Geoscience Reference
In-Depth Information
atmospheric sulfates and 25 % of nitrates. While agriculture is also a major source
of nitrates, we refer to acid rain as an inorganic contaminant of industrial and
urban origin. Discussion of acid rain in the context of groundwater acidification is
given in
Sect. 17.2
.
Rain and snow in equilibrium with carbon dioxide exhibit a pH of about 5.6.
However, as result of a strong acid atmospheric pollution, the pH can decrease to
4. Records from a 30-year period (1963-1993) show, for example, that rain and
snow in northeastern USA had an average annual pH of 4.05-4.3, and that sulfuric
acid contributed 55-75 % of this measurable acidity (Likens and Bormann
1995
).
Another case is rapid industrialization in China, which has led to a significant
increase in acid rains caused mainly by the extensive use of coal and higher
emission of pollutants. Wet and dry deposition of acidifying compounds, including
H
2
SO
4
and HNO
3
, occurs in most populated parts of China; the only exception is
the desert areas, where alkaline dust largely neutralizes the acids in the deposition
materials (Larssen et al.
2006
).
The effect of acid deposition on soil depends on effects of both intensity and
capacity (Reuss et al.
1987
). Intensity effects include changes occurring in the soil
solution as a response to strong acid anion concentration in the acid deposition,
which is buffered by the properties of the natural soil. The main capacity effect
occurs as a result of sulfate adsorption and the increased leaching of base cations
associated with strong acid anion mobility. As a ''mobile'' anion from acidic
deposition, SO
4
2-
can be retained by soils from solution, regardless of whether or
not this retention results in the stoichiometric displacement of another anion. In
general, this retention is considered as ''SO
4
adsorption.'' In many forest soils from
the northeastern USA and some regions in Canada, it has been found that as a
result of acid rain, the amount of SO
4
2-
loading is presently higher compared with
pre-industrial levels. Harrison et al. (
1989
) reviewed the degree of SO
4
2-
adsorption reversibility following a decrease in anthropological inputs and sug-
gested the pathway of soil solution sulfate concentration with and without
adsorption (Fig.
18.29
). It can be observed that an important portion of retained
sulfate remains completely irreversible in the polluted soil.
Based on this observation, Harrison et al. (
1989
) examined, in a laboratory
study, the relative SO
4
2-
adsorption capacities and reversibility of a large number
of soil samples. The samples were collected from a wide variety of forest sites in
the USA, Canada, and Norway and were subject to different environmental con-
ditions and past atmospheric deposition levels. The measure of reversibility of
SO
4
2-
adsorption was calculated from two estimates of irreversibility of adsorbed
sulfate, by comparing differences in phosphate extractable fractions, and by
comparing SO
4
2-
desorption from treated and untreated soils. The results showed
that *80 % of the tested soils irreversibly retained some of the adsorbed SO
4
2-
.
In general, soils that adsorbed higher amounts of SO
4
2-
also showed higher
amounts of irreversibly adsorbed sulfate (Fig.
18.30
). On average, *36 % of the
adsorbed SO
4
2-
was adsorbed irreversibly, for all soils studied.
Dissolution of aluminum in acidic soils has an important irreversible effect on
acid deposition. The depletion and redistribution of aluminum with depth have
