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汞元素赋存之地理环境模型(01)  

2017-11-09 07:16:59|  分类: 不归类文章 |  标签: |举报 |字号 订阅

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MERCURY GEOENVIRONMENTAL MODELS

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 particulate mercury 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 and transport 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 MERCURY CYCLE 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 both local 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 coal combustion, 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 and only 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). In mine 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 and methylmercury 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 watersheds and lakes by mercury contaminated mine wastes and waters.

The mercury species of greatest environmental concern is methylmercury, because it may become highly concentrated through bioaccumulation in fish and other fish consuming biota. Methylmercury is a neurotoxin and its pathway into humans is primarily through consumption of methylmercury-contaminated fish. A secondary pathway into humans is the direct ingestion of mercury contaminated mine wastes by children displaying pica activity (soil ingestion). Mercury released from point sources and deposited from the atmosphere into streams, wetlands, and lakes may become methylated and incorporated into the food web. Methylation of mercury and its uptake by biota is a complex process and dependent on several variables. As a result, although fisheries downstream from mercury mine sites are generally contaminated, the levels of methylmercury do not always exceed the federal action level of 1.0 ppm (for example see Gray et al., 1996, for fisheries below mercury mines in Alaska). Formation of methylmercury (CH3Hg+) is favored by the presence of ionic mercury and high concentrations of dissolved organic carbon and sulfate. Mercury methylation is a co-metabolic reaction and sulfate-reducing bacteria are the primary mediators in the biotic methylation process (Campeau and Bartha, 1985, Summers, 1986). Acid mine drainage from mercury deposits commonly has both high sulfate and mercury concentration and introduction of these waters into streams, wetlands, and lakes increases the methylation process (Rytuba and Enderlin, 1999). Although methylmercury can be formed at mine sites in mercury-contaminated soils and mine waters, most mercury methylation occurs downstream from the mine site in wetlands and larger aquatic bodies. Detrital particles, such as clays, organic phases, and iron oxyhydroxide, adsorb Hg2+ from the water column in streams and lakes, and sedimentation of these particles reduces the methylation process. Sequestration of mercury in sediment is the primary mode of removing mercury from the environment.

MERCURY PRODUCTION

Mercury has been mined in North America since the early 1800's with over 6 million flasks (a flask equals 76 pounds of mercury) of mercury being produced from ten major mercury mineral belts (fig. 1). The mercury mineral belts consist of mercury deposits with significant production (greater than 1000 flasks of mercury), small mercury occurrences having small or no production, and areas of country rock containing elevated concentrations of mercury. In North America, the California Coast Range mercury mineral belt has been the largest producer of mercury and contains fifty-one mines that have produced over 1000 flasks of mercury. The New Almaden mine in central California is the largest mercury mine in North America having produced about four million flasks.

 

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