Subsurface physical barriers have been recognized as effective in mitigating seawater intrusion in coastal aquifers, although mainly 2D (cross‐sectional) barrier effects have been considered. In this study, impermeable barriers with finite shore‐parallel lengths are investigated through 3D numerical simulation, thereby extending previous analyses. Two scenarios are considered: (a) barrier‐only and (b) barrier‐well systems; and three available barrier types are analyzed and compared: (1) subsurface dam, (2) cutoff wall, and (3) fully penetrating barrier. Barrier location, length, and height are investigated, and barrier effectiveness is evaluated from seawater volumes, seawater wedge toe positions, and maximum safe pumping rates. In the barrier‐only system, a better performance in preventing seawater intrusion was achieved by cutoff walls rather than subsurface dams. Finite‐length subsurface dams may slightly enhance seawater extent along parts of the coastline that are beyond the dam's length. Cutoff walls performed best when located at relatively small distances from the coast in the barrier‐only system, whereas with a well at 450 m from the shoreline, the subsurface dam located at a critical distance from the sea (i.e., 300 m in the current study) performed optimally (from the tested cases) and was superior to cutoff walls in terms of the maximum safe pumping rate. A fully penetrating barrier outperformed cutoff walls and subsurface dams, as expected. Our investigation indicates that subsurface barrier design should consider the effect of the shore‐parallel length, because barrier benefits may otherwise be significantly overestimated.

Salt is a key solute in salt marshes and under the influence of evapotranspiration can accumulate to a high concentration level in the marsh soil and precipitate in the solid form to become a significant stressor for plants, affecting marsh plant productivity and ecological zonation. Numerical simulations of coupled pore water flow and salt transport were conducted to examine how spring‐neap tides and evaporation combine to influence salt dynamics and distribution patterns in marshes. The salt pan formation was simulated with a sandy loam marsh soil subjected to a medium rate of potential evaporation. The critical condition for the salt pan formation was underpinned by hydraulic connection between the marsh surface and water table to sustain evaporation in the supratidal zone. Both low soil permeability and overly high potential evaporation were found to break the hydraulic connection. In this case, the surface soil salinity increased gradually over the intertidal zone to a maximum around the spring high tide mark followed by a rapid decrease to a lower constant level across the supratidal zone. This salinity distribution pattern has also been observed in the field. In both salt marshes with and without salt pans, excessive salt accumulated on the marsh surface due to evaporation was removed by tidally induced circulating flow and/or flow driven by density gradients associated with the accumulated salt. The salt dynamics and distribution patterns revealed here, especially the salt pan formation simulated for the first time, have important implications for studies of marsh plant growth and overall eco‐functions.

Unsaturated flow is an important factor that affects groundwater motion. Among various drainage models, the nonlinear Hillslope‐storage Boussinesq (HSB) model has been commonly used to predict water flux along a slope. In this study, we improved this model by considering lateral flow in the unsaturated zone. Using modified van Genuchten functions, we analytically expressed the concept of equivalent propagation thickness in the vadose zone. This analytical expression was then incorporated into the HSB model to reflect two different stages of the drainage process and to simulate the hillslope drainage process more accurately. The model results indicated that lateral flow has significant effects in the unsaturated zone during the hillslope drainage process. Even in sandy aquifers, the amount of water contributed by the unsaturated zone is a key factor that enables a decrease in the water table during the middle and late stages of the process. A comparison between the measured and simulated results based on both convergent‐type and divergent‐type hillslope drainage processes revealed that the thickness of the saturated zone decreases as the unsaturated flow increases. This study emphasizes the necessity of considering unsaturated flow in the HSB model to improve the accuracy of predicting groundwater outflow rates and develop more accurate hydrographs. The concept of equivalent propagation thickness also provides a criterion for assessing the importance of unsaturated lateral flow for future drainage research.

Unsaturated flow influences both the seawater extent under steady‐state conditions and the propagation of tides in coastal aquifers. However, its effects on salt distributions in tidally influenced coastal aquifers are little investigated. The present study used numerical simulations and data from laboratory experiments to analyze the effects of unsaturated flow on density‐dependent solute transport in coastal unconfined aquifers. The effects of the inland boundary condition (i.e., constant‐head or constant‐flux) were tested. Compared to a stable sea level, the results show that unsaturated flow has a more pronounced influence on salt distributions in coastal unconfined aquifers when tides are considered, regardless of the type of inland boundary condition. Neglect of unsaturated flow effects leads to expansion of the upper saline plume (USP), shrinkage of the saltwater wedge (seaward movement of saltwater wedge), and overestimation of water and salt exchange across the aquifer‐ocean interface. This is caused by a lower head in the nearshore area during high‐tide periods with the unsaturated zone effects removed. Thus, without the unsaturated zone, stronger head gradients within the nearshore aquifer occur at high tide, leading to stronger tidally driven seawater infiltration and hence a larger USP. Counterintuitively, ignoring unsaturated flow effects leads to greater average inland head over a tidal period, which shifts the saltwater wedge seaward. It is concluded that unsaturated zone effects should not be neglected for modeling tide‐affected seawater intrusion, especially if quantification of near‐shore conditions is important.