ICE SHEET DYNAMICS DRIVE PRONOUNCED, LONG-TERM CHANGES IN THE FRESHWATER-SALTWATER INTERFACE

Along coastlines worldwide, saltwater is migrating landward into freshwater aquifers, a process termed saltwater intrusion (SWI), with implications for water quality, biogeochemical cycling, and water resource management. Present knowledge of SWI is focused on mid- and low-latitude environments where sea-level rise and fluctuations in terrestrial groundwater head due to changes in recharge or groundwater withdrawal have been identified as dominant SWI drivers. SWI in high-latitude coastal systems is less well understood, particularly in glaciated regions such as along the coast of Greenland and Antarctica. Recent sub-ice sheet groundwater measurements in Greenland reveal that groundwater head is driven by ice sheet forcings, highlighting the intimate connectivity between groundwater systems and ice sheet dynamics. This link raises the question: how do pressure changes during ice sheet thinning impact subsurface salinity distributions? In this modeling study, we explore the effect of ice sheet thinning on nearshore groundwater head and salinity distributions using a novel numerical model that simulates aquifer loading/unloading and variable-density groundwater flow and solute transport. We find that when ice sheet thinning diminishes groundwater head, the resulting shift in subsurface pressure gradients enables the freshwater-saltwater interface to migrate landward. This work highlights a previously unconsidered mechanisms of SWI that affects glaciated coastlines over long timescales and underscores that this mechanism needs to be considered in long term studies of coastal water quality and biogeochemistry near ice sheets.