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Applied Geochemistry 27 (2012) 1–14
Contents lists available at SciVerse ScienceDirect
Applied Geochemistry
journal homepage: www.elsevier.com/locate/apgeochem
Review
Acidi?cation of Earth: An assessment across mechanisms and scales
Karen C. Rice a,?, Janet S. Herman b
a
b
U.S. Geological Survey, University of Virginia, Charlottesville, P.O. Box 400123, VA 22904-4123, USA
Department of Environmental Sciences, University of Virginia, Charlottesville, P.O. Box 400123, VA 22904-4123, USA
a r t i c l e
i n f o
Article history:
Received 5 April 2011
Accepted 1 September 2011
Available online 10 September 2011
Editorial handling by R. Fuge
a b s t r a c t
In this review article, anthropogenic activities that cause acidi?cation of Earth’s air, waters, and soils are
examined. Although there are many mechanisms of acidi?cation, the focus is on the major ones, including emissions from combustion of fossil fuels and smelting of ores, mining of coal and metal ores, and
application of nitrogen fertilizer to soils, by elucidating the underlying biogeochemical reactions as well
as assessing the magnitude of the effects. These widespread activities have resulted in (1) increased CO2
concentration in the atmosphere that acidi?es the oceans; (2) acidic atmospheric deposition that acidi?es
soils and bodies of freshwater; (3) acid mine drainage that acidi?es bodies of freshwater and groundwaters; and (4) nitri?cation that acidi?es soils. Although natural geochemical reactions of mineral weathering and ion exchange work to buffer acidi?cation, the slow reaction rates or the limited abundance
of reactant phases are overwhelmed by the onslaught of anthropogenic acid loading. Relatively recent
modi?cations of resource extraction and usage in some regions of the world have begun to ameliorate
local acidi?cation, but expanding use of resources in other regions is causing environmental acidi?cation
in previously unnoticed places. World maps of coal consumption, Cu mining and smelting, and N fertilizer
application are presented to demonstrate the complex spatial heterogeneity of resource consumption as
well as the overlap in acidifying potential derived from distinctly different phenomena. Projected population increase by country over the next four decades indicates areas with the highest potential for acidi?cation, so enabling anticipation and planning to offset or mitigate the deleterious environmental effects
associated with these global shifts in the consumption of energy, mineral, and food resources.
Published by Elsevier Ltd.
Contents
1.
2.
3.
4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.
Acidifying reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Acidification of the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.
Emissions of S and N compounds and acidic atmospheric deposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1.
Decreasing emissions in the Western hemisphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.2.
Increasing emissions in developing countries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.
Other emissions to the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Elevated atmospheric CO2 and ocean acidification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.
Neutralizing reactions important in the oceans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Acidification of freshwaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.
Acid mine drainage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.1.
Metal ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.2.
Coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2.
Acidic atmospheric deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3.
Neutralizing reactions important in freshwaters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Acidification of soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1.
Fertilizer use for crop production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2.
Neutralizing reactions important in soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
? Corresponding author. Tel.: +1 434 243 3429.
E-mail addresses: kcrice@usgs.gov (K.C. Rice), jherman@virginia.edu (J.S. Herman).
0883-2927/$ – see front matter Published by Elsevier Ltd.
doi:10.1016/j.apgeochem.2011.09.001
2
K.C. Rice, J.S. Herman / Applied Geochemistry 27 (2012) 1–14
6.
Global aggregate effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Natural sources of acidity in the environment range from volcanic emissions to drainage from newly exposed, sul?de-enriched
igneous rocks to decomposing organic matter. A collection of
reports on natural low-pH environments recently was published
in this journal (Eppinger and Fuge, 2009). Natural acidic settings
described by geochemists include those associated with oxidizing
sul?de-rich mineral deposits of Cu (Verplanck et al., 2009), Zn, Pb
and Ag (Graham and Kelley, 2009), and Fe (Kwong et al., 2009).
Volcanic eruptions inject gaseous SO2 and NO into the atmosphere,
where subsequent oxidation to H2SO4 and HNO3 supports aerosol
formation (Pueschel, 1996) and causes acidi?cation of local rainfall
(Mather et al., 2004). Oxidation of natural organic matter, including petroleum (Borgund and Barth, 1994) and peat (Gorham
et al., 1986), is well known to lower the pH of freshwater with
organic acids. Unusually low-pH waters, however, are associated
with parts of the landscape disturbed by human activities far more
commonly than deriving from natural processes in pristine settings
(Langmuir, 1997; Drever, 1997).
In this review paper, the authors explore the nature and magnitude of acidi?cation from human activities that exacerbate the
oxidation and hydrolysis reactions of C, Fe, N and S. The objectives
of this paper are to (1) describe and compare the mechanisms of
anthropogenic acidi?cation of Earth’s atmosphere, waters and
soils; (2) ensure that specialists in each of the disciplinary ?elds
touched on in this paper are aware of other mechanisms of environmental acidi?cation; and (3) demonstrate that it is the many
overlapping sectors of human activity that cumulatively cause
acidi?cation of Earth. To the authors’ knowledge, no attempt has
been made to amass an assessment of all the major anthropogenic
acidifying processes. Growing specialization of the sciences has
tended to result in a lack of awareness of the useful information
that exists in allied ?elds, limiting the success of comparative studies. This effort is presented to identify and quantify the most in?uential processes acidifying the environment across a range of
spatial scales as a starting point for others to discuss and extend.
The major anthropogenic causes of acidi?cation of the environment are (1) electric power generation, whereby the combustion of
fossil fuels affects the atmosphere and the resultant acid is widely
distributed to the oceans, freshwaters and soils; (2) resource
extraction, whereby the mining and processing of mineral and
energy resources result in acid mine drainage into freshwaters
and emissions from smelting contaminate the atmosphere and
soils; and (3) food production, whereby the manufacture and
application of N-based fertilizer affect the atmosphere through
gaseous emissions as well as alter freshwaters and soils receiving
runoff from agricultural ?elds. These alterations of Earth’s environment all increase with expanding human population and resource
consumption and have direct consequences for the present and
future chemical quality of the atmosphere, waters, and soils that
support human life.
1.1. Acidifying reactions
Biogeochemical reactions involving water in contact with the
minerals of soil and bedrock and with the gases of the atmosphere
result in solutes in aqueous solution, many of which subsequently
undergo oxidation or hydrolysis (Hem, 1985). These two types of
reactions are notable for their concomitant production of acidity.
A typical oxidation reaction utilizes O2 in the atmosphere or dissolved in water as the oxidizing agent, and the electron-transfer
reaction occurs by adding H+ to the aqueous environment (Stumm
and Morgan, 1996). The abundant redox-sensitive elements C, Fe,
N and S are intrinsic to human exploitation of energy, mineral
and food resources, so oxidation of those elements is at the center
of the analysis here.
In the aquatic environment, sul?de undergoing oxidation to
sulfate generates acidity (Garrels and Christ, 1965). Exposure of pyrite-containing rocks to O2 and water through human-induced disturbances associated with mining initiates oxidative weathering,
þ
FeS2 ðsÞ þ 7=2O2 ðgÞ þ H2 O ¼ Fe2þ þ 2SO2
4 þ 2H ;
ð1Þ
and the reaction equally describes the oxidative dissolution of marcasite and pyrrhotite.
Frequently the dissolution of a mineral solid in water is
followed by hydrolysis of the aqueous cation (Baes and Mesmer,
1976). The cation coordinates with hydroxyl ions (OH ), the source
of which is the self-dissociation of water. Once OH is taken up by
the cation, H+ remains in solution, as in this hydrolysis leading to
precipitation of an insoluble ferric oxyhydroxide solid phase
Fe3þ þ 3H2 O ¼ FeðOHÞ3 ðsÞ þ 3Hþ ;
ð2Þ
or to formation of aqueous complexes of Fe(III) depending upon local geochemical conditions. The details of aqueous speciation and
its interaction with mineral solubility and solution pH have been
quantitatively presented in textbooks (Butler, 1998) and in a host
of computerized geochemical models such as WATEQF (Plummer
et al., 1976).
The growth of organisms requires the ?xation and reduction of
C into biomass. Following death, oxidation proceeds in Earth-surface environments, ordinarily accelerated by microbial respiration
CðsÞ þ O2 ðgÞ ¼ CO2 ðgÞ;
ð3Þ
although alternate electron acceptors are utilized in O2-restricted
environments (Stumm and Morgan, 1996). In aquatic systems, CO2
gas dissolves, hydrates, and dissociates to form weak carbonic acid
CO2 ðgÞ þ H2 O ¼ H2 CO3 ðaqÞ ¼ Hþ þ HCO 3 ;
ð4Þ
which drives natural weathering reactions (Drever, 1997).
The representation of decaying biomass as C in Eq. (3) is over
simpli?ed, because a typical soil organic matter composition is
closer to C115N10S1.2P3 (Walker and Adams, 1958). Ammonium
(NHþ
4 ) is the initial N-containing product of microbial decomposition of biomass, and reduced N can follow two pathways: (1)
volatilize as gaseous NH3 and be oxidized in the atmosphere or
(2) stay in the soil as NHþ
4 where it can undergo nitri?cation in
the presence of O2 in the reaction
NHþ4 þ 2O2 ðgÞ ¼ 2Hþ þ NO 3 þ H2 O;
ð5Þ
which generates acidity (van Breemen et al., 1987).
Overall, acidi?cation of the atmosphere, surface waters, and soils
results from oxidation reactions, primarily of reduced (1) C compounds, through fossil-fuel burning and a host of other anthropogenic activities; (2) Fe compounds, associated with extraction of
mineral and coal deposits; (3) N compounds, through fossil-fuel
burning and production and application of N-based fertilizers; and
(4) S compounds, associated with removal of mineral and coal
3
K.C. Rice, J.S. Herman / Applied Geochemistry 27 (2012) 1–14
Table 1
The acidi?cation processes exacerbated by human activities and their environmental effects.
Element
oxidized
Means of human
exacerbation
Environmental
effect
Scale of
effect
Nature of effect
Potential for natural
amelioration
Trends in human exacerbation
S oxidation
generates
strong acid,
H2SO4
Fossil-fuel combustion
Acidic
atmospheric
deposition
Regional
Acidi?ed
receiving waters
(freshwater);
acidi?ed soils
Rate of bedrock weathering
too slow to balance stress;
depleted exchange capacity
in soils
Western hemisphere emission
controls implemented and
emissions reduced; global fuel
consumption increasing
S oxidation
generates
strong acid,
H2SO4
Fe oxidation and
hydrolysis
releases free
protons, H+
Mining coal and basemetal sul?des; processing
and smelting metalsul?de ores
Acid mine
drainage
Local
Acidi?ed
receiving waters
(freshwater)
Bedrock weathering
restricted to geochemically
reactive substrate
Global materials and energy
consumption increasing
C oxidation
generates
weak acid,
H2CO3
Fossil-fuel combustion;
deforestation; cement
manufacturing; biofuel
development
Elevated CO2 in
atmosphere
Global
Acidi?cation of
oceans
Neutralization capacity of
oceans inadequate
Global fuel consumption increasing;
deforestation increasing
N oxidation
generates
strong acid,
HNO3
Fossil-fuel combustion;
production and use of N
fertilizers
Acidic
atmospheric
deposition
Regional
Acidi?ed
receiving waters
(freshwater);
acidi?ed soils
Rate of bedrock weathering
too slow to balance stress;
depleted exchange capacity
in soils
Global food production and
consumption increasing;
transportation sector increasing
deposits and the burning of fossil fuel. The nature and scales of effect
of each means of human exacerbation of acidifying processes are
summarized in Table 1. Although the complex inter-relations among
these numerous processes and the receiving media prevent a neat
separation into individual components of the Earth-surface environment, this paper is roughly organized along the hydrological pathway of acidi?cation of the atmosphere, followed by effects on
surface waters, ?rst the ocean and then freshwaters, and ?nally
the effect on soils.
2. Acidi?cation of the atmosphere
Combustion of fossil fuels and various industrial and agricultural emissions of reduced C, N, and S compounds support oxidation reactions and the resulting acidi?cation of the atmosphere
(Table 1). The largest source of CO2 and SO2 released to the atmosphere is from combustion of coal for generation of electric power
(Energy Information Administration, 1998). Additionally, signi?cant atmospheric emissions derive from combustion of natural
gas and petroleum, re?ning of crude oil, smelting of ores, burning
of forests, and manufacture of chemicals, pulp and paper, steel, Al,
and cement, all of which have different spatial scales and location
of effect. For example, in Canada, the largest portion of SO2 emissions (>30%) derives from the base-metals smelting sector,
whereas in the USA, 69% of total SO2 emissions derive from electric
power generation. The largest source of NOx (NO plus NO2) compounds released to the atmosphere is exhaust emissions from
motor vehicles, aircraft, marine vessels, and other forms of transportation, but electric utilities and industrial combustion also contribute (USEPA, 1990). Here, the effect of the major processes
acting to acidify the atmosphere are considered.
2.1. Emissions of S and N compounds and acidic atmospheric
deposition
Water in equilibrium with an unpolluted atmosphere containing CO2 and natural organic acids has a pH of about 5.66, which
is slightly acidic. Atmospheric deposition with a pH of less than 5
is considered acidic atmospheric deposition, commonly referred
to as acid rain (Drever, 1997). The primary cause of acid rain is
emission of SO2 and NOx to the atmosphere during the combustion
of fossil fuels. The S and N compounds undergo oxidation and
hydration in the atmosphere in the reactions (Langmuir, 1997)
SO2 ðgÞ þ 1=2O2 ðgÞ þ H2 O ¼ 2Hþ þ SO2
4 ;
ð6Þ
2NO2 ðgÞ þ 1=2O2 ðgÞ þ H2 O ¼ 2Hþ þ 2NO 3 ;
ð7Þ
to form strong acids, H2SO4 and HNO3, which return to Earth as H+,
SO2
and NO
4
3 through ‘‘atmospheric deposition,’’ i.e., rain, snow,
fog, cloud water, gases, as well as dry deposition. Schemenauer
(1986) presents a summary of the pH of fog and cloud waters, measured at Earth’s surface and by aircraft at locations around the
world, indicating that pH values range from 2.1 to 7.5. The higher
pH values likely are in?uenced by the marine environment. Because
cloud water/fog almost always has a lower pH than precipitation
and is in contact with vegetation for greater lengths of time, it
has the potential to do more harm to the terrestrial ecosystem than
precipitation alone. One of the lowest fog–water pH values ever
measured, 1.69, was in southern California (Schemenauer, 1986).
Deleterious effects of acid rain on plants, animals, and humans
were observed as long ago as the middle of the 17th century in
England, when the atmospheric transport of pollutants between
England and France was recognized (Bricker and Rice, 1993). By
increasing the height of chimneys, pollution was displaced downwind of the source of emissions. The average height of industrial
smokestacks in the USA has tripled since 1950 (Patrick et al.,
1981). Comparable height increases in most industrialized countries in the 20th century has transformed acid rain from a local
urban problem into one of global scale. Indeed, monitoring of
wet-deposition chemistry during the 1980s and 1990s at remote
locations indicates that even Mt. Fuji, Japan (Dokiya et al., 1995),
the eastern shore of the Baltic Sea (Milukaite et al., 1995), and Torres del Paine National Park, Chile …
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