Permafrost hydrology in an Arctic tundra ecosystem: quantifying water sources using stable water isotopes

Global warming in the Arctic can lead to the alteration of the hydrological cycle and the thawing of permafrost. In this study, we utilized stable water isotope techniques (δ²H and δ¹⁸O) to infer the sources mixing of stream water at two sites (HV and PS) along the Sag River on the North Slope, Alaska (USA) during August 2022. The isotopes of snow and rain samples were plotted above the LMWL and reflected the isotopic characteristics of seasonal precipitation in Alaska. The porewater collected within the active layer showed enriched isotope values than other samples, indicating summer precipitation. Using δ¹⁸O and deuterium excess in a Bayesian mixing model, we estimated the contribution rates of summer precipitation, seasonal ice, and ice wedge. The results indicated a substantial contribution from melted seasonal ice (HV: 96.8% and PS: 74.1%), formed by frozen precipitation from the previous year, to surface water in August. Additionally, it was observed that the contribution of ice wedge was relatively greater in the downstream (PS: 21.8%) compared to the upstream (HV: 2.1%). Furthermore, we observed that the isotopic compositions of surface water in the PS site revealed evidence of evaporation, as indicated by a characteristic isotopic fractionation slope. Summer precipitation (HV: 1.1% and PS:4.1%) did not contribute substantially to surface waters. This research provides insight into fundamental processes related to sources and mixing of waters in permafrost hydrology.