Environmental Engineering Reference
In-Depth Information
Just as DOC drives fi ltered Hg export, POC drives par-
ticulate Hg export (Figure 8.4). The Hg-POC association
originates in the vegetation (foliage) or soil, and particulate
Hg in the stream refl ects erosion of near-surface organic-
rich soil particles into the channel—for example, dur-
ing high fl ows, plus any direct deposition of litter in the
channel. Particulate Hg in streamwater may also be resus-
pended from streambed sediment (Hurley et al., 1995;
Quémerais et al., 1999) or form in situ from dissolved Hg,
as there is greater affi nity of Hg for the solid phase (log K
d
from nine studies averaged 5.1; Grigal, 2002). Mason and
Sullivan (1998) showed that the K
d
for Hg increased as the
organic content of particles increased in an urban stream.
Streamwater typically displays a strong relation between
particulate Hg and total suspended solids (TSS) (Balogh
et al., 1998a; Shanley et al., 2008). For this reason, continu-
ous monitoring of turbidity, a proxy for TSS, has enabled
highly accurate calculation of stream Hg fl ux (Whyte and
Kirchner, 2000; Wall et al., 2005).
The relation of dissolved MeHg to DOC is less straightfor-
ward, as seen in the studies cited earlier in which MeHg did
not track DOC (Lee et al., 2000; Allan et al., 2001; Schwesig
and Matzner, 2001). Export of MeHg may be controlled
more by its net production rate than by the availability of
DOC for transport.
6
Sleepers, VT
Icacos, PR
Andrews, CO
Allequash, WI
5
4
3
2
1
0
0
4
8
12
16
DOC (mg L
-1
)
60
Sleepers, VT
Icacos. PR
Andrews, CO
Allequash, WI
50
40
30
20
In-Stream Mobility and Bio-availability
10
Once in the stream, THg and MeHg tend to remain mobile
until they reach a larger water body. In support of in-
stream mobility is the analysis by Grigal (2002) showing
that Hg export per unit area of the landscape decreases
only slightly with increasing watershed size. Particulate
Hg may settle out of the water column to the streambed,
particularly in response to lessening stream gradients with
decreasing elevation. However, the Hg-DOC complex is
stable and unlikely to be removed from the water column,
although dissolved Hg may partition on to particles that
ultimately settle. MeHg is subject to photooxidation and
demethylation and may be less stable in streamwater (Bradley
et al., 2011), but nonetheless there is net delivery of MeHg
from the terrestrial landscape to downstream water bodies.
In-stream transport of particulate Hg and its suscepti-
bility to settle out brings up the question of the bioavail-
ability of particulate Hg, which dominates the Hg fl ux in
some streams and rivers. Hurley et al. (1995) suggested
that particulate Hg may be available for methylation, as
organic matter is mineralized in shallow sediments. But in
one assessment, Munthe et al. (2007) questioned the bio-
logic signifi cance of this fraction. The reconstruction of
credible historic rates of Hg deposition and watershed Hg
delivery from lake sediment Hg profi les with depth (Lorey
and Driscoll, 1999; Kamman and Engstrom, 2002; Mast
et al., 2010) points to sediment as the ultimate Hg reposi-
tory, but that does not preclude some in-lake cycling prior
to deposition. In fact, research from METAALICUS shows
0
0
10
20
30
40
POC (mg L
-1
)
FIGURE 8.4
Filtered THg vs. DOC and unfi ltered THg vs. POC at
four U.S. catchments. Modifi ed from Shanley et al., 2008.
thiol groups. The binding strength of Hg with thiols is
orders of magnitude greater than that with inorganic
anions, and there are generally far more thiol binding sites
than Hg atoms present (Haitzer et al., 2002; Skyllberg et al.,
2003). Hg associates primarily with aromatic, refractory
DOC comprised of hydrophobic acids (HPOA), commonly
known as the “humic fraction.” At a stream in Vermont,
fi ltered Hg correlated strongly with DOC (r
2
0.96), but
0.99) (Shanley et al.,
2008). HPOA typically comprises about half of the DOC
in natural waters, and its percentage tends to range nar-
rowly (40-60%) (Aiken et al., 1992), thus most waters
exhibit a high correlation between fi ltered Hg and DOC,
even though the HPOA fraction is responsible for the Hg
complexation. Analysis of Hg and HPOA are costly, but UV
absorbance is a promising and inexpensive alternative to
HPOA as a proxy for dissolved Hg concentration (Dittman
et al., 2009, 2010). Using another approach, Petterson et al.
(1995) also found a larger amount of MeHg associated with
the humic fraction of organic matter in boreal streams.
even more strongly with HPOA (r
2
