DETERMINATION OF THE MAGNITUDE OF MERCURY METHYLATION IN THE WATER COLUMN OF A RIVER WITH HIGH DISSOLVED ORGANIC CARBON, LOWER OUACHITA RIVER, UNION AND ASHLEY COUNTIES, ARKANSAS

Conventional approaches to mercury (Hg) methylation research in riverine systems have focused on processes below the sediment-water interface, where redox conditions are most favorable for the conversion of inorganic Hg (Hg II) to methylmercury (MeHg) by anaerobic bacteria; however, recent studies in marine systems have shown a positive correlation between dissolved organic carbon (DOC) and MeHg concentrations in the water column, suggesting the important role of organic carbon in the production of MeHg and hinting at methylation within the water column. The Ouachita River in southern Arkansas contains five river segments listed as impaired by the Arkansas Department of Environmental Quality because of Hg concentrations in fish tissue. Previous studies in Arkansas have used GIS technology and statistical methods with Hg data in fish tissue to identify and characterize rivers with high levels of mercury contamination, determining that there is a higher frequency of occurrence of the highest MeHg concentration class (> 1 mg/kg) in the Gulf Coastal Plain region where the lower Ouachita River is located. Few data are available for Hg and MeHg concentrations in water in Arkansas, so a comprehensive characterization of Hg and MeHg is necessary to assess the controls on toxic MeHg production in this river. Historical data reflecting high concentrations of DOC (> 6 mg/L) for the lower Ouachita River and high concentrations of MeHg in fish tissue (> 1 mg/kg) make this system an important candidate for investigation of the potential link between DOC and MeHg. To assess the distribution and production of MeHg in the lower Ouachita River, water and bed sediment samples will be collected at three sites during the summer of 2010 and analyzed for Hg, MeHg, and DOC. Hg and MeHg concentrations will be assessed within the water column at the epi-benthic zone from the sampling locations and compared to DOC to determine water column contributions of MeHg and DOC influence on methylation. Bed sediment contributions of MeHg will be assessed by comparing water column concentrations of MeHg to bed sediment concentrations. The study will use state-of-the-science low-level Hg sampling protocols and analytical methods capable of detecting total Hg and MeHg in water to 0.04 ng/L.