Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 376

10.6.2 Biodegradation and Biotransformation

The common perception is that biological activities in the subsoil will degrade organic
chemical compounds. Not always understood or perceived is whether this does in fact
contribute to attenuation of the concentration and toxicity of the contaminants in the sub-
soil. Figure 10.12 provides the overall view of various attenuating mechanisms in the soil.
The top left-hand corner of the diagram shows the biologically mediated transfer mecha-
nisms participating in the attenuation process. Not well illustrated or sometimes not fully
acknowledged are the redox reactions (this includes both abiotic and biotic redox reac-
tions) listed under the Chemical Mass Transfer box in the bottom right corner of the diagram.
The discussions in the latter sections of this chapter will provide case histories dealing
with treatments using various procedures and techniques. Among these will be the use
of intrinsic bioremediation in management of contaminant transport. Before embarking
on these discussions, it is useful to clarify the use of the term substrate in the discussions
to follow. In the context of biological activities in the subsoil, the term substrate is used to
mean the food source for microorganisms. This term should not be confused with the
term substrate used in some soil mechanics and geotechnical engineering literature to
mean subsurface soil stratum. The use of substrate as a term here is consistent with usage
in microbiology, and is meant to indicate that it ( substrate ) serves as a nutrient source and
also a carbon or energy source for microorganisms.

Biologically mediated
transfer
Redox and
biological
transformations

mediated
er
and
cal
ations

Contaminant influx
as liquid waste in
combination with
groundwater

Chemical mass transfer Chemic

Sorption—co
partitioning
Dissolution/p
solids
Acid/base rea
transfer
Abiotic and b
Hydrolysis/su
Complexation
ligand-cation

Sorption—contaminant
partitioning
Dissolution/precipitation—new
solids
Acid/base reactions—proton
transfer
Abiotic and biotic redox reactions
Hydrolysis/substitution
Complexation/speciation—
ligand-cation complexes

Attenuated
contaminant flux
leaving unit of
soil mass

t flux
of

“Exploded”
view of unit of
soilmass

“Exploded”
view of unit of
soil mass

FIGURE 10.12
Processes involved in attenuation of contaminants in leachate transport through a soil element. Dilution of con-
taminants is not included in the schematic illustration. (Adapted from Yong, R.N. and Mulligan, C.N., Natural
Attenuation of Contaminants in Soils , CRC Press, Boca Raton, FL, 310 pp., 2004.)