The Transport, Fate and Cycling of Mercury in Watersheds and Air Sheds

The potential consequences of mercury (Hg) contamination of aquatic food webs were first recognized in the 1950s and 1960s in Minamata and Niigata, Japan, where human consumers of contaminated fish were severely poisoned. These and other tragic incidents prompted widespread reductions in direct releases of Hg into surface waters in many countries. Hg levels in fish in affected waters typically declined during the years after point-source loads declined, leading to a widespread perception that the “Hg problem” had been solved. Since about 1985, however, widespread Hg contamination of aquatic biota has become evident in systems remote from obvious anthropogenic Hg sources. Investigations at these sites have shown that in most cases atmospheric transport and low rates of Hg deposition are responsible for the observed Hg contamination levels, and virtually any aquatic ecosystem is potentially affected. In some cases, concentrations in fishes from these remote sites have equaled or exceeded those in fishes from waters heavily contaminated by direct industrial discharges. As the scientific understanding of what leads to pronounced, wide-spread mercury contamination has improved, it has become obvious that cycling and transport of atmospherically derived mercury in watersheds is central to understanding this problem more generally. For example mercury concentrations in aquatic biota are often elevated in fish from low-alkalinity or humic freshwaters, newly flooded reservoirs, and surface waters that adjoin wetlands. However, we lack sufficient information to predict reliably which aquatic ecosystems will contain Hg-contaminated biota, and the effectiveness of recently proposed Hg emissions reduction strategies in the US and elsewhere. Although our scientific understanding of mercury sources, cycling and fate in the environment has improved greatly in the past 10 years, we still lack a robust understanding of the role of watersheds in controlling response times to changes in mercury loads, and as sources to methylmercury (MeHg) generation in aquatic ecosystems. Methylmercury is the most toxic and bioaccumulative form of methylmercury, and is the focus of much research today, including USGS research programs. For the past sixteen years, the USGS Mercury Research Lab and the University of Wisconsin-Madison have partnered on various research projects to further our scientific understanding of mercury sources, cycling, fate, and toxicity in the environment. These collaborative research efforts have resulted in numerous fundamental scientific discoveries that have had transfer value across the globe. Through collaborative research efforts that capitalized on the differing but complimentary the capabilities and expertise’s of the USGS and the University of Wisconsin mercury research teams, substantial efficiencies are gained, and duplication in staffing and effort is reduced. The University of Wisconsin, Aquatic Sciences Center is a recognized leader in Hg research, and has developed many of the low-level trace metal techniques used by many agencies, including the USGS. At the same time, the USGS, Wisconsin Water Science Center has developed widely recognized expertise in mercury research, but the strengths and emphasis of the USGS Hg Team are complimentary to the University of Wisconsin group. For example, the University of Wisconsin group has developed sophisticated methods for the isolation of specific dissolved organic carbon (DOC) fractions (Babiarz and others, 2000; Babiarz and others, 2002) that are largely responsible for the transport of Hg and MeHg in the environment. In addition, the University of Wisconsin research team has significant expertise in performing critical aspects of atmospheric mercury research, such as: quality assurance and quality control checks, data archiving, as well as graphical presentation and interpretation of the vast amount of atmospheric data collected by the USGS group. Recent collaborations on atmospheric mercury research have lead to a joint publication (Engle and others, 2008) and a second paper is planned for fiscal year 2009. The USGS group, on the other hand, has expertise setting up field systems to collect data on mercury in water, air and sediment, but not the experienced staff to assemble the final data bases for publication. Combined, the two research groups offer a complete set of research tools to examine mercury cycling in critical environments (watersheds and air sheds) where we need a better understanding of the complex environmental mercury cycle. For this proposal, the University of Wisconsin will provide technical assistance and leadership for the Mercury Experiment To Assess Atmospheric Loadings In Canada and the United States (METAALICUS) project, and for the Coastal Zone Atmospheric Mercury Deposition project.