Environmental Engineering Reference
In-Depth Information
TRANSPORT OF MERCURY FROM SOILS
The two wetlands studied at the Huntington Forest inter-
cept a majority of the upland runoff to the Arbutus Lake
watershed and have a signifi cant impact on downstream
MeHg fl uxes (Selvendiran et al., 2008b). The percent of THg
as MeHg (%MeHg) in the upland runoff increased 2-6%
from the inlet to the outlet of the wetlands. The average
concentration of MeHg in the upland stream above the
wetlands was near quantifi able limits (0.04 ng/L) and much
lower than in the stream below the wetland (0.17 ng/L).
MeHg in the stream below was strongly driven by seasonal
processes. During the growing season, MeHg concentra-
tions were three times higher (0.27 ng/L) as compared
with the nongrowing season (0.10 ng/L). There was also
a strong coupling of increases in MeHg with decreases in
sulfate concentrations during the growing season, suggest-
ing the linkage of sulfate reduction to MeHg production in
wetlands. Moreover, transport of Hg species from the wet-
lands was facilitated by DOC, as indicated by its signifi cant
positive relations with THg and MeHg. Both wetlands were
net sources of THg to the downstream lake ecosystem. The
calculated annual fl uxes of THg and MeHg draining the
wetlands at the outlet of the watershed were 2.3 µg m
2
yr
1
and 0.092 µg m
2
yr
1
, respectively.
Routine monthly sampling at Huntington Forest indi-
cated that hydrologic conditions infl uence THg concentra-
tions in upland and wetland streams. Detailed sampling of
Hg species during elevated discharge events confi rmed that
concentrations of Hg species increased during storm events
(Bushey et al., 2008; Dittman et al., 2009). Event sampling
also showed that relationships of Hg with DOC and total
suspended solids (TSS) were not simple and varied over
storm events. Marked increases in Hg
2+
were evident during
the rising limb of the hydrograph, suggesting wash-off of
Hg from the canopy or the near-stream channel (i.e., ripar-
ian area) early in the storm event. Positive relationships of
THg and DOC were not evident until later in the hydro-
graph, likely associated with soil fl ushing of DOC and THg
and a shift in water hydrology to more shallow hydrologic
fl ow paths.
Evasion is an important Hg loss from upland forests. However,
this fl ux varies highly with space and time and is diffi cult
to quantify. Soil Hg evasion fl uxes have been measured at the
Huntington Forest, with values ranging between -2.5 ng
m
2
hr
1
(i.e., deposition) and 27.2 ng m
2
hr
1
and positively
correlated with temperature and solar radiation (Choi and
Holsen, 2009). The measured GEM emission fl ux was high-
est in spring and summer, and lowest in winter. During leaf-
off periods, the GEM emission fl ux was highly dependent
on solar radiation and less dependent on temperature. Dur-
ing leaf-on periods, the Hg emission fl ux was fairly constant
because of canopy shading of the forest fl oor. The annual
GEM emission fl ux was estimated to be 7.0
g GEM m
2
yr
1
.
Note that this value is comparable to throughfall Hg fl uxes.
Understanding mobilization/immobilization processes
from upland soils is important in assessing the fate of
Hg. Upland soils represent the largest Hg ecosystem pool
that could potentially supply large quantities of Hg to the
atmosphere and/or to downstream aquatic ecosystems. At
the Huntington Forest, THg concentrations in upland for-
est soil decreased with depth, coinciding with decreases in
soil organic matter (Driscoll et al., unpublished data). Soil
solution THg concentrations, measured using lysimeters,
also decreased from 8.0 ng L
1
to 1.9 ng L
1
from the forest
fl oor to the lower mineral soil, respectively, and were sig-
nifi cantly correlated with DOC concentrations. The stoi-
chiometric pattern of Hg and DOC within soil solutions
suggest that Hg retention/mobilization is controlled by
the dynamics of soil organic matter. Conversely, soil water
concentrations of MeHg were relatively constant, near the
analytical detection limit (0.05-0.06 ng L
1
) and were not
correlated signifi cantly with DOC. While annual drain-
age losses from upland soil (THg, 2.3
μ
μ
g m
2
yr
1
; MeHg,
g m
2
yr
1
) were a small fraction of the soil pool
remaining in soil (THg, 73 mg m
-2
; MeHg, 0.29 mg m
2
),
these values were comparable to measured fl uxes in the
stream watershed, demonstrating the potential importance
of the mobilization of upland Hg species in watershed
budgets.
0.039
μ
MERCURY FATE IN ADIRONDACK LAKES
ROLE OF WETLANDS
Compared to fi eld studies in the Adirondacks of larger
organisms (namely, fi sh), those investigating concentra-
tions of Hg in seston are limited. The Hg concentrations of
water and seston were measured in nine lakes in the Adiron-
dacks exhibiting a range of physical and chemical charac-
teristics (Adams et al., 2009). Consistent with other studies
of remote temperate region lakes with forested watersheds
(Driscoll et al., 1994), dissolved (
The fl uxes and pools of THg and MeHg were also evaluated for
two northern temperate forest wetlands at the Huntington
Forest in the Adirondacks: an abandoned beaver meadow
and a riparian wetland (Selvendiran et al., 2008a). Mass bal-
ance calculations reveal that the wetlands were net sources
of THg and MeHg, although the magnitude of the source
is a function of wetland connectivity to stream water. The
storage of THg and MeHg in wetland soil is a large pool that
appears coupled with organic carbon and sulfur accumula-
tion. In the current scenario of decreasing Hg emissions and
atmospheric deposition, the large “active” soil pool in wet-
lands is a potential short-term and long-term source of Hg
and MeHg to downstream aquatic ecosystems.
0.45 µm) Hg concen-
trations from the mid-epilimnion of the study lakes was
88
16% of THg, and ranged from 1.15 to 5.42 ng L
1
. Dis-
solved Hg concentration was strongly related to lake DOC.
Mercury concentrations in each seston size class were pro-
portional to the dissolved surface water Hg concentration
of each lake. Increased dissolved surface water Hg resulted
