Abstract Subterranean estuaries (STEs), where groundwater interacts with seawater, influence surface and subsurface coastal ecology and biogeochemistry. In Arctic‐STEs overlying permafrost, groundwater flow and heat transport determine the fate of organic matter. Yet, direct observations of groundwater flow and heat and solute transport processes in Arctic STEs remain limited. This study characterized groundwater flow paths and fluxes and heat transport within an Arctic‐STE along Alaska's Beaufort Sea coast during thawing, summer, and freeze‐up. Intertidal seabed temperature‐depth profiles collected along a 10‐m transect captured the active groundwater flow period, from thaw and flow onset in mid‐June to freeze‐up in late‐September. During this period, aquifer geometry evolved non‐uniformly due to spatially varying thaw rates across the STE (mean (m) thaw depths–beach: 0.25 to 0.55–0.6 m on 20 June, 25 July, 1 October; seabed: 0.6–0.9 m from 25 July to 1 October). Groundwater and surface water levels, salinity, and subsurface temperature profiles measured over tidal time scales were interpreted alongside groundwater flow‐heat transport numerical simulations. Fresh groundwater discharge was sporadic during thawing (m: 0.32 m 3 /day/m), abundant in summer (m: 0.45 m 3 /day/m), and was largely absent during freeze‐up. During freeze‐up, groundwater flow was driven exclusively by seawater recirculation via tidal pumping (from thawing to summer to freeze‐up: 0.00025–0.15–0.5 m 3 /day/m) and convection. Heat advection dominated near aquatic interfaces (shaping intertidal ice), and conduction controlled vertical temperature gradients in low‐flow and unsaturated sediments. These findings will help predict how prolonged summers will alter Arctic‐STE cryo‐hydrology and biogeochemistry.
Demir et al. (2026) studied this question.