Long-range atmospheric transport of mercury to ecosystems, and the importance of anthropogenic emissions—a critical review and evaluation of the published evidence

Abstract

Literature on the long-range atmospheric transport of both anthropogenic and naturally occurring mercury (Hg) to terrestrial and aquatic ecosystems was reviewed for the purpose of assessing the quantitative importance and environmental significance of the anthropogenic emissions. The weight of evidence, comprising many different kinds of data that corroborate each other independently, supports the following conclusions. (i) Approximately 5000 t of anthropogenic Hg are introduced into the atmosphere every year, both by direct emission from sources of pollution and by reemission of previously deposited Hg from diffuse secondary sources in the environment. The primary emissions ( 4000 t) result from various human activities, especially the combustion of fossil fuels (notably coal) and solid wastes. Natural emissions amount to about 2000 t/year. (ii) Although some of the annual anthropogenic Hg output (about half the quantity emitted by primary sources) is deposited near its point sources, the rest (a total of 3000 t) is subject to transport over great distances by atmospheric circulation, resulting in measurable contamination of terrestrial and aquatic environments and organisms up to several thousand kilometers from the points of origin. Indeed, a number of remote ecosystems receive most of their Hg input from the atmosphere. (iii) The available evidence supports the generally accepted conclusion that the Hg enrichment commonly seen in the uppermost horizons of sediment cores from remote lakes is due primarily to contamination by airborne anthropogenic Hg in the recent past, rather than postdepositional redistribution of Hg. Although postdepositional alteration may result in detectable remobilization of sedimentary Hg, its effects on total Hg profiles in lake sediment cores have been found, thus far, to be negligible. (iv) Atmospheric transport of anthropogenic Hg to aquatic and terrestrial ecosystems is a cause for concern, as the Hg is accumulating, to a greater or lesser extent, in organisms (e.g., fish in remote lakes). Moreover, the Hg is associated with, and interacts with, other by-products of fossil fuel combustion, including the strong acids responsible for acid precipitation. The acids aggravate the effects of Hg pollution by furthering the accumulation of methyl Hg in fish inhabiting ill-buffered lakes. Contamination of the atmosphere with Hg and associated pollutants is a serious international problem that calls for reduction or elimination of emissions.