Current Methods of Estimating Atmospheric Mercury Fluxes in Remote Areas Pat E. Rasmussen'
Earth Sciences Department, University of Waterloo, Ontario, Canada Evaluation of the impact of anthropogenic mercury (Hg) emissions to the atmosphere requires an understanding of natural background levels and cycling processes. Baseline geochemical surveys indicate that Hg is a significant and highly variable natural constituent of bedrock, surficial sediments, and vegetation. To evaluate the geological contribution of Hg to aquatic systems and to the atmosphere, methods are needed to translate existing spatial geochemicaldata into annual flux estimates. At the global scale, estimates of the geological component of the atmospheric Hg cycle vary widely, depending on which sources are considered, the magnitude of the Hg emission factors used, and the geographic area to which these emission factors are applied. Plate tectonic theory offers a useful framework for interpreting Hg distribution patterns on the earth's surface and a means of quantifying the magnitude of geologicalloadingto the oceans, the latter being a critical component of the global Hg cycle. Introduction
Reliable data describing mercury (Hg) fluxes in natural systems are scarce. In the absence of geological flux measurements, Hg cycling models at local, regional, and global scales tend to rely heavily on assumptions about the nature and significance of geological sources. Watershed budgets, for example, are commonly based on the assumption that there is a negligible contribution of Hg from local geological sources to surface water and groundwater (1-3). Certain models encompassing larger areas such as the whole of Sweden (1)or the Atlantic Ocean ( 4 ) rely on the assumption that background Hg levels are constant and conclude that spatial or temporal variations uniquely identify anthropogenic influences. A significant number of studies are being published that calculate atmospheric Hg deposition rates based on the Hg content of natural samples (organic soil, vegetation, peat bogs, lake sediments) collected in landscapes remote from industrial point sources (5-7). Based on some implicit assumptions about geological sources, these studies invariably conclude that a significant portion of the Hg in remote areas is anthropogenic in origin, derived from longrange atmospheric transport. As the above assumptions are critical to the interpretation of Hg cycling in remote landscapes, their validity needs careful examination. The first section of this paper briefly summarizes the literature on geological sources and pathways of Hg. The second section examines case studies in which atmospheric Hg deposition rates are inferred from the distribution of Hg in natural samples. The third
* Present address: Geological Survey of Canada, 601 Booth St., Ottawa, Canada K1A OE8. 0013-936X/94/0928-2233$04.50/0
0 1994 American Chemical Society
section examines estimates of the geological component of the global Hg cycle. Geological Sources and Pathways of Mercury in Environment Sources of Information. Lockeretz (8) commented that once accurate data are available to quantify anthropogenic Hg inputs, the most valuable information for assessing the impact of anthropogenic activity is an understanding of natural processes. In this context, the body of research by Jonasson and colleagues (9-15)remains the most comprehensive review of geological sources and pathways of Hg in the environment. Information is otherwise found in the literature pertaining to mineral and geothermal exploration, geophysics and seismicity, structural geology, and volcanology. These areas of research consider the distribution of Hg as an indicator of geothermal zones, sulfide mineralization, oil and gas reservoirs, and deep geologicalstructures such as rift zones and regional faults. Case studies and review papers have been published in the last decade on this subject in North America (16-22),in Australia (231,in Russia (24,25),and in China (26-28). Extensive geochemical surveys have been conducted in remote regions of Canada to investigate natural spatial variations of Hg in lake sediment and stream sediment, soil, muskeg, glacial drift, and bedrock (13-15, 29-36). Geochemical mapping at the reconnaissance scale identifies areas where concentrations of Hg and other metals are naturally elevated (33,37),which has proven valuable in understanding the geological contribution of Hg to hydroelectric reservoir systems (38, 39). Another goal of geochemical mapping is to evaluate the relative contribution of atmospheric deposition and geological sources of Hg to natural aquatic systems (37). For example, a survey of glacial drift geochemistry (34, 351,consisting of 17 000 sites sampled over an area of 40 000 km2 in the Frontenac Arch region of Ontario, Canada, identifies Hg as a natural constituent of glacial overburden, commonly occurring at levels above 200 ppb in the
