Precipitation of Fe (hydr)oxides (such as Fe(OH)3) in subterranean estuaries can form a low-permeable zone, referred to as the “iron curtain”, of which the genesis, distribution, and influencing factors remain unclear. This study investigated the abiotic formation of iron curtains under variable seawater and freshwater conditions using the numerical model code TOUGHREACT. Results indicate that Fe(OH)3 accumulates on the freshwater side of the freshwater-seawater mixing zone, whereas the precipitation on the seaward side is inhibited by locally high H+ concentrations. Fe(OH)3 initially precipitates in the shallow layer of the aquifer and subsequently extends to deeper layers, owing to the increased residence time of both freshwater and seawater with depth. The accumulation amount and the spatial distribution of Fe(OH)3 precipitation are influenced by the water flux and the O2(aq) to Fe2+ concentration ratio. Consistent with field observations, these findings enhance a better understanding of iron transformation and distribution in subterranean estuaries.

Low-permeability layer (LPL), formed by natural deposit or artificial reclamation and commonly found below the intertidal zone of coastal groundwater system, can retard the ingress of seawater and contaminants, and shorten the travel time of the land-sourced contaminant to the marine environment compared with a homogenous sandy coastal aquifer. However, there is limited understanding on how an intertidal LPL, a condition occurred in a coastal aquifer at Moreton Bay, Australia, influences the groundwater and contaminant transport across the shallow beach aquifer system. We characterized the aquifer hydrological parameters, monitored the in situ groundwater heads, and constructed a 2-D numerical model to analyses the cross-shore hydrological processes in this stratified system. The calibrated model suggests that in the lower aquifer, the inland-source fresh groundwater flowed horizontally towards the sea, upwelled along the freshwater-saltwater interface, and exited the aquifer at the shore below the LPL. Whereas in the upper aquifer, the tidally driven seawater circulation formed a barrier that prevented fresh groundwater from horizontal transport and discharge to the beach above the LPL, thereby directing its leakage to the lower aquifer. A contaminant represented by a conservative tracer was 'released' the upper aquifer in the model and results showed that the spreading extent of the contaminant plume, the maximum rate of contaminant discharge to the ocean, and its plume length decreased compared with a simulation case in a homogenous sandy aquifer. Sensitivity analysis was also conducted to investigate the characteristics of the LPL, including its continuity and hydraulic conductivity, which were found to vary along the beach at Moreton Bay. The result shows that with a lower hydraulic conductivity and continuous layer of LPL reduced the groundwater exchange and contaminant transport between upper and lower aquifer. The findings from the combined field and modelling investigations on the impact of an intertidal LPL on coastal aquifer systems highlight its significant implications to alter the groundwater and mass transport across the land-ocean interface. The intertidal low-permeability layer (LPL) could affect the groundwater dynamics and restrict the development of salt fingers in coastal aquifers. The presence of the LPL could reduce peak flux of the land-derived conservative contaminant across the aquifer-ocean interface with increased travel time in the aquifer, only when the contaminant could move across the layer. The form of the LPL outcrop reduced the exchange between the groundwater and seawater; however, the net flux across the shoreline remained similar. image

The interactions between saline seawater and fresh groundwater occurs in subterranean estuaries (STEs), triggering 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 estimations 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, confirming 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 exchange 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.

Salt flats are bare soil surfaces with elevated salinity levels which inhibit vegetation. They are commonly found in areas of coastal wetlands where salt in surface soil accumulates due to evaporation from the shallow saline water table. The expansion of these vegetation-free zones may severely affect the bio-ecological function of these coastal wetland systems. In this study, sand flume experiments and numerical modeling were carried out to investigate the mechanisms underpinning the development of salt flats in the elevated areas of coastal wetlands. We found that salt precipitation occurred on the soil surface where water content is low, but the hydraulic connection to the saline groundwater table is maintained. This connection is required for evaporation to promote upward porewater flow and bring salt to the surface. However, the surface salinity level decreases with increasing surface saturation due to the density-driven removal of salt. As a result, salt efflorescence is absent on the surface with higher water saturations, despite strong evaporation. Density-driven flows can be triggered by the evaporation-induced upward salinity gradient and transfer salt from the surface back to the water table. Numerical simulations confirmed this salt removal process and suggested the intensity of the density-driven flow is mainly dictated by the soil permeability in the unsaturated zone. The findings from this study increase understanding of the processes involved in the formation and evolution of salt flats. Salt precipitated on the soil surface with continuous evaporation and low water saturationEvaporation-induced upward salinity gradient can trigger salt fingersSalt fingers tended to form near the soil surface with higher water saturation and inhibited surface salt accumulation

•PFOS transport in the coastal aquifer with salting-out effect considered is numerically studied.•With salting-out effect included, PFOS injected from inland tends to be intercepted by the upper saline plume and discharges from aquifer with the tide-induced saltwater circulation.•Salting-out reduces both peak discharge rate and plume dispersion of the land-derived contaminants

Tidal wetland ecosystems are sensitive to porewater salinity dynamics. However, it is unclear how salts move and distribute in these wetlands, particularly how the salts accumulated by evaporation get removed from the wetland soil, so that the salinity levels may stabilize to accommodate vegetation. We conducted a combined field and modeling study to identify the porewater flow and salinity patterns in a subtropical wetland subjected to tidal inundation and evaporation. Measured and simulated salinity contours indicated the formation of hypersaline porewater plumes in the upper intertidal zone and the fresher porewater zones with salinity close to that of seawater near the creek and in the supratidal zone. Simulations indicated the discharge of the hypersaline upper intertidal porewater to the creek with a discharge pathway developed under the fresher near‐creek porewater zone, and this was further confirmed by the field‐observed significant salinity gradient under the creek. Our model suggested that both water and salt discharge from the wetland soil occurred predominantly at the creek bank and creek bed. The porewater discharge is more intensive through the creek bank than the creek bed, while the salt discharge across both the creek bank and creek bed was comparable due to the much higher salinity level under the creek bed. Salt discharge driven by density gradients and tidal‐induced porewater circulation provides a mechanism for removing salts accumulated in the upper intertidal zone due to evaporation and could prevent salt flat formation and marsh plants dieback.