Mercury dynamics in sediments of the St Lawrence River near Cornwall, Ontario

The St. Lawrence River near Cornwall, Ontario was designated an Area of Concern by the International Joint Commission in 1985. Sediments from the Area of Concern have historically been contaminated with mercury (Hg), and although concentrations have decreased since the 1970s they still remain high. Several sediment cores were collected from three sites within the Area of Concern in 2004/05, to determine whether sediments act as a source or sink of mercury. The three sites were chosen based on their proximity to former Hg dischargers, including a chlor-alkali plant, a pulp and paper mill, and a textile mill. Sediment and pore water phases were analyzed for total mercury (THg) and methyl mercury (MeHg) and cores were dated using radioisotope analaysis to determine sedimentation and Hg accumulation rates, and to study the record of Hg contamination over the past half century. Total mercury in sediments from the three sites ranged from 0.7 to 60 mug g-1 and methyl mercury ranged from 0.02 to 0.7 mug g-1. Surface sediments had much lower Hg concentrations than deeper sediments, however they still exceeded sediment quality guidelines for the protection of aquatic organisms. The diffusional flux from the sediment to the overlying water ranged from 1.2--48.2 ng cm-2 yr-1 for THg and 1.2--14.6 ng cm-2 yr-1 for MeHg, but was small when compared to the flux of particulate bound mercury being deposited to the sediment (70--309 ng cm-2 yr-1). These results showed that post-depositinal redistribution did not alter the Hg profile and trends in sediment Hg concentration corresponded well with source histories. Dated cores showed that mercury accumulation rates prior to 1970 were up to 60 times higher than current rates. Recent collections have indicated that fish from one specific site (Lamoureux Park) have higher Hg levels than fish collected from other sites along the Cornwall waterfront. This could not be explained by differences in the Hg flux or THg concentrations between sites however, the highest levels of sediment MeHg, the highest proportions of MeHg to THg in both sediment and pore water, and the highest rate of methane production in sediments were observed at the this site. At present, the contaminated sediments appear to act as a net sink for total mercury, yet they remain a potential source of MeHg to the food web and the river system