Environmental Engineering Reference
In-Depth Information
Airshed and
watershed
indicators
Airshed and
watershed
indicators
Water and
sediment
indicators
Water and
sediment
indicators
Site status
Site status
Site trends
Site trends
Integrated
assessment of
trends and
identification of causes
Integrated
assessment of
trends and
identification of causes
Ancillary data
Ancillary data
Modeling and
analysis
Modeling and
analysis
Regional status
Regional status
Regional trends
Regional trends
Sources of
trends
Sources of
trends
Wildlife
indicators
Wildlife
indicators
Aquatic biota
indicators
Aquatic biota
indicators
FIGURE 6.1
Diagram representing the factors that need to be integrated into the network design. (
Source:
Saltman et al., 2007. Used with
permission.)
In the table, the indicators for a site type are listed as well as
the proposed frequency of sampling. In addition, it is noted
whether the indicator is useful for determining trends in
Hg concentration/deposition over time only, indicative of
causal relationships between changes in input and changes
in MeHg production and accumulation, or indicative of
both. The relationship between indicators for the various
media, modeling, and integration is shown in Figure 6.1
(Saltman et al., 2007).
Measurements of Hg in wet deposition are relatively
easily accomplished and are currently monitored at the
national level through the MDN, which has around 100
active sites (MDN, 2008). This weekly collection program
has limitations in coverage, and does not provide data suit-
able for some computer-model simulations. Even given
these concerns, the recommendation is for a widely dis-
tributed weekly Hg wet deposition monitoring program
at the cluster sites (Table 6.1). At intensive sites, event-
based wet deposition collection is recommended (Driscoll
et al., 2007). The intensive sites, ranging from background
to urban environments, and including continental and
coastal locations, should measure Hg atmospheric specia-
tion. Atmospheric Hg
0
concentration is strongly refl ective
of the global atmospheric Hg pool and does not necessar-
ily provide a sensitive local indicator of short-term regional
change (Slemr et al., 2003). In contrast, the concentrations
of RGHg and PHg show a higher regional variability and
will likely show a response to changes in emissions, as
these species have a relatively short residence time in the
atmosphere, are easier to control at the emission source,
and have a strong anthropogenic signal (Schroeder and
Munthe, 1998; Hedgecock and Pirrone, 2001; Ryaboshapko
et al., 2007a, 2007b). They are, however, relatively diffi cult
to measure (Landis et al., 2002; Munthe et al., 2001; Sheu
and Mason, 2001) and are formed in situ to some extent
via atmospheric chemical reactions (Laurier and Mason,
2007). There is large uncertainty in the current estimates
of Hg dry deposition (Ryaboshapko et al., 2007a), and
methods for the measurement of atmospheric Hg speciation
and dry deposition will need to be standardized and rigor-
ously calibrated for this program; some new approaches are
being developed (Skov et al., 2006).
Atmospheric Hg speciation measurements at intensive
sites would be coupled with estimates of deposition and
ecosystem fl uxes, including litterfall, throughfall, and
event-based wet deposition, as well as measurements of
other atmospheric compounds in both the atmosphere and
in wet deposition (e.g., atmospheric ozone and NO
x
, sulfate
and major ions in precipitation) (Table 6.2) (Driscoll et al.,
2007). Therefore, benefi t would be obtained from choos-
ing locations that are already making measurements. For
example, in the United States, there are existing NADP net-
works, the MDN, the National Trends Network (NTN) and
the Atmospheric Integrated Research Monitoring Network
(AIRMON) and other atmospheric sampling programs (e.g.,
the Clean Air Status and Trends Network [CASTNET]). Such
intensive site measurements would serve two primary pur-
poses: (1) generate atmospheric data in support of regional
and global-scale atmospheric modeling efforts; and (2) col-
lect data for local-scale modeling of atmospheric deposi-
tion, and specifi cally dry deposition to complex surfaces,
such as forests. Soil, groundwater, and surface water mea-
surements would also be made (Table 6.1) to examine the
role of air-surface exchange of Hg
0
, and the fl uxes of Hg
and MeHg within the watershed, in impacting Hg trans-
port to methylation sites, in conjunction with the other
indicators discussed below.
Mercury export from watersheds is typically a small frac-
tion of the yearly input from the atmosphere (Grigal, 2003)
and is infl uenced to some extent by changes in Hg input,
although the timescale of response is very slow (Harris
et al., 2007a). Larger responses are possible over shorter
timescales from other disturbances, such as changes in
land use. Rainfall amount and other climatic variables also
