Geoscience Reference
In-Depth Information
Chapter 13
Abiotic Contaminant Transformations
in Subsurface Water
13.1 Abiotic Contaminant Transformations in Natural
Subsurface Water
Natural subsurface water is considered here as the (bulk) body of water that
completely or partially fills the porous matrix, from the soil surface to the
groundwater zone. This water may be flowing or relatively static, having accu-
mulated above an impermeable subsurface.
Abiotic transformation of contaminants in subsurface natural waters results
mainly from hydrolysis or redox reactions and, to lesser extent, from photolysis
reactions. Complexation with natural or anthropogenic ligands, as well as differ-
ential volatilization of organic compounds from multicomponent liquids or mixing
with toxic electrolyte aqueous solutions, may also lead to changes in contaminant
properties and their environmental effects. Before presenting an overview of the
reactions involved in contaminant transformations, we discuss the main chemical
and environmental factors that control these processes.
13.1.1 Factors Affecting Contaminant Transformations
The chemistry of subsurface water is a main factor to be considered when dealing
with contaminant transformation processes. The acidity (pH) of subsurface
aqueous solutions can vary over a wide range of values. For example, the pH of
swamp water can be as low as 3.5, because of a high concentration of humic and
fulvic acids in a eutrophic lake, but may reach a pH as high as 9.5 (Hutchinson
1957 ). In general, climatic conditions and properties of the surrounding porous
media greatly affect the pH of subsurface water. For contaminants containing acid
or basic functional groups, the pH of water controls species dissociation-associ-
ation behavior and thus affects contaminant half-life (Bender and Silver 1963 ).
Variations in pH induce acid-base mediated hydrolysis. For base-catalyzed
hydrolysis, for example, the rate equation for an organic contaminant is
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