Modelling cation transport and pH buffering during unsaturated flow through intact subsoils
B. LUDWIG, F. BEESE & K. MICHEL
Eur. J. Soil Sci. 56:635-645 (2005)
The rate of percolating water in soils may considerably affect the kinetics of transport processes. Undisturbed samples of a subsoil of a Luvisol which differed in their hydraulic conductivities (k1: 4 to 6 cm d-1 ; k2: 20 to 21 cm d-1; k3: 42 to 59 cm d-1) were leached under saturated conditions with 4, 20, 102 and 205 mM BaCl2 at a hydraulic gradient of 6.5 and the Darcy velocities were q1 (270 to 360 mm d-1), q2 (1280 to 1360 mm d-1) and q3 (2730 to 3830 mm d-1). Objectives were to investigate how different flux rates affect the amount of desorbed cations in an undisturbed soil when percolated with BaCl2 solution of different concentrations and whether ion concentrations in the percolates can be predicted. The model PHREEQC was used for the calculation of one-dimensional transport, inorganic complexation and multiple cation exchange and the program UCODE was used for the parameter optimization. The best prediction performance of ion concentration and the sum of cations desorbed at q1 (73 % satisfactory or good predictions) was obtained when the parameters were 0.6 cm (dispersivity ) , 0.74 (fraction of mobile water), 3.4e-5 s-1 (mass transfer coefficient alpha) and 0.91 (proportion of cation exchange capacity in contact with the mobile water fCEC-m ). At q2, the optimum performance (71 % satisfactory or good predictions) was achieved with alpha = 6.2e-4 s -1 and fCEC-m = 0.58. At q3, the optimum parameters were alpha = 6.8e-4 s -1 and fCEC-m = 0.38 which resulted in 58 % satisfactory or good predictions. This study suggests that alpha increases with increasing pore water velocity and that fCEC-m decreases with increasing hydraulic conductivity. The application of UCODE and PHREEQC was useful to get an improved understanding of cation exchange reactions in undisturbed soil .
Key words: cation exchange, CXTFIT, dispersivity, hydraulic conductivity, kinetics, mass transfer coefficient, mobile water, PHREEQC, pore water velocity, transport, UCODE.