GEOCHEMICAL CONTROLS ON MERCURY METHYLATION IN THE WATER COLUMN OF BACKWATERS OF A GULF COASTAL PLAIN RIVER SYSTEM, LOWER OUACHITA RIVER, ARKANSAS

Mercury methylation processes were investigated in the lower Ouachita River in the summer of 2010 in an effort to identify and determine the magnitude of methylation in the water-column in backwater locations. Filtered methylmercury was positively correlated with dissolved organic carbon (r²=0.76) for water samples taken from the bottom 1 ft of the water column at all three study sites, suggesting the importance of dissolved organic carbon in mercury methylation. Concentrations of filtered methylmercury and filtered total mercury taken from the bottom 1 ft of the water column were significantly higher (P=0.024 and P=0.012 respectively) than samples collected at the mid-point of the water column. Oxidation-reduction potential and sulfide concentrations of 65-143 micrograms/liter indicate sulfate reduction in the bottom 1-ft of the water column, yet filtered and particulate methylmercury concentrations were not correlated to sulfide concentrations. This finding is of particular interest as sulfate reducing bacteria are commonly associated with mercury methylation. Water chemistry results for one site including iron (39.8 milligrams/liter), high dissolved organic carbon (13.52 milligrams/liter), the highest filtered methylmercury concentration observed for the study (1.90 nanograms/liter), and no detectable sulfate suggests the predominance of iron reduction at this site. Iron-reducing bacteria can be important in driving mercury methylation as an alternative to sulfate reducers. Total mercury concentrations for two of seven samples collected from all three study sites exceeded the Arkansas numeric mercury 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 study sites, and that iron reduction may contribute to mercury methylation at one of the sites. Sulfate reduction, which was expected to be the major control on mercury methylation, was not found to correlate with dissolved methylmercury, suggesting the presence of other controls on mercury methylation in this river system.