Geoscience Reference
In-Depth Information
Chapter 6
Contaminant Partitioning in the Aqueous
Phase
One way that contaminants are retained in the subsurface is in the form of a
dissolved fraction in the subsurface aqueous solution. As described in
Chap. 1
, the
subsurface aqueous phase includes ''retained water,'' near the solid surface, and
''free'' water. If the ''retained water'' has an apparently static character, the sub-
surface ''free'' water is in a continuous feedback system with any incoming source
of water. The amount and composition of incoming water are controlled by natural
or human-induced factors. Contaminants may reach the subsurface liquid phase
directly from a polluted gaseous phase, from point and nonpoint contamination
sources on the land surface, from already polluted groundwater, or from the release
of toxic compounds adsorbed on suspended particles. Moreover, disposal of an
aqueous liquid that contains an amount of contaminant greater than its solubility in
water may lead to the formation of a type of emulsion containing very small
droplets. Under such conditions, one must deal with ''apparent'' solubility, which
is greater than ''handbook'' contaminant solubility values.
It is understood that contaminant solubility in an aqueous solution may be
affected by environmental factors, such as ambient pressure, temperature, and
composition of the aqueous solution. However, reference data usually found in the
literature are related to ''pure'' water and a conventionally accepted temperature of
25 C. These are considered standard conditions for a standard state of the
chemicals. Any deviation from standard conditions might be explained by defining
the effect of each isolated factor on the amount and rate of chemical solubility.
The solubility of most inorganic and organic contaminants in water increases
with temperature. Changes in the ''real'' concentration of a solute during changes
in ambient temperature should be considered when dealing with the partitioning of
pollutants among subsurface phases. Temperature has a direct effect on chemical
solubility, but it also has an indirect effect on various reactions occurring in the
subsurface. Moreover, the seasonal variations in temperature might affect the
solubility of toxic chemicals in subsurface solutions; an observed solubility
equilibrium therefore only reflects the solubility of a compound at a given time and
ambient temperature.
