3 research outputs found
Organic Matter in Rain: An Overlooked Influence on Mercury Deposition
The importance of Hg emissions for
deposition will be scrutinized
in the future as new legislation to control emissions of Hg to the
atmosphere comes into effect. We show that mercury (Hg) concentrations
in rainfall are closely linked to organic matter (OM) with consistent
Hg/TOC ratios over large spatial scales decreasing from that in an
open field (OF, 1.5 μg g<sup>–1</sup>) to that in throughfall
(TF, 0.9 μg g<sup>–1</sup>). The leaf area index was
positively correlated with both TF [Hg] and total organic carbon ([TOC]),
but not the Hg/TOC ratio. This study shows that the progression in
the Hg/TOC ratio through catchments starts in precipitation with Hg/TOC<sub>bulk dep</sub> > Hg/TOC<sub>soil water</sub> > Hg/TOC<sub>streamwater</sub>. These findings raise an intriguing question about
the extent to which it is not just atmospheric [Hg] but also OM that
influences [Hg] in precipitation. This question should be resolved
to improve the ability to discern the importance of changing global
Hg emissions for deposition of Hg at specific sites
Eight Boreal Wetlands as Sources and Sinks for Methyl Mercury in Relation to Soil Acidity, C/N Ratio, and Small-Scale Flooding
Four years of catchment export and wetland input–output
mass balances are reported for inorganic Hg (Hg<sub>inorg</sub>),
methyl mercury (MeHg), dissolved organic carbon (DOC), and sulfate
in eight Swedish boreal wetlands. All wetlands had a history of artificial
drainage and seven were subjected to small-scale flooding during the
complete study period (two sites) or the two last years (five sites).
We used an approach in which specific runoff data determined at hydrological
stations situated at a distance from the studied sites were used in
the calculation of water and element budgets. All wetlands except
one were significant sinks for Hg<sub>inorg</sub>. Seven wetlands
were consistent sources of MeHg and one (an <i>Alnus glutinosa</i> swamp) was a significant sink. The pattern of MeHg yields was in
good agreement with previously determined methylation and demethylation
rates in the wetland soils of this study, with a maximum MeHg yield
obtained in wetlands with an intermediate soil acidity (pH ∼5.0)
and C/N ratio (∼20). We hypothesize that an increased nutrient
status from poor to intermediate conditions promotes methylation over
demethylation, whereas a further increase in nutrient status and trophy
to meso- and eutrophic conditions promotes demethylation over methylation.
Small-scale flooding showed no or moderate changes in MeHg yield,
maintaining differences among wetlands related to nutrient status
Impact of Forestry on Total and Methyl-Mercury in Surface Waters: Distinguishing Effects of Logging and Site Preparation
Forestry operations can increase
the export of mercury (both total
and methyl) to surface waters. However, little is known about the
relative contribution of different forestry practices. We address
this question using a paired-catchment study that distinguishes the
effects of site preparation from the antecedent logging. Runoff water
from three catchments, two harvested and one untreated control, was
sampled biweekly during one year prior to logging, two years after
logging, and three years after site preparation. The logging alone
did not significantly increase the concentrations of either total
or methyl-mercury in runoff, but export increased by 50–70%
in one of the harvested catchments as a consequence of increased runoff
volume. The combined effects of logging and site preparation increased
total and methyl-mercury concentrations by 30–50% relative
to preharvest conditions in both treated catchments. The more pronounced
concentration effect after site preparation compared to logging could
be related to site preparation being conducted during summer. This
caused more soil disturbance than logging, which was done during winter
with snow covering the ground. The results suggest that the cumulative
impact of forest harvest on catchment mercury outputs depends on when
and how forestry operations are implemented