History
Introduction
The effects of acid deposition on surface water quality in the Adirondack
Mountains have been studied intensively since the 1970's. Findings
show that the acidification status of streams and lakes is strongly
influenced by the geology and hydrology of watersheds. Deposition input
quantity and quality, the mineralogy and depth of surficial materials,
the hydrological properties of soils, groundwater flow paths, wetland
processes, snowmelt, etc., all contribute to the final chemical composition
of surface waters. Acid Rain
The term ‘acid rain’ was first coined in 1872 in Air and Rain: The
Beginnings of Chemical Climatology, a book published by Angus Smith,
an English chemist, who was the first to systematically analyze the
chemistry of precipitation in industrialized Britain. The effects of
acidic deposition on aquatic and terrestrial ecosystems have been studied
intensively since the late 1960’s and early 1970’s, first in Scandinavia,
then in Europe, and eventually in the U.S when acid rain emerged as
an important ecological issue (Oden, 1968; Likens et al., 1972).
The Adirondack Mountain region of New York State receives elevated
inputs of sulfur and nitrogen in the form of ‘acid rain.’ Prevailing
winds from west to east carry pollutants emitted in the Midwest, mainly
from coal burning electric utilities, over the northeastern United
States and Canada. Acid rain forms when SO2 and NOx emissions derived
from the combustion of fossil fuels transform in the atmosphere to
sulfuric and nitric acids. Long-range transport and deposition of these
strong acids over time has resulted in the acidification of surface
waters in northeastern North America, and in the Adirondacks in particular.
Because components of ‘acid rain’ may enter terrestrial ecosystems
as precipitation (both rain and snow), fog or mist (wet deposition),
or as gases or particles (dry deposition), a more appropriate term
used for ‘acid rain’ is acid deposition.
Precipitation in the Adirondacks averages 100 – 150 cm / yr, with
about 30% falling as snow (Johannes et al. 1985). This large quantity
of precipitation results in high acid loadings. H+ deposition averaged
about 500 eq/ha/yr over the Adirondacks in the early 1990s, but decreased
to about 300 eq/ha/yr by 2002. Average deposition of H+ and SO4-2 in
the Adirondacks has been declining over the past few decades due to
significant decreases in SO2 emissions following implementation of
the Clean Air Act Amendments (CAAA) of 1970 and passage of Title IV
of the Acid Deposition Control Program by Congress in 1990. Average
annual pH values of precipitation have risen over this period from
about 4.2 to 4.5. Because precipitation amounts decrease from the western
to eastern Adirondacks due to orographic effects, H+ deposition decreases
by about 10% across the mountain range, a distance of about 150 km
(Charles 1991, Driscoll et al. 2003a). There has been no significant
change in nitrogen deposition during this same period because CAAA
legislation did not specify limits for NOx emissions (Driscoll et al.
2001).
Surface Water Acidification
Over the past century, inputs of acid deposition have
led to the acidification of surface waters in the Adirondacks. Lakes,
ponds, and streams with
low ANC were the first affected and continue to be the most severely
impacted today. Effects of acid deposition on surface water chemistry
may vary with season and with the hydrologic processes prevalent in
watersheds. For example, during winter months acidic components of
precipitation are stored in the snowpack, which may reach depths of
1-2 m. During spring snowmelt, normally covering about a two-week period
in April, large quantities of acid are released and cause surface waters
to become more acidic than at other times during the year. This is
called an episodic acidification event, and pH and ANC values may drop
to less than 5.0 and 0 µeq/L, respectively, for days or weeks
at a time. Usually, it is just the upper meter or two of a lake that
acidifies during the event, but in streams the entire water column
is affected. Episodic acidification of surface water can also occur
after a particularly heavy rainfall, for example, from an air mass
that might have originated in the Midwest where it encountered high
levels of SO2 and NOx emissions.
Brief Summary of Effects of Acid Deposition on Aquatic and
Terrestrial Ecosystems
Introduction
The literature is rich with studies on the effects of
acidic deposition on terrestrial and aquatic ecosystems, therefore,
for more detail and
further reading on this topic we refer you to both the scientific and
popular literature that has been produced over the past three decades
on this topic. Several excellent summaries worth mentioning here are
those written by: Driscoll et al., 2003a & b, and 2001; Sullivan,
2000; Lawrence and Huntington, 1999; Charles, 1991; and Schindler,
1988. Brief Summary of Effects
Acid deposition results in the mobilization of Al in soil solution,
which subsequently enters streams and lakes. Spodosols in the Adirondacks
are naturally acidic, but elevated concentrations of inorganic monomeric
Al are enhanced by strong mineral acid additions to soils from atmospheric
deposition (Cronan and Schofield, 1990). The source of the aluminum
in soils is organically bound Al, exchangeable Al, Al-hydroxides and
oxides, paracrystalline Al compounds, interlayer Al (in phyllosilicates),
and ultimately, primary mineral weathering. Low pH and concomitant
elevations of Al in surface waters contribute to the decline of fish,
zooplankton and macroinvertebrate species in affected systems (Schindler
et al., 1985; Schindler, 1988). Al concentrations in surface waters
may reach toxic levels that are sustained throughout the year, or may
rise only during episodic acidification events. In the latter case,
elevated Al levels may coincide with critical biological events, such
as the hatching of fry in the spring. High rates of fish mortality
have been linked with acidic water and elevated aluminum concentrations
(Baker and Schofield, 1982; Baker et al., 1996).
High levels of Al in soil solution along with the leaching of essential
nutrients from soils may lead to reduced tree growth and dieback of
forests (Shortle et al., 1997; DeHayes et al., 1999). Adirondack soils
naturally have low base saturation and, therefore, any process that
accelerates the removal of base cations from exchange sites decreases
the ability of the soil to sustain plant growth. Replenishment of exchangeable
bases for nutrient uptake depends heavily on primary mineral weathering,
but in acid-sensitive soils mineral weathering may be sluggish. In
soils that are composed of fairly resistant minerals, such as quartz,
K-feldspar and muscovite, base cation supply is low and exchange sites
may become occupied by H+ and Al, rather than Ca2+, Mg2+, and K+. Minerals
such as Ca-plagioclase, biotite, hornblende, diopside, and calcite
are much more susceptible to chemical weathering and can provide base
cations at rates comparable with depletion rates accelerated by acidic
deposition. Important here is consideration of the quantity of weatherable
minerals present in the soil, the residence time of the subsurface
water, and the flowpath of water through the soil. Calcium depletion
seems to be a critical factor in Adirondack and other northeastern
forest soils and studies are currently under way to assess the effect
of Ca depletion on the dieback of tree species, such as red spruce
and sugar maple among others (Driscoll et al., 2001; Lawrence et al.,
1999).
Over time, soils receiving acidic deposition will accumulate both
S and N, mainly in the form of SO4-2 and NO3-. Driscoll et al. (2001)
suggest that even though S deposition is declining because of emission
controls on SO2, the slow release of previously accumulated SO4-2 from
soils will delay the recovery of surface waters. NO3- concentrations
are usually low in surface and soil waters because it is generally
considered to be a growth-limiting nutrient in forest ecosystems. However
continued nitrate deposition can lead to nitrate saturation of the
ecosystem. At this point nitrogen deposition exceeds nutrient uptake
and excess nitrate is exported to surface waters. In the Adirondack
region there does not seem to be any significant regional change in
NO3- in surface waters, or in atmospheric deposition.
Processes Influencing Surface Water Chemistry
Although much of the Adirondack region is underlain by acid-sensitive
bedrock, the pH of surface waters varies widely across the region from
about neutral to acidic.
The extent of neutralization of acidic inputs to surface waters is
determined by the interaction of a complex series of factors, including
soil, hydrology, vegetation, geology, climate and atmospheric deposition.
The relative contribution of these factors in regulating the acid-base
status of surface waters is highly variable, even within very small
regions. For example, hydrologic factors may dictate drainage water
chemistry in one watershed, while an adjoining watershed may be largely
influenced by geologic factors (e.g., the presence of carbonate minerals).
Base cations are derived primarily from cation exchange and mineral
weathering reactions occurring in the soil and in the surficial materials
within a watershed (April et al., 1986; Newton et al., 1987). The rate
at which these are supplied largely determines the acid-base status
of surface water in the watershed.
For most watersheds in the Adirondacks, the routing of water through
the soils and geologic materials is the major factor determining the
base cation supply rate (Figure 4). The relative routing of water,
or flow path, is a function of both the nature of the surficial material
within the watershed, as well as the hydrologic retention time, or
residence time, within the deposits. Surficial materials in the Adirondacks
range from highly acidic upper soil horizons to more base-rich, relatively
unweathered till and stratified drift. Rarely do drainage waters in
the Adirondacks contact carbonate minerals. However, when carbonate
minerals are present in the bedrock, the resulting surface water is
enriched in base cations (particularly Ca2+ and Mg2+) and ANC.
Figure 1. How flow paths of water through
unconsolidated glacial sediments
influence lake
water chemistry.
The flow path of water moving through a watershed can be a function
of a number of lake/watershed characteristics including thickness of
unconsolidated sediments, hydraulic conductivity, and land slope. However,
for most Adirondack watersheds, the dominant flow path is determined
by the thickness of the unconsolidated glacial sediments overlying
the bedrock (Newton et al., 1987). The thickness is also important,
as it defines the size of the potential groundwater reservoir (Figure
1). Watersheds with thick surficial deposits have a large groundwater
storage capacity. During precipitation events water infiltrates through
soil and moves downward to the groundwater table where it is slowly
discharged to streams and lakes. In these basins, deeper flow paths
dominate and result in surface waters with higher ANC. In contrast,
those watersheds with thin deposits of surficial sediments, or high
proportions of bedrock outcrop, have only a small groundwater reservoir,
which is rapidly filled during the early part of precipitation and
snowmelt events. Subsequent rainfall or snowmelt is forced to move
rapidly as shallow interflow through the upper acidic soils horizons,
or as overland flow to streams and lakes, resulting in low ANC surface
water.
References
April, R.H. and Newton, R.M., 1985, Influence of geology on lake acidification
in the ILWAS watersheds: Water Air Soil Poll., v. 26, p.373-386.
April, R.H., Newton, R.M., and Coles, L.T., 1986, Chemical weathering
in two Adirondack watersheds: Past and present-day rates: Geol. Soc.
Am. Bull., v. 97, p.1232-1238.
April, R.H., Keller, D.M. and Driscoll, C., 2004, Smectite in Spodosols
from the Adirondack Mountains of New York. Clay Minerals, v. 39, 99-113.
Baker, J.P. and Schofield, C.L., 1982, Aluminum toxicity to fish in
acidic waters: Water Air Soil Poll., v. 18, p. 289-309.
Baker, J.P., Van Sickle, C.J., Gagen, C.J., DeWalle, D.R., Sharpe,
W.E., Carline, R.F., et al., 1996, Episodic acidification of small
streams in the northeastern United States: Effects on fish populations:
Ecological Applications, v.6, p. 422-437.
Charles, D.F., ed., 1991, Acidic Deposition and Aquatic Ecosystems:
Regional Case Studies: New York, Springer-Verlag.
Cronan, C.S. and Schofield, C.L., 1990, Relationship between aqueous
aluminum and acidic deposition in forested watersheds of North America
and Northern Europe: Environmental Science & Technology, v. 24,
no. 7, p. 1100-1105.
DeHayes, D.H., Schaberg, P.G., Hawley, G.J., and Strimbeck, G.R.,
1999, Acid rain impacts calcium nutrition and forest health: BioScience,
v. 49, p.789-800.
Driscoll, C.T. and Newton, R.M., 1985, Chemical characteristics of
Adirondack lakes: Environmental Science & Technology, v. 19, p.1018-1024.
Driscoll, C.T., Lawrence, G.B., Bulger, A.J., Butler, T.J., Cronan,
C.S., Eager, C., Lambert, K.F., Likens, G.E., Stoddard, J.L., and Weathers,
K.C., 2001, Acidic deposition in the northeastern United States: Sources
and inputs, ecosystem effects, and management strategies: BioScience,
v.51, no.3, p.180-198.
Driscoll, C.T, K.M. Driscoll, M.J. Mitchell and D.J. Raynal. 2003a.
Effects of acidic deposition on forest and aquatic ecosystems in New
York State. Environmental Pollution 123: 327-336.
Driscoll, C.T, K.M. Driscoll, K.M. Roy and M.J. Mitchell. 2003b.Chemical
response of lakes on the Adirondack Region of New York to declines
in acidic deposition. Environmental Science & Technology, v. 37,
p.2036-2042.
Johannes, A.H., Altwicker, E.R., and Clesceri, N.L., 1985, The Integrated
Lake-Watershed Acidification Study: Atmospheric inputs: Water Air Soil
Poll., v. 26, p.339-353.
Kretser, W.A., Gallagher, J., and Nicolette, J., 1989, Adirondack
Lakes Study 1984-1987: An evaluation of fish communities and water
chemistry: Adirondack Lakes Survey Corporation, Ray Brook, NY.
Larson, D.P., Thornton, K.W., Urquhart, N.S., and Paulsen, S.G., 1994,
The role of sample surveys for monitoring the condition of the nation’s
lakes: Environmental Monitoring and Assessment, v. 32, p. 101-134.
Lawrence, G.B. and Huntington, T.G., 1999, Soil-calcium depletion
linked to acid rain and forest growth in the eastern United States:
U.S. Geological Survey WRIR 98-4267.
Lawrence G.B., David, M.B., Lovett, G.M., Murdoch, P.S., Burns, D.A.,
Baldigo, B.P., Thompson, A.W., Porter, J.H., and Stoddard, J.L., 1999,
Soil calcium status and the response of stream chemistry to changing
acidic deposition rates in the Catskill Mountains of New York: Ecological
Applications, v. 9, p. 1059-1072.
Likens, G.E., Bormann, F.H., and Johnson, N.M., 1972, Acid Rain: Environment,
v. 14, no.3, p.33-40.
McLelland, J. and Chiarenzelli, J., 1990, Isotopic constraints on
emplacement age of anorthositic rocks of the Marcy massif, Adirondack
Mtns., New York: Jour. Geology, v.98, p. 19-41.
McLelland, J., 2001, Rock of Ages: From volcanic islands to Himalayan
ranges: the geologic evolution of the Adirondack Mountains: Adirondack
Life, July/August, 2001.
Newton, R.M., Weintraub, J. and April, R.A., 1987, The relationship
between surface water chemistry and geology in the North Branch of
the Moose River: Biogeochemistry, v. 3, p. 21-35.
Oden, S., 1968, The acidification of air precipitation and its consequences
in the natural environment: Bull. Ecological Research Communications
NFR. Ecology Comm. Bull. no. 1, Stockholm.
Schindler, D.W., Mills, K.H., Malley, D.F., Findlay, S., Shearer,
J.A., Davies, I.J., Turner, M.A., Lindsey, G.A., and Cruikshank, D.R.,
1985, Long-term ecosystem stress: Effects of years of experimental
acidification: Canadian Jour. Fisheries and Aquatic Science, v. 37,
p. 342-354.
Schindler, D.W., 1988, Effects of acid rain on freshwater ecosystems:
Science, v. 239, p. 149-157.
Shortle, W.C., Smith, K.T., Minocha, R., Lawrence, G.B., and David,
M.B., 1997, Acid deposition, cation mobilization, and stress in healthy
red spruce trees: Jour. Environmental Quality, v. 26, p. 871-876.
Stevens, D.L., 1994, Implementation of a national monitoring program:
Jour. Environmental Management, v. 42, p.1-29.
Stumm, W. and Morgan, J.J., 1981, Aquatic Chemistry: 2nd edition,
New York, Wiley-Interscience.
Sullivan, T.J., 2000, Aquatic Effects of Acidic Deposition: Boca Raton,
Lewis Publishers/CRC Press LLC.
|