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A Review of Published Literature on the Effects of Permafrost on the Hydrogeochemistry of Bedrock

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A Review of Published Literature on the Effects of Permafrost on the Hydrogeochemistry of Bedrock FI0000082 POSIVA 2000-09 A review of published literature on the effects of permafrost on the hydrogeochemistry of bedrock M. Gascoyne Gascoyne GeoProjects Inc. Ca nada June 2000 POSIVA OY Toolonkatu 4. FIN-00100 HELSINKI, FIMLAND Phone (09) 2280 30 (nat.), ( + 358-9-) 2280 30 (int.) Fax (09) 2280 3719 (nat.), ( + 358-9-) 2280 3719 (int.) ISBN 951-652-095-2 ISSN 1239-3096 The conclusions and viewpoints presented in the report are those of author(s) and do not necessarily coincide with those of Posiva. Please be aware that all of the Missing Pages in this document were originally blank pages Posiva-raportti - Posiva Report Rapor1intunnu s -Report code POSIVA 2000-09 P0Siva °y lulkaisuaika - Date T66!6nkatu 4, FIN-00100 HELSINKI, FINLAND JuiKaisuaiKa uate Puh. (09) 2280 30 - Int. Tel. +358 9 2280 30 June 2000 Tekija(t) - Author(s) Toimeksiantaja(t) - Commissioned by M. Gascoyne Posiva Oy Gascoyne GeoProjects Inc. NimeklT- Title A REVIEW OF PUBLISHED LITERATURE ON THE EFFECTS OF PERMAFROST ON THE HYDROGEOCHEMISTRY OF BEDROCK Tiivistelma -Abstract Salt-rejection into the aqueous phase from permafrost growth ('aggradation') during the onset of cold-climate conditions in the Pleistocene period is a mechanism that could account for the presence of saline groundwaters in the Fennoscandian Shield. This report describes the results of a review of scientific literature on the subject of permafrost, to search for and evaluate information which may indicate whether this mechanism is feasible for sites such as Olkiluoto and Aspo on the Baltic Sea coast. The geomorphological characteristics of permafrost (such as development of patterned ground, ice wedging, pingo growth) have been studied in detail in the literature and provide an understanding of the effects of pore water expulsion and saline water formation. Evidence of salt-rejection during permafrost aggradation is found in results of analyses of the chemical and isotopic compositions of water in pingos and open taliks published in North American, Chinese and Russian literature over the last fifty years. While most studies have concentrated on shallow permafrost in soils and sediments, deep-drilling by the oil and gas industry has shown that permafrost may extend both laterally and to considerable depth. For instance, permafrost on the north slope of Alaska is laterally continuous over an area of at least 1000 km2 and is associated with fluids of salinities up to 130 g/L. Also, in northern Siberia, permafrost has been observed to depths of over 900 m. Saline waters are ubiquitous in coastal areas that are currently underlain by permafrost. However, it is not clear how much of the salinity has been produced by the freezing process and how much is simply due to leaching of saline soils and sediments by groundwaters and the presence of residual seawater in the sediments. Possible indicators of concentration by freezing include the presence of brines (i.e. waters of greater salinity than seawater), mineral precipitates (e.g. mirabilite) that are formed on freezing of seawater, enhanced or depleted sulphate concentrations in certain groundwaters, and lighter isotopic signature of the saline waters caused by the ice-water isotopic fractionation. This review has found that salt-rejection processes undoubtedly will have occunred in groundwaters in the marine sediments and bedrock of the Baltic coast during the Pleistocene. Deeply penetrating permafrost in the bedrock would cause relatively pure water to form as ice in fractures and displace residual saline fluids, under density flow, to greater depths. The process could have occurred to a sufficient extent that large volumes of saline water were generated, some of which may currently remain in the fractured rock. In these waters, loss of sulphate by mirabilite precipitation would be expected to have occurred but, on warming and degradation of the permafrost, lower-salinity meltwaters would re-dissolve the mirabilite, giving rise to a SO4-rich groundwater. This may be the origin of groundwater that is currently identified as Litorina Sea water at the Aspo and Olkiluoto sites. This mechanism differs from that suggested by Israeli workers who propose freezing of open seawater and infiltration of residual brines into the bedrock followed by lateral migration inland. The hypothesis presented here, of formation of saline waters and brines by permafrost aggradation and salt-rejection is more acceptable from a hydrogeological standpoint because the saline waters are formed in situ and need not migrate laterally. Further field evidence, coupled with modelling of depths of permafrost penetration, could be used to assess the volume and concentration of saline groundwater formed as a result of downward advancement of permafrost in the crystalline bedrock. Avainsanat - Keywords permafrost, groundwater chemistry, salinity, salt-rejection, isotopes, glaciation ISSN ISBN 951-652-095-2 ISSN 1239-3096 Sivumaara - Number ot pages weli - Language 53 English Posiva-raportti - Posiva Report Raportintumus ~Report oode POSIVA 2000-09 POSiva Oy Julkaisuaika - Date Töölönkatu 4, FIN-00100 HELSINKI, FINLAND Julkaisuaika Date Puh. (09) 2280 30 - Int. Tel. +358 9 2280 30 Kesäkuu 2000 Tekijä(t) - Author(s) Toimeksiantaja(t) - Commissioned by M. Gascoyne Posiva Oy Gascoyne GeoProjects Inc. Nimeke - Title IKIROUDAN VAIKUTUS KALLIOPERÄSSÄ VALLITSEVIIN HYDROGEOKEMIALLISIIN OLOSUHTEISIIN - KIRJALLISUUSKATSAUS Tiivistelmä - Abstract Suolan jääminen vesifaasiin maa- ja kallioperän jäätyessä ja konsentroituminen permafrostin kasvuvaiheessa kylmän ilmastovaiheen alussa Pleistoseenikaudella on mekanismi, joka voi olla syynä suolaisten pohjavesien esiintymiseen Fennoskandian kilpialueella. Tässä työssä esitetään tulokset katsauksesta permafrostista julkaistuun tieteelliseen kirjallisuuteen. Erityisenä tavoitteena on ollut etsiä ja arvioida tietoa, joka voisi antaa viitteitä siitä, onko tämän mekanismin esiintyminen todennäköistä Pohjanmeren rannikolla sijaitsevilla Olkiluodon ja Äspön tutkimus- alueilla. Kirjallisuudesta löytyy perusteellista tietoa permafrostin geomorfologisista ominaisuuksista (kuten ikiroudalle ominaisten kuviomaiden muodostuminen, jääkalojen esiintyminen, pingojen kasvu), joka luo hyvän pohjan ymmärtää huokosveden erottumista ja suolaisen veden muodostumista. Selviä viitteitä suolan erottumisesta permafrostin kasvuvaiheessa löytyy pingoista ja avoimista taliikeista viimeisen 50 vuoden aikana julkaistuista Pohjois-Amerikassa, Kiinassa ja Venäjällä tehtyjen kemian ja isotooppianalyysien tutkimusten tuloksista. Vaikka useimmat tutkimukset koskevat maanpinnan läheistä permafrostia maaperässä ja sedimenteissä, öljy- ja kaasu- teollisuuden tarkoituksiin kairatut syvät reiät ovat osoittaneet että permafrost saattaa ulottua laajoille alueille ja huomattavan syvälle. Esimerkiksi, Alaskan pohjois-osassa permafrost ulottuu jatkuvana yli 1000 km2 alueen ja siihen liittyy pitoisuuteen 130 g/l asti olevia suolaliuoksia. Pohjois-Siperiassa permafrostin on havaittu ulottuvan yli 900 m syvyyteen. Suolaisia vesiä esiintyy nykyisin ikiroudan alla kaikkialla rannikkoalueilla. Kuitenkin on epäselvää, mikä osuus suolaisuudesta on muodostunut jäätymisprosessin aikana ja mikä osuus on yksinkertaisesti liuennut pohjaveteen suolaisesta maaperästä tai sedimenteistä ja sedimenttien merivesijäänteestä. Jäätymisen aiheuttaman konsentroi- tumisen indikaattoreita ovat brine-vesien esiintyminen (valtamerta suolaisempia vesiä), mineraali saostumat (esim. mirabiliitti), jotka muodostuvat meriveden jäätyessä, sulfaatin suhteen rikastunut tai köyhtynyt pohjavesi, sekä isotooppien jäähän ja veteen tapahtuvan erottumisen seurauksena suolaisten vesien kevyemmät stabiilit isotoopit. Tämän katsauksen perusteella suolan erottumisprosesseja on epäilemättä esiintynyt Pohjanlahden rannikolla merisedimenttien ja kallioperän pohjavesissä Pleistoseenikaudella. Syvälle kallioperään tunkeutuva ikirouta kallion raoissa muodostaa jäätä melko puhtaasta vedestä (alhainen ionivahvuus) ja syrjäyttäisi suolaista, tiheämpää jäännösnestettä syvemmälle kalliossa. Ilmiö on saattanut esiintyä riittävän laajasti niin, että on syntynyt suuria määriä suolaista vettä. Tästä suolaisesta vedestä osa saattaa vielä olla jäljellä rakoilleessa kalliossa. Näissä vesissä on oletettavasti esiintynyt sulfaatin köyhtymistä mirabiliitin saostumisen seurauksena, mutta ilmaston lämmetessä ja permafrostin vetäytymisvaiheessa mirabiliitti on voinut liueta alhaisemman suolaisuuden omaavan sulamisveden suotautuessa kallioon ja näin on voinut muodostua SO4-rikkaita pohjavesiä. Tämä saattaa olla tällä hetkellä Litorina- meri vesityyppinä tunnetun veden alkuperä sekä Äspössä että Olkiluodossa. Tämä mekanismi poikkeaa israelilaisten tutkijoiden tulkinnasta. He ovat ehdottaneet mekanismia, jossa avoin merivesi jäätyy ja syntynyt jäännös brine-vesi suotautuu kallioperään sekä virtaa sisämaahan päin. Tässä esitetty permafrostin etenemisvaiheen aikainen suolaisten vesien muodostumiseen ja suolan hylkimiseen perustuva hypoteesi on helpommin hyväksyttävissä hydrogeologiselta näkökannalta, koska suolaiset vedet muodostuvat in-situ eikä lateraalivirtausta tarvitse tapahtua. Kattavampaa kenttätutkimustietoa permafrostin syvyystunkeutumisen mallinnuk- sen kanssa voitaisiin käyttää arvioitaessa suolaisen veden määrän ja konsentraation arvioinnissa, kun tarkastellaan ikiroudan syvemmälle kallioon tapahtuvan etenemisen vaikutuksia. Avainsanat - Keywords permafrost, ikirouta, pohjavesikemia, suolaisuus, suolan erottuminen, isotoopit, jääkausi ISBN T55FT ISBN
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