Environmental Engineering Reference
In-Depth Information
chaP ter 7
the role of soils in storage and cycling of Mercury
ar ia aM irbah Man and ivan J. Fernande z
1999) in the upper Midwestern United States and the Adiron-
dacks (New York) watersheds, respectively. The Hg content
of forested watersheds in the United States is more than
two orders of magnitude greater than the annual anthro-
pogenic Hg emissions in this country. Approximately 90%
of watershed Hg is in the soil rather than the aboveground
vegetation, with the bulk of the Hg storage in the mineral
soil (Grigal, 2003). Soils can also be a direct source of Hg to
the associated surface waters (Swain et al., 1992; Lorey and
Driscoll, 1999). It is estimated that 5-25% of atmospherically
derived Hg reaches downstream lakes, and in some instances
can account for up to 85% of the total Hg loading to these
ecosystems (Mierle and Ingram, 1991; Swain et al., 1992;
Krabbenhoft et al., 1995; Grigal, 2002; Shanley and Bishop,
this topic, chapter 8). In light of the ability of soils to store
Hg, it might be argued that soils buffer downstream aquatic
ecosystems against acute Hg contamination.
Soils play a large role in determining the fate and trans-
port of Hg within a watershed. Cycling and speciation of
Hg in watersheds is largely dependent on characteristics
such as percentage of wetland, slope, soil thickness, veg-
etation type, pH, and carbon content (Branfireun et al.,
1998). Hg retention can vary among watersheds having
similar precipitation and discharge rates because of differ-
ences in watershed characteristics, especially soils (Mierle
and Ingram, 1991). Hg retention in soils varies depending
on the type of soil. That soil Hg is largely associated with
the organic matter is now axiomatic (Grigal, 2003). Soil
mineralogy, however, can have an influence on the fate
of Hg (Andersson, 1979). Soils that consist largely of alu-
minosilicate minerals, such as those common in northern
climates, behave differently from soils that are dominated
by Al and Fe hydroxides, such as those found in tropical
regions. Waterlogged or hydric soils, such as those found
in wetlands, also behave differently from well-drained
The Role of SoilS in MeRcuRy SpecieS BudgeTS
total Mercury
Methylmercury
Modes of Mercury entry into soil
ASSociATion of MeRcuRy SpecieS wiTh SoilS And Soil
conSTiTuenTS
solid-solution interface
soil-air interface: Mercury emission from soils
Role of VegeTATion in MeRcuRy AccuMulATion
in foReST SoilS
Role of foReST fiReS And oTheR diSTuRBAnceS
Forest Fires
other disturbances
concluSionS
Mercury (Hg) cycling in soils is critical to our understanding
of Hg accumulation and loss in the environment. Soils have
a large capacity to store Hg, and small changes in Hg dynam-
ics can have large effects on ecosystem function and bio-
logic exposure. In most watersheds, the terrestrial compo-
nents receive and retain considerably more atmospheric Hg
deposition than the associated freshwater bodies. This can
be attributed to: (a) the typically larger areal extent of the
watershed as compared with the water body, (b) the role of
a vegetative canopy in expanding the effective surface area,
and (c) soil's high capacity to store Hg. These characteristics
cause soils to act as a long-term source of Hg to freshwater
systems and can uncouple the temporal linkage between
atmospheric deposition of Hg and its delivery to surface
waters. Studies have estimated the present-time annual
retention of atmospheric Hg deposition at 74% (Engstrom
et al., 1994) and 78% (Driscoll et al., 1998; Lorey and Driscoll,
