Modeling Cadmium Transport in Neutral and Alkaline Soil Columns at Various Depths

Human health has been potentially threatened by cadmium (Cd) contained in sewage irrigation water. Previous studies of Cd transport in soils were mainly conducted using small soil cores with pH values less than 6. The objectives of this study were to determine the parameters of the convection-dispersion equation (CDE) for Cd transport in relatively larger columns with neutral and alkaline soils, and to investigate the parameters' variability with depth. The soil columns were 50 cm in length and 12.5 cm in diameter. Ceramic suction lysimeters were buried at depths of 2.5, 7.5, 17.5, 27.5, and 37.5 cm to abstract soil solution. Cd concentration in the soil solution samples were subsequently analyzed to obtain breakthrough curves (BTCs). Equilibrium and nonequilibrium models in CXTFIT program were used to estimate parameters of the CDE. The results suggested that both equilibrium and non-equilibrium models performed well in modeling Cd transport. The hydrodynamic dispersion coefficient (D) ranged from 0.18 to 10.70 cm2 h−1, showing large differences among different depths. The retardation factor (Rd) ranged from 25.4 to 54.7 and the standard deviation of Rd value was lower than 30% of the mean value. Precipitation coefficient (Rp) decreased consistently with increasing depth, varying from 1.000 × 10−10 to 0.661 h−1. Sensitivity tests showed that D was less sensitive than Rd. These results would be helpful in understanding the transport and retention of Cd in non-acidic soils.