Effects of heat and moisture transfer on the transient dynamic of solute transport in unsaturated soil under isothermal and thermal conditions

The mobility of solute in unsaturated soil column from the surface to the groundwater induced with moisture and heat transfer processes is investigated using the spectral element method. The problem is mathematically described using partial differential equations. The heat and moisture are assumed to be one-dimensional processes and the unsaturated soil has variable thermal, hydraulic and convective properties. The goal is to provide analysis concerning the solute transport in unsaturated soil. The investigation quantifies the solute spatial-temporal variations and studies the effects of the related characteristic parameters. The established highly nonlinear equations describing the heat and mass transfer processes are solved using a developed MATLAB program. The numerical prediction of solute movement is compared with the analytical benchmark solution from the literature and good performance is exhibited. The fully coupled solute transport with heat and water flow is conducted and the corresponding transient flowing of temperature and moisture distributions are estimated. The results show that the thermal model developed has a little influence on soil solute prediction at low upper surface boundary with temperature valued of 20 degrees C when compared to the isothermal model. Affected with the soil gravity, isothermal and thermal solute predictions are equivalent for small time variations, non-large soil depth as well as for high solute diffusivity. However, a great discrepancy is observed between isothermal and thermal predictions when increasing the input variation of the soil surface temperature to 30 degrees C. At this relatively high soil temperature changes, the output solute breakthrough magnitude decreases which generally leads to its fast transportation in soil.