Environmental Engineering Reference
In-Depth Information
The high proportion of Hg inputs that are retained have
protected freshwater ecosystems from the full impact of
Hg pollution. But society is confronted with a contami-
nated landscape where “legacy” Hg has accumulated pref-
erentially in the organic fraction of soils, and especially
wetlands. Relative to newly deposited Hg, the availability
of this legacy Hg for transport and methylation remains
in question. Even if the legacy Hg should prove to be less
available for biotic uptake, there are still two major impli-
cations for the role of the terrestrial landscape as a source
of THg and especially MeHg to freshwater ecosystems.
The fi rst implication is that reductions in the atmospheric
inputs of Hg to catchments are unlikely to bring about a
corresponding decrease in the output of THg and MeHg
from catchments in the coming decades. This does not
mean that deposition reductions will not bring some rapid
benefi ts to freshwater ecosystems. For example, Hg deposi-
tion directly to lakes enters the aquatic food chain rapidly
(Munthe et al., 2007). For lakes, the importance of this
direct atmospheric loading relative to the catchment load-
ing varies with factors such as catchment to lake area and
the methylation potential of the landscape as compared
with the lake (Rudd, 1995).
The second implication pertains to another compo-
nent of atmospheric deposition, sulfate, which may regu-
late the rate of net methylation in the landscape. If the
“sulfur rain” hypothesis is correct, then elevated S depo-
sition in the latter half of the 20th century has stimu-
lated the SRB responsible for much of the transformation
of Hg to MeHg, the form that bio-accumulates most rap-
idly. If so, then the dramatic decrease in S deposition that
has occurred since about 1990 across much of Europe
and North America could lead to a decrease in net meth-
ylation in the landscape, and a corresponding decrease
in the loading of MeHg to surface water ecosystems. At
least one study, in the north-central United States, dem-
onstrates a decrease in fi sh Hg from decreasing S deposi-
tion alone (Hg deposition was constant) (Drevnick et al.,
2007). However, the lack of widespread declines in the Hg
of freshwater biota suggest that this effect is either small,
delayed, or has escaped detection because monitoring
networks are too sparse. Alternatively, widespread trends
of increasing DOC concentrations (Monteith et al., 2007)
may be countering the effects of decreasing S. Increasing
DOC will result in mobilizing additional Hg from terres-
trial to aquatic systems.
While changing deposition inputs of Hg to the land-
scape appears unlikely to have a strong infl uence on Hg
and MeHg outputs from catchments in the near future,
changes in climate may act more rapidly. This climatic
infl uence can work by changing hydrology and/or the
pools of carbon in the soil. Since carbon binds most of the
Hg in the soil, increasing those pools will increase the abil-
ity of catchments to hold Hg, while factors that reduce the
C pool in the soil will increase the output of legacy Hg and
reduce the ability to retain new inputs. The removal of Hg
from catchments occurs either by volatilization of elemen-
tal Hg back to the atmosphere, or by fl ushing from the
catchment in runoff. Thus, changes in hydrology, as are
widely expected from climate change (Huntington, 2006),
will likely affect the outputs of carbon and Hg. Changes in
hydrology would also alter water-table levels and soil satu-
ration and redox potential, another key control on the net
methylation rates in the landscape.
In the long term, society will reduce the risk of contami-
nation from terrestrial Hg if it reduces Hg emissions. In the
short term, however, society must contend with the legacy
of Hg on the landscape, and weigh its management options.
Herein lies a conundrum. Some management activities that
otherwise have strong environmental or societal benefi ts
may exacerbate Hg contamination. These include wetland
restoration and storm water retention, both of which cre-
ate favorable sites for Hg methylation. Another example is
managing nutrient runoff to reduce eutrophication. The
resulting reduction in algal growth, while clearly desir-
able for restoring dissolved oxygen and for aesthetic values,
actually increases Hg uptake in fi sh by concentrating avail-
able Hg into a more limited algal biomass that forms the
base of the food web. Managing hydropower-generating
facilities to limit water-table fl uctuations may reduce Hg
methylation in those water bodies, but these measures may
be at odds with optimal power production. The one man-
agement strategy universally favorable, with the caveat that
it will reduce the eutrophication that dilutes Hg, is erosion
control. Management of human activities, including land
development, agriculture, and forestry, to reduce sediment
movement to streams, will help to keep the mercury on the
terrestrial landscape and out of the water.
References
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Å. Iverfeldt. 1991. Occurrence and transport of mercury
within a small catchment area.
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Organic
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