Natural and Anthropogenic Mercury Sources
33
in Virginia. The mercury concentrations in the surface waste deposits averaged
160 pg g -I. The authors reported the highest fluxes yet published, on the order of
120 000 ng m -2 h -I to 170 000 ng m -2 h -I at 35°C and estimated an annual
atmospheric emission from this one area alone of about 40 kg year-I. Fluxes
increased exponentially with surface temperature, and resulted in ambient air
concentrations near the area of over 950 ng m- 3 in summer. Globally, hundreds
of such sites still exist and their contribution to regional mercury emissions
could be significant.
Elevated mercury emiSSIOns have also been reported for soils near the
Almaden mercury mine in Spain (Lindberg et al. 1979). Emission rates over
naturally Hg-enriched soils (concentrations averaged 97 pg g -I) ranged from 600
to 750 ng m -2 h -I at 35°C and also increased exponentially with soil temperature. For the first time direct plant uptake of this emitted airborne mercury was
measured, leading to elevated levels in leaves, although root uptake from the soil
pool was limited.
More recently, mercury fluxes from highly contaminated mill tailings from
former gold and mercury mines in the western US have been crudely estimated
from atmospheric concentration gradients combined with a simple onedimensional diffusion model. Fluxes were estimated to be 5-120 pg m -2 h -I,
and, surprisingly, no clear correlations with any meteorological parameter could
be identified; only total Hg content in the soil samples seemed to always correlate
with the estimated fluxes. The largest fluxes were usually derived from early
morning measurements, but sometimes increased fluxes were obtained at
nighttime or in the afternoon. Such results may indicate that some of the
measured gradients were artifacts of advection of contaminated air from local
sources, and cannot be used to derive fluxes. Scaling of the results to the whole
Carson River Superfund site, Nevada, USA, yielded emissions on the order of 150
to 400 kg year-I. These studies also showed elevated atmospheric Hg concentrations over a naturally mercury-enriched geothermal area, but these were much
smaller than over the mine tailings and no fluxes were derived (Gustin et al.
1996). More extensive measurements over a nearby geothermal site confirmed air
concentrations as high as 50 ng m - 3 while fluxes measured with portable soil
chambers ranged from approximately 10 to 1000 ng m -2 h -I, much lower than
seen over the mine tailings (Gustin and Lindberg 1997). More data will be
necessary to confirm that fluxes over naturally enriched soils are generally below
those over contaminated soils.
Another type of Hg-contaminated soils are wetlands adjacent to Hg-contamina ted waterbodies which have stored large amounts of Hg as a result of
sedimentation of contaminated particulate matter during periodic floodings. It is
a well-established remediation technique to construct wetlands in order to
remove contaminants from polluted river systems due to the wetlands' high
organic carbon accumulation (Patrick 1994). In the case of Hg, however, there are
indications that floodplains are not only initial sinks of deposited Hg, but also
long-term diffuse sources of atmospheric Hg since the high water content and
high productivity of wetlands seem to generate ideal conditions for the
transformation of nonvolatile into volatile Hg species (Wallschlager et al.
1997C). There are two prominent case studies for this kind of diffusely Hg-
33
in Virginia. The mercury concentrations in the surface waste deposits averaged
160 pg g -I. The authors reported the highest fluxes yet published, on the order of
120 000 ng m -2 h -I to 170 000 ng m -2 h -I at 35°C and estimated an annual
atmospheric emission from this one area alone of about 40 kg year-I. Fluxes
increased exponentially with surface temperature, and resulted in ambient air
concentrations near the area of over 950 ng m- 3 in summer. Globally, hundreds
of such sites still exist and their contribution to regional mercury emissions
could be significant.
Elevated mercury emiSSIOns have also been reported for soils near the
Almaden mercury mine in Spain (Lindberg et al. 1979). Emission rates over
naturally Hg-enriched soils (concentrations averaged 97 pg g -I) ranged from 600
to 750 ng m -2 h -I at 35°C and also increased exponentially with soil temperature. For the first time direct plant uptake of this emitted airborne mercury was
measured, leading to elevated levels in leaves, although root uptake from the soil
pool was limited.
More recently, mercury fluxes from highly contaminated mill tailings from
former gold and mercury mines in the western US have been crudely estimated
from atmospheric concentration gradients combined with a simple onedimensional diffusion model. Fluxes were estimated to be 5-120 pg m -2 h -I,
and, surprisingly, no clear correlations with any meteorological parameter could
be identified; only total Hg content in the soil samples seemed to always correlate
with the estimated fluxes. The largest fluxes were usually derived from early
morning measurements, but sometimes increased fluxes were obtained at
nighttime or in the afternoon. Such results may indicate that some of the
measured gradients were artifacts of advection of contaminated air from local
sources, and cannot be used to derive fluxes. Scaling of the results to the whole
Carson River Superfund site, Nevada, USA, yielded emissions on the order of 150
to 400 kg year-I. These studies also showed elevated atmospheric Hg concentrations over a naturally mercury-enriched geothermal area, but these were much
smaller than over the mine tailings and no fluxes were derived (Gustin et al.
1996). More extensive measurements over a nearby geothermal site confirmed air
concentrations as high as 50 ng m - 3 while fluxes measured with portable soil
chambers ranged from approximately 10 to 1000 ng m -2 h -I, much lower than
seen over the mine tailings (Gustin and Lindberg 1997). More data will be
necessary to confirm that fluxes over naturally enriched soils are generally below
those over contaminated soils.
Another type of Hg-contaminated soils are wetlands adjacent to Hg-contamina ted waterbodies which have stored large amounts of Hg as a result of
sedimentation of contaminated particulate matter during periodic floodings. It is
a well-established remediation technique to construct wetlands in order to
remove contaminants from polluted river systems due to the wetlands' high
organic carbon accumulation (Patrick 1994). In the case of Hg, however, there are
indications that floodplains are not only initial sinks of deposited Hg, but also
long-term diffuse sources of atmospheric Hg since the high water content and
high productivity of wetlands seem to generate ideal conditions for the
transformation of nonvolatile into volatile Hg species (Wallschlager et al.
1997C). There are two prominent case studies for this kind of diffusely Hg-