Propagation of Ocean Water Level Signals Through a Dynamic Arctic Coastal Active Layer

ABSTRACT

In the Arctic, shallow groundwater is conveyed in supra‐permafrost aquifers within the seasonally thawed active layer. Due to data paucity in the North, the dynamics of these high‐latitude aquifers remain poorly understood in coastal settings, despite recent intensified interests in low‐latitude coastal aquifers. Addressing short‐ and long‐term dynamics controlling groundwater flow begins with a comprehensive characterization of seasonal aquifer properties. This study investigates seasonal and spatial variability of the inland propagation of ocean level signals through supra‐permafrost aquifers via paired meteorological and groundwater level time series at a fresh and saline groundwater site along Simpson Lagoon, Alaska. Despite progressive mid‐to‐late summer active layer thawing, the difference between groundwater level and thaw depth reveals that the saturated thickness remained relatively constant at both sites, but seasonal changes in aquifer function were caused by depth‐varying hydraulic conductivity and specific yield. At the saline site, hydraulic diffusivity (transmissivity/storativity) decreased over time, likely due to lower hydraulic conductivity at depth, whereas diffusivity remained stable or slightly increased at the fresh site. Spatial variability in diffusivity was apparent, which is attributed to salinity‐induced thaw enhancement modifying aquifer properties. Groundwater systems were more responsive to ocean level fluctuations and storm events under higher air temperatures and low atmospheric pressure, especially at the salinized site. Given that both sites exhibited strong ocean‐groundwater connectivity, they are sensitive to changes in ocean level dynamics driven by climate change or storm events. Climate change‐driven permafrost thaw and saltwater intrusion may mutually enhance future ocean‐aquifer interactions, driving broader coastal biogeochemical impacts.