Sulfur and carbon controls on methyl mercury in St. Louis River Estuary sediment Phase II

The presence of methyl mercury (MeHg) in freshwater aquatic systems is a concern to public health due to its bioaccumulative properties in aquatic food webs and neurotoxicity in humans. Transformation of inorganic mercury to MeHg is primarily driven by sulfate and iron reducing bacteria (SRB and FeRB) in anoxic sediment and water columns. The first objective of this research is to examine the production of MeHg in a sulfate-impacted freshwater estuary in the context of mercury-related geochemical parameters including organic carbon content and lability, sulfate concentrations, and bioavailability of inorganic mercury to methylating microbes. The second objective is to determine the significance of MeHg transport from sediment to the St. Louis River Estuary relative to upstream sources. A laboratory sulfate addition experiment was performed using 20 cm intact sediment cores obtained from three characteristic sites in the St. Louis River Estuary. The intact cores from each site were exposed to a high (50 mg/L), medium (15 mg/L), and low (5 mg/L) overlying water sulfate treatment for an incubation period of six months. Over the six month laboratory study, MeHg/THg ratios in surface sediment (0-4cm) appeared to be insensitive to overlying water sulfate concentrations in all sites. However, at one site, MeHg/THg ratios in deeper sediment (4-10 cm) did appear to be related to overlying water sulfate concentration. Methyl mercury was strongly correlated with total mercury in sediment from all sites and all sulfate amendment conditions. Analysis of the bulk geochemical differences among sites suggests that MeHg concentrations are related to the total mercury in sediment and the quantity and type of organic carbon. Laboratory flux measurements provided a means to compare MeHg loading from sediment relative to MeHg loading from the upstream St. Louis River. The estimated MeHg loading from habitat zones represented by the three sites included in this study (45% of total estuary area) exceeds that of upstream sources during median and low flow conditions. These MeHg loading estimates suggest that MeHg transport from sediment could influence overlying water MeHg concentrations in the St. Louis River Estuary.