Natural and Anthropogenic Mercury Sources
19
sources. The other three processes are all (more or less) influenced by human
activities and are both sources and sinks of atmospheric mercury. In contrast to
the former, they could be characterized as "surface-linked" processes for they
occur only at the very surface of the Earth's crust. The big question for those
interactions is: are they net sources or sinks? In the following sections, we will
attempt to quantify the first three processes and to evaluate the flux direction and
magnitude for the three natural air-surface exchange processes. Their absolute
and relative importance for regional and global Hg budgets will then be discussed
in Section 6.
It seems to be the general opinion that whenever elevated mercury
concentrations are encountered in any ecosystem compartment in the absence
of obvious local or direct anthropogenic sources, this can be interpreted as proof
of an anthropogenic influence via atmospheric long-range transport and
deposition. While this may be true in many cases, some of these anomalies
may be caused, at least partially, by natural local emissions due to underlying
geologic anomalies. For example, in the case of elevated Hg concentrations in
plants, foliar atmospheric uptake of crustal mercury emissions or root uptake
from geologically enriched soils are possible alternative explanations to longrange transport of pollutants (Lindberg et al. 1979). Also, surface enrichment
mercury in depth profiles measured in sediment cores may be caused by upward
migration due to diagenetical processes or groundwater movement rather than
solely by increased discharges or atmospheric deposition to the aquatic systems
(Rasmussen 1994). However, the direct evidence for such sediment redistribution
of mercury has yet to be published. Since these features may be overlooked in
ecosystem mass balances, they will be explored in more depth hereafter. The flux
estimates presented in the following sections are based on limited direct
measurements, simple concentration measurements, or modeling results.
4.1
Erosion and Oegasing of Mercury from Mineralized Surface Soils
Various estimates assume mercury volatilization rates between 1 and
5 ng m -2 h -I for background soils in the mercuriferous belts and mineralized
areas compared to below 1 ng m- 2 h- ' in totally unimpacted areas (e.g. Lindqvist
et al. 1991). Far higher emission rates (10-50 ng m- 2 h- ') have actually been
recently measured over temperate forest soils impacted by atmospheric
deposition (Lindberg et al. 1992; Kim et al. 1995) and over seismic zones
(Varekamp and Buseck 1986). Flux chamber measurements over cinnabar rich
soils in Almaden, Spain, yielded much higher fluxes, on the order of
330 ng m- 2 h- ' (Lindberg et al. 1979), and fluxes on the order of 501000 ng m- 2 h- ' were recently measured over geothermal zones in Nevada
(Gustin and Lindberg 1997), so, locally, these evaporation rates can be of
increased importance. Estimates of global natural fluxes from continents based
on lower flux estimates suggest a total of 700 t year-I degassing from soils, with
500 t year-I being contributed from the mercuriferous belts (Lindqvist et al.
1991). Clearly, the newer measurements may suggest larger natural emissions.
19
sources. The other three processes are all (more or less) influenced by human
activities and are both sources and sinks of atmospheric mercury. In contrast to
the former, they could be characterized as "surface-linked" processes for they
occur only at the very surface of the Earth's crust. The big question for those
interactions is: are they net sources or sinks? In the following sections, we will
attempt to quantify the first three processes and to evaluate the flux direction and
magnitude for the three natural air-surface exchange processes. Their absolute
and relative importance for regional and global Hg budgets will then be discussed
in Section 6.
It seems to be the general opinion that whenever elevated mercury
concentrations are encountered in any ecosystem compartment in the absence
of obvious local or direct anthropogenic sources, this can be interpreted as proof
of an anthropogenic influence via atmospheric long-range transport and
deposition. While this may be true in many cases, some of these anomalies
may be caused, at least partially, by natural local emissions due to underlying
geologic anomalies. For example, in the case of elevated Hg concentrations in
plants, foliar atmospheric uptake of crustal mercury emissions or root uptake
from geologically enriched soils are possible alternative explanations to longrange transport of pollutants (Lindberg et al. 1979). Also, surface enrichment
mercury in depth profiles measured in sediment cores may be caused by upward
migration due to diagenetical processes or groundwater movement rather than
solely by increased discharges or atmospheric deposition to the aquatic systems
(Rasmussen 1994). However, the direct evidence for such sediment redistribution
of mercury has yet to be published. Since these features may be overlooked in
ecosystem mass balances, they will be explored in more depth hereafter. The flux
estimates presented in the following sections are based on limited direct
measurements, simple concentration measurements, or modeling results.
4.1
Erosion and Oegasing of Mercury from Mineralized Surface Soils
Various estimates assume mercury volatilization rates between 1 and
5 ng m -2 h -I for background soils in the mercuriferous belts and mineralized
areas compared to below 1 ng m- 2 h- ' in totally unimpacted areas (e.g. Lindqvist
et al. 1991). Far higher emission rates (10-50 ng m- 2 h- ') have actually been
recently measured over temperate forest soils impacted by atmospheric
deposition (Lindberg et al. 1992; Kim et al. 1995) and over seismic zones
(Varekamp and Buseck 1986). Flux chamber measurements over cinnabar rich
soils in Almaden, Spain, yielded much higher fluxes, on the order of
330 ng m- 2 h- ' (Lindberg et al. 1979), and fluxes on the order of 501000 ng m- 2 h- ' were recently measured over geothermal zones in Nevada
(Gustin and Lindberg 1997), so, locally, these evaporation rates can be of
increased importance. Estimates of global natural fluxes from continents based
on lower flux estimates suggest a total of 700 t year-I degassing from soils, with
500 t year-I being contributed from the mercuriferous belts (Lindqvist et al.
1991). Clearly, the newer measurements may suggest larger natural emissions.