Environmental Engineering Reference
In-Depth Information
FIGURE 8.7 An upland forested-lake landscape showing methylation hot spots. The wetland and pond sediments (stippled areas) are favored
sites for methylation. Dots indicate the possibility for localized methylation.
in more gentle terrain. Wetlands are generally, but not always,
hydrologically connected to the stream network. Wetlands
that are hydrologically connected to the stream system
or lake play a disproportionately strong role in Hg and
MeHg mobility on the landscape (Watras et al., 1995;
Kramar et al., 2005). Wetlands along the margins of lakes
have similar importance (Driscoll et al., 1995).
The importance of wetlands to Hg cycling was noted by
Mierle and Ingram (1991), who showed that the mass of Hg
exported from wetlands was an order of magnitude greater
than that from other units of the landscape. St. Louis et al.
(1994) and Branfi reun et al. (1996) showed that different types
of wetlands had differing effects on THg and MeHg export.
Selvendiran et al. (2008) showed that a wetland created by a
beaver dam was a net source of THg and MeHg, while a nearby
riparian wetland, despite high porewater THg and MeHg
concentrations, was not a source because it had low water
throughput. In a broad landscape study, Balogh et al. (1998b)
showed higher THg and MeHg in forested areas with wetlands
relative to nearby agricultural areas that lacked wetlands.
Transformation/Methylation Processes
An early advance in the understanding of Hg cycling in
terrestrial/freshwater ecosystems was the recognition of
MeHg as the form responsible for most bio-accumulation
of Hg. The abundance of MeHg is controlled by two
counteracting microbiologic processes: Hg methylation
(Jensen and Jernelov, 1969) and MeHg demethylation
(Spangler et al., 1973). The pattern of net methylation
in the landscape (i.e. the balance of these processes in
space and time) determines the risk of biota to Hg bio-
accumulation. Here we address terrestrial methylation,
including wetlands. Until recently, Hg methylation was
viewed primarily as an aquatic process affecting primar-
ily the aquatic food web. However, aquatic MeHg fi nds
its way into terrestrial food webs (Cristol et al., 2008),
and high MeHg has been documented in songbirds nour-
ished from purely terrestrial food webs (Rimmer et al.,
2005, 2010), making it clear that terrestrial Hg meth-
ylation cannot be ignored (this topic, chapter 16). The
importance of terrestrial relative to aquatic methylation
can be expected to vary with their relative areas on the
landscape (e.g., the ratio of catchment to lake surface
area), as well as the relative strength of the methylation
sources (such as wetlands), as explored by Rudd (1995).
Typical methylation sites on the landscape are depicted
in Figure 8.7.
There is great variability in MeHg concentrations in soil
water and groundwater (Åkerblom et al., 2008). They are
generally low (up to a few tenths of a nanogram per liter)
where there are low DOC concentrations and/or oxic condi-
tions. However, in waters with more than a few milligrams
per liter of DOC, MeHg concentrations are often higher, with
concentrations of one to several nanograms per liter pos-
sible, but with considerable variations. These patterns, which
refl ect the local balance between methylation and demeth-
ylation, vary in space but are less well characterized in time.
Riparian Zones
Riparian zones are important to Hg and MeHg mobility for
two reasons. First, they often have wetland-like character,
with highly organic surface horizons and water tables near
the surface, and they may be sources of MeHg production.
Second, the proximity to the stream facilitates the ultimate
movement of aqueous Hg to the channel, whether by shal-
low groundwater fl ow or saturation overland fl ow (Bishop
et al., 1995c), or erosion of particulate Hg (Hurley et al., 1995;
Schuster et al., 2008). Bradley et al. (2010) demonstrated that
contrasting fl oodplain groundwater hydrology explained why
only one of two adjacent South Carolina coastal streams—
the one where upward hydraulic gradients transported
MeHg to the stream—had elevated fi sh Hg levels.
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