Salt‐Freshwater Mixing and Submarine Groundwater Discharge in Sloping Unconfined Tidal Aquifers

Abstract Previous studies have revealed the groundwater flow and salt transport in unconfined coastal aquifers in response to numerous factors. However, previous studies assumed a flat aquifer base, and it is unclear how a sloping aquifer base would affect the subsurface flow and salinity distributions. In reality, the bases of many coastal aquifers are inclined toward either the sea or the inland. This study presents numerical simulations that examine the subsurface hydrodynamics in sloping unconfined coastal aquifers. The results show that, when the aquifer base is seaward sloping, the salt‐freshwater mixing zone is wider and the seawater intrusion distance is longer. Also, a seaward base slope reduces/increases the percentagewise contribution of tide‐/density‐driven recirculation to the total submarine groundwater discharge (SGD), extends the transport pathway and transit times of particles. While a landward aquifer base reverses these trends. These findings may assist more accurate estimation of total SGD and strategies for mitigating marine contamination.