Environmental Engineering Reference
In-Depth Information
Grigal (2003) discussed the changing Hg:SOM stoichi-
ometry with depth. Based on data reported in the litera-
ture, Grigal estimated an average of 0.7 mg Hg g
1
C in
the O horizon and 1.5, 2.5, and 3.8 mg Hg g
1
C in the
A, B, and C horizons, respectively. Other authors have also
reported Hg and organic C concentrations by depth or
morphologic increments. For example, Amirbahman et al.
(2004) reported 0.3, 1.4, and 2.3 mg Hg g
1
C in the O, Bh,
and Bs horizons, respectively. Aastrup et al. (1991) reported
2.7, 4.6, 4.3, and 3.4 mg Hg g
1
C in the O, upper B, lower B,
and C horizons, respectively, and Nater and Grigal (1992)
reported 0.29 and 0.44 mg Hg g
1
SOM (~0.58 and 0.88 mg
Hg g
1
C) for the forest fl oor and 0-25 cm mineral horizon,
respectively. The fact that the Hg:SOM ratio increases with
depth from the organic horizon to the mineral horizon is
largely a function of the higher mineralization rate of SOM
in the organic horizon. It may also be partly attributed to
the higher reactive surface area of the more humifi ed OM
in the mineral horizon that would enhance Hg retention
per gram of C as compared with the OM in the organic
horizon. The increases in Hg:SOM ratio with depth has
also been attributed to the increasing Fe and Al hydroxide
content with depth (Schlüter, 1997). As these minerals dis-
solve, soil pH increases, allowing for less competition with
Hg from H
for available surface binding sites. It should be
noted, however, that for spatially extensive soil data, little
or no correlation between Hg concentrations and soil prop-
erties such as SOM, clay content, or pH can result because
of confounding factors such as parent materials and depo-
sition history (Tack et al. 2005). Grigal (2003) notes that
factors such as historical Hg loading and the quality of
SOM can affect variations in the Hg:SOM ratio in soils in
the same landscape.
of the deciduous forests results in rapid turnover of the
SOM (Johnson, 1995), with little evidence for signifi cant
organic horizon development, which limits Hg accumula-
tion in the forest fl oor. Depth distributions of Hg in min-
eral soils (SOM
10%) of oxisols were similar to those of
extracted Fe and Al, suggesting that Hg accumulation is
controlled by soil Fe and Al concentrations (Roulet and
Lucotte, 1995; Roulet et al., 1998). As a result, soil Hg bur-
dens in a given system with similar external Hg input may
be predominantly a function of the degree of soil pedo-
genesis (Roulet et al., 1998). Roulet et al. reported that soil
differences between the plateau and valley manifest in the
progressive podzolization from oxisols to ultisols to spodo-
sols and corresponds to the leaching of soil Fe and Al and,
consequently, Hg. Emphasizing the role of ferrallitic soils
in Hg sequestration and the degradation of these soils in
Hg release into freshwaters, Roulet et al. (1999) have main-
tained that gold mining or deforestation through large-
scale burning does not explain the high Hg concentrations
in these waters. They indicated that
97% of the Hg bur-
den in the surface of central Amazonian soils is natural
and not due to anthropogenic activity. Deforestation is
important insofar as its role in accelerating the degradation
of ferrallitic soils is concerned (Oliveira et al., 2001). Gold-
mining operations and artisanal gold extractions have
important consequences for soil Hg at the local level. Soils
in the vicinity of these operations show Hg concentrations
considerably higher than those observed in tropical forests
(Lacerda et al., 1991, 2004; Rodrigues and Maddock, 1997).
Methylmercury
Processes leading to MeHg production in freshwater and
terrestrial environments are reviewed by Chen et al. (this
topic, chapter 9). In these environments, biotic Hg meth-
ylation is the most important process in MeHg production
and is largely catalyzed by sulfate-reducing bacteria. As a
result, Hg methylation requires the existence of anaero-
bic conditions. In forested watersheds, hydric soils and
wetlands have been identifi ed as methylation hot spots
because of the predominance of anaerobic conditions
(Branfi reun et al., 1996; Gilmour et al., 1998; Heyes et al.,
1998; Marvin-DiPasquale et al., 2003; McClain et al., 2003;
Mitchell et al., 2008). High concentrations of dissolved
MeHg have been observed in waters draining wetlands
and poorly drained upland soils (Krabbenhoft et al., 1995;
Rudd, 1995; Branfi reun et al., 1996). Low MeHg concentra-
tions and fl uxes from well-drained soils indicate that the
rate of methylation in these soils is insignifi cant, except for
recently harvested forests (Porvari et al., 1993; Driscoll
et al., 2007).
In 2008, Mitchell et al. observed that there is
a signifi cant spatial heterogeneity in MeHg distribution in
wetlands, with methylation hot spots concentrated close to
the upland-peatland interface. They attributed this partly
to the transport of sulfate and labile DOM from the adja-
cent upland to the peatland, enhancing methylation in
TOTAL MERCURY IN TROPICAL SOILS
Roulet et al. (1998) presented a comparison of Hg concen-
tration and cycling between the tropical soils of the cen-
tral Amazon and those of temperate environments. They
reported total Hg concentrations in the range of 93-171 ng
g
1
in the organic horizons, similar to those found in tem-
perate soils, and 88-209 ng g
1
in mineral horizons, higher
than those found in temperate soils. The concentrations
reported by Roulet et al. were similar to those reported
for other tropical soils (e.g., Aula et al., 1994; Roulet and
Lucotte, 1995; Oliveira et al., 2001; Lacerda et al., 2004). As
a result, the soils of the central and northeastern Amazon
possess Hg burdens in the upper 60 cm of the mineral soil,
ranging between 36 and 147 mg m
2
(Roulet et al., 1998;
Fostier et al., 2000), up to 10 times the Hg burden in tem-
perate and boreal forest soils (Aastrup et al., 1991; Grigal
et al., 1994).
In tropical soils, the generally advanced stage of pedo-
genesis and accompanying Fe and Al enrichment results
in elevated retention and accumulation of Hg. In these
soils, relatively high temperatures and high litter quality
