Environmental Engineering Reference
In-Depth Information
less throughout the Everglades (Restoration, Coordination
and Verifi cation, 2007). However, sulfur is applied to EAA
soils to decrease pH and improve nutrient availability for
agricultural use. Using sulfur-stable isotope measurements,
it appears that sulfur applied for agriculture is a major
contributor to the excess sulfate concentrations in the
Everglades (Bates et al., 2002). The relative contribution of
recent versus legacy sulfur additions to sulfate concentra-
tions in the Everglades is not clear.
Sulfate concentrations vary spatially throughout the
Everglades depending on the proximity to the EAA and the
relative contribution of water sources from precipitation,
storm water, and groundwater. The highest sulfate concentra-
tions
WCA have decreased 30-70% from concentrations observed
in the late 1980s and early 1990s. However, concentrations
have not changed appreciably since 1998 to the present
(Lange, 2006). The REMAP study showed declines in the Hg
concentrations of mosquitofi sh from 1995 to 2005 in the
Everglades. In contrast, concentrations of Hg in largemouth
bass have increased in the ENP since 1999. It is not clear
what is driving these complex changes in fi sh Hg concentra-
tions across the Everglades. The recent declines in fi sh Hg
in the WCAs do not appear to coincide with decreases in
atmospheric deposition of Hg. Rather, it is hypothesized that
decreases in fi sh Hg in the WCAs, that formerly exhibited
very high concentrations, are due to restoration associated
water fl ow increases resulting in a decrease in sulfate concen-
trations to below values that were optimum for MeHg pro-
duction by sulfate-reducing bacteria (Axelrad et al., 2005).
In summary Hg contamination is widespread in the
Everglades. Concentrations of Hg in fi sh widely exceed
established criteria for the health of humans and wildlife.
The extent of this contamination is likely due to elevated
inputs of Hg from atmospheric deposition coupled with the
high sensitivity of the Everglades ecosystem (due to exten-
sive wetlands, elevated sulfate inputs, warm conditions and
low productivity). There are distinct spatial patterns of Hg
contamination across the Everglades due to variations in
source waters, and the supply of sulfate, DOC, and possi-
bly phosphorus. There have also been temporal variations
in fi sh Hg concentrations across the Everglades. Declines in
fi sh Hg from the 1980s through 1990s were probably driven
by controls on local incinerator sources. Since the late
1990s, changes in fi sh Hg have not coincided with changes
in atmospheric deposition of Hg. Rather, it is likely that
recent changes in Hg dynamics are due to changes in sulfate
concentrations, and possibly changes in other constituents
that affect Hg transport and trophic transfer. One pattern
that emerges from this synthesis of Hg in the Everglades
is that land use is central to the Hg problem in south Florida
because it infl uences the supply of critical chemical con-
stituents (i.e., sulfate, DOC, nitrate, and total phosphorus)
and possibly alters the pathways of water that transport Hg
and the critical chemical constituents infl uencing Hg trans-
formation. These spatial and temporal patterns are indica-
tive of the complexity of Hg dynamics and the myriad
factors that infl uence the extent of contamination in the
Everglades. Future management of Hg contamination will
need to consider these factors that control Hg transport,
methylation, bio-availability, and bio-accumulation, and
not solely Hg inputs.
100 mg/L are observed in canals within the EAA and
in WCA-2A (USEPA, 2007b). The storm water treatment areas
remove modest and variable quantities of sulfate, ranging
from 5% to 67% (USEPA, 2007). From this source, concentra-
tions of sulfate in the Everglades decrease toward the south
and west. Sulfur dynamics appear to be an important spatial
controller of MeHg production. In the northern Everglades,
the high supply of sulfate coupled with reducing conditions
result in high concentrations of sulfi de in wetland porewa-
ters, often exceeding 1 mg/L (USEPA, 2007b). Sulfi de concen-
trations
0.3 mg/L appear to restrict the production of MeHg
(Benoit et al., 2003). The elevated production of sulfi de may
limit MeHg concentrations in the northern Everglades. With
decreases in sulfate and sulfi de concentrations, there is an
increase in MeHg production rate in WCA-2B and WCA-3A,
with subsequent decreases through the ENP toward the
south (see Figure 9.4; Gilmour et al., 2007).
An additional factor that likely infl uences the spatial
patterns in fi sh Hg in the Everglades is phosphorus sup-
ply. Water concentrations of phosphorus exhibit a distinct
decreasing gradient north to south due to inputs from the
EAA (see Figure 9.5; USEPA, 2007b). This elevated supply
of phosphorus increases aquatic productivity, which may
result in the biodilution discussed above (Pickhardt et al.,
2002; Chen and Folt, 2005). However, to our knowledge
this hypothesis has never been tested for the Everglades.
TEMPORAL PATTERNS
An ambitious REMAP plan is in place to track changes in
Hg contamination in south Florida (Axelrad et al., 2007;
USEPA, 2007). The major sources of Hg emissions in south
Florida in the early 1990s were municipal and medical-
waste incinerators. Since that time there has been an
approximately 90% decline in these sources due to reduc-
tions in the waste stream and emission controls (Atkeson
et al., 2005). Wet Hg deposition decreased from 1994 to
2002. Since that time, deposition has been highly variable,
with high deposition from early 2003 to mid-2004 due to
elevated precipitation (Axelrad et al., 2007).
The REMAP study (USEPA, 2007) showed slight changes
in concentrations of THg and MeHg in water from 1995 to
2005. Mercury concentrations of largemouth bass in the
The Adirondacks
The Adirondack region of New York is one of the bio-
logic Hg hot spots identifi ed by Evers et al. (2007), with
consistently widespread contamination of Hg in fi sh and
wildlife as compared with other areas in the region. It is
located in northern New York state and encompasses a
