Environmental Engineering Reference
In-Depth Information
THE RESERVOIR EFFECT
MeHg production was not signifi cantly different between
the middle- and high-carbon reservoirs, but was consider-
ably elevated in these relative to the low-carbon reservoir.
In the FLUDEX project, MeHg also peaked at year 3 of the
study. The FLUDEX study indicated that lability of the
carbon was a more important driver of methylation than
the simple presence of higher quantities of organic carbon.
In both the ELARP and FLUDEX projects, MeHg photo-
reduction (Sellers et al., 1996) was likely limited because
of decreased light penetration from increases in dissolved
organic matter in the water column. The limitation of this
pathway of MeHg loss would serve to perpetuate higher
MeHg concentrations.
Similar fi ndings were noted from experimental studies
of soils of varying types in the Three Gorges Dam project
area (Zhang et al., 2006; Yu et al., 2007). Paradoxically,
while the highest methylation potential was observed in
the highest-carbon cambisols (e.g., paddy soils) the second-
highest methylation potential was observed in well-drained
soils primosols, which are low in carbon, but may allow for
more movement of soil-based DOC. This fi nding further
emphasizes not only the importance of soil carbon content
of parent soil material, but also the subsequent lability of
the organic ligands that are produced by that soil.
In every case, creation of new reservoirs begins by the
impoundment of waters in low-lying terrain, with resul-
tant reservoirs ranging in size from small “fl owage” ponds
to major projects such as the Three Gorges Dam (China),
Tucurui Reservoir (Brazil), or the Bourassa-La Grande com-
plex (James Bay, Quebec). The initial impoundment of a
new reservoir yields a large fl ux of organic and inorganic
material to the water in the form of decomposing plants,
woody material, and organic and mineral soils (Figure 9.6a).
This material includes legacy Hg from prior atmospheric
deposition or legacy Hg from industrial sources. This
occurs in a decompositional environment characterized
by high organic loading and low dissolved oxygen, which
favors methylation. Muresan et al., (2006) describe a newly
constructed reservoir in simple terms as a “reactor” that
creates favorable conditions for MeHg production, both at
the sediment-water interface, and on organic particles in
water. This pattern is known colloquially as the “reservoir
effect,” which is well described by current and emerging
literature. For example, in the reservoirs of the Bourassa-La
Grande project, aqueous MeHg concentrations increased
to 1.3 times their initial values within the fi rst 13 years
after reservoir creation (Shetagne and Verdon, 1999). Much
higher increases have been observed in one experimental
reservoir system (up to 13 times, Bodaly et al., 2004; see
also, St. Louis et al., 2004). Reservoir systems with interme-
diate increases in MeHg have been described in Amazonia
(Albuquerque Palermo et al., 2006a: Niklasson et al., 2006;
Muresan et al., 2006) and in chains of reservoirs in the
Wujiang River, China (e.g., Guo et al., 2006). The typical
increase in aqueous MeHg concentrations in reservoirs is
2-4 times those observed before fl ooding.
METHYLATION SITES WITHIN RESERVOIRS
As with lakes and rivers, the processes by which MeHg in
reservoirs is incorporated into biota is governed by factors
desc r ibed in t his chapter, as well as t hose su m ma r i zed by Evers
et al. (2007) and Driscoll et al. (2007). There is little about the
existence or operation of a reservoir alone that results in a dif-
ferential uptake of Hg into biota, at least within the reservoirs
themselves. However, the sources of the MeHg may indeed
differ. In reservoir systems, as has also been shown for natural
lakes, methylation can occur in marginal wetlands and litto-
ral soils (Branfi reun et al., 1996, 2006), upon particles within
open waters (St. Louis et al., 2004) or at the sediment-water
interface (e.g., Kuwabara et al., 2003; Albuquerque-Palermo
et al., 2006a; Malm, 2006). Verta et al. (1986) initially attrib-
uted high fi sh-tissue Hg concentrations to MeHg production
in marginal peats. While the supply of MeHg from marginal
wetlands to natural lakes is well documented (e.g., Branfi reun
et al., 1996), the importance of these wetlands in open-water
reservoirs remains the topic of considerable research (Bran-
fi reun et al., 2006).
In the ELARP, the production of MeHg in both open
waters and marginal peatlands was studied. Despite gener-
ally higher rates of MeHg production in the wetlands, the
predominant source of MeHg in biota was that produced
in the open water areas of the pond on the suspended
organic matter, which served as a methylation substrate.
Because of alteration of hydrology and depositional envi-
ronments, ongoing erosion of soil material to open waters
can perpetuate the supply of substrates for methylation,
prolonging the so-called reservoir effect (St. Louis et al.,
Organic Matter and Methylation
The single most important factor controlling the increase
in MeHg in reservoir waters is the organic content of the
impounded soils and detrital material. While the relation-
ship between the organic content of sediment and water and
methylation potential is well described in the literature (see
the review in Driscoll et al., 2007), this simple relationship is
more nuanced in reservoir systems. In a study of an experi-
mentally impounded wetland/pond system in Ontario (the
ELARP project, Kelly et al., 1997; St. Louis et al., 2004), the
methylation promoted by the initial fl ooding peaked in
intensity 2-3 years after the fl ood. These high MeHg con-
centrations began to decline thereafter because of increased
demethylation (St. Louis et al., 2004), likely by active
demethylation via the “mer” detoxifi cation pathway used by
sulfate-reducing bacteria (Marvin-DiPasquale et al., 2000).
Similarly, Bodaly et al. (2004, the FLUDEX project)
describe the creation of three experimental impoundments
in watersheds characterized by low, medium, and high
concentrations of soil organic content. In these watersheds,
