Modeling of 36Cl transport and water flow in amended gypsiferous calcareous soil

Dissolution of gypsum has great influence on water flow and solute transport occurring in gypsiferous-calcareous soils. The objectives of this study, were to model the transport of solute (³⁶Cl) in gypsiferous-calcareous soil treated with fuel oil (FO) based on the convection-dispersion transport model (CDE), and determine the effect of FO on dissolution of gypsum associated with calcium carbonate content. Radiotracer of chloride as carrier free of ³⁶Cl was applied to surfaces of saturated soil columns that have 20, 250 and 500 g kg⁻¹ gypsum treated with 0, 1, 2, and 4% of fuel oil and leached with water until complete displacement of chloride. The application of 1 to 2% FO improved the transport properties due to modification in soil structure. The applied models gave good fit between measured and predicted breakthrough curves of Cl with significant linear correlation coefficient (r) that ranged between 0.972 and 0.999. They gave a calculated dispersion coefficient (D) ranging from 2.1 to 79.2 cm²/day, and retardation factor (R) ranging from 0.92 to 1.58. It was found that D was also linearly related to pore water velocity (v). Also, the results indicate the possibility of predicting the distribution of chloride in gypsiferous-calcareous soil for different time periods using correct boundary conditions. Experimental results show that gypsum dissolution in the soil columns is mainly determined by the flow velocity, soil saturation and then partially coating with FO.