When a solvent carries cations through a soil with fixed capacity for adsorption or ion exchange, the evolution of concentration fronts, which propagate through the soil at characteristic fractions of the solvent flow speed, is well described by classical theory.  Under two conditions -- when the soil contains hydrophilic minerals such as iron oxyhydroxide, quartz, and clays which readily adsorb and desorb protons, and when the pH of solvent differs from the pH of the soil -- nonclassical concentration fronts can arise, in addition to the classical. We outline the necessary conditions for all classical and nonclassical behaviors and describe it in the context of classical method of characteristics. We then compare two environmental applications.<br/>The first application exhibits classical pH-dependent transport, arising when low pH solvent (e.g., brine saturated with CO<sub>2</sub>) enters a soil containing adsorbed cations. The acidified brine favors the formation of a desorbing front moving slower than the flow speed, but characterized by a peak whose maximum concentration, controlled by the brine pH, can be much larger than the initial value presenting a health hazard.<br/>The second exhibits nonclassical behavior (anomalously rapid solute transport) that occurs when a solvent at high pH enters a low pH  iron oxyhydroxide-bearing soil. In addition to a classical retarded front, a nonclassical pulse forms which travels at the flow speed much faster than expected. It is due to the combination of hydrodynamic dispersion and pH-dependent adsorption. Dispersion creates a low pH mixing zone within the initial front where cation adsorption is negligible; it detaches from the retarded front and travels unretarded ahead of it.

Solute transport with pH variations through soil

249th National Meeting (2015)

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