Environmental Engineering Reference
In-Depth Information
soils, such as those found in uplands, with respect to Hg
cycling. In terrestrial environments, wet soils that develop
anoxia are largely responsible for the methylation of Hg to
its highly toxic form, methylmercury (MeHg), which is bio-
magnifi ed in the food chain. In general, well-drained soils
do not produce and export signifi cant concentrations of
MeHg (Grigal, 2003).
Soils act as a source of Hg for associated freshwater bod-
ies: (a) via transport of soluble Hg-dissolved organic mat-
ter (DOM) complexes, and (b) as particulate matter due to
mechanical soil erosion or in-stream mineral (Al and Fe
hydroxides) formation, especially during high-discharge
events. The extent of watershed Hg contribution to a lake
depends on the nature of the lake; seepage lakes receive
most of their Hg loading directly from the atmosphere or
groundwater, whereas drainage lakes can receive a signifi -
cant fraction of their Hg from the associated watershed.
There have been several review articles specifi cally
devoted to the role of soils in Hg retention and cycling
(Andersson, 1979; Schuster, 1991; Schlüter, 2000; Grigal,
2003; Gabriel and Williamson, 2004), with each study
emphasizing different aspects of the issue. Andersson,
Schuster, and Gabriel and Williamson (2004) reviewed
geochemical processes involved in Hg-soil surface associa-
tion, Schlüter (2000) reviewed Hg volatilization from soils,
and Grigal (2003) reviewed the role of soils in Hg storage.
In this chapter, the emphasis is on Hg storage and cycling
in forest soils, particularly in the freely drained upland
ecosystem compartments. Wetlands that are key areas to
MeHg production in terrestrial environments are discussed
by Shanley and Bishop (this topic, chapter 8). We review
the geochemical processes leading to the binding, trans-
port, and volatilization of Hg. Hg transformation processes
are reviewed mainly with respect to its reduction from the
ionic to the elemental form, which is the species most ame-
nable to volatilization. We present the existing data on soil
MeHg storage, but since well-drained forest soils do not
contribute signifi cantly to MeHg production, methylation
and demethylation processes are not emphasized. We also
review the role of vegetation type and soil disturbance on
soil Hg storage and cycling.
mass of Hg in the forest fl oor (~1 mg m
2
), the mineral soil
(~10 mg m
2
), and peat (~20 mg m
2
) are considerably larger
than that in forest vegetation (~0.1 mg m
2
). In a survey
of Flemish soils of various origins (agricultural, forest, and
pasture soils, n
494), Tack et al. (2005) observed median
baseline concentrations for Hg of 100 ng g
1
, with the 90th
percentile value of 300 ng g
1
. Hg content in parent materi-
als is approximately estimated at
50 ng g
1
in igneous rocks
and minerals, sandstones, and limestones, with somewhat
higher concentrations in shales because of the presence of
fi ner-grained materials (Andersson, 1979).
Total Hg concentrations in surface organic horizons
(e.g., O horizons) are signifi cantly higher than those in
the mineral horizons. Nater and Grigal (1992) reported a
mean total Hg concentration of 140 ng g
1
in the organic
horizon and 20 ng g
1
in the mineral soil in forests across
the north-central USA (n
133). In the surface organic
horizons, mean total Hg concentrations of 190 ng g
1
in Norway, 250 ng g
1
in Sweden, and 300-900 ng g
1
in Central Europe have been reported (Godbold, 1994;
Schwesig et al., 1999; Grigal, 2003). These regional varia-
tions may be attributed to the historical loading to indi-
vidual soils (Grigal, 2003). Hissler and Probst (2006)
reported total Hg concentrations ranging from 16 to 399
ng g
1
in the mineral layer of 11 grasslands in a moun-
tainous watershed in France. Their mean lithogenic Hg
concentration of 32
9 ng g
1
corresponded to those of
other studies in temperate environments (e.g., Gracey and
Stewart, 1974; McKeague and Kloosterman, 1974; Dudas
and Pawluk, 1976; Nater and Grigal, 1992; Grigal et al.,
1994; Amirbahman et al., 2004)
.
In temperate soils, it is widely accepted that inorganic Hg
and MeHg are largely associated with soil organic matter
(SOM). SOM in an O horizon is composed of a combina-
tion of freshly deposited litter and partially decomposed
and humifi ed material, typically with a larger and highly
variable particle size as compared with SOM in mineral
horizons. This holds true, in general, when comparing O
and mineral soils whole or after sieving. The larger average
particle size of SOM in the O horizon provides less surface
area and fewer sites available to bind Hg than the SOM in
the mineral horizons (Andersson, 1979). Hissler and Probst
(2006) found a strong positive correlation between soil
Hg concentrations and SOM (slope, 13.3) in brown acidic
and brown podzolic soils. Brown acidic soils with a lower
SOM (
The Role of Soils in Mercury Species Budgets
Total Mercury
4%) had lower Hg concentrations (
100 ng g
1
),
Given the dominance of soil pools for Hg in upland water-
sheds, it is critical to understand soil Hg dynamics when
establishing watershed mass balance relationships for
Hg cycling and speciation. There is a large body of litera-
ture that reports on Hg budgets for soils of temperate and
equatorial regions. Reviews of this literature have been
published by Roulet et al. (1998) and Grigal (2003). Gri-
gal reported that the mean total Hg concentrations in 48
agricultural soils and 50 forest soils (pastures and peats
included) were similar at 140 ng g
1
. He estimated that the
and podzolic soils with a higher SOM (
6%) had higher Hg
concentrations (
100 ng g
1
). They showed that Hg enrich-
ment in soil and stream sediments, estimated by normal-
izing with respect to the soil scandium (Sc) content (Shotyk
et al., 2000) was proportional to SOM. They interpreted
this to refl ect that SOM binds with Hg deposited from the
atmosphere. The measured Hg concentration in the deeper
mineral horizon (30-60 cm) was three times higher than
the lithogenic Hg, suggesting the downward transport of
Hg in the soil column.
