Simulation of solute transport under oscillating groundwater flow in homogeneous aquifers

In this paper, we focus on the influence of temporal variations in the regional hydraulic gradient, particularly, on the impact of temporal variations in the boundary conditions on the spreading of solute plumes in homogenous aquifers. We examined the problem by numerical simulations. Two-dimensional fully implicit finite difference model TRANS_GW_2D for the unsteady groundwater flow and a random walk particle tracking model TRANS_RW_2D for solute transport have been developed to solve the governing equations without restriction on the values of aquifer storativity or on the magnitude of the temporal fluctuations. It has been shown that transient flow conditions (in terms of gradient magnitude variability) have a significant impact on contaminant transport only if the amplitude and period of the oscillations are relatively large. For relatively small oscillations, a steady state flow field can be justified. Transient conditions may be relevant in coastal aquifers with high tidal amplitudes. This tidal variation can have an effect on the spreading of solutes and on salt-water intrusion. Our numerical experiments demonstrate that in case of relatively high storativity values, the dispersion coefficient is amplified in time as the plume moves towards the fluctuating boundary.