Impact of Transient Flow on Subsurface Solute Transport with Exponentially Time-Dependent Flow Velocity

The groundwater flow velocity might be temporally variable instead of being a constant as most analytical solutions of solute transport in the subsurface commonly assume. This study investigates the impact of transient flow on solute transport in the subsurface with time-dependent groundwater flow velocity. This study is based on the analysis of breakthrough and leaching processes of solute transport in a one-dimensional (1D) setting. As an example, the flow velocity is assumed to follow an exponential function of time and eventually approaches its steady-state value. Analytical solutions of such models are obtained using the Laplace transform assuming a homogeneous media and Fickian type of dispersion, and the impacts of different parameters of the temporally exponential function of the groundwater flow velocity on solute transport are thoroughly analyzed. The results indicate that a larger power index in the temporally and exponentially decreasing velocity equation results in a faster solute transport process. A sensitivity analysis of parameters shows that the solute transport is most sensitive to the initial flow velocity for the case with exponentially increasing velocity, whereas it is most sensitive to the final steady-state velocity for the case with exponentially decreasing velocity. The general conclusion is that groundwater flow transiency usually has significant impacts on the solute transport process and should not be overlooked.