hydrology Article Hydrological Connectivity in a Permafrost Tundra Landscape near Vorkuta, North-European Arctic Russia Nikita Tananaev 1, * , Vladislav Isaev 2 , Dmitry Sergeev 3 , Pavel Kotov 2 and Oleg Komarov 2   Citation: Tananaev, N.; Isaev, V.; Sergeev, D.; Kotov, P.; Komarov, O. Hydrological Connectivity in a Permafrost Tundra Landscape near Vorkuta, North-European Arctic Russia. Hydrology 2021, 8, 106. https://doi.org/10.3390/ hydrology8030106 Academic Editors: Il-Moon Chung, Sun Woo Chang, Yeonsang Hwang and Yeonjoo Kim Received: 19 June 2021 Accepted: 17 July 2021 Published: 22 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 P.I. Melnikov Permafrost Institute, Siberian Branch, Russian Academy of Sciences, 677010 Yakutsk, Russia 2 Department of Geology, Lomonosov Moscow State University, 119991 Moscow, Russia; tpomed@yandex.ru (V.I.); kotovpi@mail.ru (P.K.); tpomed@rambler.ru (O.K.) 3 Sergeev Institute of Environmental Geoscience, 101000 Moscow, Russia; sergueevdo@mail.ru * Correspondence: TananaevNI@mpi.ysn.ru Abstract: Hydrochemical and geophysical data collected during a hydrological survey in September 2017, reveal patterns of small-scale hydrological connectivity in a small water track catchment in the north-European Arctic. The stable isotopic composition of water in different compartments was used as a tracer of hydrological processes and connectivity at the water track catchment scale. Elevated tundra patches underlain by sandy loams were disconnected from the stream and stored precipitation water from previous months in saturated soil horizons with low hydraulic conductivity. At the catchment surface and in the water track thalweg, some circular hollows, from 0.2 to 0.4 m in diameter, acted as evaporative basins with low deuterium excess (d-excess) values, from 2‰ to 4‰. Observed evaporative loss suggests that these hollows were disconnected from the surface and shallow subsurface runoff. Other hollows were connected to shallow subsurface runoff, yielding d-excess values between 12‰ and 14‰, close to summer precipitation. ‘Connected’ hollows yielded a 50% higher dissolved organic carbon (DOC) content, 17.5 ± 5.3 mg/L, than the ‘disconnected’ hollows, 11.8 ± 1.7 mg/L. Permafrost distribution across the landscape is continuous but highly variable. Open taliks exist under fens and hummocky depressions, as revealed by electric resistivity tomography surveys. Isotopic evidence supports upward subpermafrost groundwater migration through open taliks under water tracks and fens/bogs/depressions and its supply to streams via shallow subsurface compartment. Temporal variability of isotopic composition and DOC in water track and a major river system, the Vorkuta River, evidence the widespread occurrence of the described processes in the large river basin. Water tracks effectively drain the tundra terrain and maintain xeric vegetation over the elevated intertrack tundra patches. Keywords: permafrost hydrology; Russian Arctic; water tracks; hydrological connectivity; stable water isotopes; dissolved organic carbon; electrical resistivity tomography; taliks 1. Introduction Hydrologic connectivity is a complex concept referring to water transfer in the land- scape, or between landscapes, or, more generally, within or between the water cycle units, and its (dis)continuity along the major water transport pathways acting on the water- shed [1–3]. It includes both lateral water transfer along slopes, including channelized runoff, and vertical water transfer between surface and subsurface compartments, or between different groundwater aquifers at different depths [4]. Connectivity exists be- tween larger domains, e.g., surface runoff and groundwater flow, landscape elements and fluxes—structural connectivity, and between processes—functional connectivity [4,5]. Permafrost significantly alters the water cycling through the affected landscapes compared to that in temperate catchments [6–8]. In continuous permafrost, water transport is mostly confined to the active layer, which rarely exceeds 3 m depth, and to vertical and lateral talik zones. Water migration in soils is partly driven by processes related to phase transition in soils [9,10]. The hydrological system structure is simplified, and the Hydrology 2021, 8, 106. https://doi.org/10.3390/hydrology8030106 https://www.mdpi.com/journal/hydrology