The effect of high sulfate loading on methylmercury production, partitioning, and transport in mining-imapcted freshwater sediments and lakes in northeastern Minnesota

Abstract

Methylmercury (MeHg) is a highly toxic form of mercury with the ability to bioaccumulate in food webs. The bioaccumulation of MeHg leads to elevated MeHg levels in fish tissue and poses a threat to public health. Thus MeHg concentrations in surface waters - which may be a result of water column MeHg production, or sediment MeHg production and subsequent flux from sediment porewater - are of particular concern. The production of MeHg from inorganic mercury (iHg) is primarily a result of sulfate-reducing bacteria (SRB) activity in anoxic aquatic environments.Ongoing and historic mining activity on the Mesabi Iron Range (Minnesota, USA) has led to elevated sulfate levels in the downstream waters of the St. Louis River watershed. In an effort to understand the effect of mining-related sulfur-loading on the production and partitioning of MeHg, sediment samples were collected and analyzed from sulfur impacted and non sulfur-impacted lakes and wetlands within the watershed. Additionally, the water column and inlet and outlet streams of a mesotrophic lake (Lake McQuade) were sampled intensively during summer stratified conditions in order to identify the sources and sinks of MeHg to the lake system and determine the potential for MeHg export downstream.Results suggest that dissolved sulfide plays a large role in governing MeHg dynamics in sulfate-impacted freshwater sediment. Consistent with previous research, net MeHg production appeared to be inhibited in sediments with dissolved sulfide >60 uM. However, these high concentrations of dissolved sulfide were accompanied by increased partitioning of MeHg into the porewater phase, potentially increasing the fraction of MeHg available to be transported into surface waters.Sediment at sulfate-impacted sites was generally characterized by high dissolved sulfide and a low potential for long-term net MeHg production. However, the accumulation of dissolved sulfide in sediment porewaters can be limited by the availability of free labile iron (Fe2+) and consequent iron-sulfide precipitation reactions. In the results presented here, high sulfur-loading at two sites appeared to have consumed the available free labile iron and created conditions which allowed for the accumulation of dissolved sulfide and inhibition of MeHg production in the sediment. However, relatively high sulfur-loading(>100 mg/L) to a third site where iron remains in excess of sulfur in sediment may have led to robust net MeHg production, in absence of inhibitory dissolved sulfide concentrations. Accumulation of MeHg in the hypolimnion of Lake McQuade occurred during summer 2012 during a time when bottom water sulfate was being consumed. Though some uncertainty remains as to the ultimate source of the MeHg, estimates of MeHg inputs and outputs to the hypolimnion suggest that water column production was a primary source of MeHg to the hypolimnion during the stratified summer months. Following the wet spring months when inputs were dominated by upstream flows, the flux of MeHg across the limnetic surface was estimated to be the primary source of MeHg to the epilimnion during the stratified summer months. However, most of MeHg input to the epilimnion was apparently degraded prior to being exported to the outlet stream. Thus, despite mid-summer accumulation of MeHg in the hypolimnion, the combination of stratification and substantial degradation in the epilimnion acted to limit export of MeHg out of Lake McQuade.As a whole, Lake McQuade acted as small net source of MeHg to the surrounding water system during the summer months of 2012. Evidence points to a brief rise in MeHg export immediately following lake turnover in Mid-August due to the release of hypolimnetic MeHg to surface waters during lake mixing.