Toward an Assessment of the Global Inventory of Present-Day Mercury Releases to Freshwater Environments

Abstract

Aquatic ecosystems are an essential component of the biogeochemical cycle of mercury (Hg), as inorganic Hg can be converted to toxic methylmercury (MeHg) in these environments and reemissions of elemental Hg rival anthropogenic Hg releases on a global scale. Quantification of effluent Hg releases to aquatic systems globally has focused on discharges to the global oceans, rather than contributions to freshwater systems that affect local exposures and risks associated with MeHg. Here we produce a first-estimate of sector-specific, spatially resolved global aquatic Hg discharges to freshwater systems. We compare our release estimates to atmospheric sources that have been quantified elsewhere. By analyzing available quantitative and qualitative information, we estimate that present-day global Hg releases to freshwater environments (rivers and lakes) associated with anthropogenic activities have a lower bound of ~1000 Mg·a−1. Artisanal and small-scale gold mining (ASGM) represents the single largest source, followed by disposal of mercury-containing products and domestic waste water, metal production, and releases from industrial installations such as chlor-alkali plants and oil refineries. In addition to these direct anthropogenic inputs, diffuse inputs from land management activities and remobilization of Hg previously accumulated in terrestrial ecosystems are likely comparable in magnitude. Aquatic discharges of Hg are greatly understudied and further constraining associated data gaps is crucial for reducing the uncertainties in the global biogeochemical Hg budget.

Biotic formation of methylmercury: A bio–physico–chemical conundrum

“…Hg is unique among transition metals due to its high volatility as gaseous elemental Hg (Hg 0 ), with a residence time in the atmosphere of about 6–12 months, allowing for long‐range transport of Hg. Although both Hg 0 and inorganic divalent Hg (Hg II ) are released from many sources through a variety of natural and anthropogenic processes, the reported rise in Hg levels in the biosphere, and in terrestrial and marine systems is a consequence of anthropogenic emissions (Amos et al ; Lamborg et al ; Kocman et al ). In contrast to Hg 0 and Hg II , direct anthropogenic sources of organic Hg, mono‐methylmercury (MMHg, i.e., CH 3 Hg + ) or dymethylmercury (DMHg, i.e., [CH 3 ] 2 Hg) are scarce.…”

Five hundred years of anthropogenic mercury: spatial and temporal release profiles*

“…Compounds of Hg that are emitted into the atmosphere in divalent (Hg II ) form have a much shorter atmospheric lifetime (approximately 26 d against deposition) and are thus deposited much closer to emission sources [5]. In addition, much of the Hg resulting from human activities is released onto land or into water bodies close to the point of extraction or use [6]. This locally-deposited Hg can then be transported into rivers and find its way into lakes and oceans [5,6], and a substantial fraction is reduced back to Hg 0 and reemitted to the atmosphere.…”

“…In addition, much of the Hg resulting from human activities is released onto land or into water bodies close to the point of extraction or use [6]. This locally-deposited Hg can then be transported into rivers and find its way into lakes and oceans [5,6], and a substantial fraction is reduced back to Hg 0 and reemitted to the atmosphere. For example, evasion of Hg 0 from terrestrial ecosystems and the ocean is more than double present-day anthropogenic emissions [5,7].…”

Impact of Water-Induced Soil Erosion on the Terrestrial Transport and Atmospheric Emission of Mercury in China

“…5,6 Quantification of the THg amount released into the environment is essential to assess Hg global biogeochemical cycling accurately, and such evaluations have been performed at the global scale. 7–11 Nevertheless, the mechanism of horizontal terrestrial transport of THg is still poorly quantified. Previous studies have already indicated that terrestrial discharges of inorganic Hg, natural organic matter and nutrients, can significantly enhance MeHg accumulation in aquatic biota.…”