This report gives a brief description of field work on the Kuril Islands in summer 2021 carried out by staff of the Institute of Volcanology and Seismology of  FEB RAS within the framework of the Institute research theme and projects of the Russian Science Foundation (RSF) and Russian Fund for Basic Research (RFBR). To study chemical erosion of volcanic islands and to estimate hydrothermal export of magmatic volatiles, hydrological and hydrochemical works were carried out on the rivers draining the slopes and thermal fields of the Baransky volcano and the Bogdan Khmelnitsky volcanic massif (Iturup Island). Detailed hydrochemical studies with water sampling at different depths and a bathymetric survey of Lake Kipyashchey located in the caldera of Golovnin volcano (Kunashir Island) were performed. We also proceeded with studying the CO2 diffusion flux through thermal fields and volcanic lakes. In the course of ongoing regime observations on Ebeko volcano (Paramushir Island), aerial and infrared imaging of its near-crater part was carried out. For the first time since the eruption began in 2016, a quadcopter survey of the lake located in the Srednii (middle) crater of the volcano was conducted. For further analytical studies a large number of water and gas samples were taken, and the collection of sediments was replenished.

The chemical and isotopic composition of hot springs at the apical part of the active Ebeko volcano (Paramushir Island, Kuril Islands) is characterized based on the data obtained from fieldwork in 2020–2021. Thermal waters discharging from one of the sources of the Kuzminka River are ultra-acid (рН<2) Al-Ca-SO4-Cl type with mineralization up to 5 g/l and temperature up to 70°С. The anionic composition of waters is formed due to the dissolution in groundwater of acidic volcanic gases, partially «purified» in the main reservoir of the volcano's hydrothermal system. The cationic composition of waters, including rare-earth elements, is formed by isochemical dissolution of host rocks in the equivalent of 5 g per 1 liter of water. Differences in the isotopic composition and ratios of macrocomponents (SO4 /Cl, Al+Fe/Ca+Mg/Na+K) of waters of the near the summit  prings and the northwestern slope suggest the presence of different-level aquifers in the hydrothermal system confined to the edifice of the Ebeko volcano.

На основе гидрогеохимических данных, полученных в результате полевых работ, проведенных на острове Шиашкотан в 2011 г., приводится характеристика различных типов природных вод, включая конденсат фумарольных газов, разгружающихся на западном склона вулкана Кунтоминтар. В исследуемом районе распространены холодные и термальные кислые минерализованные воды, соответствующие сульфатному типу с пестрым катионным составом. Конденсат фумарольных газов относится к хлоридному кальциевому типу с высоким содержанием бора. Формирование химического состава природных вод происходит за счет взаимодействия метеорных вод с гидротермально измененными породами. Газы вулкана Кунтоминтар имеют магматическое происхождение: их конечный состав сформирован в результате подъема флюида к поверхности и значительного разбавления его метеорной водой в близповерхностных условиях.

Приведены новые данные по макро- и микрокомпонентному составу термальных вод, разгружающихся в бассейне р. Юрьевой (о. Парамушир, Северные Курильские острова). Прослежены изменения в химическом составе и физико-химических показателей вод Верхне-Юрьевских источников, произошедшие с начала их исследований в 1957 г. до 2010 г. Дана предварительная оценка геохимической работы источников по состоянию на 2010 г. Рассмотрены условия формирования термальных вод.

Based on the results of expeditionary studies (July 2021), geochemical features of thermal waters (natural manifestations and borehole waters) discharging near or directly on the Sea of Okhotsk and Pacific coasts of the central part of Iturup Island were considered: Reidovskie, Lososevye, Cape Konakov, Goryachie Klyuchi, Dachnye, water-recreational complex «Vannochki». The springs (with the exception of Lososevye) belong to subneutral Cl(Cl-HCO3)-Na waters with salinity less than 10 g/l. The Lososevye springs are of the weakly acidic HCO3-SO4-Ca-Na type with salinity of 3 g/l. The spontaneous gas is dominated by CO2 (Dachnye and Vannochki), N2 (Reidovskie and Cape Konakov) and CH4 (Goryachie Klyuchi). Against the background of a general extremely low content of microelements,  thermal waters are enriched with boron and lithium. The springs are characterized by small (0.1 l/s) to insignificant (0.02–0.05 l/s) flow rates, while wells drilled nearby uncover horizons of pressurized waters.

На основе геохимических исследований скорректированы представления об условиях формирования и разгрузки термальных вод о. Шиашкотан. Термальные источники, распространенные на острове, являются поверхностными проявлениями Северо-Шиашкотанской и Кунтоминтарской гидротермальных систем. Северо-Шиашкотанская гидротермальная система имеет классическую гидрохимическую зональность. Разгрузка Кунтоминтарской гидротермальной системы ограничена двумя термальными полями, расположенными в центральном и северо-восточном кратерах одноименного вулкана. Высокая температура газов вулкана Кунтоминтар на поверхности и повышенные прогностические отношения S/Cl, S/C, CO2/H2 в его составе свидетельствуют о возможной активизации его фумарольной деятельности.

Based on the data of field studies in different years, the macro- and microcomponent composition of the waters of the Zelenoe Lake in the crater of the Troitsky active volcano Maly Semyachik were considered. It is shown that the lake contains ultra-acidic (pH < 1) water of sulfate-chloride composition with mineralization from 8 to 42 g/L, depending on the state of the volcano. The anion composition of the lake is formed by the inflow and subsequent dissolution of acidic volcanic gases in the aquifer located directly under the lake. The cation composition of water is formed by the almost congruent dissolution of the host rocks. After a long period of dormancy, in 2008 a new stage of hydrothermal activation of the volcano began. The activity is ongoing. Against the background of a constantly increasing volume, there is an increase in the concentrations of the main macro components (SO4, Cl, Al, Fe) and in the salinity of water in general.

Приведены новые данные по геохимии термальных вод Кошелевского вулканического массива на юге Камчатки. Рассмотрены условия формирования термальных вод, возможные варианты теплового и глубинного вещественного питания Кошелевской гидротермальной системы и предложена ее концептуальная модель.

Lake Kipyashchee (Boiling) with an area of ~4.6 ha and a maximum depth of 25 m fills the explosion funnel near one of the extrusive domes of the Golovnin volcano caldera. The water of the lake is ultra-acid (рН = 2.2–2.5) of the chloride-sulfate type with a mineralization of 2.0–2.2 g/l. The water temperature on the surface varies from 30 to 100°C, the average is 37°C. The flow of the lake is carried out through the Protoka in the lake Goryachee (Hot) is 120 l/s (August 2021). The hydrothermal output of magmatic Cl and S (as SO4) from Lake Kipyashchee is 10 t/day and 5.4 t/day, respectively. For the first time, the total diffusive removal of carbon dioxide from the surface of Lake Kipyashchee was estimated as 5.4 tons/day. Obtained from the results of field work in 2020–2021 geochemical data indicate an increase (compared to 2015) in hydrothermal activity in the Golovnin volcano caldera.