Geoscience Reference
In-Depth Information
Fig. 8.13 Concentration of selected petroleum hydrocarbons (mL/100 g soil) during volatiliza-
tion of kerosene from air-dry vertisol. Reprinted from Fine and Yaron (
1993
). Copyright 1994
with permission of Elsevier
8.3.1 Acidity and Alkalinity Effects
Hydrogen ion regulation in subsurface water is provided by numerous homoge-
neous or heterogeneous buffer systems. A buffer system is characterized by a range
of pH within which it is efficient, relative to its acid- or base-neutralizing capacity.
In a natural environment, the pH of the subsurface water does not generally
correspond to an equilibrium state; this results in instability of pH values, which is
reflected in short-term fluctuations due to seasonal variations and anthropogenic
processes. The effects of low and high pH on dissolution of a number of earth
minerals are discussed here.
Aluminum dissolution under acid rain is the outcome of an anthropogenically
induced effect on the subsurface acid-base equilibrium. As a result of acidic atmo-
spheric pollutants (e.g., HCl, HNO
3
,H
2
SO
4
), rainwater becomes acidic; and when it
reaches the subsurface, where buffering by existing bases is missing, the acidity of
the subsurface water increases. Soil acidification usually is a long-term process. For
example, Johnston et al. (
1986
) report a decrease from pH 7.2 to pH 4.2 after
100 years at the Rothamsted agricultural experimental station (United Kingdom).
Dissolution of Al, which has devastating effects on soil biological populations,
is the main consequence of acidic atmospheric deposition in the forest environ-
ment in large areas of northern and central Europe and North America (Mulder
et al.
1987
). In a study of acid effects on sandy soils, van Grunsven et al. (
1992
)
observed that the logarithm of Al dissolution rates in individual earth samples
follows an inverse linear relationship with pH. Hargrove (
1986
) investigated Al
dissociation from mica-OM complexes, observing that Al concentrations in water
solutions reach minimum values in a pH range of about 4-5 (Fig.
8.14
).
Trace element dissolution from soils surrounding abandoned waste incinerators,
as a combined effect of pH and speciation, was reported by Bang and Hesterberg
(
2004
), who determined the concentration of dissolved metals eluted from soil
samples as a function of pH. In parallel, the speciations of Cu and Zn were
analyzed using X-ray absorption and near-edge structure spectroscopy analysis
