Environmental Engineering Reference
In-Depth Information
northern Sweden was comparable to that at Gårdsjön, despite
two times higher MeHg deposition at Gårdsjön.
Wetland area and location is an important factor in THg
and especially MeHg export and retention, as highlighted
by St. Louis et al. (1996). Although their estimate of dry
THg deposition as 50% of wet deposition may be low, THg
and MeHg export from various wetland types on their
Ontario landscape was relatively high, giving somewhat
low THg retention values of 55% to 85%, with wetland type
infl uencing the amount of retention. MeHg output from
the various wetland types varied greatly, with at least one
site showing net export of MeHg. Branfi reun et al. (1996)
and Selvendiran et al. (2008) have also found that THg and
MeHg retention vary by wetland type, and Kramar et al.
(2005) showed that wetland location relative to the stream
network was a critical control on THg export.
Applying Hg mass balances to specifi c components of
the landscape can help to pinpoint Hg sources, transfor-
mations, and retention mechanisms. In the few studies in
which the areal export from wetland and nearby uplands
could be identifi ed (Figure 8.6), wetlands, including ripar-
ian organic soils, showed one to two orders of magnitude
greater MeHg export than uplands. Even though there is
considerable variability in the MeHg export from differ-
ent types of peat and organic soils, the generally elevated
MeHg export from organic soils relative to mineral soils
may be a fi rst-order control on some of the large differences
in export and retention rates between catchments.
sites. Log K d is vary as the actual partitioning is controlled by
the quantity and quality of organic matter in both phases.
With Hg partitioning strongly favoring the solid phase, only
DOC with strong binding sites successfully complexes and
mobilizes Hg in solution. In the boreal and temperate ecosys-
tems where Hg has been most studied, DOC concentrations
are about six orders of magnitude greater than Hg concentra-
tions (Bishop et al., 1995b; Driscoll et al., 1995; Schwesig and
Matzner, 2001) The Hg:DOC ratio is often consistent across
diverse sites (Grigal, 2002; Shanley et al., 2002; Åkerblom
et al., 2008; Riscassi and Scanlon, 2011), but may vary at a
single stream site (Schelker et al., 2011).
Uplands
Schwesig and Matzner (2001) found that about 20% of
MeHg and 40% of THg in deposition pass through the for-
est fl oor to mineral soil. The THg-DOC relation was much
stronger in the stream than in soil water, presumably
because the stream integrates the various THg and DOC
source areas in the catchment. In freely draining upland
soils, the concentration of THg in soil solution tends to
increase toward the soil surface, mirroring the patterns of
DOC in soil solution and organic matter in the soil. MeHg
is also more abundant in organic soils, and rising water
tables facilitate transport of THg and MeHg to the stream.
In B-horizon soil water and in groundwater, DOC and THg
concentrations decrease. In well-drained upland catchments,
THg concentrations in groundwater are usually
1 ng L -1 ,
controlling the basefl ow Hg concentrations in streams. Note
that even these low THg concentrations are still controlled
by DOC, as supported by the linearity of the THg-DOC rela-
tion (Figure 8.4). THg and MeHg typically are considerably
higher in peatland groundwater (Selvendiran et al., 2008).
Groundwater may transport Hg directly to a lake or coastal
sea without any stream transport (Bone et al., 2007).
When considering THg and MeHg movement through
catchments, debate has centered on “new” Hg, and its reac-
tivity (Krabbenhoft et al., 2004). Researchers have hypoth-
esized that newly deposited atmospheric Hg is more mobile
and more biologically available to methylating bacteria
than native or aged Hg, so that lowering of Hg emissions
would have an immediate benefi t. In the METAALICUS
study, designed to directly test this idea, the hypothesis
was supported for Hg applied directly to the lake (Harris
et al., 2007). However, a surprisingly small amount of Hg
(~1%) applied to uplands appeared in runoff in the fi rst sea-
son of application, and most of the Hg export was preexist-
ing “old” Hg (Hintelmann et al., 2002).
Mercury Mobility and Flow Paths
The high soil sequestration rate for Hg is explained by
its strong preference for the solid phase. The log K d for
THg is on the order of 5.0 (Hurley et al., 1998; Brigham
et al., 2009), consistent with an O-horizon soil Hg concen-
tration of 100 ng g -1 in equilibrium with a soil solution Hg
concentration of 1 ng L -1 , typical values for uncontaminated
1.0
Uplands
Wetl an ds
3.1
0.8
0.6
0.4
0.2
Wetlands
0.0
Wetlands are hot spots on the landscape for production
of MeHg and for mobilization of THg and MeHg. Wet-
lands range from small pocket swamps or mires in oth-
erwise steep and/or well-drained terrain, to large features
FIGURE 8.6 THg export per unit area reported from several upland
and wetland areas in Europe and N. America. ( Sources: St. Louis et al.,
1994; Krabbenhoft et al., 1995; Bishop and Lee, 1997; Lee et al., 2000.)
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