78
L.D. Lacerda and W. Salomons
fraction is negligible, due to the low biological activity typical of tailings and the
high concentration of residual metallic Hgo.
In a study of gold mining tailings in central Brazil, Tumpling et al. (1993,1995)
showed total Hg concentrations in air just over tailings to range from 76 to
85 ng m- 3 , with only 3 to 6 ng m- 3 (less than 0.1%) of organomercurials. In the
same region and over the same geological formation, Hg concentrations in air
over noncontaminated soils average 2.8 ng m-3, with 0.4 ng m- 3 (ca. 15%) of
organomercurials (Tumpling et al. 1993). However, these results are only
preliminary, and detailed studies on Hg speciation in the degassing flux from
tailing should still be performed.
4.2
Mercury Export from Tailings to Adjacent Aquatic Systems
Strong gradients have been frequently found in both sediments and water Hg
concentrations from point sources of Hg, such as tailings. In general, these
gradients are very steep, indicating the low dispersability of Hg from the
releasing points. For example, in small creeks draining mine tailings in central
Brazil, Andrade et al. (1988), found that Hg contamination in the Crixas deposit,
Goias State, decreases from a maximum value of ca 16 ~lg g -I at the tailings to the
local background of ca 0.7 ~lg g-I in the first 200 m along the drainage. The same
behavior was found by Lacerda et al. (1991) at the Tanque dos Padres tailings,
Mato Grosso State. Mercury concentrations decrease from values up to 3.0 pg g-I
in the tailings to local background levels ("-'0.04 ~lg g -I) in the first 100 m along
the drainage.
Figure 1 shows Hg concentrations in sediments of rivers draining tailings
deposits in Pocone, central Brazil (Lacerda et al. 1991a). It is clear that Hg is very
refractory to transport since its concentrations drop to nearly local background
values only a few kilometers from tailings.
Similar results have been found in other tailings-affected drainages in the
Crixas mining site, Goias State, central Brazil and in the Cumaru mining site,
northern Amazon (Andrade et al. 1988; Ramos and Costa 1990).
Although Hg leaching from tailings seems a slow process, through the years
the migration of tailings themselves may be the most important Hg dispersal
mechanism. In the Carlson City mining district over the past century Hg-rich
tailings were transported over 100 km from the Six Mile Canyon and tributaries
to the Carlson River Valley, forming an Hg-rich 1.5 km
2
fan area (Lechler and
Miller 1993; Miller et al. 1993). In the Falsom-Natomas region, California,
Prokopovich (1984) suggested that groundwater migration could export Hg from
1868 dredge tailings, based on anomalous concentrations found in groundwater
samples from that area. In Nova Scotia, Canada, however, in groundwater
samples collected in last-century tailings, with Hg content ranging from 1.1 to
19.9 pg g-I, no contamination of groundwater could be detected (Lane et al.
1988). Creek waters draining tailings with up to 40 ~lg g-I of Hg in the Waverly
area showed Hg concentrations always below 50 pg 1-1 (Mudroch and Clair
1986). Surface and groundwater samples collected from over 21 stations in creeks
L.D. Lacerda and W. Salomons
fraction is negligible, due to the low biological activity typical of tailings and the
high concentration of residual metallic Hgo.
In a study of gold mining tailings in central Brazil, Tumpling et al. (1993,1995)
showed total Hg concentrations in air just over tailings to range from 76 to
85 ng m- 3 , with only 3 to 6 ng m- 3 (less than 0.1%) of organomercurials. In the
same region and over the same geological formation, Hg concentrations in air
over noncontaminated soils average 2.8 ng m-3, with 0.4 ng m- 3 (ca. 15%) of
organomercurials (Tumpling et al. 1993). However, these results are only
preliminary, and detailed studies on Hg speciation in the degassing flux from
tailing should still be performed.
4.2
Mercury Export from Tailings to Adjacent Aquatic Systems
Strong gradients have been frequently found in both sediments and water Hg
concentrations from point sources of Hg, such as tailings. In general, these
gradients are very steep, indicating the low dispersability of Hg from the
releasing points. For example, in small creeks draining mine tailings in central
Brazil, Andrade et al. (1988), found that Hg contamination in the Crixas deposit,
Goias State, decreases from a maximum value of ca 16 ~lg g -I at the tailings to the
local background of ca 0.7 ~lg g-I in the first 200 m along the drainage. The same
behavior was found by Lacerda et al. (1991) at the Tanque dos Padres tailings,
Mato Grosso State. Mercury concentrations decrease from values up to 3.0 pg g-I
in the tailings to local background levels ("-'0.04 ~lg g -I) in the first 100 m along
the drainage.
Figure 1 shows Hg concentrations in sediments of rivers draining tailings
deposits in Pocone, central Brazil (Lacerda et al. 1991a). It is clear that Hg is very
refractory to transport since its concentrations drop to nearly local background
values only a few kilometers from tailings.
Similar results have been found in other tailings-affected drainages in the
Crixas mining site, Goias State, central Brazil and in the Cumaru mining site,
northern Amazon (Andrade et al. 1988; Ramos and Costa 1990).
Although Hg leaching from tailings seems a slow process, through the years
the migration of tailings themselves may be the most important Hg dispersal
mechanism. In the Carlson City mining district over the past century Hg-rich
tailings were transported over 100 km from the Six Mile Canyon and tributaries
to the Carlson River Valley, forming an Hg-rich 1.5 km
2
fan area (Lechler and
Miller 1993; Miller et al. 1993). In the Falsom-Natomas region, California,
Prokopovich (1984) suggested that groundwater migration could export Hg from
1868 dredge tailings, based on anomalous concentrations found in groundwater
samples from that area. In Nova Scotia, Canada, however, in groundwater
samples collected in last-century tailings, with Hg content ranging from 1.1 to
19.9 pg g-I, no contamination of groundwater could be detected (Lane et al.
1988). Creek waters draining tailings with up to 40 ~lg g-I of Hg in the Waverly
area showed Hg concentrations always below 50 pg 1-1 (Mudroch and Clair
1986). Surface and groundwater samples collected from over 21 stations in creeks