Geoscience Reference
In-Depth Information
Part V
Transformations and Reactions of
Contaminants in the Subsurface
After reaching the subsurface, contaminants are partitioned among the solid,
liquid, and gaseous phases. A fraction of the contaminated gaseous phase is
transported into the atmosphere, while the remaining part may be adsorbed on the
subsurface solid phase or dissolved into the subsurface water. Contaminants
dissolved in the subsurface aqueous phase or retained on the subsurface solid
phase are subjected, over the course of time, to chemical, biochemical, and
surface-induced degradation, which also lead to formation of metabolites.
In many cases, the subsurface is contaminated by a mixture of toxic chemicals
with components having different physical and chemical properties. Therefore, a
contaminant undergoes transformations controlled by the properties of each
substance, the characteristics of the subsurface, and the ambient conditions.
Physicochemically mediated and biologically mediated degradation are the main
processes involved in such transformations.
From an environmental point of view, the transformation of contaminants is not
restricted to molecular changes. Rather, we also must consider all deviations from
the original properties of a contaminant that may be relevant to its behavior in the
subsurface. For example, in addition to degradation-induced transformation,
contaminants may change their physicochemical characteristics as a result of
specific reactions with other natural or anthropogenic chemicals found in the
subsurface environment. To be more specific, complexation of a contaminant with
existing natural ligands may lead to a change in its solubility, thus affecting its
transport and redistribution in the vadose zone. The volatilization of high vapor
pressure compounds from a mixture of immiscible (NAPL) contaminants, such as
petroleum products, leads to changes in the retention or transport behavior of
remaining constituents.
As a first approximation, we consider the main subsurface transformation
processes to comprise reactions leading to chemical transformation or degradation
and metabolite formation in the liquid phase or the solid-liquid interface and
reactions resulting in complexation of chemicals, which in turn lead to a change in
their physicochemical properties.
