelines for surface water, exceedances would be observed under stricter thresholds. These findings underscore the importance of upstream sources in PFAS contamination and highlight the need for stricter monitoring and regulation.