Geochemical controls on mercury methylation in backwaters of a Gulf Coastal Plain River System, implications for water column processes

The abundance and distribution of mercury and methyl mercury were investigated at three sites in the lower Ouachita River in the summer of 2010 in an effort to provide the first characterization of the extent of mercury contamination in this river system, and to investigate the potential for mercury methylation in the water column of backwaters off of the main channel. Results showed that filtered methyl mercury was positively correlated to dissolved organic carbon (r2=0.76) for water samples taken from the bottom 1 ft of the water column at three sites, suggesting the importance of dissolved organic carbon in mercury methylation. Concentrations of filtered methyl mercury and filtered total mercury in the bottom-water were significantly different (P=0.039 and P=0.022 respectively) at two of the sample sites located approximately 14 river miles apart. Sulfide concentrations of 74.0-142.7 micrograms/liter indicate sulfate reduction was occurring in the bottom water or at the sediment-water interface, yet filtered and particulate methyl mercury concentrations were not significantly correlated to sulfide concentrations. The occurrence of sulfides in the bottom-water is important as sulfate-reducing bacteria are most commonly associated with mercury methylation. Water chemistry results for one site including total iron (39.8 milligrams/liter), high dissolved organic carbon (13.52 milligrams/liter), the highest filtered methyl mercury concentration observed for the study (1.90 nanograms/liter), and no detectable sulfate suggests the predominance of iron reduction at this site. Microbial iron reduction is also a known mercury methylation pathway. Total mercury concentrations for two of seven samples exceeded the Arkansas numeric water quality standard for total recoverable mercury in water (12 nanograms/liter), at concentrations of 13.76 and 13.99 nanograms/liter. These data provide evidence that availability of dissolved organic carbon affects mercury methylation at all three of the sites, and that iron reduction may contribute to mercury methylation at one of the sites. No correlation between sulfide and dissolved methyl mercury was observed, suggesting sulfate reduction may not be the driving process for mercury methylation at all our study sites, and indicating the presence of multiple controls on mercury methylation in this river system.;Keywords: methyl mercury, mercury, biogeochemistry, contaminant hydrology...