Environmental Engineering Reference
In-Depth Information
Total mercury
A
Dry deposition
Litterfall + Net
throughfall
Fluxes: g m
-2
yr
-1
Stocks: g m
-2
80
Evasion
1
Net
throughfall
8
16
Litterfall
10
Wet deposition
Streamflow
Forest floor 1000
Streamflow
10
Mineral soil
10,000
8000
22,000
Wetland
Bedrock
Limited by weathering
B
Methyl mercury
Dry deposition
Litterfall + Net
throughfall
Fluxes: g m
-2
yr
-1
Stocks: g m
-2
1
Evasion
0
Net
throughfall
0.2
0.5
Litterfall
0.1
Wet deposition
Streamflow
Forest floor 20
0.15
Streamflow
Mineral soil
10
40
50
Wetland
Bedrock 0
FIGURE 8.5
Typical values for stocks and annual fl uxes in the northern temperate-boreal landscape for (a) THg and (b) MeHg.
is that it has a lower stock of Hg available for export and thus
a high net accumulation, while the unburned catchment is
closer to a steady-state condition. Presumably, aged Hg made
up a considerable portion of Hg export from the unburned
catchment (Hintelmann et al., 2002).
In another paired catchment study at Gårdsjön, Sweden,
Munthe and Hultburg (2004) examined output of THg and
MeHg from a small forested control catchment and a nearby
experimental catchment with a roof constructed below the
canopy. The roof excluded ambient throughfall, but the for-
est fl oor was irrigated with artifi cial throughfall of prein-
dustrial composition. Although THg and MeHg stream fl ux
per unit area in the control was about two times that in the
roofed catchment before roof construction, this ratio did not
change during 10 years of the roof experiment. This fi nding
convincingly demonstrates that existing Hg stores and inter-
nal catchment processes, such as organic matter mineraliza-
tion, can be more important to stream THg and MeHg export
than ambient deposition at time scales at least up to a decade.
The Gårdsjön results are supported by the METAALICUS
study in Canada, where only a minute fraction of isotopically
labeled Hg applied to an upland catchment was detected in
streamwater; rather, Hg export was dominated by native Hg
already present in the catchment (Harris et al., 2007). Also
demonstrating the importance of catchment processes, Lee
et al. (1995) showed that MeHg output at Svartberget in
