Natural and Anthropogenic Mercury Sources
25
classify here as indirect anthropogenic reemlSSlOns. This term is chosen to
describe secondary reemission of mercury from anthropogenic sources following
partial initial deposition of primary anthropogenic Hg emissions. Unfortunately
(for our understanding of global atmospheric Hg exchange), these processes
usually take place in compartments that also exhibit a natural mercury surface
exchange like waters and soils, thereby making it impossible to distinguish
between the two parallel ongoing natural and anthropogenic ally induced
processes and to separate them quantitatively. Examples of these indirect
anthropogenic emissions are discussed in this section and include emissions
from soils as a result of mercury spills, reemissions of formerly deposited
mercury in the proximity of large point sources, and emissions from anthropogenically contaminated waterbodies and their adjacent wetlands.
5.1
Reemissions from Rivers
Little is known about mercury emissions from the waterbodies of rivers. It is well
established that rivers transport mercury as a result of anthropogenic discharges
and/or natural surface runoff, and both transport and deposition behavior in the
river and especially in the estuaries with respect to discharges into the oceans
have been studied. The few studies that investigate Hg exchange between rivers
and the atmosphere all deal with the estuarine section of the rivers and are
discussed in the following section on oceans. One recent study (Bahlmann 1997),
however, deals with the occurrence of volatile Hg species along the transsect of
the highly contaminated Elbe River in Germany. This investigation shows that
volatile Hg compounds (defined as purgeable within 1 h; the exact speciation was
not investigated, but we designate them here as dissolved gaseous mercury, or
DGM) in the waterbody can be detected along the course of the river. It is shown
that the highest concentrations of "free" volatile Hg compounds are found in the
estuarine region. However, continuous purging of the waters revealed that a
much larger amount of Hg could be volatilized from the limnie samples, and this
may represent in-situ production of Hg(o) by reduction of HgH. The levels of
DGM found in this study are illustrated in Table 15, which compares DGM levels
in several waters.
This process is quite likely, since the suspended particles in the waterbody of
the Elbe are rich in both Hg (Wilken and Hintelmann 1991) and bacteria which
have been shown to be resistant to the high Hg levels. These bacteria were shown
to demethylate MeHg compounds as a detoxification mechanism and might also
be able to reduce divalent Hg compounds (Ebinghaus and Wilken 1993). On the
base of the determined concentrations and volatilization potentials, it has been
calculated that the Elbe River could emit between 100 and 500 kg of Hg to the
atmosphere anually (Bahlmann 1997). As in the case of oceans, there is a severe
lack of experimental flux measurements for rivers, so this estimate has to be
taken with great caution. One investigative experiment failed to detect a
significant Hg concentration gradient right above the water surface of the Elbe,
indicating that emission fluxes (if they occur at all) might be rather small
25
classify here as indirect anthropogenic reemlSSlOns. This term is chosen to
describe secondary reemission of mercury from anthropogenic sources following
partial initial deposition of primary anthropogenic Hg emissions. Unfortunately
(for our understanding of global atmospheric Hg exchange), these processes
usually take place in compartments that also exhibit a natural mercury surface
exchange like waters and soils, thereby making it impossible to distinguish
between the two parallel ongoing natural and anthropogenic ally induced
processes and to separate them quantitatively. Examples of these indirect
anthropogenic emissions are discussed in this section and include emissions
from soils as a result of mercury spills, reemissions of formerly deposited
mercury in the proximity of large point sources, and emissions from anthropogenically contaminated waterbodies and their adjacent wetlands.
5.1
Reemissions from Rivers
Little is known about mercury emissions from the waterbodies of rivers. It is well
established that rivers transport mercury as a result of anthropogenic discharges
and/or natural surface runoff, and both transport and deposition behavior in the
river and especially in the estuaries with respect to discharges into the oceans
have been studied. The few studies that investigate Hg exchange between rivers
and the atmosphere all deal with the estuarine section of the rivers and are
discussed in the following section on oceans. One recent study (Bahlmann 1997),
however, deals with the occurrence of volatile Hg species along the transsect of
the highly contaminated Elbe River in Germany. This investigation shows that
volatile Hg compounds (defined as purgeable within 1 h; the exact speciation was
not investigated, but we designate them here as dissolved gaseous mercury, or
DGM) in the waterbody can be detected along the course of the river. It is shown
that the highest concentrations of "free" volatile Hg compounds are found in the
estuarine region. However, continuous purging of the waters revealed that a
much larger amount of Hg could be volatilized from the limnie samples, and this
may represent in-situ production of Hg(o) by reduction of HgH. The levels of
DGM found in this study are illustrated in Table 15, which compares DGM levels
in several waters.
This process is quite likely, since the suspended particles in the waterbody of
the Elbe are rich in both Hg (Wilken and Hintelmann 1991) and bacteria which
have been shown to be resistant to the high Hg levels. These bacteria were shown
to demethylate MeHg compounds as a detoxification mechanism and might also
be able to reduce divalent Hg compounds (Ebinghaus and Wilken 1993). On the
base of the determined concentrations and volatilization potentials, it has been
calculated that the Elbe River could emit between 100 and 500 kg of Hg to the
atmosphere anually (Bahlmann 1997). As in the case of oceans, there is a severe
lack of experimental flux measurements for rivers, so this estimate has to be
taken with great caution. One investigative experiment failed to detect a
significant Hg concentration gradient right above the water surface of the Elbe,
indicating that emission fluxes (if they occur at all) might be rather small