Benthic flux of mercury between sediments and the overlying water in the St Lawrence River near Cornwall, Ontario

Sediments, located near Cornwall have been historically contaminated with Hg and other metals by local industries. Cornwall was designated an area of concern by the International Joint Commission (IJC) in 1985. The concentrations of mercury in these sediments exceeded the sediment quality guideline (SQG), of 170 ng g-1 set by Environment Canada for the protection of aquatic biota. To identify the role of these contaminated sediments on mercury dynamics in the river, I measured concentrations of total mercury (THg) and methyl mercury (MeHg) in both the porewater and solid phase of sediment cores and in the overlying water to determine whether sediments are a net source or sink for Hg. A comparison of porewater profiles in June, July and August of 2007 revealed little seasonal variation in MeHg concentrations. I also compared THg and MeHg vertical profiles in sediments with complimentary redox-dependent variables, including sulfate, sulfide, and Fe2+ distributions which showed that zones of active sulfate reduction and Fe reduction have little effect on the distribution of dissolved MeHg in the sediments. THg in sediment cores was related to the sediment accumulation rates by 210Pb radiochronology which showed the history of industrial Hg emissions to the river. MeHg contributed 4% to 100% of the THg in the porewater samples, whereas in the solid phase it contributed less than 1% of the THg. There was little to no diffusion of THg and MeHg from sediments to the overlying water. I have concluded that sediments are a major sink for THg and MeHg to the St. Lawrence River near Cornwall