Mercury in the Marine Environment - Mercury in the Environment - page 178

Dissolved and particulate MMHg concentrations reported
for estuaries are often less than 0.3 pM, and MMHg generally
represents less than 4% of the total mercury pool (Balcom
et al., 2004, 2008; Benoit et al., 1998; Choe and Gill, 2003;
Conaway et al., 2003; Cossa and Gobeil, 2000; Leermakers
et al., 1995, 2001; Mason et al., 1993, 1999). Substantially
higher MMHg concentrations may exist, but values

MMHg again constitutes ~1.5% of this 14 Mmol yr 1 , then
the riverine fl ux of MMHg to coastal areas would be ~0.21
Mmol yr 1 . Because such fl uxes are concentrated in nearshore
regions, riverine inputs represent a more important source of
MMHg to coastal regions than for the entire oceanic reservoir.
Estuaries represent an important source of MMHg to the
coastal ocean both as vectors for transporting MMHg from
upstream terrestrial environments and as sites of MMHg
production. This behavior is in contrast to the ability of
estuaries to attenuate the fl ux of total mercury to coastal
oceans through the formation and evasion of Hg(0) out
of surface waters and the scavenging of total Hg by estua-
rine sediments (Benoit et al., 1998; Macleod et al., 2005;
Mason et al., 1999). The contrasting behavior of total Hg
and MMHg result in the estuaries of the Patuxent River
(Benoit et al., 1998), Chesapeake Bay (Mason et al., 1999),
Gulf of Trieste (Faganeli et al., 2003), and San Francisco Bay
(Macleod et al., 2005) all acting as sinks for total mercury
but net sources of MMHg to adjacent coastal waters.
Despite diffi culties in determining the relative impor-
tance of internal versus upstream sources of MMHg in estu-
aries, estuary sediments are known to be a source of MMHg,
and their effi cient production of MMHg is due to a num-
ber of factors unique to these ecosystems. The oxic-anoxic
oscillations and alternating episodes of sedimentation and
resuspension experienced by tidally infl uenced estuary
sediments can facilitate both the production and export of
MMHg (Catallo, 1999; Tseng et al., 2001; Sunderland et al.,
2004). In addition to mercury methylation and demethyl-
ation, the export of MMHg from estuaries is highly depen-
dant upon hydrologic and transport processes, and at
times can be controlled largely by fl ow conditions and the
transport of organic carbon and particulate material to and
through an estuary (Mason et al., 1999).

2 pM
are only rarely reported for estuary waters (Baeyens et al.,
1998; Conaway et al., 2003; Mason et al., 1993, 1999).
Much of this riverine and estuarine MMHg is trans-
ported bound to particles. MMHg has a high affi nity for
particles, with particle distribution coeffi cients (K d ) gener-
ally in the range 10 4 —10 5 in estuarine and coastal waters
(Baeyens et al., 1998; Balcom et al., 2008; Benoit et al.,
1998; Choe and Gill, 2003; Horvat et al., 1999; Leermakers
et al., 2001; Hammerschmidt and Fitzgerald, 2006a). In
addition, a substantial portion of the operationally defi ned
“dissolved” MMHg can be associated with colloids in both
estuary and open ocean environments, as is also true for
total Hg (Choe and Gill, 2003; Mason and Sullivan, 1999;
Mason et al., 1995, 1998; Stordal et al., 1996).
Evaluating the importance of rivers, or any other input, as a
source of MMHg to the marine environment requires an esti-
mate of the amount of MMHg in the oceans. Unfortunately,
data on MMHg levels in many regions of the ocean are scarce.
Outside of coastal regions, MMHg concentrations in the water
column of the ocean are generally less than 0.2 pM (Cossa
et al., 1997; Horvat et al., 2003; Kotnik et al., 2007; Lamborg
et al., 2008; Mason and Fitzgerald, 1993; Mason and Sullivan,
1999), and it is not uncommon for them to be less than detec-
tion limits, which until very recently were ~0.05 pM for sea-
water. Using an estimate of the total amount of mercury in
the oceans of 1760 Mmol (Sunderland and Mason, 2007) and
assuming that MMHg comprises, on a weighted average basis,
roughly 4% of the total oceanic mercury pool (Cossa and
Coquery, 2005; Cossa et al., 1997; Horvat et al., 2003; Kotnik
et al., 2007; Lamborg et al., 2008; Mason and Fitzgerald,
1993; Mason and Sullivan, 1999), then the total amount of
MMHg in the oceans (water column only) is estimated to be
~70 Mmol (equivalent to a mean MMHg concentration of
~50 fM). Using a global riverine fl ux of total mercury to the
open ocean of 1.9 Mmol yr 1 (Sunderland and Mason, 2007)
and assuming that MMHg constitutes 1.5% of this, the
input of MMHg to the open oceans from rivers would be
~0.03 Mmol yr 1 . This fl uvial fl ux is ~0.03% of the estimated
standing stock of MMHg in the ocean, and is similar to the
estimated wet depositional fl ux of MMHg.
This calculation of riverine inputs includes only dis-
solved and particulate MMHg making it to the open
ocean. As described above, roughly 90% of the total mer-
cury riverine inputs are deposited in coastal and nearshore
sediments, and the overall fl ux of total mercury to coastal
waters from rivers is ~14 Mmol yr 1 (Sunderland and Mason,
2007). MMHg is not as particle-reactive as total mercury
(lower K d ), and a larger fraction of MMHg likely makes it
to the open ocean than for total mercury. Nonetheless, if

WASTEWATER

Municipal and industrial wastewater or sludge can be a
source of MMHg when discharged directly or indirectly
to coastal waters (Balcom et al., 2004; Balogh and Nollet,
2008; Bodaly et al., 1998). The most infamous example of
this occurred in Minamata, Japan, and was one of the few
cases of environmental mercury contamination in which
the mercury was methylated before it entered the environ-
ment. Most developed countries now regulate mercury lev-
els in wastewater effl uent, and various technologies have
been used to remove Hg from the waste stream (Bodaly
et al., 1998; Wagner-Dobler, 2003; Balogh and Nollet,
2008). As a result, wastewater discharges do not represent
an important source of MMHg to the marine environment
at large, although they may in isolated coastal environments.

SUBMARINE GROUNDWATER DISCHARGE

Studies of mercury dynamics in subterranean estuaries in
Massachusetts (Bone et al., 2007), northern France (Laurier