Simulating the Hydro-Geo-Chemical Processes during Submarine Groundwater Discharge by TOUGHREACT

More than 60% of the global population lives in coastal areas, especially within 100 km from the coastlines, relying mostly on shallow groundwater resources. Seawater intrusion and submarine groundwater discharge (SGD) occur in the coastal aquifer systems, threatening these critical freshwater resources. Salinity seawater and fresh groundwater complexly interact with each other via SGD and SI. The SGD drives the discharge of not only a large volume of freshwater, but also terrestrial geochemical substances into the ocean through a mixing zone between discharging freshwater and recirculating seawater. The flux of SGD may be even greater than that of surface water through rivers and estuaries. For example, the SGD was estimated to be ~40 % and 80 %~160 % of the river water discharging flux into the South Atlantic Bight and Atlantic Ocean, respectively, and as a major source of dissolved organic matter and nutrients to Arctic coastal waters and the Mediterranean Sea.

A few hydrological models, including MARUN, SEAWAT, SUTRA, and PHT3D, are commonly used for SGD studies. The recently developed TOUGHREACT is robust in simulating coupled hydrodynamic, thermodynamic, and geochemical processes. From TOUGH2 (Transport Of Unsaturated Groundwater and Heat, version 2), a multi-dimensional numerical model for simulating coupled transport of water, vapor, non- condensable gas, and heat in porous and fractured media. However, TOUGHREACT is rarely used for SGD analysis, despite it being a well-rounded model with wide applications. Additionally, relevant studies on the iron (Fe) precipitation during SGD have focused predominantly on its spatial distribution and the adsorption of dissolved species, and studies on the genesis and geochemical evolution are scarce.

Therefore, we developed a systematic method using TOUGHREACT to simulate the hydrological processes in STEs and benchmarked the estimations; and then we numerically explored the groundwater flow and salt transport dur SGD by considering the influencing factors of tidal amplitude, freshwater head, seawater diffusion coefficient, and beach slope ratio. Consequently, by employing TOUGHREACT simulation, we analyzed the formation and spatiotemporal distribution of the Fe precipitation in the shallow beach aquifer due to the mixing of freshwater and seawater, and identified the key influencing factors during SGD.

The results show that, freshwater-derived Fe²⁺ is oxidized by O₂(aq) in seawater during SGD, then precipitates as Fe (hydr)oxides (Fe(OH)₃) to form an Fe precipitation zone. Fe(OH)₃ tends to accumulate in the freshwater side of the mixing zone, whereas Fe(OH)₃ precipitation in the seaward side of the mixing zone is inhibited by locally high H⁺ concentrations. The Fe(OH)₃ first precipitates in the shallow aquifer, then extends to deeper layers over time, which is attributed to the increase in the residence time with the depth of both freshwater and seawater. The spatial distribution, and particularly, the extent of the iron curtain are influenced by the water flux and the concentration ratio of O₂(aq) to Fe²⁺. These results are beneficial for better understanding the formation and distribution of iron curtains, and shed light on enhancing the understanding of the hydrogeochemical processes in subterranean estuaries.