Geology Reference
In-Depth Information
used to predict the geochemical behaviour of metal ions, organic
pollutants, etc. As discussed in Section 3.2.1.2, certain equations, e.g.
Davies equation for activity coecient calculation, are contained in the
source code of these models. In contrast, a recently developed model,
ORCHESTRA (Objects Representing CHEmical Speciation and
TRAnsport), enables the user to define equations in text format, which
can then be read by the ORCHESTRA calculation kernel. 36 The object-
oriented structure of this model also allows the user to choose appro-
priate chemical models, e.g. for activity coecient calculation (see
Section 3.2.1.2), adsorption at organic surfaces (MODEL V, 37 NICA 38 ),
and adsorption at oxide surfaces (CD-MUSIC 39 ).
Overall, geochemical computer models can be extremely useful in the
description of chemical equilibria occurring in the aquatic environment.
In some cases, predictions about reaction kinetics and transport of
species can also be made. The application of geochemical models is not
limited to natural aquatic systems but has been usefully extended to
predict the effectiveness of certain remediation strategies in the treat-
ment of waters emanating from contaminated sites. 40
3.3 CASE STUDIES
3.3.1 Acidification
3.3.1.1 Diatom Records. The onset and flourishing of the Industrial
Revolution, with its dependence for energy upon the combustion of
fossils fuels, especially coal, released huge quantities of sulfur dioxide
and nitrogen oxides to the atmosphere, which ultimately were returned
to the land surface in the form of acid precipitation (H 2 SO 4 , HNO 3 ).
The effect of this upon freshwater lakes, especially in poorly buffered
catchment areas (e.g. low-carbonate soils, granitic bedrock) was a
significant decrease in pH and a concomitant decline in biological
productivity, often leading to clear acid waters devoid of fish. That
the acidification of freshwater lakes is a comparatively recent (post-
1850) phenomenon linked to acid deposition has been demonstrated by
the diatom record in the sediments at the bottom of acidified lakes. 41
Diatoms are algae with silicified cell walls. Depending upon the acidity
of the water, some species flourish more than others. Upon deposition,
these siliceous remains are preserved in the sediment column. The
dramatic change in pH after 1850 inferred from the diatom record in
dated sediments from the Round Loch of Glenhead in the Galloway
region of south-west Scotland rules out long-term acidification processes
as the cause. 42
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