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Abstract Saltwater‐freshwater mixing zones in beach aquifers support biogeochemical reactions that moderate chemical loads in fresh groundwater discharging to marine ecosystems. Existing laboratory and numerical modeling studies have demonstrated that fluid density gradients in the mixing zone can lead to free convection and the formation of density instabilities, or salt fingers, under a range of hydrologic, morphologic, and hydrogeologic conditions. However, salt fingers have rarely been observed in real‐world beach aquifers despite a growing body of field studies investigating intertidal mixing zones. In this study, we used geostatistical methods to generate randomly distributed assemblages of fine and medium sand and incorporated those geologic realizations into variable‐density variably‐saturated flow and salt transport simulations to explore the influence of geologic structure on mixing zone stability in tidally‐influenced beaches. Ensemble‐averaged model results show that geologic heterogeneity inhibits salt finger formation and promotes a stable intertidal mixing zone due to enhanced dispersion. This effect is highest for high degrees of heterogeneity and for more laterally connected geologic architecture. Compared to hydraulically equivalent homogeneous models, sediments with moderate to high heterogeneity produce mixing zones that are on average 19%–29% smaller and 3–10 times more stable due to the absence of the downward convection and seaward movement of salt fingers. The models indicate that geologic heterogeneity may explain the lack of field observations of salt fingers in real‐world intertidal mixing zones. The findings have implications for predicting the onset of free convection in beaches and for understanding intertidal pore water biogeochemistry and chemical fluxes to the ocean. 

Abstract Future increases in the frequency of tidal flooding due to sea level rise (SLR) are likely to affect pore water salinities in coastal aquifers. In this study, we investigate the impact of increased tidal flooding frequency on salinity and flow dynamics in coastal aquifers using numerical variable‐density variably‐saturated groundwater flow and salt transport models. Short (sub‐daily) and long (decadal) period tides are combined with SLR projections to drive continuous 80‐year models of flow and salt transport. Results show that encroaching intertidal zones lead to both periodic and long‐term vertical salinization of the upper aquifer. Salinization of the upper aquifer due to tidal flooding forces the lower interface seaward, even under SLR. System dynamics are controlled by the interplay between SLR and long period tidal forcing associated with perigean spring tides and the 18.6‐year lunar nodal cycle. Periodic tidal flooding substantially enhances intertidal saltwater‐freshwater mixing, resulting in a 6‐ to 10‐fold expansion of the intertidal saltwater‐freshwater mixing area across SLR scenarios. The onset of the expansion coincides with extreme high water levels resulting from lunar nodal cycling of tidal constituent amplitudes. The findings are the first to demonstrate the combined effects of gradual SLR and short and long period tides on aquifer salinity distributions, and reveal competing influences of SLR on saltwater intrusion. The results are likely to have important implications for coastal ocean chemical fluxes and groundwater resources as tidal flooding intensifies worldwide. 

This resource contains the data required to reproduce the results of Heiss et al. (2022). Heiss, J. W., Mase, B., & Shen, C. (2022). Effects of Future Increases in Tidal Flooding on Salinity and Groundwater Dynamics in Coastal Aquifers. Water Resources Research, 58. https://doi.org/10.1029/2022WR033195 

This resource contains example model input and output data for Olorunsaye and Heiss (2024). Olorunsaye, O., & Heiss, J. W. (2024). Stability of saltwater‐freshwater mixing zones in beach aquifers with geologic heterogeneity. Water Resources Research, e2023WR036394, 1–22. https://doi.org/10.1029/2023WR036056 

The interactions between the atmosphere, ocean, and beach in the swash zone are dynamic, influencing water flux and solute exchange across the land-sea interface. However, the integrated role of these interactions in governing transport processes within the swash zone remains unexplored. This study employs groundwater simulations to examine the combined effects of waves and evaporation on subsurface flow and salinity dynamics in a shallow beach environment. Our simulations reveal that wave motion generates a saline plume beneath the swash zone, where evaporation induces hypersalinity near the sand surface. This leads to the formation of a hypersaline plume beneath the swash zone during periods of wave recession, which extends vertically downward, driven by the resulting vertical density gradients. This hypersaline plume moves landward and down the beachface due to wave-induced seawater infiltration and is subsequently diluted by the surrounding saline groundwater. Furthermore, swash motion increases near-surface moisture, leading to an elevated evaporation rate, with dynamic fluctuations in both moisture and evaporation rate due to high-frequency surface inundation caused by individual waves. Notably, the highest evaporation rates on the swash zone surface do not always correspond to the greatest elevations of salt concentration within the swash zone. This is because optimal moisture is also required – neither too low to impede evaporation nor too high to dilute accumulated salt near the surface. These insights are crucial for enhancing our understanding of coastal groundwater flow, biogeochemical conditions, and the subsequent nutrient cycling and contaminant transport in coastal zones. 

This archive contains field data used to investigate swash velocities by Britt Raubenheimer, Steve Elgar, and Alexandra Muscalus. The quality controlled cross-shore velocity time series are in the .zip folder "u", and the quality-controlled alongshore velocity time series are in the .zip folder "v". Details about the files and data are provided in README.txt. For further questions, please contact Britt Raubenheimer at braubenheimer@whoi.edu, Steve Elgar at elgar@whoi.edu, or Alexandra Muscalus at alexandra.muscalus@whoi.edu Support was provided by the National Science Foundation, the US Coastal Research Program, and the WHOI Build the Base Program. 

This resource includes the data from the Journal of Hydrology manuscript Hydraulic head fluctuations in an intermediate depth aquifer. The data set includes the 3-year time series of hydraulic heads presented in the manuscript as well as the .hds files from the MODFLOW-NWT runs. 

Alongshore array of ADVs in 2-m water depth and one ADV in inner surfzone