Environmental Engineering Reference
In-Depth Information
large (~2.4 million ha) predominantly forested portion of
the Canadian Shield underlaid by metasedimentary rocks
and granite gneisses. The soils of the Adirondacks are
largely Spodosols developed from glacial surfi cial materials
that cover much of the region (Driscoll et al., 1994; Sul-
livan et al., 2006). There are approximately 2800 lakes in
the Adirondack Park (Driscoll et al., 1991). Moreover, wet-
lands are a prominent feature of the Adirondack landscape,
covering about 5% of the land area. An important charac-
teristic of the Adirondacks is that it receives elevated acidic
deposition and contains sensitive watersheds and sur-
face waters that have been impacted by acidic deposition
(Driscoll et al., 1991). Inputs of acidic deposition have been
decreasing in recent years resulting in limited recovery of
surface water acidity (Driscoll et al., 2003).
Wet Hg deposition has been monitored at the Mercury
Deposition Network (MDN) site at the Huntington Forest
in the central Adirondacks (48.98°N, 74.23°W) since 2000,
with an annual mean value of 6.7 µg m
2
yr
1
. Levels of
wet Hg deposition in the Adirondacks are modest com-
pared to other regions of the United States (http://nadp
.sws.uiuc.edu/mdn). Unfortunately, there are no long-term
measurements of Hg deposition. However, paleolimno-
logic measurements of lake sediments provide a record
of historical changes in Hg deposition. Studies of the
sediment record of eight lakes in the Adirondacks suggest
that preindustrial Hg deposition to the region was approx-
imately 3 µg m
2
yr
1
(Lorey and Driscoll, 1999). Around
1900, there was a marked increase in Hg deposition to
about six times background levels. Mercury deposition
peaked around the 1970s-1980s and has declined about
25% in more recent decades.
Mercury contamination in the region is widespread; New
York has issued 66 fi sh-consumption advisories for elevated
Hg in the Adirondacks. Considerable research has been
conducted on the deposition, transport, and effects of Hg
in the Adirondack region of New York. In particular there
have been detailed process-level studies of Hg dynamics at
the Huntington Forest in the central Adirondacks, which
includes Arbutus Lake and its watershed at Archer Creek,
and at Sunday Lake watershed in the western Adirondacks
(43.85°N, 74.10°W). These detailed studies have been com-
plemented by regional surveys. A synthesis of this research
is provided in this case study.
mer (see below), and oxidation of GEM largely by ozone,
which is also more prominent in the summer months. Choi
et al. (2008a) used potential source contribution function
analysis and back trajectory analysis to determine potential
Hg sources to the Adirondacks. Their analysis showed that
regional source areas, including Pennsylvania, West Vir-
ginia, Ohio, Kentucky, Indiana, and Texas, were important
in contributing Hg deposition to the Adirondacks.
Studies from the Adirondacks and other remote forest eco-
systems have shown that the canopy can greatly enhance
atmospheric Hg deposition (Rea et al., 2002; Miller et al.,
2005). Throughfall is the water leached from the forest can-
opy that enters the forest fl oor. Choi et al. (2008b) observed
a slight enrichment in throughfall Hg in comparison with
wet Hg deposition (11.6 µg Hg m
2
yr
1
vs. 12.0 µg Hg m
2
yr
1
)
from a hardwood site at Huntington Forest in the central
Adirondacks. Similar levels of throughfall enrichment were
evident for a hardwood stand in the western Adirondacks
(Sunday Lake watershed; Demers et al., 2007). In contrast,
much greater enrichment in throughfall Hg fl uxes occurred
from a conifer stand. These increases were attributed to a
greater leaf area index and the year-round canopy of the
coniferous vegetation (Demers et al., 2007).
Plant litter is an also important pathway of Hg inputs to
forest ecosystems. At the Huntington Forest, the THg con-
centration of leaf tissue increased approximately 10-fold
over the growing season for three hardwood species, from
4.4
12.4 ng g
1
(Bushey et al., 2008). This
linear increase in concentration over the growing season
suggests that accumulation in leaves is regulated by mass
transfer processes between the atmosphere and leaf tis-
sue. Conversely, the MeHg concentration of leaf tissue
(0.17
2.7 to 37.3
0.18 ng g
1
) and accumulation were low. Annual
litterfall Hg at Huntington Forest was 17.2
g m
2
yr
1
,
representing the largest input of Hg to this forest ecosys-
tem. Litter input of MeHg was 0.12
μ
g m
2
yr
1
. Demers
et al. (2007) observed comparable rates of litterfall Hg from
the hardwood stand at Sunday Lake (14.9
μ
g m
2
yr
1
) but
much lower rates at the conifer stand (9.7
μ
g m
2
yr
1
).
Modeling estimates of transpiration uptake and GEM depo-
sition suggest that Hg in leaf tissue is largely derived from
atmospheric sources (Bushey et al., 2008). Thus, litterfall
Hg probably represents largely a new, rather than recycled,
input to the forest ecosystem. The pattern that emerges
from this work is that the forest canopy greatly ampli-
fi es Hg deposition. Dry deposition (estimated as the sum
of net throughfall plus litterfall Hg) is the predominant
pathway of Hg inputs to the forest fl oor, representing
72% for the hardwood stands and 82% at the conifer
stand (Demers et al., 2007). In addition, the mecha-
nism of dry inputs appears to vary with forest vegetation
type. Total Hg inputs were greater at the conifer stand
(35.5
μ
ATMOSPHERIC DEPOSITION, THROUGHFALL,
AND LITTERFALL
Air concentrations of Hg in the Adirondacks are typical of
remote regions and relatively low as compared with values
reported for more urban and industrial regions (Choi et al.,
2008a). Signifi cant diurnal patterns were apparent in warm
seasons for all Hg species, whereas diurnal patterns were
weak in cold seasons. These diurnal patterns of Hg species
may be due to reemissions of GEM from the forest fl oor,
which largely occur during the daylight period in the sum-
g m
2
yr
1
in deciduous
stands), and Hg inputs occurred largely as throughfall,
while for the hardwood stands litterfall is the major Hg
input pathway.
μ
g m
2
yr
1
vs. 22.3
μ
