Do wood fibers from a pulp mill affect the distribution of total and methyl mercury in river sediments?

The St. Lawrence River near Cornwall Ontario is affected by industrial mercury contamination of sediments and biota. It has been suggested that pulp and paper mill effluents may stimulate bacterial mercury methylation in these sediments, leading to contamination of aquatic biota. To test this hypothesis, we examined sediment–porewater dynamics of total mercury (THg) and methyl mercury (MeHg) at a site with high concentrations of wood fibers from a pulp and paper mill effluent and a nearby reference site with low wood fiber content. Dissolved phase THg (THgdiss) and MeHg (MeHgdiss) in porewater profiles showed that 38±30.9% (SD) of THg in porewaters was in the methylated form regardless of wood fiber content. MeHgdiss and THgdiss concentrations were homogeneous between porewater and overlying water, indicating (a) that there is minimal net diffusion of MeHgdiss and THgdiss and (b) that redox-dependent processes such as sulfate reduction and Fe reduction were not associated with MeHgdiss distribution in these sediment profiles. MeHg and THg in solid phase showed coincident subsurface peaks at depths>40cm suggesting either that historical deposits of MeHg on particles (MeHg(p)) are preserved in deep sediments, or that Hg methylation is active in deep sediments.