HYDROLOGIC AND GEOLOGIC DRIVERS OF REDOX DYNAMICS, BIOGEOCHEMICAL HOTSPOTS, AND SOLUTE FLUXES IN COASTAL AQUIFERS (Invited Presentation)

Coastal zones are hydrologically dynamic due to forcing on multiple timescales from waves to glacial cycles. The hydrologic transience drives changes in biogeochemistry that have important implications for chemical cycling and land-sea material fluxes. We give two examples of settings where both long- and short-timescale hydrologic forcing combined with geologic heterogeneity drive redox dynamics that lead to biogeochemical reactions that affect solute fluxes to the coastal ocean via submarine groundwater discharge. In the beach aquifer at Cape Henlopen, DE, measurements and modeling show that redox potential and the location and intensity of redox reactions such as denitrification shift on tidal to seasonal timescales, impacting N loads to coastal waters. Even in mildly heterogeneous sandy aquifers, spatial variability in hydraulic conductivity substantially influences reaction rates. Another setting strongly influenced by coastal hydrology and geology is the transition zone between marsh and upland, which is migrating due to sea-level rise and climate change. There, both long (i.e. sea-level rise and seasons) and short (periodic high tides and storms) hydrologic shifts alter redox conditions in the shallow subsurface, leading to changes in the concentration and composition of dissolved organic carbon that translate to shifts in carbon fluxes across the upland-marsh interface. Improving understanding of these complex, dynamic interfaces between land and sea will improve our ability to predict how these systems and their ecosystem services may change in the future in response to anthropogenic and climatic change.