Occurrence and Mobility of Mercury in Groundwater

6. Conceptual model for processes influencing mercury fate and transport
in groundwater
Overall, the studies of the Hg-contaminated groundwater in New Jersey and Cape Cod, in
particular, have provided opportunities to investigate mechanisms for releasing Hg from the
land surface to the water table, and to suggest further avenues to explore biogeochemical
reactions that mobilize Hg from the subsurface and within aquifers. A conceptual model for
Hg mobilization has been developed by New Jersey researchers (Reilly et al., 2011) and is
shown in figure 2.
Mercury, either naturally occurring, of anthropogenic origin, or both, is released to the water
table as Hg(II) from surface soils and subsoils by weathering or by inputs of anthropogenic
chemicals such as road salt or fertilizers, or by subsurface inputs of septic-system effluent.
Under oxidizing conditions, dissolved THg is mobile as a complex with DOM, and sorbed
Hg(II) is mobile on Fe hydroxide particles. Effluent discharges provide electron donors and
sorbed Hg(II) is released as Fe hydroxides reductively dissolve, and Hg(II) may be reduced
either by DOM or by Fe(II). Where anoxic conditions are present, sulfate reduction is an
important terminal electron accepting process, and methylation may take place. Additionally,
sulfides may precipitate, removing Hg from the aqueous phase. Hg(0) may be re-oxidized to
Hg(II) should groundwater become more enriched in oxygen farther down a flowpath.
Current Perspectives in Contaminant Hydrology and Water Resources Sustainability
134

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7. Conclusions
Mercury is relatively rare compared to most other elements, but owing to its toxicity at low
concentrations, Hg is an important potential contaminant. There is a large reservoir of
inorganic Hg in the environment—much of it derived from human activities, most associated
with industrialization. Some of that Hg enters freshwater supplies where conditions may be
conducive to methylation and thus the production of MeHg that readily bioaccumulates.
Relatively less Hg is mobilized to groundwater than to surface water, in part because Hg can
be attenuated by sorption to clays, iron oxides, and residual soil organic matter. Studies of the
fate and transport of Hg in the subsurface are beginning to reveal how transport from land
surface to groundwater might occur and how Hg remains mobile within aquifers.
In none of the above studies of Hg discharge to coastal waters have the sources of the Hg in
SGD been identified, nor have the mechanisms for maintaining Hg mobility in groundwater
discharging to the coasts been discerned. Given the recent discoveries of Hg and MeHg inputs
from SGD, there clearly are avenues for further investigations into this phenomenon. Uptake
into estuarine biota in the biodiverse and biomass rich estuarine and coastal waters is of key
concern for these sensitive ecosystems and for human health, given the great potential for
biomagnification. The importance of groundwater inputs thus cannot be ignored.