past two decades. Mercury exists in the environment in three oxidation states — Hg(0), Hg(I), and Hg(II) — and for each valence many chemical forms can occur in the solid, aqueous, and gaseous phases. The environmental chemistry of mercury is very complex, and subtle changes in chemical, physical, biological, and hydrologic conditions can cause substantial shifts in its physical form and valence state over time scales ranging from hourly to seasonal.136,138,152 Here we briefly summarize selective aspects of mercury speciation in the atmospheric, aquatic, and terrestrial environments, focusing on aspects most pertinent to the ecotoxicology of mercury, such as the formation and abundance of methylmercury.
16.4.1 Atmosphere
In most locations, mercury in the atmosphere is mostly (> 95%) gaseous elemental Hg0, with the remainder composed largely of particulate ionic Hg(II), gaseous divalent mercury (commonly termed “reactive gaseous mercury”), and, on occasion, trace amounts of methylmercury.141,145 Particulate and reactive gaseous mercury have relatively short travel distances (up to tens of kilometers) and residence times in the atmosphere, whereas gaseous elemental mercury has global- scale transport and an average atmospheric residence time of about 1 year.48 Recent analyses of air in northern Europe showed that total gaseous mercury averaged 1.98 ng/m3, whereas particulate mercury and reactive gaseous mercury averaged 56 and 22 pg/m3, respectively.154,155 Monovalent mercury is stable only as the dimer (Hg22+), which rapidly disproportionates to Hg0 and Hg,2+ and is probably only detectable in atmospheric samples at extremely low levels.135 Over the open oceans concentrations of gaseous elemental mercury increase from the southern hemisphere (~1 ng/m3 at 60° south) to the northern hemisphere (~3 ng/m3 at 60° north),156,157 reflecting the stronger sources of mercury in the northern hemisphere, which is more industrialized and heavily populated than the southern hemisphere.
Reactive gaseous mercury is generally assumed to be HgCl2, although recent research has shown the existence of Hg(NO3)2ãH2O in the gas phase.158 After polar sunrise gaseous Hg0 in the Arctic and Antarctic atmospheres is rapidly depleted via oxidation to reactive gaseous mercury, which increases rapidly in abundance as Hg0 is depleted.139,140,159,160 During April and May 2000, reactive gaseous mercury often comprised more than 60% of the total gaseous mercury measured in air over Barrow, Alaska.160 Reactive gaseous mercury is rapidly removed from the atmosphere via both wet and dry deposition140,155,160 and is considered to be available for methylation once deposited.140 16.4.2 Aquatic Environments
The methylation of mercury and subsequent exposure of biota to methylmercury are greater in aquatic environments than in terrestrial environments. Many recent investigations of mercury in surface waters have determined methylmercury, gaseous elemental mercury (Hg0), and total mercury (defined as the sum of all mercury species recovered from a strongly oxidized sample).161 A fraction termed “reactive mercury,” which is generally equivalent to mercury reducible by stannous chloride, has also been measured; however, such fractions are often poorly defined and difficult to relate to other environmental factors or processes. The recent development of methods for separating col- loidal and truly dissolved fractions of inorganic mercury and methylmercury should advance understanding of aqueous-solid phase partitioning of mercury species and possibly bioavailable fractions.162 Dimethylmercury has been observed in the marine environment, but only at extremely small concentrations (averaging 0.016 ng/L in the North Atlantic).163 Dimethylmercury has not been confirmed in fresh waters, however, and its overall importance in the mercury cycle is unknown. We limit this discussion to the three fractions — total mercury, Hg0, and methylmercury
— that are most commonly reported for water.
Except under rare geochemical conditions, or in the vicinity of strong geologic or anthropogenic mercury sources, the concentrations of all forms of mercury in most natural waters are very low
(picograms to nanograms per liter). Most naturally occurring mercury compounds have very low solubility, although mercury complexes with dissolved organic matter are much more soluble.164 Among surface waters or within a given lake or stream, the abundances of methylmercury and total mercury can vary widely, and the accurate quantification of their aqueous concentrations requires the steadfast application of trace-metal clean techniques to minimize sample contamination during collection, handling, and analysis, coupled with the application of highly sensitive analytical methods.132,165 When proper sample collection and preservation protocols are followed, inter- comparisons among laboratories that use accepted analytical methods for total mercury and meth- ylmercury yield similar results.166
The speciation of mercury in water is most strongly influenced by the aqueous chemical conditions — most notably redox, pH, organic ligands, and inorganic ligands.165 Inorganic divalent mercury, Hg(II), and methylmercury are strongly influenced by the chemical makeup of the host water and almost entirely form ion pairs with ligands in the aquatic environment.167 In most oxic, circumneutral surface waters, ion-pair formation for Hg(II) and methylmercury is dominated by dissolved organic matter and chloride.168,169 Under anoxic conditions, which can occur in sediment porewaters and in the hypoliminia of certain lakes, or anywhere reduced sulfur species are appre- ciable, inorganic Hg(II) and methylmercury will dominantly be present as sulfide or sulfhydryl complex ion pairs.50,168,170 The complexation of Hg(II) with sulfide can substantially affect the availability of mercury for methylation by microbes.171
Concentrations of total mercury in unfiltered water samples from lakes and streams lacking substantive, on-site anthropogenic or geologic sources are usually in the range of 0.3 to 8 ng/L.133,172,173 In waters influenced by mercury mining or industrial pollution, concentrations of total mercury are greater, often in the range of 10 to 40 ng/L.33,133,173–175 Surface waters with high concentrations of humic substances can also have high concentrations of total mercury, demon- strating the importance of natural organic material on solubility and aqueous transport of the metal.115,123,127,176,177 Surface waters draining areas with high geologic abundances of mercury or with contaminated tailings from mercury or gold mining can exceed 100 or even 1000 ng/L in total mercury.3,26–28,33,38,41,165,178
In oxic waters, concentrations of methylmercury are typically within the range of 0.04 to 0.8 ng Hg/L.33,41,68,93,97,123,124,133,173,179,180 However, concentrations of 1 to 2 ng Hg/L can occur in surface waters affected by either industrial pollution (e.g., chlor-alkali plants)174,181 or mercury mine drain- age.28,133 The fraction of total mercury present as methylmercury is generally higher in fresh waters than in estuarine or marine systems,182 which may result from inhibition of methylation by the abundant sulfide in pore waters of brackish water systems171 or from the generally low level of dissolved organic matter in marine settings.127 Within a given drainage basin or geographic area the concentrations and yields of methylmercury, as well as the fraction of total mercury present as methylmercury, are typically highest in surface waters that drain wetlands.68,93,123–125,133,183 The biogeochemical processes contributing to the methylmercury-wetland association are under inves- tigation; however, it is evident that biogeochemical conditions in wetlands are favorable for meth- ylation and that complexation of methylmercury with the abundant natural organic matter in wetlands can facilitate its export to waters downstream. Methylmercury generally accounts for about 0.1 to 5% and seldom exceeds 10% of the total mercury present in oxic surface water.123,133,179
Under anoxic conditions, however, methylmercury can be one of the dominant species of mercury present, and concentrations can exceed 5 ng Hg/L.96,100,179,184
Early measurements of Hg0 in fresh waters showed concentrations ranging from about 0.01 to 0.10 ng/L, which led to a conclusion of pronounced super saturation of Hg0 in the water column, usually by a factor of 100 to 500, relative to the overlying air,185 yielding high estimated rates of Hg0 volatilization to the atmosphere. More recent investigations involving diel measurements have generally shown strong correlations between, on the one hand, instantaneous Hg0 in the water column and solar intensity and, on the other, a reequilibration with the atmosphere after sundown, with much lower concentrations of Hg0 in water at night (about 0.005 ng/L).138,186 Moreover, the
rapid reoxidation of Hg0 in surface water has also been demonstrated187 and, when taken into account, greatly decreases estimated volatilization rates of Hg0 from surface waters.188 In marine ecosystems, the evasion of Hg0 appears to be a geochemically significant efflux of mercury.48,189 16.4.3 Terrestrial Environments
Comparatively few data are available on the abundances of total mercury and methylmercury in soils and groundwater in upland settings relative to the substantive information available for surface water, sediment, and peat in aquatic environments. Yet recent estimates indicate that terrestrial soils contain the largest inventories of mercury from natural and anthropogenic emis- sions.48,131 In addition, the toxicity, solubility, and volatility of mercury depend highly on its speciation, and such information for soils is scant. Various reductive processes can yield appreciable emissions of Hg0 from contaminated soils, and the mercury in soils may be cycling more actively than previously thought.147,153,190,191
The speciation of mercury in most upland soils is probably dominated by divalent mercury species that are sorbed primarily to organic matter in the humus layer and secondarily to mineral constituents in soil.131,147 Nater and Grigal,148 who studied forest soils across the upper Midwest of the United States, found that concentrations of total mercury in humus ranged from about 100 to 250 ng/g dry weight, whereas the mineral horizon just below the humus layer contained about 15 to 30 ng/g. In locations near point sources, especially cinnabar (HgSs) deposits or abandoned placer mines, mercury concentrations can be considerably higher — generally in the àg/g range.26,153,192,193
The speciation of mercury in such highly contaminated soils depends on the origin of the mercury itself (most likely Hg0 used for placer mining and chlor-alkali plants, or HgSs) as well as the chemistry and texture of the soil. Barnett et al.,194 for example, observed that liquid Hg0 released to anaerobic, hydric soils resulted in the formation and long-term stabilization of mercuric sulfide.
Cinnabar, on the other hand, seems to be more stable when exposed to the surface as mine tailings or transported down gradient from mining operations, generally maintaining its HgSs stoichiometry, although surface coatings of secondary mercury compounds have also been observed on weathered cinnabar.195 Little is known about the relative stability and reactivity of mercury amalgam; however, it probably behaves similarly to elemental mercury in the environment.
Published information on concentrations and speciation of mercury in upland soils is sparse, especially for methylmercury. Forest soils have been rarely analyzed for methylmercury, and reported concentrations are generally low — about 0.2 to 0.5 ng/g in the humus layer and <
0.05 ng/g in the mineral-trophic layer.150,196,197 Although data are few, the very low concentrations of methylmercury in soils, runoff, and groundwater in upland environments suggest that little methylmercury is produced in upland landscapes.125,131,198