Contaminants and Land Environment Sustainability Indicators - Sustainable Practices in Geoenvironmental Engineering - page 330

Unit volume containing a group of
particles assembled as a
microstructural unit
Unit volume containing a
large number of
individual particles in a
dispersed state
Dispersed soil particles
FIGURE 9.14
Illustration of inluence of soil structure on availability of exposed reactive soil particle surfaces for sorption of
contaminants. The dispersed particles are typical of soil suspensions used for batch equilibrium test for deter-
mination of adsorption isotherms, whereas the microstructural unit is typical of soil structures in subsurface
soils.
is representative of a soil structure in a natural soil subsurface environment (top illustra-
tion). The schematic shows that although the two identical unit volumes do not necessarily
contain the same number of particles, a relatively small proportion of the reactive particle
surfaces in the microstructural unit is available for interaction with the contaminants in
the porewater.
Distribution coeficients k d obtained from adsorption isotherms using the batch equi-
librium with soil solutions and prepared target contaminants are very useful in that they
deine the upper limit of partitioning of the target contaminant. Problems or pitfalls aris-
ing from the application of k d values reported in the literature for use in models to predict
actual fate and transport of contaminants in the natural subsoil can be traced to
• Inappropriate use of the coeficient, i.e., using the reported k d value to represent
partitioning effects in a natural compact subsoil. This can arise from a lack of
appreciation or knowledge of the particulars of the batch equilibrium tests. If
the tests were conducted to provide the upper limit of partitioning, as has been
described, the model would overpredict sorption and therefore underpredict
transport.
• Major differences in the composition of the leachate and contaminants in the leach-
ate plume being modeled. Unless the batch equilibrium tests were conducted with
actual leachates from the site under consideration, it is inappropriate to use results
from single-species contaminant tests to represent multispecies behavior. In any