EFFECT OF GEOLOGIC HETEROGENEITY ON THE DEVELOPMENT OF SALT FINGERS IN BEACH AQUIFERS

Fresh submarine groundwater discharge can contribute excess nutrient loads to coastal marine ecosystems. However, waves and tides drive infiltration of seawater across sandy beachfaces, forming a saltwater-freshwater mixing zone below the intertidal zone that can attenuate excess nutrient loads prior to discharge. Existing laboratory and numerical modeling studies have demonstrated that unstable flow conditions in beach aquifers can lead to the formation of density instabilities (salt fingers) under certain hydrologic, morphologic, and hydrogeologic conditions. However, salt fingers have not been observed in real-world beach aquifers despite a growing body of field studies investigating intertidal mixing zones. We hypothesize that the lack of field evidence of salt fingers in beaches is due to geologic heterogeneity which reduces concentration gradients through enhanced saltwater-freshwater mixing. In this study we used geostatistical methods to generate realizations of aquifer heterogeneity for different degrees of geologic connectivity. The realizations were incorporated into variable-density variably-saturated flow and salt transport simulations to explore the influence of geologic structure on mixing and salt finger formation in tidally-influenced beaches. Results show that heterogeneity creates more stable conditions in the intertidal subsurface due to greater mixing between freshwater and saltwater. Compared to equivalent homogeneous models, this enhanced mixing reduces salt finger formation for intermediate proportions of fine and medium sand. The models suggest that aquifer heterogeneity may explain the absence of observations of salt fingers in intertidal mixing zones. The findings have implications for understanding intertidal pore water biogeochemistry and chemical fluxes to the ocean.