Environmental Engineering Reference
In-Depth Information
2004). Erosion control in reservoirs and their associated
watersheds is therefore one management option that can
mitigate prolonged MeHg production and bio-accumula-
tion. He et al. (2008) also described open water methyla-
tion as the predominant source of MeHg to the large and
highly eutrophic Hongfeng Reservoir, Guizhou Province,
China.
In many reservoirs, methylation at the sediment-water
interface is the major source of bio-available MeHg. In the
Baihua Reservoir, China, and in some large Amazonian
reservoirs, migration and focusing of sediments toward the
deepest reservoir forebay areas have resulted in a migration
of the most active MeHg-producing zones. (Han and Feng,
2006; Malm, 2006). Kuwabara et al. (2003) also found the
greatest methylation potential along the thalweg sediments
of Camp Far West Reservoir.
THE ROLE OF WATER-LEVEL FLUCTUATIONS
Water-level fl uctuation also infl uences fi sh Hg concen-
trations (Figure 9.6b). In reservoirs, reservoir managers
fl uctuate water stage primarily for hydropower genera-
tion or fl ood control, but also for irrigation and, in lim-
ited instances, recreation. Bodaly et al. (1994) and Verta
et al. (1986) identifi ed water-level fl uctuation in Manitoba
and Finnish Reservoirs, respectively, as the key variable
explaining high fi sh-tissue Hg concentrations. In the Finn-
ish study, the elevated fi sh Hg concentrations were attrib-
uted to the expulsion of MeHg-rich waters from marginal
peats due to pressing by ice following winter drawdown.
Numerous studies since have also implicated water-level
fl uctuations. For example, depth and hydroperiod have
been shown to be strongly associated with increased fi sh
Hg concentrations in depression wetland ponds of the
Southeastern United States (Snodgrass et al., 2000). In an
experimental study, fi sh-tissue Hg concentrations were
signifi cantly elevated in manipulated reservoirs in Maine
(annual drawdown, 3-7 m) relative to control lakes (annual
fl uctuation, 1-1.2 m), and the MeHg:Hg ratio in sediments
was shown to increase considerably, then remain elevated,
after the onset of reservoir fl uctuation (Haines and Smith,
1998). Tissues of fi shes and avian piscivores nesting on
large open water impoundments with large dewatered and
reinundated littoral areas have also been shown to display
elevated Hg concentrations, particularly where littoral
sediment substrates are more enhanced in organic matter
(Evers et al., 2007).
A study of 14 Minnesota lakes subject to varying water-
level manipulation regimens provides the best empiri-
cal evidence of the role played by water-level fl uctuation
(Sorensen et al., 2005). In that study, variation in the sea-
sonal magnitude and timing of fl uctuations was highly and
positively correlated to Hg concentrations in young-of-year
yellow perch. The work of these authors, along with fi nd-
ings of ELARP, clarify the importance of redox cycling at
the dewatered and re-inundated sediment-water interface
in reservoirs. During periods when reservoirs are at full
capacity, methylation proceeds in sediments until the avail-
able sulfate has been reduced to sulfi des. Upon drawdown,
the sulfi des within the interstitial soils of exposed littoral
areas undergo reoxidation to sulfate. When the water level
increases again, this newly “re-available” sulfate, in addition
to atmospherically deposited sulfate and Hg, facilitate the
reestablishment of the methylation cycle. Assuming that
the transformation of sulfi de to sulfate is not instanta-
neous, then the amount of time the exposed littoral surface
is dry will also enhance methylation potential in the next
reinundation cycle, due to the presence of progressively
more available sulfate. Therefore, where drawdowns are
longer and/or affect greater area, ongoing water-level fl uc-
tuations in systems subject to continuing Hg deposition or
stream inputs explain why Hg concentrations in fi sh and
other biota often remain elevated in “old” reservoirs.
RESERVOIRS: A SOURCE OF DOWNSTREAM
METHYLMERCURY
The production of MeHg near the sediment-water interface
of forebay waters presents a particular problem where res-
ervoir discharges are from hypolimnetic depth zones. In
this instance, the MeHg that is produced in one reservoir
becomes immediately available to downstream biota in
the receiving tailraces. This idea is well supported by the
ELARP and FLUDEX projects (Bodaly et al., 2004; St. Louis
et al., 2004) and from several cascading reservoir systems
studied in Brazil and China. In the ELARP and FLUDEX
projects, the reservoir systems became sources of MeHg for
downstream receiving waters for periods of several years.
The same phenomenon has been observed by He et al.
(2008) in the Hongfeng Reservoir. In the Tucurui Reservoir
system of Brazil, fi shes of several trophic guilds were found
Hg levels that were elevated by a factor of 3 in downstream
receiving waters over in-reservoir fi shes of similar trophic
position (Albuquerque Palermo et al., 2006b).
RESERVOIR AGE AND FISH MERCURY
The enhanced production of MeHg in reservoirs is readily
bio-accumulated in fi shes and other piscivorous biota. This
effect is an unavoidable consequence of creating a reservoir,
but one with a fi nite lifespan. The age of the reservoir has a
large infl uence on net MeHg production. In the Bourassa-
La Grande project, increases in fi sh Hg concentrations of
1.5 to 4 times natural lake background levels were observed,
with concentrations peaking ~10-15 years postconstruc-
tion and declining thereafter (Shetagne and Verdon, 1999).
Where reservoirs are not further manipulated or managed,
fi sh Hg concentration typically declines to background lev-
els between 20 and 40 years after construction (Anderson
et al., 1995; Shetagne and Verdon, 1999). In a series of
Amazonian reservoirs, Malm (2006) observed a 2-3 times
decreases in fi sh Hg concentrations during the period of
6-21 years after construction.
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