Occurrence and Mobility of Mercury in Groundwater

3. Biogeochemistry of mercury: Field and experimental studies in soil and
surface-water environments
3.1. Impact of mercury reactivity on transport
Oxidation-reduction, precipitation-dissolution, aqueous complexation, and adsorption-
desorption reactions will strongly influence the fate and transport of Hg in groundwater, and
Occurrence and Mobility of Mercury in Groundwater
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in the environment, generally. At the land surface, Hg participates in photochemical reactions
(see the review of Zhang, 2006), but these reactions are not relevant to groundwater. Biogeo‐
chemical reactions in soils are of great importance to the fate and transport of Hg, however.
Characteristics of soils, which include pH, carbon content, mineralogy, drainage properties,
slope, and texture, all play a role in Hg retention or mobility and whether THg inputs to the
land surface reach the water table. Concentrations of THg typically are higher in organic soil
horizons than in the deeper mineral horizons because Hg typically is closely associated with
organic matter (Amirbahman & Fernandez, 2012), and Andersson (1979) reports sorption to
iron oxides (typical of some temperate-climate subsoils, and also tropical soils) at pH > 5.5. The
reactions described below can occur in soils, in the surface-water environment, and, appa‐
rently, in groundwater as well.
3.2. Oxidation-reduction and sorption reactions
The three stable oxidations states of Hg in low-temperature aquatic systems are Hg(II), Hg(I)
and Hg(0). The mercurous (Hg(I)) species is stable over a more limited range of conditions in
sulfidic aqueous systems than it is when sulfur is absent (Hem, 1970). The Hg species vary in
their solubility, complexation, adsorption (Stumm and Morgan, 1995) and their availability for
microbial processes. Therefore, oxidation-reduction (redox) reactions will have a profound
influence on Hg concentrations and mobility in groundwater. Both abiotic and biotic (primarily
microbial) processes can drive Hg redox transformations.
Iron geochemistry is intimately associated with that of Hg. Anaerobic column experi‐
ments showed transport of Hg(II) retarded by sorption (as a Hg-Cl complex) to pyrite (FeS
2
)
(Bower, et al., 2008),  and Hg(II)) has been shown to sorb to iron oxides at pH > 5.5
(Andersson, 1979). Given a positive association of Hg with iron (Fe) in iron-hydroxide-
rich sub-soils in the New Jersey Coastal Plain, USA, sorption of Hg to Fe hydroxides
appears to be a mechanism for attenuating Hg (Barringer & Szabo, 2006).  The same
mechanism appears present at some mining sites (Rytuba, 2000;  van Staaten, 2000),
although formation of aqueous and solid-phase sulfides controls Hg(II) concentrations in
tailings-contaminated sediments from California, USA, mines (Rytuba et al., 2005). Fe(II)
hydroxides can be reductively dissolved by sulfide, resulting in the release of sorbed Hg.
Experiments showed that, in the presence of sulfide (S
2-
), Fe (III) was reduced and
concentrations of dissolved Hg increased (Slowey & Brown, 2007). It appears that these and
the experiments of Bower et al. (2008) were not done in the dark, however. Consequent‐
ly, applicability of results to a groundwater setting is not clear.
Field examples also demonstrate that oxygen-depleted conditions caused by septic-system-
effluent releases led to reductive dissolution of Fe hydroxides, resulting in release of sorbed
Hg(II) (Barringer & MacLeod, 2001). Further, the Fe(II) generated in such a reaction may adsorb
to minerals where it can then reduce Hg(II) to Hg(0) (Charlet et al., 2002). Recent experiments
show that Fe(II) in minerals also can reduce Hg(II) to Hg(0). For example, in sealed, dark
bottles, magnetite was found to reduce Hg(II) to Hg(0) within minutes (Wiatrowski et al.,
2009; Yee et al., 2010). Mercury (Hg(II)) was also rapidly reduced in anoxic solutions by Fe(II)
under varying pH conditions, with aqueous Fe(OH)
+
being the species that best described the
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electron transfer that occurred in the experiments (Amirbahman et al., 2012). Metals other than
iron, such as tin, are also known to reduce Hg(II) to Hg(0) (e.g., Biester et al., 2000).
Natural organic matter has been shown to abiotically reduce Hg(II) to Hg(0) (Allard & Arsenie,
1991). Experiments under dark anoxic conditions by Gu et al. (2011) showed that dissolved
organic matter (DOM) reduced Hg(II) to Hg(0) when low concentrations of DOM were present.
At higher DOM concentrations, however, complexation with Hg inhibited Hg reduction
reactions.
Microbially mediated redox reactions involving Hg also have been demonstrated. Hg(II) was
reduced to gaseous Hg(0) by a Pseudomonas strain (Baldi et al., 1993). A newly isolated merA-
carrying Bradyrhizobium bacterium recently was found by Wang et al. (2012) to also reduce
Hg(II) to Hg(0). The recent work by Wang et al. (2012) shows that Hg inhibits denitrification
in groundwater in direct proportion to the concentration of Hg.