Submarine groundwater discharge and its components in response to negative hydraulic barriers

Submarine groundwater discharge (SGD) plays a critical role in local and regional hydrological cycles, and has a significant impact on the quality and ecology of coastal waters. However, our current understanding of the influence of negative hydraulic barriers (NHB), commonly used in coastal aquifers to mitigate seawater intrusion, on SGD and its components is still lacking. This study aims to fill this gap by developing a three-dimensional variable density groundwater flow model to assess the contribution of NHB to the fresh and saline SGD fluxes. The stability and reliability of our model were validated through rigorous comparisons with previous work by our group. The simulation results indicate that although the NHB is effective in desalinating areas with high salt concentrations, it does not sufficiently eliminate low-concentration saline groundwater within the 20-year simulated period. The saltwater wedge initially advances landward and then recedes, with the rate of retreat decreasing over time. In general, the implementation of NHB disrupts the exchange flux between the aquifer and the ocean, resulting in reduced fluxes of SGD compared to scenarios without barriers. The reduction in fresh SGD (Qf) ranges from 3.12 % to 21.87 %, while for density-driven circulation fluxes (Qs), it varies from 2.22 % to 29.86 %. Moreover, the sensitivity of Qs to changes in NHB location decreases in the case of inland pumping wells. Specifically, when the distance between the well and the shoreline (diswell) increases from 30 to 40 m, Qs experiences a decrease of 10.82 %. However, as the diswell increases from 60 to 70 m, the decrease in Qs is only 2.81 %. In addition, it was found that the time required for Qf to reach a stable state is shorter than that of Qs. As the pumping intensity and distance from the ocean increase, both Qs and Qf decrease, resulting in an overall reduction in total discharge.