IMPACTS OF SEDIMENT GEOCHEMISTRY ON MERCURY SPECIATION AND METHYLATION IN A FRESHWATER ESTUARY

The St. Louis River Estuary (SLRE) is a highly utilized freshwater estuary at the mouth of the largest tributary to Lake Superior. The prevalence of favorable biogeochemical conditions in the estuary facilitates the microbial production of toxic and bioaccumulative methylmercury (MeHg). Inorganic mercury (Hg) is deposited from mostly atmospheric sources to the SLRE where it accumulates in sediments and transforms into many different inorganic species depending, largely, on the presence or absence of sulfide and organic carbon. Many of these inorganic Hg species are bioavailable for methylation since they preferentially partition into the aqueous phase where microorganisms initiate methylation. As a result, fish and other aquatic biota in the estuary often contain elevated levels of MeHg. In this study, we focus on the impact of sediment geochemistry and solid-phase Hg speciation on Hg bioavailability and partitioning in relation to MeHg production in the estuary. Over two field campaigns in 2016 and 2017, sediment cores and porewater were collected from five geochemically distinct locations within the estuary and analyzed for total Hg and MeHg, organic carbon (solid and dissolved), and sulfide. Additionally, sequential extractions were used to quantify solid phase mercury species by behavioral classes. Initial results indicate that legacy industrial Hg contamination is present in some locations, primarily in the lower harbor; however, MeHg production is limited. MeHg production is highest in sediments with intermediate organic carbon content; however, porewater total Hg is negatively correlated to organic carbon. Additionally, MeHg production and porewater MeHg was lowest in sediment with the highest solid phase sulfide, suggesting both inorganic and methylmercury availability are partially controlled by solid phase organic carbon and sulfide. Sequential extractions will help elucidate the solid phases involved in binding mercury by separating the solid-phase mercury present into water soluble, stomach acid soluble, organo-chelated, elemental, and mercuric sulfide classes. Coupled with total and methylmercury data, selective extraction results allow for the assessment of bioavailable mercury pools throughout the SLRE.