Environmental Engineering Reference
In-Depth Information
As an example, Figure 1.3 shows lake sediment profi les
of Hg enrichment from four lakes in the Western United
States. Here, enrichment is the Hg enhancement, relative to
another element that is assumed to be purely lithospheric
in origin, titanium. These data are from a study of Hg and
persistent organic pollutants from eight National Parks
in the western United States and Alaska that receive only
atmospheric input (Landers et al., 2008). Enrichment is
relative to titanium concentrations in the sediment cores
and is defi ned by:
Atmosphere
2600
(850)
1500
(900)
1600
(810)
1900
(950)
1500
900
(local)
Surface
Ocean
Land
Industry
2400
90
(90)
200
(90)
(M
x
Ti
x
)
(M
b
Ti
b
)
__
Percent Sediment Enrichment
Ti
b
)
100
(M
b
Deep
Reservoirs
where
Hg
x
mercury concentration (ng/g) at interval depth x
Ti
x
titanium concentration (ng/g) at interval depth x
FIGURE 1.2
A simplifi ed global geochemical mercury cycle. All
values are tons per year. Preindustrial values are given in parenthe-
ses below the modern values. The inner, dashed circle indicates the
perturbation of industrial activities that signifi cantly increased the
extraction of mercury from deep reservoirs. This results in signifi -
cantly greater local deposition and also increased input to the global
atmospheric pool, which increases global deposition. Upward arrows
from the land and ocean are net evasion and downward arrows are
wet and dry deposition. (
Source:
Adapted from Mason and Sheu,
2002.)
Hg
b
mercury concentration (ng/g) at interval closest
to year 1870
Ti
b
titanium concentration (ng/g) at interval closest to
year 1870
These lake sediments show a clear enhancement in
mercury deposition from the preindustrial era to the pres-
ent, consistent with other studies (e.g., Swain et al., 1992;
Schuster et al., 2002). Increases in the deposition rate, aver-
aging about threefold, are widely observed, although there
are signifi cant variations between sites. Individual lakes
and cores can vary because of local geography, geology,
and local emissions. These factors are not well understood
and limit our overall understanding of the Hg cycle. For
example, once deposited, Hg can be sequestered by organic
carbon. Thus, a change in organic carbon in the air or lake
can change the fraction of Hg that is permanently cap-
tured. Nonetheless, the increased atmospheric deposition
to the catchment, from both regional and global sources, is
required to explain the enhancements in Hg deposition in
the lake sediment cores. (Swain et al., 1992; Fitzgerald et al.,
1998; Schuster et al., 2002; Landers et al., 2008).
Observations of wet and dry deposition and large-scale
modeling of the global Hg cycle indicate that input from
the atmosphere is the primary cause of the increased accu-
mulation in the sediments. There is, nonetheless, still sig-
nifi cant uncertainty in some of the mechanisms involved
(e.g., Calvert and Lindberg 2005; Lin et al., 2006; Lindberg
et al., 2007). The fraction of the mercury deposition that
can be attributed to local (or regional) sources versus the
global background is an area of continued research. A num-
ber of modeling studies have apportioned local and regional
deposition to various sources, including anthropogenic
(local and global), natural, and recycled. For example, in
the United States, Seigneur et al., (2004) estimate that 30%
of total deposition is from North American anthropogenic
sources, 40% is due to anthropogenic sources outside North
America, and 33% is from natural sources. Selin et al., (2008)
100 greater than the annual fl ux, indicating a much lon-
ger lifetime for Hg. Therefore, for any given change in the
global cycle (e.g., anthropogenic emissions), we can expect
that the atmosphere will respond much more rapidly than
the ocean. Further, since the mixing and circulation of the
atmosphere occurs much more rapidly than that of the
ocean, we can expect that changes in the atmospheric bur-
den will be seen much more rapidly throughout the globe
than changes in the ocean. The net lifetime of mercury
against long-term burial (i.e. lifetime in the atmosphere,
surface ocean, and land surface system) is on the order of
1000 years (Selin et al., 2008).
Evidence of Recent Changes
in Deposition of Mercury
Signifi cant advances in mercury detection and sampling
have occurred in the past 20 years. These advances have
allowed for reliable determination of current and his-
torical levels of Hg deposited in sediments and ice cores.
Despite some initial contradictory reports, there is now
a good consistency between studies of deposition from
a large number of locations in North America, South
America, New Zealand, and Europe. These studies show a
compelling pattern of increasing deposition to lake sedi-
ments and glacial ice (Swain et al., 1992; Fitzgerald et al.,
1998; Lamborg et al., 2002; Landers et al., 2008) and a
generally consistent pattern in peat cores (Biester et al.,
2002, 2007).
