76
1.D. Lacerda and W. Salomons
of site, due to the similarity of mining operations and the fact that only a single
mercury species enters the process (HgO).
4
Mercury Emissions from Contaminated Tailings
4.1
Mercury Degassing from Contaminated Land and Tailings
Emission from contaminated soils, waters, and tailings involves the degassing of
Hg vapor to the atmosphere. The evasional fluxes of Hgo have an important
geochemical significance, since they can reach up to 15% of the atmospheric
deposition over natural areas (Vandal et al. 1993). Tailings are by far the most
important site of Hg degassing, since they can reach Hg concentrations of over
5000 ~lg g -I. In certain areas such as the Folsom Canal, California, degassing
from tailings is a potential source of Hg to humans living in urban developments
built on last-century dredge tailings from gold mining (Prokopovich 1984).
Rates of Hg degassing from natural, noncontaminated areas are in general less
than 10-20 ng m- 2 day-lor 1-3 ~lg m- 2 year- I (Fitzgerald et al. 1991; Table 1).
Wind speed is a key factor controlling Hg degassing from natural soils and water
surfaces, the absence of plant cover on tailings may therefore increase Hg
degassing.
Degassing from silver and gold mine tailings typically ranges from 100 to
1500 ~Lg m -2 year -I, much higher than over mineralized areas containing
cinnabar deposits, where degassing rates can reach up to 60 ~lg Hg m 2 year- I
(Rasmussen 1994). These rates, however, are similar to those measured over Hg
mining tailings, such as Almaden, Spain, where degassing may reach extremely
high values of up to 2600 ~lg Hg m2
year1
(Ferrara and Maserti 1994).
Apart from gaseous Hg, tailings may also emit to the atmosphere heavily
contaminated dust particles. Extremely high Hg concentrations have been
reported in this compartment. For example, reported values from the Amazon
gold mining, reach 36 ~lg g-I and 250 ~lg g-I of dust (MaIm et al. 1991; Thornton
et al. 1992, respectively). This compartment shows a very short residence time in
the atmosphere. However, it has been reported as a significant source of human
Hg exposure, through inhalation, in gold mining centers in central Brazil (Hacon
1966 ).
Contaminated soils and tailings tend to show degassing rates linearly
proportional to Hg concentrations in the first 15 cm of soil (Lechler 1993),
similar to soils over geological anomalies (Rasmussen 1994). Surface temperature
also exhibits a very strong influence on Hgo degassing from contaminated soils.
At the Carson River tailings, Nevada, USA, Lechler and Miller (1993) estimated
Hg efflux to the atmosphere in excess of 500 ng m- 2 day-I (c.a.
180 ~lg m- 2 year-I) (Table 1). These authors found a proportion between
degassing rates and Hg concentration in tailings close to linearity from
1.D. Lacerda and W. Salomons
of site, due to the similarity of mining operations and the fact that only a single
mercury species enters the process (HgO).
4
Mercury Emissions from Contaminated Tailings
4.1
Mercury Degassing from Contaminated Land and Tailings
Emission from contaminated soils, waters, and tailings involves the degassing of
Hg vapor to the atmosphere. The evasional fluxes of Hgo have an important
geochemical significance, since they can reach up to 15% of the atmospheric
deposition over natural areas (Vandal et al. 1993). Tailings are by far the most
important site of Hg degassing, since they can reach Hg concentrations of over
5000 ~lg g -I. In certain areas such as the Folsom Canal, California, degassing
from tailings is a potential source of Hg to humans living in urban developments
built on last-century dredge tailings from gold mining (Prokopovich 1984).
Rates of Hg degassing from natural, noncontaminated areas are in general less
than 10-20 ng m- 2 day-lor 1-3 ~lg m- 2 year- I (Fitzgerald et al. 1991; Table 1).
Wind speed is a key factor controlling Hg degassing from natural soils and water
surfaces, the absence of plant cover on tailings may therefore increase Hg
degassing.
Degassing from silver and gold mine tailings typically ranges from 100 to
1500 ~Lg m -2 year -I, much higher than over mineralized areas containing
cinnabar deposits, where degassing rates can reach up to 60 ~lg Hg m 2 year- I
(Rasmussen 1994). These rates, however, are similar to those measured over Hg
mining tailings, such as Almaden, Spain, where degassing may reach extremely
high values of up to 2600 ~lg Hg m2
year1
(Ferrara and Maserti 1994).
Apart from gaseous Hg, tailings may also emit to the atmosphere heavily
contaminated dust particles. Extremely high Hg concentrations have been
reported in this compartment. For example, reported values from the Amazon
gold mining, reach 36 ~lg g-I and 250 ~lg g-I of dust (MaIm et al. 1991; Thornton
et al. 1992, respectively). This compartment shows a very short residence time in
the atmosphere. However, it has been reported as a significant source of human
Hg exposure, through inhalation, in gold mining centers in central Brazil (Hacon
1966 ).
Contaminated soils and tailings tend to show degassing rates linearly
proportional to Hg concentrations in the first 15 cm of soil (Lechler 1993),
similar to soils over geological anomalies (Rasmussen 1994). Surface temperature
also exhibits a very strong influence on Hgo degassing from contaminated soils.
At the Carson River tailings, Nevada, USA, Lechler and Miller (1993) estimated
Hg efflux to the atmosphere in excess of 500 ng m- 2 day-I (c.a.
180 ~lg m- 2 year-I) (Table 1). These authors found a proportion between
degassing rates and Hg concentration in tailings close to linearity from