Modeling the movement of volatile organic chemicals in columns of unsaturated soil

Mechanisms affecting the fate of nondegradable volatile organic chemicals in soils include (1) advection in air and water, (2) dispersion in air and water, (3) air-water mass transfer and equilibrium, (4) diffusion in immobile water, (5) mass transfer between mobile and immobile water, and (6) sorption. A deterministic model was developed to account for these processes in laboratory columns of unsaturated soil. The general form of the model was solved numerically. The numerical solution was verified with analytic solutions for simplified conditions. Column experiments were conducted to validate the model and to determine the relative importance of each mechanism in two soil types. The movement of trichloroethene was measured in a column packed with a uniform sand and one packed with uniformly sized aggregates that were made from clay. Parameter values for the model predictions were independently determined from direct measurements and literature correlations. Bromide tracer studies were performed to determine parameter values that could not be measured directly or were not estimated accurately by literature correlations. For the sand column the amount of immobile water, the rate of liquid diffusion, and the liquid dispersion coefficient were measured in a tracer study. A batch rate study was used to measure the rate of intraaggregate diffusion in the clay aggregates. The liquid dispersion coefficient for the column containing aggregates was measured in a tracer study. These parameter values were used in the model to predict the breakthrough and elution of trichloroethene in the two columns. To describe the column data, however, Henry's constant was increased from a literature value of 0.4 to 0.7, and the predicted gas dispersion coefficient was reduced by a factor of 10.

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