Impact of transient non-Darcian flow on subsurface solute transport in low- permeability media

Abstract

Abstract Solute transport in low-permeability geological formations such as clay is a complex and ubiquitous phenomenon. Traditional solute transport models have been proved to be inadequate for describing the processes of solute transport in low-permeability media due to transient non-Darcian flow. In this study, we developed a numerical model with non-Darcian flow, aquifer storage effect and temporally exponential hydraulic head to investigate the features of solute transport. The impacts of different parameters of the threshold pressure gradient, the specific storage and time-dependent hydraulic head on breakthrough curves (BTCs) were thoroughly analyzed. New insights indicate, firstly, for non-Darcian flow, a slight change of the threshold pressure gradient will lead to considerable change of BTCs, meaning that the impact of non-Darcian flow on solute transport in low-permeability media should not be ignored; secondly, aquifer storage effect leads to a large flow velocity near flow inlet; thirdly, a larger power index β means that hydraulic head increases faster, also resulting in faster solute transport; finally, the sensitivity analysis shows that the threshold pressure gradient plays a crucial role in solute transport of low-permeability porous. In general, the effects of transient non-Darcian flow are not negligible for describing the solute transport process in low-permeability media.