A MULTIDISCIPLINARY APPROACH TO CHARACTERIZING ICE-DISTAL GLACIOFLUVIAL DELTAS TO GAIN INSIGHT OF SUBMARINE GROUNDWATER DISCHARGE FROM HIGH-LATITUDE ACTIVE MARGINS

Coastal aquifers play an important role in marine ecosystems by providing disproportionately high fluxes of nutrients and solutes via near-shore and submarine groundwater discharge pathways. Physical characterization of these dynamic coastal systems is foundational to understanding the extent and magnitude of hydrogeologic processes and their subsequent chemical contributions. We describe a 100 m-scale experimental field site located in an ice-distal glaciofluvial delta on the peripheries of the Wosnesenski River watershed entering Kachemak Bay, Alaska. We combine a suite of approaches to better comprehend and represent the complexity of coastal aquifers in proglacial environments. To characterize the study aquifer in detail we apply geologic (sample grain size analyses and lithologic descriptions), geophysical (electrical resistivity tomography, seismic refraction tomography, and horizontal to vertical spectral ratio analyses), and hydraulic (slug and tidal response tests observed in drive-point piezometers) methods. The geologic and geophysical characterization revealed thick (~100 m) glaciofluvial sediments dominated by sands and gravels with intermittent thin layers of silt and delineated zones of freshwater and saltwater at both high and low tide. The hydraulics tests yielded a hydraulic conductivity of 22 m/d, with a substantial tidal response signal driven by tidal loading with pronounced connectivity of shallow groundwater to the sea. Hydraulic conductivity values from slug tests correlate well with mean values estimated from Hazen and Kozeny-Carman equations of grain-size analyses of the dirty sands and gravels that constitute much of the proglacial coastal aquifer, although these estimates ranged from 1-250 m/day. This information is critical for subsequent numerical simulations that model groundwater flow, tidal pumping, and chemical reactions and transport in these understudied environments. Our approach is transferable to similar areas that lack valuable subsurface data. Submarine groundwater discharge estimates are severely lacking in high-latitude glacierized landscapes, physical characterization of coastal aquifers is a necessary first step in addressing this issue.