Proceedings World Geothermal Congress 2020+1 Reykjavik, Iceland, April - October 2021 1 The Impact of Hydrothermal Activity on the Geological Environment, Kamchatka Peninsula Julia V. Frolova 1 , Michael S. Chernov 1 , Sergey N. Rychagov 2 , Oleg V. Zerkal 1 1 Faculty of Geology, Lomonosov Moscow State University, Leninskie Gory 1, GSP-1, Moscow 119234, Russia 2 Institute of Volcanology and Seismology, Far East Branch, Russian Academy of Sciences, 9 Piipa Avenue, Petropavlovsk- Kamchatsky, 683006 Russia ju_frolova@mail.ru Keywords: hydrothermal activity, Kamchatka, alteration, landslides, impact on the environment ABSTRACT Geothermal exploration and development of geothermal sites requires the investigation of their engineering geological setting and an assessment of the hydrothermal activity impact on the environment. Hydrothermal alteration of rocks and changing in their physical- mechanical properties promotes a broad range of geological phenomena that is indicated by many researchers on various geothermal sites all over the world. There are slope instability with the initialization of landslides or rock avalanches, changes in relief, hydrothermal explosions, migration of thermal manifestations, changes in temperature and the hydrodynamic regime of a hydrothermal system, reduction of well production, surface deformation and subsidence, discharge of wastewater from production wells to the creeks with formation of silica deposits etc. These processes have an impact on the environment and influence on exploitation of the geothermal field, and in some cases, they could be hazardous for the economic or tourist development of the geothermal site. In the paper we discuss the environmental impact problems induced by hydrothermal activity, which may occurred during natural evolution of hydrothermal systems or due to the geothermal development and utilization, and describe the geological phenomena associated with hydrothermal activity on the most well-known hydrothermal systems of Kamchatka Peninsula (Far East, Russia). 1. INTRODUCTION Geothermal exploration and development of geothermal sites requires the investigation of their engineering geological setting, including hydrothermal alteration and an assessment of the hydrothermal activity impact on the environment. Progressive hydrothermal alteration of rocks and changing the physical-mechanical properties promotes a broad range of geological phenomena that is indicated by many researchers. There are instability and failure of slopes with the initialization of landslides or rock avalanches, hydrothermal explosions, surface deformations etc. These processes have an impact on the development and exploitation of the geothermal field, and in some cases, they could be hazardous for the economic or tourist development of the geothermal site. Hydrothermal eruptions are common phenomena in geothermal fields that is noted in papers of Hedengvist and Henley (1985), Lawless and Browne (2001), Handal (2004), Montanaro et al. (2015), Shulyupin et al (2016). They can be natural events due to evolution of hydrothermal system or had artificial origin induced by geothermal exploration. They took place in prehistoric or historic times. Geological product of hydrothermal eruption is breccia or brecciated rocks. After the eruption, secondary minerals deposited from the filtering fluids quickly cement the formed debris. Lawless and Browne (2001) insisted that many hydrothermal eruptions start close to the ground surface and result from the rapid formation of steam due to a sudden pressure reduction. This steam provides the energy necessary to brecciate, lift and eject fragments of the host rocks as a flashing front descends and water nearby in the reservoir boils. A hydrothermal eruption continues until the steam is produced too slowly to lift the brecciated rocks. There is no genetic difference between the small eruptions induced by exploitation and those, which occur as a geothermal system evolves naturally and whose effects may penetrate to much greater depths. Lawless and Browne (2001) summarized data about hydrothermal eruptions in New Zealand but also mentioned examples all over the world. They justified the importance of studying hydrothermal explosions. The main reason is that they are destructive events that can cause human victims loss and damage or destroy structures of geothermal plants and pipelines. The information about prehistoric events and eruptions frequency will assist to avoid potentially dangerous sites when planning pipelines, power plants, and reinjection systems. The second reason is that hydrothermal eruptions may cause major changes in the hydrogeological structure of the hydrothermal system and thus influence on production regime and volume. The third reason has scientific importance. The material ejected by a hydrothermal eruption provides knowledge about the lithology of the geothermal reservoir from which it derives. In addition, hydrothermal minerals, which cement debris in breccia, bring information about the subsurface conditions such as temperatures, fluid types and salinities. One more reason is that hydrothermal breccia can be the host rocks for epithermal mineral and ore deposits. Slope processes are widespread in geothermal areas. They are often hazardous for the economic, social and tourist development of the regions that mentions by Reid et al. (2001) and Suemnicht et al. (2007) for Long Valley caldera, USA; Huang and Tian (2006) for geothermal fields in China; Dvigalo and Melekestsev (2009), Kiryukhin (2010, 2012), Gvozdeva et al. (2015), Zerkal and Gvozdeva (2018), Zerkal et al. (2019) for Geysers Valley on Kamchatka, Russia; Pioquinto and Caranto (2005), Pioquinto et al. (2010) for Philippines; Kristianto et al. (2013), Wijaya et al. (2014), Yuhendar et al. (2016) for Indonesia; Della Seta et al. (2015), Marmoni et al. (2017) for Ischia Island, Italy. A significant impact to the study of the catastrophic slope processes in geothermal areas was given by a landslide of more than 800 thousand m 3 , formed in 1991 on the Zunil I hydrothermal field (Western Guatemala), which operated 10.8 MW power plants that described by Flynn et al. (1991) and Voight (1992). Twenty-three people were killed in this landslide; several farms were destructed.