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.
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