Wave, tidal, and seasonal dynamics of groundwater flow, saltwater-freshwater mixing, and reactive transport in beach aquifers

Michael, HollyNutrient, metal, and carbon fluxes in submarine groundwater discharge can adversely impact the chemistry of nearshore marine ecosystems. These solutes can undergo biogeochemical transformations in saltwater-freshwater mixing zones that form when fresh groundwater flowing toward the sea meets with saline groundwater of marine origin in sandy beaches. This dissertation focuses on the driving mechanisms of flow, the time scales of mixing, and biogeochemical processes that are key in altering the fate and fluxes of nutrients prior to discharge to the sea. ☐ A field and numerical modeling study investigated intertidal salinity dynamics in a sandy tidally-influenced beach aquifer across a wide range of temporal scales and hydrologic forcings. Seasonal variations in the terrestrial freshwater gradient were primarily responsible for controlling the shape and size of the intertidal saltwaterfreshwater mixing zone, followed by spring-neap variability in tidal amplitude, and tidal stage. Intertidal salinities decreased as the seasonal freshwater hydraulic gradient increased in spring and winter, and increased as freshwater forcing decreased in summer. The effects of Hurricane Sandy and seasonal sea level anomalies on the subsurface salinity distribution were minor compared to seasonal freshwater forcing. ☐ Coupled surface and subsurface measurements reveal for the first time the motion and areal extent of infiltration, recharge, and discharge zones at swash and tidal time scales under and across the beachface. Infiltration was controlled by the location of wave runup and occurred across a part of the beach widely accepted in literature to be a zone of groundwater discharge, while recharge occurred at both swash and tidal time scales. The results demonstrate that identification of infiltration, recharge, and discharge zones can be achieved only through coupled measurements of the swash location and water content conditions in the beach. A more accurate conceptual model of groundwater-surface water interactions in the intertidal zone is developed that will require that sediment transport models be reevaluated to properly represent zones of groundwater-surface water exchange. ☐ A variable-density groundwater flow and reactive solute transport model of a beach aquifer was used to investigate the influence of 5 physical factors, including tidal amplitude, freshwater flux, hydraulic conductivity, beach slope, and dispersivity on the biogeochemical reactivity of the intertidal zone for mixing-dependent and mixing-independent reactions, modeled as denitrification and sulfate reduction, respectively. A sensitivity analysis of nitrate and sulfate removal efficiencies demonstrates that tidal forcing promotes denitrification along the boundary of the intertidal saltwater circulation cell between 1 and 10 ppt. Denitrification increases with the size of the mixing zone, while sulfate supply is the main factor controlling sulfate reduction. The results reveal the type of beaches that are most chemically active and have the largest role in moderating chemical fluxes to the sea. ☐ These studies demonstrate that hydrologic and biogeochemical processes interact across time and space, and that these interactions moderate chemical fluxes to the sea. Thus, the findings have important implications for managing marine ecosystems, and the recreational and economic resources they provide.University of Delaware, Department of Geological SciencesPh.D