Determination of solute transport parameters is fundamental in numerical modelling and estimation of contaminant movement in the soil. Usually the transport parameters are obtained in the laboratory by columns tests in disturbed samples. However, disturbed samples may not accurately reflect real situations that are subjected to the presence of natural imperfections, influencing the transport parameters values, mainly the hydrodynamic dispersion coefficients (D_h). In this paper we determined D_h values of ammonium (NH_4~+) in undisturbed soil columns and observed the possible effects of the natural imperfections on these values. Column tests in undisturbed samples were performed to obtain the average linear velocity, Darcy velocity and hydraulic conductivity. Later, three solutions of (NH_4)_2HPO_4 were percolated through the columns. The NH_4~+ breakthrough curves were plotted and the D_h values were determined by the methods proposed by Singh (1998) and Bear (1961). We found that both the soil chemical characteristics and macroporosity strongly influenced D_h values. Our overall conclusion was that soils with intact structure are subjected to the presence of natural variations that may powerfully affect D_h values. Moreover, we concluded that a method that uses directly the velocity values in the D_h calculation, cannot be appropriate when there are macropores in the column. Finally, these values obtained from undisturbed columns can result in more reliable data and greater accuracy in prediction of contaminant movement.
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