THE EFFECTS OF TRANSIENT TEMPERATURE-GRADIENTS ON SOIL-MOISTURE DYNAMICS

The remediation of non-mobile non-aqueous phase liquid (NAPL) from the unsaturated zone involves a number of physical processes that may be functions of temperature. The coupled mechanism of soil moisture (liqu id and vapour) and thermal energy transfer in a partially saturated po rous medium has been investigated through the use of mathematical mode lling. The physical flow processes involved in this analysis are liqui d, vapour and adsorbed liquid flow, heat conduction, heat convection, latent heat and heat of wetting transfer. A mixed form of both the moi sture and the thermal energy equations is utilized, ensuring a mass an d energy conservative formulation. The Galerkin finite element method is employed for the spatial integration of the governing equations whi le temporal integration is achieved by a fully implicit finite differe nce method with a variable time-stepping scheme. The two-dimensional c oupled equations of moisture and thermal energy flow are assembled thr ough the use of a simultaneous solution approach, resulting in a non-l inear system of equations which is linearized at each time-step using a full Newton-Raphson iteration scheme. The proposed model was tested on a variety of problems which verified that it was formulated properl y. Simulation results indicate that transient temperature gradients af fect the moisture velocity and water pressure fields, due mainly to vi scosity effects on the hydraulic properties of the medium. The effects of pressure gradients on the temperature distribution are minimal, in dicating that heat conduction is the dominant mechanism associated wit h thermal energy transport. This present study is a step toward the no n-isothermal analysis of the transport of contaminant gases in the uns aturated zone and in the dissolution kinetics of residual organics whi ch may lead to a better understanding of remedial actions that involve thermal processes.