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Sediment has been characterized as the greatest carrier of pollutant.
The sources of water pollution can be distinguished as environmental, domestic, industrial, and agricultural. Depending upon the nature of the solute, transport process in a hydrologic environment may entail advection, diffusion, dispersion, adsorption, desorption, decay of contaminants, chemical reactions, solubilization, precipitation, volatilization, particulate transport, and miscibility. The mechanisms for transport are advection, dispersion, diffusion, solute-solid interaction, chemical reactions, and decay phenomena. The process by which solute is transported by the bulk motion of flowing water is called advection. When a solute moves, it tends to spread out from its advection path. This spreading, called hydrodynamic dispersion, comprises mechanical dispersion and molecular diffusion. Mechanical dispersion is caused entirely by mixing during the motion of the fluid. In molecular diffusion, molecular constituents move under the influence of their thermal kinetic energy in the direction of their concentration gradient. Many solutes react with soil through the process of adsorption. This reaction results in partitioning of the solute into the mobile solution phase and the immobile surface phase. The reverse of adsorption is the process of dislodgement of chemicals from soil, called desorption.
References
The Engineering Handbook 1996, IEEE Press, Article on Hydrology by Singh, V. P. pp. 1003-1017
Jackson, A., Roy, D., and Breitenbeck, G. 1994. Transport of a bacterial suspension through a soil matrix using water and an anionic surfactant. Water Research. 28(4):943-949.
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