Saltwater-freshwater mixing dynamics in a sandy beach aquifer over tidal, spring-neap, and seasonal cycles

The biogeochemical reactivity of sandy beach aquifers is closely linked to physical flow and solute transport processes. Thus, a clearer understanding of the hydrodynamics in the intertidal zone is needed to accurately estimate chemical fluxes to the marine environment. A field and numerical modeling study was conducted over a 1 year timeframe to investigate the combined effects of tidal stage, spring‐neap variability in tidal amplitude, and seasonal inland water table oscillations on intertidal salinity and flow dynamics within a tide‐dominated, microtidal sandy beach aquifer. Measured and simulated salinities revealed an intertidal saline circulation cell with a structure and cross‐sectional mixing zone area that varied over tidal, spring‐neap, and seasonal time scales. The size of the circulation cell and area of the mixing zone were shown for the first time to be most affected by seasonal water table oscillations, followed by tidal amplitude and tidal stage. The intertidal circulation cell expanded horizontally and vertically as the inland water table declined, displacing the fresh discharge zone and lower interface seaward. Over monthly spring‐neap cycles, the center of the circulation cell shifted from beneath the backshore and upper beachface to the base of the beach. Salinity variations in the intertidal zone over semidiurnal tidal cycles were minimal. The dynamics of the circulation cell were similar in simulations with and without a berm. The highly transient nature of intertidal salinity over multiple time scales may have important implications for the types and rates of chemical transformations that occur in groundwater prior to discharge to the ocean. Key Points Beach mixing zone varied over tidal, spring‐neap, and seasonal cycles Salinity varies most strongly in response to seasonal inland water table changes Beach groundwater dynamics have implications for biogeochemical cycling