Spatial and temporal variation in mercury methylation in sediment and water of Farmington Bay of the Great Salt Lake, Utah.

摘要

The bioavailable form of mercury, methyl mercury (MHg), has been shown by previous studies to be relatively high in the main body of the Great Salt Lake (GSL). A consumption advisory for several species of waterfowl from the GSL and surrounding wetlands was recently released due to findings of toxic mercury levels in those species. The GSL is a system of vertically and spatially connected "compartments," including shallow and deep brine layers in the main bays, as well as bays on the eastern side where fresh water is delivered from corresponding rivers. It is presently unknown which compartments and locations are predominant in transforming mercury to methyl mercury. One method used to investigate Hg methylation in sediment and water is to spike the sample with inorganic mercury enriched in a minor isotope and observe the evolution of methyl mercury concentrations in response, which was implemented in this study. Sediment and water samples were collected from sites along two east-west trending transects at the north and south ends of Farmington Bay in summer and fall. Water temperature, pH, conductivity, dissolved oxygen, sulfide and sulfate concentrations, and ambient total Hg and MHg concentrations were measured at each site. Total solids content and organic matter were evaluated for the sediment. Subsamples of the sediment and water from each site were spiked with 204Hg2+ and were incubated for 12, 24, 48, and 72 hours. Mercury isotopic signals for the incubated subsamples were detected using cold vapor atomic fluorescence spectrometry in line with inductively couple plasma mass spectrometry and were used to develop first order conditional methylation rate constants (kmeth) for each site. The methylation rates were considered conditional because a simplified first order rate equation was assumed and not all parameters affecting methylation (e.g., sulfate loading, sediment organic matter, etc.) were included in the model. Methylation was observed in the sediment, but not in the water column. Methylation rates ranged from 1.1E-4 hrs-1 to 5.1E-8 hrs-1. The highest methylation rates were observed for the three westernmost sites from the fall sampling of the north transect, while the lowest were found at the easternmost sites of the south transect during fall. A positive correlation between methylation rate and sediment organic matter was shown when samples were grouped by transect.