Environmental Engineering Reference
In-Depth Information
In addition to affecting controls on mercury emissions,
linking mercury in fi sh to human activities has implica-
tions for how mercury concentrations in fi sh are perceived
and regulated. Risk perception likely changes depending on
whether one considers mercury in fi sh as natural or instead
as an insidious by-product of human industrial activity. The
former description is far less likely to arouse a call for gov-
ernment regulation. And indeed, the proposed link between
human activity and mercury concentrations in fi sh has been
the basis of litigation for health warnings on some seafood.
“MFM” model (Mason et al., 1994), the Global/Regional
Interhemispheric Mercury Model “GRIMM” (Lamborg
et al., 2002), the Mason and Sheu (2002) global mer-
cury cycle model, the Strode et al. (2007) GEOS-Chem
global mercury atmospheric/mixed slab ocean model, the
Sunderland and Mason (2007) multicompartment open
ocean box model, and the Selin et al. (2008) GEOS-Chem
global 3-D ocean-atmosphere-land model. Based on mea-
sured changes in sources, reservoir size, concentrations,
and knowledge of reaction rates and exchange processes,
these models attempt to recreate past and current con-
centrations in various reservoirs over time and predict
future trends in mercury concentrations and fl uxes.
Table 10.6 presents results from those six models related to
past and current increases in total mercury concentrations
in surface and deep waters of the ocean.
The predominance of atmospheric exchange with the
ocean is a key feature of global mercury cycling models
(Lamborg et al., 2002; Mason and Sheu, 2002; Mason et al.,
1994; Selin et al., 2008; Strode et al., 2007; Sunderland
and Mason, 2007). However, Sunderland and Mason's
model suggests that for surface waters of the North Pacifi c,
Mediterranean, and Atlantic Oceans, the input of mercury
via rivers is substantially more important than estimated
in previous models. While vertical mixing and exchange
with the deep ocean was previously assumed to be unim-
portant and was sometimes excluded from older models
(e.g., Mason et al., 1994), more recent models of mercury in
the ocean consider both deep ocean exchange and lateral
movement (e.g., Sunderland and Mason, 2007).
The global mass budget of Sunderland and Mason (2007)
suggests that reservoirs of mercury in the surface ocean
(to a depth of 1500 m) and deep ocean have increased on
average by ~25% and ~11%, respectively, compared with
preindustrial values. They note that global mean values
obscure regional-scale changes. For example, mercury
concentrations in surface waters of the Mediterranean have
increased by ~68% relative to preindustrial values, whereas
those in surface waters of the North Pacifi c have increased
by only ~9% in their model.
However, these estimates differ in both absolute and rela-
tive amounts from those made by others (Lamborg et al.,
2002; Selin et al., 2008; Strode et al., 2007), with respect to
s u r f ac e w ate r s i in s ome c a s e s a nd d e e p w ate r s i in ot he r s ( Ta ble
10.6), and their accuracy has been questioned (Fitzgerald
et al., 2007). Despite these differences, there is gener-
ally agreement that the marine environment is no longer
in steady state with respect to mercury fl uxes, and the
ocean is now a net sink for mercury, with a current rate
of accumulation of between 2.4 and 9.2 Mmol yr
1
, rep-
resenting an increase of 0.1-0.5% per year when averaged
across the entire ocean. Calculations predict that at cur-
rent rates of anthropogenic emissions, concentrations of
mercury in the oceans will continue to increase: up to
~80% of current levels in surface oceans and ~170% in deep
oceans in one model (Sunderland and Mason, 2007). Such
Long-Term Changes of Mercury
in the Marine Environment
Humans have undeniably had a measurable and profound
effect on mercury in the environment (see previous chap-
ters). Mercury depth profi les in lake sediment, peat, and
ice cores demonstrate global and regional increases in mer-
cury deposition from human activity (Biester et al., 2002;
Gobeil et al., 1999; Roos-Barraclough and Shotyk,
2003;
Vandal et al., 1993). Lake sediments appear to provide the
most reliable archive of past mercury deposition (Biester
et al., 2007), and these studies collectively show a threefold
to fi vefold increase in atmospheric deposition of mercury
since the advent of the Industrial Revolution. The observed
increases in mercury in these sediment records are consis-
tent with human uses of mercury and estimated losses to
the environment (as also discussed in previous chapters),
and generally refl ect the change in the magnitude of the
atmospheric mercury reservoir since preindustrial times.
Nearshore and estuarine sediment cores near urban-
ized areas show similar or greater temporal increases in
mercury contamination, demonstrating more local and
regional effects of human pollution. In San Francisco Bay,
for example, a tidal salt marsh downstream of a mercury
mine showed mercury concentrations that had increased
10 times over preindustrial values, peaking after the 1950s,
then returning to 5 times above baseline levels in modern-
day sediments (Conaway et al., 2004). Mercury deposition
histories recorded in Connecticut wetlands and Long Island
Sound sediments exhibit similar temporal trends of enrich-
ment, generally with an increase of threefold to fi vefold.
However, in these areas mercury contamination appears to
be caused by both local point sources and regional atmo-
spheric fl uxes (Varekamp et al., 2003).
Models of Mercury Cycling
While the increase in atmospheric mercury levels and
deposition due to human activity is well documented, it
is less clear how mercury concentrations in the ocean have
changed since preindustrial times. Because there have
been no global measurements of mercury over the past
150 years, models must be used to reconstruct human-
caused changes in the global distribution of mercury.
Some recent models include Mason, Fitzgerald, and Morel's
