William F. Fitzgerald

The biogeochemical cycling of elemental mercury: Anthropogenic influences☆

Abstract A review of the available information on global Hg cycling shows that the atmosphere and surface ocean are in rapid equilibrium; the evasion of Hg0 from the oceans is balanced by the total oceanic deposition of Hg(II) from the atmosphere. The mechanisms whereby reactive Hg species are reduced to volatile Hg0 in the oceans are poorly known, but reduction appears to be chiefly biological. The rapid equilibrium of the surface oceans and the atmosphere, coupled with the small Hg sedimentation in the oceans makes deposition on land the dominant sink for atmospheric Hg. About half of the anthropogenic emissions appear to enter the global atmospheric cycle while the other half is deposited locally, presumably due to the presence of reactive Hg in flue gases. We estimate that over the last century anthropogenic emissions have tripled the concentrations of Hg in the atmosphere and in the surface ocean. Thus, two-thirds of the present Hg fluxes (such are deposition on land and on the ocean) are directly or indirectly of anthropogenic origin. Elimination of the anthropogenic load in the ocean and atmosphere would take fifteen to twenty years after termination of all anthropogenic emissions.

Determination of volatile mercury species at the picogram level by low-temperature gas chromatography with cold-vapour atomic fluorescence detection

Abstract Alkylmercury compounds were preconcentrated from air on a Carbotrap (graphitized carbon black) column at room temperature. The species were then transferred by thermal desorption to a U-tube chromatographic column packed with 15% OV-3 on Chromosorb WAW-DMSC, held at −196°C in liquid nitrogen. The compounds were clearly separated and eluted in order of increasing polarity using a simple, ramped heating step to 180°C over 20 min. After thermal decomposition of the eluant, the resultant mercury vapour was detected by cold-vapour atomic fluorescence spectrometry. The detection limits (as Hg) for the system were approximately 0.3 pg for mercury and dimethylmercury, 0.4 pg for diethylmercury, and 2.0 pg for methylmercury chloride. A study of the Long Island Sound atmosphere showed Hg0 to account for 95–100% of the total mercury present, with the remainder being monomethylmercury.

A Synthesis of Progress and Uncertainties in Attributing the Sources of Mercury in Deposition

Abstract A panel of international experts was convened in Madison, Wisconsin, in 2005, as part of the 8th International Conference on Mercury as a Global Pollutant. Our charge was to address the state of science pertinent to source attribution, specifically our key question was: “For a given location, can we ascertain with confidence the relative contributions of local, regional, and global sources, and of natural versus anthropogenic emissions to mercury deposition?” The panel synthesized new research pertinent to this question published over the past decade, with emphasis on four major research topics: long-term anthropogenic change, current emission and deposition trends, chemical transformations and cycling, and modeling and uncertainty. Within each topic, the panel drew a series of conclusions, which are presented in this paper. These conclusions led us to concur that the answer to our question is a “qualified yes,” with the qualification being dependent upon the level of uncertainty one is willing to accept. We agreed that the uncertainty is strongly dependent upon scale and that our question as stated is answerable with greater confidence both very near and very far from major point sources, assuming that the “global pool” is a recognizable “source.” Many regions of interest from an ecosystem-exposure standpoint lie in between, where source attribution carries the greatest degree of uncertainty.

Atmospheric cycling and air-water exchange of mercury over mid-continental lacustrine regions

Atmospheric mobilization and exchange at the air-water interface are significant features of biogeochemical cycling of Hg at the Earths surface. Our marine studies of Hg have been extended to terrestrial aquatic systems, where we are investigating the tropospheric cycling, deposition and air-water exchange of Hg in the mid-continental lacustrine environs of northcentral Wisconsin. This program is part of a multidisciplinary examination into the processes regulating the aquatic biogeochemistry of Hg in temperate regions. Trace-metal-free methodologies are employed to determine Hg and alkylated Hg species at the picomolar level in air, water and precipitation. We have found Hg concentrations and atmospheric fluxes in these fresh water systems to be similar to open ocean regions of the Northern Hemisphere. A well constrained mass balance for Hg has been developed for one of the lakes, Little Rock Lake, which is an extensively studied clear water seepage lake that has been divided with a sea curtain into two basins, one of which is untreated (reference pH: 6.1) while the other is being experimentally acidified (current pH: 4.7). This budget shows that the measured total atmospheric Hg deposition (ca. 10 µg m−2 yr−1) readily accounts for the total mass of Hg in fish, water and accumulating in the sediments of Little Rock Lake. This analysis demonstrates the importance of atmospheric Hg depositional fluxes to the geochemical cycling and bioaccumulation of Hg in temperate lakes. It further suggests that modest increases in atmospheric Hg loading could lead directly to enhanced levels of Hg in biota. Analogous modeling for monomethylmercury (MMHg) is as yet limited. Nevertheless, preliminary data for the atmospheric deposition of MMHg indicate that this flux is insufficient. to account for the amounts of MMHg observed in biota. An in-lake synthesis of MMHg is implicated. The importance of volatile Hg which is principally in the elemental form, and its evasion to the atmosphere is also illustrated. We suggest that the in-lake production of Hg° can reduce the Hg (II) substrate used in the in-lake microbiological synthesis of MMHg.

Picomolar mercury measurements in seawater and other materials using stannous chloride reduction and two-stage gold amalgamation with gas phase detection

Abstract Sampling and analytical methodologies were developed and tested which are non-contaminating, accurate, and sensitive, permitting the reliable determination of picomolar levels of Hg in natural waters. Mercury was isolated from solution using SnCl2 reduction and gas phase stripping with collection and concentration onto Au utilizing Class 100 clean laboratory conditions and practices. Mercury detection was conducted using a two-stage Au amalgamation gas train to introduce elemental Hg0 vapor into the gas cell of a flameless atomic absorption spectrophotometer. By carefully controlling and precisely estimating the procedural blank, a detection limit of 0.21 pM was achieved using a 2-1 sample volume for analysis. An analytical precision of about 10% was obtained for solutions with Hg contents between 2 and 20 pM using 500-ml aliquots for sample analysis. Verification of the analytical accuracy and precision of the method was demonstrated using aqueous laboratory and NBS standard reference materials spiked into acidified natural water samples at picomolar levels. Sample exposure to laboratory air containing elevated Hg was identified as a potentially serious source of Hg contamination to acidified natural water collections containing picomolar levels of Hg. Additional studies revealed that the bulk of Hg in open ocean and coastal seawater (≥88%) consists of labile species which are immediately reactive to SnCl2 reduction under acidic conditions.

Mercury biogeochemical cycling in the ocean and policy implications

Anthropogenic activities have enriched mercury in the biosphere by at least a factor of three, leading to increases in total mercury (Hg) in the surface ocean. However, the impacts on ocean fish and associated trends in human exposure as a result of such changes are less clear. Here we review our understanding of global mass budgets for both inorganic and methylated Hg species in ocean seawater. We consider external inputs from atmospheric deposition and rivers as well as internal production of monomethylmercury (CH₃Hg) and dimethylmercury ((CH₃)₂Hg). Impacts of large-scale ocean circulation and vertical transport processes on Hg distribution throughout the water column and how this influences bioaccumulation into ocean food chains are also discussed. Our analysis suggests that while atmospheric deposition is the main source of inorganic Hg to open ocean systems, most of the CH₃Hg accumulating in ocean fish is derived from in situ production within the upper waters (<1000 m). An analysis of the available data suggests that concentrations in the various ocean basins are changing at different rates due to differences in atmospheric loading and that the deeper waters of the oceans are responding slowly to changes in atmospheric Hg inputs. Most biological exposures occur in the upper ocean and therefore should respond over years to decades to changes in atmospheric mercury inputs achieved by regulatory control strategies. Migratory pelagic fish such as tuna and swordfish are an important component of CH₃Hg exposure for many human populations and therefore any reduction in anthropogenic releases of Hg and associated deposition to the ocean will result in a decline in human exposure and risk.

A non-steady-state compartmental model of global-scale mercury biogeochemistry with interhemispheric atmospheric gradients

A box model of mercury (Hg) cycling between the atmosphere and ocean is described and used to estimate Hg fluxes on a global scale (The Global/Regional Interhemispheric Mercury Model, GRIMM). Unlike previous simulations of this system, few assumptions are made concerning the rate of prominent marine biogeochemical processes affecting Hg (e.g., evasion, particle scavenging, and deep ocean burial). Instead, consistency with two observed atmospheric distributions was required: the interhemispheric gradient in total atmospheric Hg and the value for changes in the deposition of Hg from the atmosphere since industrialization observed in both hemispheres. Sensitivity analyses underscore the importance to modeling of the atmospheric lifetime of Hg, the magnitude of the interhemispheric gradient, the historical changes in Hg concentrations of various reservoirs, and vertical exchange between the surface ocean and the permanent thermocline. Results of the model indicate: lower evasional fluxes of Hg from the global ocean than previous estimates; a prominent role for particle scavenging as a removal mechanism from the surface ocean; a modest influence of dry processes (dust and gas) on Hg removal from the atmosphere; and an estimate of natural land-based sources of Hg to the atmosphere that is no more than about half that of anthropogenic sources.

11.4 – Geochemistry of Mercury in the Environment

This chapter focuses principally on the low-temperature environmental biogeochemistry of mercury. The current understanding of mercury cycling at Earths surface (soils, sediments, natural waters, and the atmosphere) is presented. The coverage, as appropriate, includes the anthropogenic interferences (i.e., mercury pollution) and biological mediation, which affect significantly the speciation, behavior, and fate of mercury in the environment. Some of the gains made in the understanding of the environmental biogeochemistry of mercury since Goldschmidts groundbreaking work are summarized. Much of this advancement has come since the early 1970s, and the development in mercury research continues at breakneck pace.

Mercury in the North Atlantic

Abstract Reactive mercury (Hg) concentrations (0.80±0.44 pM), measured during a cruise in the sub-polar North Atlantic (50–70°N) in August 1993 were lower those of more southerly regions of this ocean. A large fraction of the Hg in the surface waters was elemental Hg (Hg°; 88% of the reactive Hg, on average; 0.65±0.50 pM). Little dimethylmercury (DMHg) or monomethylmercury (MMHg) was found in surface waters but higher concentrations were found at depth. The high surface water Hg° concentrations likely reflect production of Hg° in concert with the short season of primary production in these waters. Overall, gas exchange plays a dominant role in the cycle of Hg in the upper waters of this region. For the sub-thermocline waters, a comparison of the Hg speciation in different water masses suggests that surface waters accumulate reactive Hg and methylated Hg—due to input of reactive Hg via remineralization and conversion of reactive Hg to methylated Hg—after sinking during winter to form North Atlantic deep water. Reactive Hg concentrations varied between water masses with the lowest concentrations being found in the waters north of Iceland (the source waters for deep water formation). DMHg was found throughout the sub-thermocline water column and the limited data set for MMHg suggests that this species was also prevalent. These observations suggest that methylated Hg production in the ocean is not confined to low oxygen or anoxic regions, as has been found in lakes. Concentrations of DMHg (0.08±0.07 pM, on average) were lower than those previously measured in the equatorial Pacific. Total Hg concentrations averaged 2.4±1.6 pM but little of this was particulate Hg ( −1 , typically). There was evidence of colloidally-bound Hg in surface waters. Overall, the data strengthen our hypotheses concerning Hg biogeochemical cycling in the ocean and confirm the importance of gas exchange and Hg methylation in the fate and transport of Hg in the ocean.

Sea-Air Partitioning of Mercury in the Equatorial Pacific Ocean

The partitioning of gaseous mercury between the atmosphere and surface waters was determined in the equatorial Pacific Ocean. The highest concentrations of dissolved gaseous mercury occurred in cooler, nutrient-rich waters that characterize equatorial upwelling and increased biological productivity at the sea surface. The surface waters were supersaturated with respect to elemental mercury; a significant flux of elemental mercury to the atmosphere is predicted for the equatorial Pacific. When normalized to primary production on a global basis, the ocean effluxes of mercury may rival anthropogenic emissions of mercury to the atmosphere.