Implication of density-dependent flow on numerical modelling of SW-GW interactions

With the growing interest in the last decades in the modelling of hydrogeological processes involved in the water resources management, it has been recognized that the assumption of constantproperties water is no longer adequate in the analysis and simulation of the flow considered in these cases. In recent years, many studies used simplistic approaches that may not represent the aquifer flow dynamics realistically by not accounting for changing fluid density. This study explore the importance of understanding the impact of density-dependent flow on SW-GW interactions. To this aim two synthetic models was developed at large and small scales and various scenarios were defined to explore the impact of density-dependent flow on drivers including river and aquifer salinity ratio, hydraulic gradient and river geometry. The results shows that simplifying by excluding density-dependent flow leads to overestimation of solute mass accumulation, and eventually groundwater salinity and limited freshwater lens. Also, the simulated model without density-dependent flow is not able to represent the unsaturated zone properly. However, these impacts are limited to the river banks. In the small scale, when simulated with density-dependent flow, large salinity ratio between river and aquifer can significantly influence both solute and flow dynamics. Moreover, mixed-convention was observed when hydraulic gradient was towards river. Overall, it was concluded that density-dependent flow play an essential role in SW-GW interaction and needs to be taken in to account where the river and aquifer have significant salinity difference, particularly at the vicinity of the river banks.