Natural and Anthropogenic Mercury Sources
41
The assessment of truely "natural" mercury sources and their relative
importance compared to direct anthropogenic emissions and indirect (re)emissions is a fundamental problem in studying the global balance and cycling of
mercury in the environment. This knowledge is critical to our understanding of
the fate of mercury since so-called natural emissions cannot be controlled or
reduced in most cases.
In general, the distinction between natural and "quasinatural reemissions" of
mercury (that which was formerly deposited from the atmosphere to surfaces) is
difficult to discern and may be regarded as an unresolved problem. In addition,
the so-called natural emissions also include mercury that was previously
deposited from natural sources. It can be assumed that mercury that evades from
these types of sources is generally in the elemental form. Estimates of global
natural fluxes suggest a total of 700 t year-I degassing from soils, with
500 t year-I originating from the mercuriferous belts. However, more recent
measurements may suggest larger natural emissions. Degassing of mercury from
mineralization zones is a major contribution to the total continental mercury
emissions and seems to be comparable in magnitude to emissions from volcanic
and geothermal activity.
Soils located close to large atmospheric mercury 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. Globally,
hundreds of such sites exist and their contribution to regional mercury emissions
could be significant. It has been suggested that fluxes over contaminated soils are
generally higher from those over naturally enriched soils. The existence of a
"compensation point" for soils has been proposed, meaning that soils below a
certain Hg(o) concentration (or probably: soil gas concentration) tend to adsorb
mercury, while soils above that concentration emit mercury depending on the
Hg( 0) concentration in the overlying air. For forest soils, a recent study has
demonstrated that fluxes could change from net emission to net deposition
depending on the soil moisture status. Solar radiation, temperature, and soil
moisture were identified as key parameters affecting mercury emissions from
soils. A recent study gives an estimate for mercury emissions from background
soils of 1000 t year-Ion a global basis, with two thirds coming from sunlightexposed open-field soils. There is evidence that the treatment of agricultural soils
with municipal sewage sludge can be an important factor for mercury emissions
into the atmosphere at least on a regional scale in otherwise uncontaminated
areas where this practice is common. Emissions of mercury from forests may
exceed similar emissions from background soils on a global basis. It has further
been suggested that terrestrial emissions and re-emissions may be comparable to
or exceed those from oceans.
The emission of mercury from oceans to the atmosphere is certainly occurring;
a number of studies have measured volatile mercury species (primarily DGM) in
ocean waters well above saturation levels. However, since deposition is also
occurring at the same time, a major question is whether or not emissions exceed
deposition. More detailed and larger-scale studies are necessary for the
characterization of the oceans or the ocean margins as either absolute sinks or
sources for atmospheric mercury. It is especially important to finally obtain real
41
The assessment of truely "natural" mercury sources and their relative
importance compared to direct anthropogenic emissions and indirect (re)emissions is a fundamental problem in studying the global balance and cycling of
mercury in the environment. This knowledge is critical to our understanding of
the fate of mercury since so-called natural emissions cannot be controlled or
reduced in most cases.
In general, the distinction between natural and "quasinatural reemissions" of
mercury (that which was formerly deposited from the atmosphere to surfaces) is
difficult to discern and may be regarded as an unresolved problem. In addition,
the so-called natural emissions also include mercury that was previously
deposited from natural sources. It can be assumed that mercury that evades from
these types of sources is generally in the elemental form. Estimates of global
natural fluxes suggest a total of 700 t year-I degassing from soils, with
500 t year-I originating from the mercuriferous belts. However, more recent
measurements may suggest larger natural emissions. Degassing of mercury from
mineralization zones is a major contribution to the total continental mercury
emissions and seems to be comparable in magnitude to emissions from volcanic
and geothermal activity.
Soils located close to large atmospheric mercury 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. Globally,
hundreds of such sites exist and their contribution to regional mercury emissions
could be significant. It has been suggested that fluxes over contaminated soils are
generally higher from those over naturally enriched soils. The existence of a
"compensation point" for soils has been proposed, meaning that soils below a
certain Hg(o) concentration (or probably: soil gas concentration) tend to adsorb
mercury, while soils above that concentration emit mercury depending on the
Hg( 0) concentration in the overlying air. For forest soils, a recent study has
demonstrated that fluxes could change from net emission to net deposition
depending on the soil moisture status. Solar radiation, temperature, and soil
moisture were identified as key parameters affecting mercury emissions from
soils. A recent study gives an estimate for mercury emissions from background
soils of 1000 t year-Ion a global basis, with two thirds coming from sunlightexposed open-field soils. There is evidence that the treatment of agricultural soils
with municipal sewage sludge can be an important factor for mercury emissions
into the atmosphere at least on a regional scale in otherwise uncontaminated
areas where this practice is common. Emissions of mercury from forests may
exceed similar emissions from background soils on a global basis. It has further
been suggested that terrestrial emissions and re-emissions may be comparable to
or exceed those from oceans.
The emission of mercury from oceans to the atmosphere is certainly occurring;
a number of studies have measured volatile mercury species (primarily DGM) in
ocean waters well above saturation levels. However, since deposition is also
occurring at the same time, a major question is whether or not emissions exceed
deposition. More detailed and larger-scale studies are necessary for the
characterization of the oceans or the ocean margins as either absolute sinks or
sources for atmospheric mercury. It is especially important to finally obtain real