1 Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec H4B 1R6, Canada.
2 International Joint Research Center for Persistent Toxic Substances (IJRC-PTS), State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China.
3 Brock-Niagara Validation, Prototyping, and Manufacturing Institute, Brock University, St. Catharines, Ontario L2S 3A1, Canada.
4 Department of Earth Sciences, University of Toronto, 22 Ursula Franklin Street, Toronto, Ontario M5S 3B1, Canada.
5 School of the Environment, University of Toronto, 33 Willcocks Street Suite 1016 V, Toronto, Ontario M5S 3E8, Canada.
6 Environmental Monitoring & Reporting Branch, Ontario Ministry of the Environment, Conservation and Parks, Toronto, Ontario M9P 3 V6, Canada.
7 Department of Chemistry, Memorial University of Newfoundland, St. John's, Newfoundland and Labrador A1C 5S7, Canada.
8 Lab Service Branch, Ontario Ministry of the Environment, Conservation and Parks, Toronto, Ontario M9P 3 V6, Canada.
9 Technical Assessment and Standards Development Branch, Ontario Ministry of the Environment, Conservation and Parks, Toronto, Ontario M4 V 1M2, Canada.
10 Environment and Climate Change Canada, Water Science and Technology Directorate, Burlington, Ontario L7S 1A1, Canada.

Coal tar-related products as a source of polycyclic aromatic compounds (PACs) are particularly concerning due to high PAC concentrations and inadequate source management. Benzo[b]carbazole, a benzocarbazole isomer exclusively found in coal tar-derived products, acts as an ideal marker to distinguish coal tar sources from others, enabling more robust quantification of coal tar contributions to PACs. To evaluate the historical and recent contributions of coal tar-related sources to the levels of PACs in Lake Ontario and associated ecological risk, we analyzed 31 PACs and 3 BCBz isomers in surface sediments and a sediment core. In the surface sediments, summed concentrations of the PACs ranged from 170 to 11,000 ng/g, dry weight, 63-90% of which were attributed to the 16 EPA polycyclic aromatic hydrocarbons (PAHs). Our results suggest the contribution by coal tar-related sources to PAC contamination has increased over the past decades and reached over 40% in Lake Ontario surface sediments. Employing the toxicological-priority-index scheme for the field data, high molecular weight EPA PAHs were identified as priority PACs. Cumulative PAC risk assessments indicate that with the exception of Mimico Creek, all other sampling sites exhibited elevated risk values.