Benchmarking water and salt dynamics at subterranean estuaries using TOUGHREACT

The interactions between saline seawater and fresh groundwater occurs in subterranean estuaries (STEs), trig-gering complex hydrological and geochemical processes. These processes are influenced by multiple hydrological factors including tidal amplitude (A), freshwater head (h), seawater diffusion coefficient (d), and slope ratio. Besides, accurate predictions of the geochemical processes in STEs require validating the hydrological estima-tions by the geochemical models against that by conventionally used hydrological models. TOUGHREACT is employed to numerically study the effects of these hydrological factors on groundwater flow and salt transport in STEs. At the quasi-static state, the spatiotemporal distributions of the upper saline plume (USP) and the saltwater wedge (SW), and the water fluxes at the surface water-groundwater interface agree with previous results, con-firming the applicability of TOUGHREACT for submarine groundwater discharge (SGD) simulations. Increasing tidal amplitude, inland freshwater head, seawater diffusion coefficient, and beach slope ratio enhances the ex-change of groundwater and surface water, strengthens the density-driven circulation (DDC) of seawater, and alleviates the saltwater intrusion (SI) by shrinking the SW. The rise of tidal amplitude and seawater diffusion coefficient, and the reduction of freshwater head and beach slope ratio intensify the seawater exchange across the beach mainly by increasing the flux of tide-driven circulation (TDC) of seawater, but reducing the freshwater flux. These findings are beneficial for better understanding the hydrological processes of SGD and serve as a benchmark for predicting the water and salt flow across STEs using TOUGHREACT. This validation expands the numerical toolsets used for quantifying the hydrological processes, and enables future analysis of the geochemical processes occurred in STEs.

Automated summary

The interactions between saline seawater and fresh groundwater in subterranean estuaries (STEs) trigger complex hydrological and geochemical processes. These processes are influenced by multiple hydrological factors. Numerical experiments using TOUGHREACT were conducted to study the effects of these factors on groundwater flow and salt transport in STEs. The results show that increasing tidal amplitude, inland freshwater head, seawater diffusion coefficient, and beach slope ratio enhances the exchange of groundwater and surface water, strengthens the density-driven circulation of seawater, and alleviates saltwater intrusion.