Environmental Engineering Reference
In-Depth Information
Methylation in the solid-phase peat was more limited but
still rapid, with 2% of both applied and native Hg methylated
after 90 days. Transport of spike Hg both laterally and verti-
cally was quicker than expected. In an analogous study in
METAALICUS uplands, Hintelmann et al. (2002) showed less
methylation overall in soils but preferential methylation of
spike Hg. In the upper 3 cm of forest fl oor, about 1.5% of
spike Hg was methylated as compared with 0.4% of the native
Hg. With the upland Hg spike, in contrast, most Hg stayed
in vegetation and did not migrate far vertically into upland
soils (Hintelmann et al., 2002). The preferential methyla-
tion of newly deposited Hg in wetlands suggests an immedi-
ate benefi t of reduced Hg emissions. However, it is unclear
whether differences in the availability of new and old Hg are
suffi cient for reduced deposition to translate into reduced Hg
methylation and MeHg export, at least on a decadal scale.
(Garcia and Carignan, 1999) and THg levels in pike (Garcia
and Carignan, 2000) were observed in adjacent waters
after clear cutting. Desrosiers et al. (2006) found that log-
ging decreased periphyton biomass in Canadian boreal
lakes but increased its MeHg content by a factor of 2 to 10.
Garcia and Carignan (2000, 2005) found elevated levels in
fi sh after lake catchments were harvested, and Garcia et al.
(2007) found signifi cantly elevated MeHg in zooplankton
in lakes with forest harvesting in their basins, correlated
with increases in DOC. In all cases, the effect lasted for at
least 3 years following the cessation of logging.
Although there are still relatively few studies of forestry
effects on Hg cycling, they have raised awareness that for-
estry practices may contribute to Hg bio-accumulation in fi sh.
Minimizing disturbances in riparian zones and peatlands,
where much of the methylation potential lies, appears to be
a prudent measure to try to reduce the impacts of forestry on
methylation and export of Hg. With the information avail-
able today though, it is diffi cult to assess the magnitude of
that eventual forestry contribution or the possibilities for mit-
igating that contribution through altered forestry practices.
Fire is another feature of the landscape that infl uences
Hg. Garcia et al. (2007) found that increased MeHg in zoo-
plankton after forest harvest had no signifi cant effect on
lakes whose basins had wildfi res during the same time.
Few studies have investigated Hg release from fi re, though
Hg release has been documented from western United
States forest fi res (Friedli et al., 2003). Watershed studies at
Acadia National Park, Maine, were conducted on a water-
shed known to have burned in 1938. THg fl ux from the
burned watershed was only about half of the THg fl ux from
a nearby control watershed, possibly because of the loss of
Hg volatilized during the fi re, lower DOC release from the
redeveloping forest fl oor, or both (Kahl et al., 2007; Nelson
et al., 2007) (see also, this topic, chapter 7).
Land-use changes alter Hg cycling not only during the
initial conversion, but also in a more permanent way by
changing land surface properties and land-atmosphere
interactions. For example, forest removal causes a signifi cant
reduction in Hg inputs via dry deposition. Countering this
factor, however, is a greater susceptibility to erosion in agri-
cultural and developing landscapes, favoring Hg mobility.
Thus, Hg export from agricultural streams is often greater
than forested streams in the same reg ion (Hurley et al., 1995,
1998; Babiarz et al., 1998; Balogh et al., 1997, 1998b). Urban
runoff has some of the highest Hg export per unit area
(Mason and Sullivan, 1998, Eckley and Branfi reun, 2008;
Shanley and Chalmers, 2012). However, MeHg export in
urban landscapes is relatively small, presumably because of
lack of landscape features that foster methylation.
Effects of Perturbation
The large pool of Hg in the surface soil leaves it vulnerable
to release on perturbation. This release may be as particu-
late THg or MeHg, where disturbance causes surface erosion,
as dissolved THg or MeHg where fl ow paths change, or as
gaseous Hg in the case of fi re. Perturbations may be natural
(hurricanes, forest fi res, tree throw) or human-induced (for-
est harvest, agriculture, land development).
Forestry is the predominant land use in much of the
boreal zone, where elevated Hg in fi sh is widespread. In a
synthesis of the literature, Bishop et al. (2009) estimated
that between 10% and 25% of the Hg in the fi sh of high-
latitude, managed forest landscapes can be attributed to
forest harvesting. Specifi c catchment studies have demon-
strated THg and MeHg mobilization as a result of forest dis-
turbance. Munthe and Hultberg (2004) measured large and
enduring releases of THg and MeHg when a logging road
was constructed through a forested research catchment
in Sweden. Porvari et al. (2003) documented increased Hg
export from a logging operation in Finland.
These effects of forestry can be attributed to an increased
net methylation of natural and anthropogenic Hg stored
in soils. Removal of trees will raise the water table, and the
disturbance associated with logging can also increase the
hydrologic connectivity along superfi cial fl ow pathways as
well as the output of DOC, which is associated with THg-
MeHg. However, Sorensen et al. (2009) demonstrated that
harvesting alone caused only a 20-30% increase in Hg fl ux
and had an indeterminate effect on MeHg. They implicated
soil disturbance in association with site preparation as the
primary driver of Hg and MeHg release following logging.
In any case, the evidence is clear that forest harvest can lead
to both increased Hg outputs and increased net Hg meth-
ylation, but more work needs to be done to identify the
degree of sensitivity of forested catchments to disturbance.
Forestry practices that release Hg and MeHg to surface
waters have been linked to Hg bioaccumulation. In Canada,
elevated concentrations of MeHg levels in zooplankton
Conclusions and Future Scenarios
Anthropogenic emission of Hg to the atmosphere and its
subsequent deposition has increased the concentrations
of THg and MeHg in the superfi cial soils of catchments.
