Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 284

• Esters, compounds containing a modified carboxylic acid group (−COOH), where
the acid hydrogen atom has been replaced by a different organic functional group.
The process of hydrolysis within this group converts the ester compound into the
“parent” organic acid and an alcohol (Equation 8.5):

H O CH COOC H + → CH COOH C H OH + (8.5)

2 3 2 5 3 2 5

In reduction-oxidation degradation reactions (redox), electrons are transferred from one
atom to another. Chemical reduction is defined as the addition of electrons and chemical
oxidation is defined as the loss of electrons (Hemond and Fechner-Levy 2000). In a reac-
tion involving atoms A and B, if atom A gains an electron, it is reduced, and atom B, having
donated an electron, is the reductant . Because atom B loses an electron, B is oxidized and
atom A is the oxidant . Each reaction involving the loss or gain of an electron is termed a
half reaction .
The oxidation of contaminants can occur very rapidly through combustion or incin-
eration. Here, fire transforms the contaminants through oxidation at greatly elevated
temperatures and is represented by applications in cooking, heating, and transportation
(Hemond and Fechner-Levy 2000).

8.3.3 Fate and Transport of Contaminants in Soil

Folklore holds that the presence of soil protects groundwater quality by filtering contami-
nants before they reach and impact groundwater (Hornsby 1990). Soil does have a limited
ability to filter contamination; however, it does not do a perfect job of holding, filtering,
degrading, transforming, or destroying contaminants. These capabilities also depend
upon a number of factors related to the chemistry of the contaminant and the geological
environment where the contaminant is released.
Soil is defined as the unconsolidated mineral matter on the immediate surface of Earth
(Soil Science Society of America 1987). Basic to an understanding of soil are the factors
affecting its development and ultimate physical structure. The composition, texture, and
thickness of soil are influenced by its source material, plant growth, micro- and macro-
organisms, climate, topography, process of formation (e.g., alluvial, fluvial, and glacial),
and physical and chemical weathering since original formation (Brady and Well 1999).
Structurally, soil is composed of three phases: soil gases, soil water, and organic and inor-
ganic solids. The gas and water phases may comprise 25%-50% of the total volume of a
surface soil, especially at shallow depths (USEPA 1999).
Contaminants released into the soil can migrate within all three phases. Once a contam-
inant is resident in soil, these factors determine its migration rate (Schnoor 1996; USEPA
1999; Kaufman et al. 2009):

• Contaminant mass released
• Duration of the release
• Physical chemistry of the contaminant
• Physical chemistry of the soil (e.g., pH, redox potential and mineralogy)
• Amount of water present
• Permeability of the soil
• Retention capacity of the soil