32
R. Ebinghaus et al.
sediments, as well as with the open ocean, also seem to balance. Atmospheric
exchange in general seems to be a net emission process. They also attempted a
mass balance for methylmercury in ocean margins and came to the conclusion
that no boundary exchange process had significant impact on the MeHg pool in
coastal zones, except for upwelling processes that imported about half of the
annual biomass uptake of MeHg into ocean margin areas, with the other half
being produced in situ.
Considering the enormous part of the Earth's surface covered by oceans, it is of
vital importance to understand if oceans only reemit mercury that is transported
by rivers or deposited from the atmosphere, or if they actually contribute actively
to the global Hg budget as a net source. There is evidence that Hg from oceanic
sediments is released to the overlying water during diagenetic processes (Cossa
et al. 1996). These investigations suggest that Mn oxides bind most of the Hg in
oceanic sediments, and as these oxides become reduced and dissolved during
bacterial degredation of organic matter, Hg concentrations in sediment
porewaters are elevated by 1 order of magnitude compared to the overlying
waters. Fluxes calculated from these concentrations based on molecular diffusion
equal 26 ng m- 2 day-I (9.5 ~lg m- 2 year-I). Although this flux apparently accounts for only 3% of the total input into these systems (Cossa et al. 1996), when
scaled to the total ocean sediment surface area of 3.6 x 10" km 2 (= 71% of the
Earth's total surface area) (Fairbridge 1966), it yields over 3 x 10° t year-I. It is
therefore vital to investigate if this flux estimate is accurate and globally
representative, how it compares to direct sedimentation (i.e., if there is a net
release of Hg from oceanic sediments to the water column) and to what extent
this flux is carried through the water column to lead to atmospheric emission.
5.4
Reemissions from Terrestrial Systems and Wetland Areas
Compared to the seas, there are more data available regarding mercury exchange
between the atmosphere and several types of soils with respect to both the origin
and the level of their Hg contamination. Beginning with impacted sites, studies in
the vicinity of a strong industrial point source of mercury in eastern Germany
revealed wet deposition fluxes of 1200 ~lg m -2 year -I to the soils surrounding the
factory premises. For comparison, the combined source strength of the four
individual production sites (one active, three shut down) including their highly
contaminated vicinity was backmodeled to be 800 to 1600 kg year-I and the
model indicates that as much as 30% of the total annual emission is deposited
within 5 km from the spot of emission (Ebinghaus and Kruger 1996). Thus, soils
located close to large atmospheric Hg sources are clearly sinks while these
sources are active. In the long run, however, these soils may turn into important
sources when the original emissions are discontinued and direct flux measurements will be important at such sites.
The first direct flux measurements of mercury at an industrially impacted site
used a laboratory flux chamber (Lindberg and Turner 1977). Emissions were
measured over waste deposited in holding ponds from a former chlor-alkali plant
R. Ebinghaus et al.
sediments, as well as with the open ocean, also seem to balance. Atmospheric
exchange in general seems to be a net emission process. They also attempted a
mass balance for methylmercury in ocean margins and came to the conclusion
that no boundary exchange process had significant impact on the MeHg pool in
coastal zones, except for upwelling processes that imported about half of the
annual biomass uptake of MeHg into ocean margin areas, with the other half
being produced in situ.
Considering the enormous part of the Earth's surface covered by oceans, it is of
vital importance to understand if oceans only reemit mercury that is transported
by rivers or deposited from the atmosphere, or if they actually contribute actively
to the global Hg budget as a net source. There is evidence that Hg from oceanic
sediments is released to the overlying water during diagenetic processes (Cossa
et al. 1996). These investigations suggest that Mn oxides bind most of the Hg in
oceanic sediments, and as these oxides become reduced and dissolved during
bacterial degredation of organic matter, Hg concentrations in sediment
porewaters are elevated by 1 order of magnitude compared to the overlying
waters. Fluxes calculated from these concentrations based on molecular diffusion
equal 26 ng m- 2 day-I (9.5 ~lg m- 2 year-I). Although this flux apparently accounts for only 3% of the total input into these systems (Cossa et al. 1996), when
scaled to the total ocean sediment surface area of 3.6 x 10" km 2 (= 71% of the
Earth's total surface area) (Fairbridge 1966), it yields over 3 x 10° t year-I. It is
therefore vital to investigate if this flux estimate is accurate and globally
representative, how it compares to direct sedimentation (i.e., if there is a net
release of Hg from oceanic sediments to the water column) and to what extent
this flux is carried through the water column to lead to atmospheric emission.
5.4
Reemissions from Terrestrial Systems and Wetland Areas
Compared to the seas, there are more data available regarding mercury exchange
between the atmosphere and several types of soils with respect to both the origin
and the level of their Hg contamination. Beginning with impacted sites, studies in
the vicinity of a strong industrial point source of mercury in eastern Germany
revealed wet deposition fluxes of 1200 ~lg m -2 year -I to the soils surrounding the
factory premises. For comparison, the combined source strength of the four
individual production sites (one active, three shut down) including their highly
contaminated vicinity was backmodeled to be 800 to 1600 kg year-I and the
model indicates that as much as 30% of the total annual emission is deposited
within 5 km from the spot of emission (Ebinghaus and Kruger 1996). Thus, soils
located close to large atmospheric Hg sources are clearly sinks while these
sources are active. In the long run, however, these soils may turn into important
sources when the original emissions are discontinued and direct flux measurements will be important at such sites.
The first direct flux measurements of mercury at an industrially impacted site
used a laboratory flux chamber (Lindberg and Turner 1977). Emissions were
measured over waste deposited in holding ponds from a former chlor-alkali plant