SUBMARINE GROUNDWATER DISCHARGE ACROSS SCALES FROM MARSH TO SHELF (Invited Presentation)

Discharge of groundwater into estuarine and marine waters changes ocean chemistry on both short and long timescales, with impacts on ecosystems and solute mass budgets. Submarine groundwater discharge (SGD) occurs in a range of coastal environments and all along the continental shelf. Each setting has different geologic characteristics, timescales of hydrologic influence, and biogeochemical environments that influence discharge rates and variability, as well as the geochemical and ecological impacts. We review SGD studies in a range of hydrogeologic environments that illustrate these differences and emphasize the importance of geologic and hydrologic variability. In salt marshes, groundwater-surface water exchange is enhanced by crab burrows and drives spatial and temporal variations in redox fluctuations and associated carbon fluxes. In beaches, hydrologic variations control the spatial distribution of biogeochemical reactions and net reaction rates in the intertidal aquifer. In estuaries, large-scale geologic features such as paleochannels and small-scale sediment heterogeneity impact discharge rates and patterns and fluxes of nitrate and ammonium. All along continental shelves, both short- and long-timescale processes promote aquifer-ocean exchange. Wave-drive benthic exchange promotes ecologically important reactions in seafloor sediments and varies globally due to bathymetry, sediment characteristics, and temporal wave patterns. On 1000-year timescales, variations in salinity within continental shelves result in density-driven seawater circulation offshore that can contribute large loads of solutes produced from water-rock interactions. These examples are only a few of the diverse settings in which SGD plays an important role in biogeochemical and ecological processes.