Geoscience Reference
In-Depth Information
1.2.2 Subsurface Aqueous Solutions
The aqueous solution here refers to free water in the subsurface having a com-
position affected by the interaction between the incoming water and the solid and
gaseous phases. This composition is achieved under a dynamic equilibrium with
natural processes and may be disturbed by anthropogenic activities. The chemical
composition of the subsurface aqueous solution at a given time is the end product
of all the reactions to which the liquid water has been exposed.
The thermodynamic properties of subsurface aqueous solutions are expressed in
terms of a single-species solution activity coefficient for each molecular constit-
uent. Its composition, however, should be considered on the basis of molecular
speciation in the aqueous solution, which in turn is related to biological uptake
exchange reactions and transport through the subsurface.
Yaron et al. (
1996
) summarize the characteristics of aqueous solutions as
follows:
• Acidity-alkalinity of the solution, measured as pH, is affected by the quality of
the incoming water (generally acidic for rain, neutral or alkaline for irrigation
and effluents) and buffered by the environmental system.
• Salinity, or total salt concentration, is usually expressed in terms of total dis-
solved solids (TDS) or as the electrical conductivity (EC) of the solution. The
major fractions of anions are composed of Cl
-
,SO
4
2-
, and NO
3
-
and the
common cations are Ca
2+
,Mg
2+
,Na
+
, and K
+
. The composition of the sub-
surface solution varies between the composition of water entering the system
and that of the solution in equilibrium with the solid phase.
• Trace elements of natural or anthropogenic origin may enter in the composition
of the subsurface aqueous solution. Alkali and cationic materials, transition
metals, nonmetals, and heavy metals are inorganic trace elements potentially
found in the composition of the subsurface solution. Adsorption is the most
significant mechanism for distributing trace elements between the solid and
liquid phases in the subsurface.
• Organic ligands found in solution cause complexation of inorganic trace ele-
ments, which influences the equilibrium status between solid and aqueous
phases and affects their concentration in the subsurface solution. The presence
of organic trace compounds of natural or anthropogenic origin in the aqueous
phase is controlled by the nature and properties of subsurface colloids, the
chemical and physicochemical characteristics of the organic molecules, and the
nature of the environmental system.
The volume of solution in the subsurface, under partially saturated conditions,
varies with the physical properties of the medium. In the soil layer, the compo-
sition of the aqueous solution fluctuates as a result of evapotranspiration or
addition by rain or irrigation water to the system. Changes in the solution con-
centration and composition, as well as the rate of change, are controlled by the
buffer
properties
of
the
solid
phase.
Because
of
the
diversity
in
the
