Mercury geoienvironmental models

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James J. Rytuba INTRODUCTION Mercury mines and mines that produce byproduct mercury can impact the environment through the release of mercury-enriched sediment, mercury-rich mine drainage, and mercury vapor released to the atmosphere. Most mercury mines contain mercury contaminated mine wastes and soils that are a source for both soluble and particulatemercury species. Environmental concerns related to mining and processing of mercury-bearing ores consist of contamination of soils, sediments, and waters by mine wastes; mercury vapor released during ore processing and from mine tailings; and mercury mine drainage and toxic metal release into watersheds. Significant environmental impacts on watersheds also are associated with the release andtransport of elemental mercury from past and present gold placer operations, and from tailings generated during the processing of precious metal ores where the amalgamation process was used. Although geoenvironmental models of mercury deposits can be used to establish the environmental effects caused by these deposits, the models must be used in the context of global cycling of mercury. GLOBAL MERCURYCYCLE AND ENVIRONMENTAL The global atmospheric pool of mercury contributes mercury to watersheds and lakes through both wet and dry depositional processes that are mediated by leaf uptake of mercury by plants (Mason and others, 1994). Because of this atmospheric contribution of mercury, the magnitude of the flux of mercury to the environment from mineralized areas and mine sites reflects bothlocal and global contributions of mercury, although the local source typically predominates in mineralized areas. The primary natural sources of mercury to the atmosphere include, in decreasing importance, the oceans, soil degassing, volcanoes, and geothermal systems (Varekamp and Buseck, 1986, Mason and others, 1994). Anthropogenic sources of mercury to the atmosphere are primarily from coalcombustion, waste incineration, and smelters (Nriagu and Pacyna, 1988). Since the beginning of industrial period in the middle 1800s, the global atmospheric mercury deposition increased until about 1970 and then decreased slightly in the past three decades (Fitzgerald and others, 1997). This increase in atmospheric mercury deposition is reflected in the sedimentary record of lakes, estuaries, and bogs.Cores of recently deposited sediments from all these environments record an increase in mercury concentration of about 2 to 5 times over background mercury concentrations established prior to the industrial period (Fitzgerald and others, 1997, Hurley and others, 1994; Verta and others, 1990). For this reason mercury background levels in various media such as soils generally cannot be established andonly baseline concentrations can be determined now. Unlike most metals, plants uptake mercury primarily through leaves rather than through the root system. Under high ambient air mercury concentration, plants uptake and concentrate mercury in their leaves, and conversely, under low ambient air concentrations of mercury, plants give off mercury through their leaves (Lindberg and others, 1992). Inmine areas where ambient air concentrations of mercury are high either due to roasting of mercury ores or from emission of mercury vapor from contaminated or naturally anomalous soils, plant communities down wind from these sites concentrate both mercury and methylmercury in their leaves. For example, at mercury contaminated mine sites in southwest Alaska, plant leaves contain elevated mercury andmethylmercury concentration, up to 970 ppb and 37 ppb respectively, as compared to baseline values of 190 ppb, and 1.5 ppb, respectively, in unmineralized areas (Bailey and Gray, 1997, Bailey et al., 1999). Wash off and litter fall are the primary routes for introduction of mercury into creeks and lakes from these plant communities. This process will augment any direct contamination of...
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