Control of mercury methylation in wetlands through iron addition

The San Francisco Bay-Delta System lost an estimated 85-95% of its historical tidal marshes to urban development, agriculture, and commercial salt production since the middle of the nineteenth century. Fortunately, there are many recent initiatives underway throughout the estuary to re-establish the important ecosystem functions and critical wildlife habitat that these tidal wetlands offer. However, there is a significant potential drawback to these restorations, as wetlands have been shown to play a major role in the production and export of methylmercury (MeHg), which is a potent neurotoxin that affects both humans and wildlife. While mercury pollution is a global problem, it is of special concern in the San Francisco Bay-Delta, where substantial additional inputs of inorganic mercury from historical mining activities have resulted in increased mercury levels in ecosystem. The potential exacerbation of MeHg health effects due to wetland restoration and construction is a serious concern that has been recognized in recent mercury regulations. However, restoration and management technologies have not yet been developed to control MeHg production and export from wetlands. The research described in this report tested the efficacy of one such potential control: the application of an iron sediment amendment to tidal wetland microcosms. The conversion of inorganic mercury to MeHg is predominantly a biologically-driven process under typical wetland sediment conditions. The net production of MeHg is controlled by both bacterial activity and the bioavailability of inorganic mercury species to the microbial community. Under the reducing conditions typical of wetland sediments, dissolved mercury speciation and concentration is controlled by the presence of reduced sulfur, and it has been hypothesized that it is the uncharged dissolved Hg-S species that are readily available for methylation, as they are the species able to diffuse into bacterial cells. In this research, we tested the hypothesis that amending wetland sediments with iron will reduce net methylmercury production by decreasing dissolved porewater sulfide concentrations through the formation of insoluble iron-sulfur minerals, which correspondingly decreases the pool of mercury available to the methylating bacteria. Two laboratory microcosm experiments were conducted using in-tact sediment cores collected from a tidal salt marsh in the San Francisco Bay estuary, where one experiment used sediments that had the vegetation removed and the second included live wetland plants. Microcosms in the devegetated experiment were split into four dosing groups (0, 180, 360, and 720 g- Fe/m2) and were monitored for a period of 17 weeks. Shortly after iron addition, porewater S(-II) concentrations decreased significantly at all iron doses relative to the control, and net MeHg production and export to the overlying surface water decreased by over 90% at the highest iron doses. Despite some conversion of FeS(s) to pyrite, the effects persisted for at least 12 weeks. The inclusion of wetland vegetation substantially increased the amount of variation between triplicate cells, but general trends were similar to those found in the devegetated experiment. This project was the first work to demonstrate that an iron sediment amendment has the potential to be an effective control of methylmercury production in tidal wetland sediments at the microcosm scale. These results have laid the groundwork for future studies to evaluate the efficacy of an iron amendment at the field scale, which could demonstrate that this technique is a viable landscape-scale control on methylmercury production in restored and constructed tidal wetlands.