Ignoring the pre-Darcy flow phenomenon in low permeability media may lead to great deviation in contaminant transport prediction

Abstract

This study focuses on the impact of pre-Darcy flow on solute transport in low-permeability media. Most existing studies on solute transport in low-permeability media typically neglect the effects of pre-Darcy flow and adopt Darcy’s law to describe fluid flow for simplification, which may lead to inaccurate estimations of solute transport behavior and concentration distribution. Based on experiments and numerical simulations, we evaluated the applicability of Darcy’s law in simulating solute transport in low-permeability media. The results indicate that the use of Darcy’s law overestimates solute transport rates in low-permeability media. As an alternative approach, the pre-Darcy flow model improves the agreement between simulation and experimental results. The maximum absolute percentage errors of solute concentration obtained from simulations using Darcy’s law and the pre-Darcy model are 18.22% and 6.76%, respectively. We also analyzed the effects of the nonlinearity between flow velocity and hydraulic gradient, permeability, and dispersion coefficient on solute transport. The deviation caused by Darcy’s law increases with the enhancement of velocity-hydraulic gradient nonlinearity. A decrease in permeability and dispersion coefficient also amplifies the deviation. Among these influencing factors, the nonlinearity between velocity and hydraulic gradient has the most significant impact on simulation results, followed by permeability and then dispersion coefficient. The smaller the hydraulic gradient, the greater the potential deviation caused by neglecting pre-Darcy flow. In a case study on radionuclide transport in a geological nuclear waste repository considered in this research, neglecting the pre-Darcy flow effect resulted in a maximum concentration deviation of up to 20.41% when the hydraulic gradient ranged from 1.6 to 3.3.