2.2.2 Different soil-water-contaminant interaction mechanisms
           The contaminant that can pose serious threat to humans persist in short or long interval of time. These contaminants can be naturally occurring ones such as arsenic, fluoride, traces of mercury or anthropogenic substances such as chlorinated organics, dissolved heavy metals etc. The major role of a geoenvironmental engineer is to predict the fate of contaminants in the subsurface and minimize its migration towards groundwater. Fate prediction is very essential to understand the presence of contaminants in groundwater sources or subsurface for long term (50 to 200 years). This would essentially depend on different interaction mechanisms between contaminant and soil solids and also between contaminant and dissolved solutes present in pore water. This knowledge is required to assess the risk or threat posed by these contaminants to humans and other organisms.
            Fate of contaminant in geoenvironment is decided by retention and transport of contaminants. The important mechanisms governing these factors are as follows (Yong 2001):
(A) Chemical mass transfer and attennuation
(a) Sorption- contaminant partitioning
(b) Dissolution/ precipitation- addition or removal of contaminants
(c) Acid-base reaction- proton transfer
(d) Redox reaction- electron transfer
(e) Hydrolysis/ substitution/ complexation/ speciation- ligand-cation complexes.
(B) Mass transport
(a) Advection- fluid flow
(b) Diffusion- molecular migration
(c) Dispersion- mixing
(C) Other factors
(a) Biological transformations
(b) Radioactive decay
An adequate knowledge of these mechanisms is required to predict the fate of contaminant. When the contaminated pore fluid passes through the soil mass, it is bound to undergo weak or strong reactions. Sorption process in which the contaminants clings on to the soil solids is one of the predominant reactions. Such reactions does not ensure permanent removal of contaminants from the pore fluid, rather attenuation takes place. Attenuation is the reduction in contaminant concentration during fluid transport due to retardation, retention and dilution. The extent of interaction between the contaminants and soil fraction determines reversible or irreversible nature of contaminant partitioning. The term retention is used for strong sorption of contaminants on the soil particles such that the concentration of pore fluid decreases with time. The amount of contaminant concentration reaching a particular target is considerably less than the source concentration. Chemical mass transfer and irreversible sorption removes the contaminants from the moving pore fluid. This is a very important aspect for a contaminant barrier system, where in the contaminants reaching ground water is minimized. Retardation is mainly governed by reversible sorption and hence release of contaminant would eventually occur. This will ensure the delivery of the entire contaminant load to the final target (example ground water), but with much delay. In nature, the effect of contaminated pore fluid is reduced when it interacts with fresh water (especially during precipitation). This process of dilution also delays the contaminant migration. However, the process of dilution is mostly independent of soil interaction.
            The process of retention and retardation is depicted in Fig. 2.14. From the figure, it can be noted that for retention process, the area under the curves (concentration) goes on reducing. For retardation, the area remains constant (mass conservation), however the concentration of a particular contaminant reduces.