Environmental Engineering Reference
In-Depth Information
et al., 2003). Many chemical, biologic, and physical fac-
tors—such as bacterial community structure, pH, redox,
nutrients, and sulfate—infl uence MeHg production, and
human impacts such as eutrophication and other ter-
restrial disturbances also affect the extent of Hg meth-
ylation. Because MeHg can be degraded (demethylated) to
Hg
II
, by both biotic and abiotic processes, there is rapid
cycling between the Hg and MeHg pools, and measured
concentrations represent short-term steady-state standing
stocks (Benoit et al., 2003).
In designing a monitoring program, it is necessary
that the program accurately measures and quantifi es the
sources of the changes being documented. Because of dif-
ferences in the lifetime of the various Hg species in surface
waters, and their different source profi les and their trans-
formation in the atmosphere, the tracking and detecting
of changes in atmospheric Hg deposition relative to reduc-
tions in emissions of Hg from anthropogenic sources can
be complex (Morel et al., 1998; Ryaboshapko et al., 2007b).
The atmospheric transformations are also infl uenced by
the concentration and reactivity of atmospheric oxidants
(Schroeder and Munthe, 1998; Hynes et al., 2009). Overall,
the ability to detect the response to changes in anthropo-
genic Hg emissions in the continental United States, for
example, will be confounded by changes in natural emis-
sions, in the rate of reemission of previously deposited Hg
(Mason, 2009), as well as by changes in Hg emissions in
other countries (Bullock and Brehme, 2002, Ryaboshapko
et al., 2002, 2007a). Thus, any framework for monitoring
atmospheric wet and dry deposition needs to measure
or estimate the contribution from all forms of Hg in the
atmosphere.
The timescale of a particular ecosystem response to Hg
emission reductions is not well known, and it will most
likely involve an initial rapid change followed by a slower
response, and these rates of change will be different for
different ecosystems (Saltman et al., 2007). Therefore,
the indicators used must respond on different timescales.
Concentrations in upper-trophic-level organisms change
relatively slowly (over years) because of growth dilution, as
MeHg is only slowly depurated. Conversely, lower-trophic-
level organisms, such as zooplankton, respond rapidly
(within weeks) to changes in water-column MeHg concen-
tration, and their concentration refl ects a transient signal
refl ecting short-term variability of Hg dynamics and other
factors rather than longer-term changes in Hg (Hudson
et al., 1994; Harris et al., 2007a). Clearly, the careful
choice of Hg monitoring indicators will help reduce these
confounding factors due to short-term variability, but
they must also be able to integrate the signal so that the
direction of change can be determined. Therefore, short-
term measurements will be needed to assess variability
imparted by ancillary ecosystem changes; however, the
monitoring program should be maintained for a suffi -
cient time (at least 10 years) to assess longer-term trends
in atmospheric Hg input. Furthermore, a baseline of infor-
mation in addition to Hg speciation and distribution is
needed to allow for the detection of change. Given that
change is already occurring (Pacyna et al., 2006; Pirrone
et al., 2009, 2010), the monitoring program needs to be
instituted as soon as possible to ensure that adequate
background information is gathered. In this context,
therefore, there is much to be gained by using existing
sites where measurements are being made, as these will
have the necessary and required background information,
even if only some of the indicators and ancillary informa-
tion are being measured.
As noted, factors other than changes in Hg deposition
also impact Hg methylation and the bio-accumulation
(Schmeltz et al., 2011) and fate of MeHg (Benoit et al.,
2003), and ancillary data must be collected to prop-
erly assess the impact of these confounding variations
to ensure that the interpretation of the indicators is
scientifi cally robust and defensible. Confounding fac-
tors include land-use changes, global warming impacts,
changes in food-web structure and species, existing
point-source discharges, changes in climate and atmo-
spheric chemistry and acidic deposition, in situ chemi-
cal and physical properties, and hydraulic retention
time. Such changes impact all environments; therefore,
a cross-section of systems with varying sensitivities to
loading needs to be examined, as well as sampling loca-
tions that are impacted by local/regional atmospheric
Hg emissions. Clearly, while the environmental settings
(e.g., water-body type, geographic location) most respon-
sive to short-term changes in atmospheric Hg deposition
need to be examined, less responsive and “background”
environments should also be monitored. Each ecosystem
is unique, and the ability to discern trends in Hg concen-
trations in indicators will depend on a detailed and clear
understanding of the factors that infl uence Hg biogeo-
chemical cycling.
The Network Design
A regional network should preferably be continental in
scale and should include a fi nite number of study sites that
are intensively monitored for all the required variables and
indicators (so-called intensive sites) as well as a broad range
of colocated monitoring locations (called cluster sites)
across different ecosystems, where a more limited num-
ber of measurements are made (Mason et al., 2005; Harris
et al., 2007a). As it is not possible to provide details for all
locations, the proposed network design for North America
will be presented here as an example. Depending on the
fi nal approach and rationale for choosing sites, it appears
that 10-20 intensive sites would provide (Schmeltz et al.,
2011) suffi cient spatial coverage for North America in
terms of Hg deposition levels and ecosystem types. North
America consists of four ecosystem domains (polar, desert,
