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Permafrost: Occurrence and Physicochemical Processes

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Permafrost: Occurrence and Physicochemical Processes •11111111111 FI0200007 POSIVA 2001-05 Permafrost: occurrence and physicochemical processes Lasse Ahonen Geological Survey of Finland .33/15 October 2001 POSIVA OY Toolonkatu 4, FIN-00100 HELSINKI, FINLAND Phone (09) 2280 30 (nat.), ( + 358-9-) 2280 30 (int.) Fax (09) 2280 3719 (nat.). ( + 358-9-) 2280 3719 (int.) Posiva-raportti - Posiva Report Raportin tunnus - Report code POSIVA 2001-05 Posiva Oy Toolonkatu 4, FIN-00100 HELSINKI, FINLAND Julkaisuaika - Date Puh. (09) 2280 30 - Int. Tel. 4358 9 2280 30 October 2001 Tekija(t) - Author(s) Tbimeksiantaja(t) - Commissioned by Lasse Ahonen Geological Survey of Finland Posiva Oy Nimeke-Title PERMAFROST: OCCURRENCE AND PHYSICOCHEMICAL PROCESSES Tiivistelma-Abstract Bedrock of the Northern Hemisphere areas to the north of about the 60th latitude are nowadays dominated by permafrost conditions. Fennoscandia is a major exception being characterised by temperate climate. In studying deep geological disposal of long-living nuclear waste, long-term climatic changes have to be taken into account. One of the scenarios to be studied is the extension of the deep permafrost conditions to the disposal site. Quaternary climatic fluctuations and their possible reasons are discussed shortly. The author's conclusion is that future climatic changes cannot be undoubtedly derived from the past variations, mainly because of the current anthropogenic involvement and of the poorly known dynamics of the major climate-affecting factors like ocean currents, which cannot be treated in a deterministic way. In low-porosity crystalline rocks permafrost may propagate to the depth of about 500 metres in some thousands to ten thousands of years. On the other hand, the major effects of permafrost are related to the freezing of water in the pores. Water expands about 9 percent in freezing, and the increasing stress may lead to pressure melting of ice. Dissolved salts in water do not accommodate into the solid ice, but they form saline water or brine segregations having freezing point of even less than minus ten degrees. A front of saline water may develop beneath the frozen bedrock. Pockets of saline water may also occur in ice, and unfrozen adsorption water may occur on the grain boundaries. With respect to the radionuclide transport processes, permafrost as such is a barrier, while the unfrozen domains (taliks) beneath major lake and river systems are potential flow paths. Avainsanat - Keywords Climatic variations, nuclear waste, freezing of water, cryopegs, gas hydrates, taliks ISBN ISSN ISBN 951-652-106-1 ISSN 1239-3096 Sivumaara - Number of pages Kieli - Language 42 English Posiva-raportti - Posiva Report Raportin tunnus - Report code POSIVA 2001-05 Posiva Oy Töölönkatu 4, FIN-00100 HELSINKI, FINLAND Julkaisuaika - Date Puh. (09) 2280 30 - Int Tel. -(558 9 2280 30 Lokakuu 2001 Tekijä(t)-Author(s) Toimeksiantaja(t) - Commissioned by Lasse Ahonen Geologian tutkimuskeskus Posiva Oy Nimeke -1 itle IKIROUTA: ESIINTYMINEN JA FYSIKAALIS-KEMIALLISET PROSESSIT Tiivistelmä-Abstract Pohjoisella pallonpuoliskolla likimain 60 leveyspiirin pohjoispuolisten alueiden kallioperä on tällä hetkellä suurelta osin ikijään vallassa. Fennoskandian alue muodostaa poikkeuksen kuuluen lauhkean ilmaston vyöhykkeeseen. Tutkittaessa pitkäikäisen ydinjätteen loppusijoitusta kallio- perään, mahdolliset pitkien aikavälien ilmastomuutokset on kuitenkin huomioitava. Yksi tutkittavista tulevaisuus-skenaarioista on syvälle kallioon ulottuvan ikiroudan leviäminen loppusijoituspaikalle. Raportissa käsitellään lyhyesti Kvartäärikauden ilmastovaihtelua ja sen mahdollisia syitä. Kirjoittajan johtopäätös on, etta tulevia ilmastomuutoksia ei voida varmuudella johtaa menneistä ilmastovaihteluista, koska ihmisen vaikutusta ja ilmastoon ratkaisevasti vaikuttavien merivirtojen dynamiikkaa ei voida käsitellä ennalta määrättyinä prosesseina. Vähähuokoisissa kiteisissä kivissä ikirouta voi edetä noin 500 metrin syvyyteen muutamien tuhansien - kymmenen tuhannen vuoden ajanjaksona. Toisaalta ikiroudan pääasialliset vaikutukset kallioperässä liittyvät huokostilassa olevan veden jäätymiseen. Vesi laajenee jäätyessäan noin 9 prosenttia ja tästä johtuvan jännityksen kasvu voi purkautua esimerkiksi painesulamisena. Veteen liuenneet suolai eivät jäädy, vaan ne muodostavat suolavesierkaumia, joiden jäätymispiste voi olla jopa alle miinus kymmenen astetta. Jäätyneen kallion alapuolelle voi muodostua suolaisen veden rintama. Myös jäan sisässä voi esiintyä suolataskuja ja raerajoilla voi olla jäätymätöntä adsorptiovettä. Radionuklidien kulkeutumisen kannalta ikirouta on sinänsä este, kun tåas vesistöjen alapuolelle syntyvät jäätymättömät alueet (talikit) muodostavat mahdollisen kulkeutumisreitin. Avainsanat - Keywords Ilmastonvaihtelu, ydinjäte, veden jäätyminen, suolavesierkaumat, kaasuhydraatit, talikit ISBN ISSN ISBN 951-652-106-1 ISSN 1239-3096 Sivumåärä - Number of pages Kieli - Language 42 Englanti PLEASE BE AWARE THAT ALL OF THE MISSING PAGES IN THIS DOCUMENT WERE ORIGINALLY BLANK PREFACE This study is based on research contract between Posiva Oy and Geological Survey of Finland (GTK). Contact persons were Margit Snellman from Posiva Oy and Lasse Ahonen from GTK. The author wishes to thank Margit Snellman, Liisa Wikstrom and Aimo Hautojarvi for their comments. TABLE OF CONTENTS 1 INTRODUCTION 3 2 OCCURRENCE AND DISTRIBUTION OF PERMAFROST 4 3 FACTORS AFFECTING FORMATION OF PERMAFROST : 11 3.1 Depth and growth rate of permafrost 11 3.2 Climatic requirements and scenarios of permafrost growth 16 3.2.1 Climatic prerequisites of permafrost growth 16 3.3 Processes and scenarios related to the past and future climates 18 4 FLOW AND TRANSPORT IN PERMAFROST CONDITIONS 23 4.1 Physical and chemical processes related to ground freezing 23 4.1.1 Volume/pressure conditions in freezing process 23 4.1.2 Dissolved components in freezing 26 4.1.3 Formation and behaviour of gas hydrates 28 4.2 Flowpaths and transport processes in permafrost conditions 32 4.2.1 General 32 4.2.2 The presence of unfrozen water at the small scale 33 4.2.3 Cryopegs... 33 4.2.4 Taliks 34 4.3 Frost cracking and other periglacial processes 35 5 DISCUSSION AND CONCLUSIONS 36 5.1 Future climatic conditions 36 5.2 Effects of permafrost on a deep nuclear waste repository 37 6 REFERENCES 39 1 INTRODUCTION Deep geological disposal of nuclear waste aims at permanent isolation of the hazardous material from the ground-surface biosphere. Long half-life of many radionuclides of the waste implies that the performance of the repository must be assessed in time perspective of hundreds of thousands years. During the geological history, especially the last two million years, Fennoscandia has been subjected to repeated glacial and periglacial periods. It is reasonable to assume that such climatic changes may occur also in the future. Periglacial conditions are typical to the areas around glaciated terrains. Perennially frozen bedrock - permafrost - is a characteristic feature of the periglacial environment. Deep permafrost may affect the performance of a deep nuclear waste repository through different mechanisms and, consequently, features, events and processes related to the permafrost scenario must be examined carefully. Nurmi (1985) made a comprehensive study of the possible long-term environmental changes in the future, effects of permafrost on the bedrock and groundwaters were discussed in the report. Recently, Gascoyne (2000a) studied the effects of permafrost on the hydrogeochemistry of bedrock. The aim of the present report is to • Evaluate factors and processes leading to the formation of permafrost. • Study depth extent and growth rate of the freezing front. • Study physical and chemical processes related to the ground freezing. 2 OCCURRENCE AND DISTRIBUTION OF PERMAFROST Permafrost is defined as soil or rock that remains below 0°C throughout the year, and forms when the ground cools sufficiently in winter to produce a frozen layer that persists throughout the following summer. Permafrost conditions prevail in predominant parts of the upper ground of the Northern Hemisphere, north of the 60th latitude (Figure 2.1). The main exceptions are Fennoscandia, Iceland and northwestern parts of Russia, which - due to the warming effects of Gulf Stream and predominant southwestern Atlantic winds - belong to the Temperate Zone. Greenland at the same latitudes is covered by thick continental ice. Mountain permafrost may exist at high altitudes also at low latitudes (e.g., in Alps). Mountain permafrost reaching the depth of about 60 metres is also known to occur in the northern side of the hill Yllas in Finnish Lapland (Kukkonen and Safanda 2001). In the continuous permafrost zone, permafrost occurs everywhere beneath the ground surface except large bodies of water. The discontinuous permafrost zone is divided to the widespread permafrost zone, where permafrost underlies 50-90% of the land area, and to the sporadic permafrost zone, where it occurs mostly in peatlands and underlies 10-50% of the land area. The continuous permafrost area of the Northern Hemisphere is divided into the High Arctic (largely barren with some tundra vegetation), Middle Arctic (tundra with some bare ground) and Low Arctic (continuous tundra cover) (Gascoyne 2000a). The zone of discontinuous permafrost may be covered with boreal forests and wetlands and the snow cover persists for at least six months of the year. Permafrost underlies an estimated 20%-25% of the world's land surface; it occupies the land surface of more than 50% of Russia and
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