Mercury transport between sediments and the overlying water of the St. Lawrence River area of concern near Cornwall, Ontario.

Contaminated sediments in the St. Lawrence River remain a difficult problem despite decreases in emissions. Here, sediment and pore water phases were analyzed for total mercury (THg) and methyl mercury (MeHg) and diffusion from the sediment to the overlying water was 17.5 + or - 10.6 SE ng cm(-2) yr(-1) for THg and 3.8 + or - 1.7 SE ng cm(-2) yr(-1) for MeHg. These fluxes were very small when compared to the particle-bound mercury flux accumulating in the sediment (183 + or - 30 SE ng cm(-2) yr(-1)). Studies have reported that fish from the westernmost site have higher Hg concentrations than fish collected from the other two sites of the Cornwall Area of Concern, which could not be explained by differences in the Hg flux or THg concentrations in sediments, but the highest concentrations of sediment MeHg, and the greatest proportions of MeHg to THg in both sediment and pore water were observed where fish had highest MeHg concentrations.

Sediment mercury dynamics and historical trends of mercury deposition in the St. Lawrence River area of concern near Cornwall, Ontario, Canada.

The St. Lawrence River near Cornwall, Ontario was designated an Area of Concern by the International Joint Commission in 1985. Sediments from this area have historically been contaminated with mercury (Hg), and although concentrations have decreased since the 1970s, they still remain high. Nine sediment cores were collected from three sites within the Area of Concern in 2004/05 to determine the variability in historical profiles of Hg deposition to the river. Sediment and pore water phases were analyzed for total mercury (THg) and methyl mercury (MeHg) and cores were analyzed for 210Pb to determine chronologies of sedimentation at these sites. Mercury diffusion rates in pore waters within the sediment column were determined to be very low (between 0 and 2.15 ng cm(-2) year(-1), n = 3) compared to the recent Hg sedimentation rates at these sites (183+/- 30 ng cm(-2) year(-1) SE, n = 9) determined by multiplying surface Hg concentrations with 210Pb-derived sedimentation rates. These results indicate that Hg profiles in these cores accurately depict historical releases of Hg to the river bed. The influence of federal regulations in the early 1970s to restrict Hg emissions to the river was apparent in these dated sediment cores, as were the closures of several local industries in the mid 1990s. Mercury accumulation rates prior to 1970 were 60 times higher than those occurring after 1995. Methyl mercury showed surface enrichment in most of these sediment cores providing evidence that mercury methylation occurred most rapidly near the sediment surface.