Mixing dynamics of groundwater-seawater systems at the land-ocean interface

Subterranean estuaries are important coastal features where dissolved materials from groundwater and seawater can react and transform. Hence, they affect the quantity and quality of fluid and chemical fluxes across the sediment-water interface. Local geologic and hydrologic heterogeneities in coastal systems can modify this interaction and fluxes. I investigate groundwater-seawater mixing dynamics and groundwater discharge in geographically relevant coastal systems where groundwater upwells through permeable sediments, under small river estuaries, and at a site susceptible to episodic rise in the water table with heavy rainfall. Using numerical simulations of variably-saturated density-dependent flow in porous media coupled with solute transport, I show that freshwater plumes and seawater recirculation cells develop where groundwater upwells from discrete freshwater outlets. Mixing zones that form along the freshwater plume-recirculation cell boundary change with tides and seasonal variations in the strength of the upwelling groundwater. Brackish fluxes increase with the waning strength of groundwater upwelling and sediment thickness but to a much lesser extent. Freshwater fluxes increase with stronger upwelling. Using electrical resistivity (ER) as salinity proxy complemented with groundwater head and temperature profile measurements, I find cut banks to be persistent groundwater discharge sites whereas point bars are predominant locations of hyporheic exchange underneath small river estuaries. These trends are consistent on an estuary-wide scale where sinuous reaches show high ER indicating groundwater discharge hotspots while straighter channel segments have resistivities similar to surface water suggesting the dominance of hyporheic exchange. Time-series porewater salinity profiles along a beach cross section directly showed the flushing and freshening of the intertidal subterranean estuary within a 24 hour period. Subsurface temperature profiles, hydraulic head measurements, and ²²²Rn in seawater concentration indicate consequent elevated groundwater discharge towards the coast. These demonstrate that episodic rainfall can rapidly elevate the groundwater table in permeable beach sediments which affects the chemical and thermal regime of the subterranean estuary. This can potentially deliver materials and energy cycled within the mixing zone in episodic doses to coastal areas akin to flooding of rivers and estuaries.