The case for atmospheric mercury contamination in remote areas

Elevated levels of mercury in aquatic environments remote from industrial sources have been broadly attributed to long-range atmospheric transport and deposition of anthropogenic Hg. Evidence in support of this prevailing scientific view-global biogeochemical Hg models, sedimentary archives of historic Hg fluxes, and geographic trends in soil Hg-have been challenged as being insufficiently rigorous to rule out the alternative explanation that natural geologic sources are the principal contributors of Hg in remote locations. In this review, we examine the weaknesses in interpretation and the choice of information that has been used to argue against atmospheric Hg contamination. Analytical advances in measuring trace levels of environmental Hg have greatly narrowed estimates of natural Hg fluxes, providing a clear measure of the relative magnitude of anthropogenic Hg emissions and deposition. Recent experimental results indicate that diagenetic processes cannot explain the mounting number of lake sediment and peat profiles showing substantial increases in Hg flux during the past century. Geologic sources of Hg may be important in specific localities but cannot explain corresponding geographic trends in soil Hg and industrial emission sources. Despite uncertainties in current understanding, there is a broad and geochemically consistent data base indicating that, over large regions of the globe, human-related Hg emissions have increased relative to natural sources since the onset of the industrial period.