ABSTRACT
Potential groundwater contaminants can enter the subsurface soils in the source area in two
basic states: (1) as already dissolved in the infiltrating water and (2) as liquid, either readily
miscible with water, or hydrophobic (immiscible) such as various nonaqueous phase liquids
(NAPLs; see Section 5.2.4). In any case, before reaching the water table (saturated zone,
aquifer), the potential contaminant first has to move through the vadose, or unsaturated
zone, where it is subject to various physical and bio-geochemical processes collectively called
contaminant fate and transport (F&T) processes. Figure 6.1 and Figure 6.2 illustrate the
general concept of contaminant movement from the land surface into the subsurface and
further downward through the vadose zone. One or more F&T processes may result in the
potential contaminant never reaching the water table, which would be an ideal outcome. If
the contaminant does reach the saturated zone, it would continue its ‘‘journey’’ through the
aquifer and it will be affected by most of the same F&T processes as in the vadose zone.
Although distinction between the terms ‘‘fate’’ and ‘‘transport’’ is not always clear, it is
generally understood that fate refers to various bio-geochemical processes acting upon the
contaminant, whereas transport refers to physical movement of the contaminant. An example
of a fate process would be complete mineralization of an organic contaminant, i.e., its
conversion into inorganic substances such as carbon dioxide and water. An example of a
pure transport process would be advection, or movement of the dissolved contaminant
together with groundwater. On the other hand, some critical processes that affect contamin-
ant movement, without changing its chemical nature, may be the result of various chemical
interactions among the three media: contaminant, water, and aquifer solids. An example
would be processes collectively described as sorption of the dissolved contaminant particles
(molecules) onto solid surfaces of the porous media (grains). This general term is used to
describe immobilization of the contaminant particles by the porous media, irrespective of the
actual mechanism. It may be the result of various more specific processes caused by geo-
chemical interactions (forces) between the solids and the dissolved contaminant. Cation
exchange would be one example of sorption where the contaminant is immobilized by the
mineral (usually clay) surfaces. This immobilization may not be permanent, and the contam-
inant may be released back into the water solution by the reverse process when geochemical
conditions in the aquifer change (e.g., change of pH or inflow of another chemical species
with the greater affinity for cation exchange with the mineral surfaces). Adsorption is the term
often used to describe a process of contaminant particles or molecules ‘‘sticking’’ to aquifer
materials simply because of the affinity for each other. For example, many hydrophobic
organic contaminants are adsorbed onto particles of organic carbon present in the aquifer,
and can be desorbed if conditions change. Adsorption is commonly used interchangeably
with sorption, a more generic term, which sometimes may cause confusion. Absorption, a
rather vague term, usually refers to contaminant incorporation ‘‘deep’’ into the solid particle
structure and it has chemical connotation. The term, however, is seldom used as its net effect
would be equal to a complete destruction of the contaminant, i.e., its permanent removal
from the flow system. Precipitation is another mechanism that completely removes the
contaminant from the flow system. It is a well-understood chemical complexation reaction
in which the complex formed by two or more aqueous species is a solid. When the precipitate
is insoluble, the contaminant is permanently (irreversibly) removed from the flow system.
Precipitation is particularly important to the behavior of heavy metals in soil or groundwater
systems, and it is heavily influenced by pH and redox potential. Dissolution is the reverse of
precipitation, and it may reintroduce the same contaminant back into the flow system, or it
may introduce a new chemical species into the solution based on the changing geochemical
conditions.
