P O S I V A O Y

F I N - 2 7 1 6 0 O L K I L U O T O , F I N L A N D

P h o n e ( 0 2 ) 8 3 7 2 3 1 ( n a t . ) , ( + 3 5 8 - 2 - ) 8 3 7 2 3 1 ( i n t . )

F a x ( 0 2 ) 8 3 7 2 3 7 0 9 ( n a t . ) , ( + 3 5 8 - 2 - ) 8 3 7 2 3 7 0 9 ( i n t . )

POSIVA 2007 -04

Origin and Implications ofDissolved Gases in Groundwater

at Olkiluoto

March 2007

Pet te r i P i tkänen

Sami Par tamies

POSIVA 2007-04

March 2007

POSIVA OY

F I - 27160 OLK I LUOTO, F INLAND

Phone (02 ) 8372 31 (na t . ) , ( +358 -2 - ) 8372 31 ( i n t . )

Fax (02 ) 8372 3709 (na t . ) , ( +358 -2 - ) 8372 3709 ( i n t . )

Petter i P i tkänen

Sami Par tamies

VTT Techn ica l Resea rch Cen t re o f F i n l and

Origin and Implications ofDissolved Gases in Groundwater

at Olkiluoto

Base maps: © National Land Survey, permission 41/MYY/07

ISBN 978-951 -652 -152 -0ISSN 1239-3096

The conc lus ions and v i ewpo in ts p resen ted i n the r epo r t a r e

those o f au tho r ( s ) and do no t necessa r i l y co inc ide

wi th those o f Pos i va .

Tekijä(t) – Author(s)

Petteri Pitkänen, VTT Sami Partamies, VTT

Toimeksiantaja(t) – Commissioned by

Posiva Oy

Nimeke – Title

ORIGIN AND IMPLICATIONS OF DISSOLVED GASES IN GROUNDWATER AT OLKILUOTO

Tiivistelmä – Abstract

Olkiluoto at Eurajoki has been selected as a repository site for geological disposal of spent nuclear fuel produced in Finland. An understanding of the hydrogeochemical groundwater conditions and evolution is essential in evaluating the long-term safety of the repository. Dissolved gases are an important factor in the groundwater system and they also constitute a notable mass of dissolved species at Olkiluoto. They have significance in controlling hydrogeochemical evolution and in evaluating palaeohydrogeology or recent groundwater flow and therefore in assessing the safety of a geological repository.

The data of dissolved gas samples (71 in total) examined have been collected during 1997 to 2005 with the PAVE sampler to take groundwater samples at in-situ pressure. Dissolved gas contents are notable in deep groundwaters at Olkiluoto, however, they correspond well with other lithologically similar sites in Shield areas. Nitrogen and methane form the major portion on total gas contents. Hydrocarbons are typically high for sites with rock types of low oxygen fugacity during crystallisation, typically containing graphite and sulphides.

Gas compositions correspond well with the stratification of redox conditions in hydrogeochemical system at Olkiluoto. Methane, the most abundant gas in deep saline groundwater (below 300 m) at Olkiluoto seems to have two primary sources: thermal abiogenic hydrocarbons (probably extremely old) dominate in the deepest parts of sampled groundwaters whereas the fraction of bacterial methane increases steadily with decreasing depth and CH4 content. In SO4-rich groundwater above 300 m depth only traces of hydrocarbons are observed, which corresponds well with the theory of instability between dissolved CH4 and SO4 in a common system producing dissolved sulphide when, for instance sulphate reducing bacteria are present.

Carbonate reduction is the most probable primary process for bacterial methane. Hydrogen required in the process seems to increase below 800 m. Methanogens may metabolise it, producing a CH4-saturated system and degassing CH4 which may migrate upwards and dissolve at depths where it is clearly unsaturated (above 800 m depth). Hydrogeochemical interpretations, based on the results of stable isotopes of water, Cl and CH4 suggest that CH4

enrichment is a very slow process and saturation requires at least tens of thousands to hundreds of thousands years.

Gas phase formation has, however, substantial importance for safety and it is therefore essential to obtain more data of hydrogen, methane, higher HC, DIC, fracture calcites (all with isotopic compositions) and microbes from the deep groundwater system in order to evaluate CH4 production and potential gas phase ebullition. Corresponding, further studies should be directed to the interface between methanic and sulphidic systems in order to evaluate potential SO4

reduction.

The results of noble gases (He and Ar) and nitrogen support previous interpretations of the relative residence times, palaeohydrogeology and the concept of mixing at Olkiluoto. Helium together with its isotopes could give particular information of detailed hydrogeological conditions.

Avainsanat - Keywords

hydrogeochemistry, deep gases, methane, hydrocarbons, methanogenesis, abiogenesis, sulphate reduction, palaeohydrogeology

ISBN

ISBN 978-951-652-152-0 ISSN

ISSN 1239-3096

Sivumäärä – Number of pages

57Kieli – Language

English

Posiva-raportti – Posiva Report

Posiva Oy FI-27160 OLKILUOTO, FINLAND Puh. 02-8372 (31) – Int. Tel. +358 2 8372 (31)

Raportin tunnus – Report code

POSIVA 2007-04

Julkaisuaika – Date

March 2007

Tekijä(t) – Author(s)

Petteri Pitkänen, VTT Sami Partamies, VTT

Toimeksiantaja(t) – Commissioned by

Posiva Oy

Nimeke – Title

POHJAVETEEN LIUENNEIDEN KAASUJEN ALKUPERÄ JA GEOKEMIALLINEN MERKITYS OLKILUODOSSA

Tiivistelmä – Abstract

Eurajoen Olkiluodon kallioperä on valittu käytetyn ydinpolttoaineen loppusijoituspaikaksi Suomessa. Hydrogeo-kemiallisten prosessien tuntemus antaa perusteet arvioida loppusijoituksen pitkäaikaista turvallisuutta. Kaasut edustavat huomattavaa liuennutta ainemäärää etenkin Olkiluodon syvissä suolaisissa pohjavesissä. Ne muodostavat merkittävän pohjavesikemiallisia olosuhteita säätävän osatekijän ja niitä voidaan hyödyntää arvioitaessa niin paleohydrogeologisia kuin vallitseviakin virtausolosuhteita.

Pohjavesien kaasuaineisto käsittää 71 näytettä, jotka on otettu vuosina 1997 – 2005 aina 1000 m:n syvyydestä saakka. Näytteenotossa on käytetty Posivan kehittämää PAVE-näytteenottinta. Sillä vesinäyte ja siihen liuenneet kaasut kerätään säiliöihin, joissa vallitsee näytteenottosyvyyttä vastaava hydrostaattinen paine. Säilyttämällä in situ paine estetään kaasujenpurkautuminen ja fraktioituminen ennen analyysiä, millä pyritään varmistamaan tulosten kvantitatiivinen luotettavuus.

Typpi ja metaani ovat kaasujen pääkomponentit. Hiilivetypitoisuudet (suurimmillaan n. 1000 ml/l mitattuna NTP:ssä) eivät kuitenkaan poikkea tuloksista, joita on mitattu kivilajiolosuhteiltaan vastaavankaltaisilta pelkistäviltä (sulfideja, grafiittia)paikoilta Suomesta tai muilta kilpialueilta maailmassa. Kaasukoostumus vaihtelee ja noudattaa todettuja pohjaveden redox-vyöhykkeitä. Typpi ja CO2 dominoivat kallion yläosan SO4-pitoisessa pohjavedessä (300 m:n syvyydelle), jossa hiilivedyt eivät ole teoreettisesti pysyviä, vaan mikrobitoiminta pyrkii pelkistämään SO4:n metaania hapettamalla. Metaani ja muut hiilivedyt ovat hallitsevia puolestaan 300 m:n alapuolella SO4–vapaissa pohjavesiolosuhteissa ja niiden pitoisuus kasvaa voimakkaasti alaspäin.

Hiilivedyt näyttävät edustavan pääasiassa kahta eri syntytapaa. Syvällä valitsevin komponentti on abiogeeninen, korkeassa lämpötilassa, epäorgaanisista lähtöaineista muodostunut CH4 (oletettavasti erittäin vanhaa). Ylöspäin pohjavesisysteemissä tultaessa mikrobiologisesti karbonaatin pelkistymisen kautta muodostunut CH4 saa ylivallan. Prosessi perustuu vedyn anaerobiseen hapettamiseen. Vetyä on hyvin vähän, mutta sen määrä kasvaa 800 m:n alapuolella. Metaanipitoisuudet ovat siellä lähes kyllästymispisteessä ja on mahdollista, että esimerkiksi metaania tuottavan mikrobitoiminnan vuoksi metaania on erkaantunut liuosfaasista ja kulkeutunut ylemmäs alikylläiseen vyöhykkeeseen, jossa se taas on liuennut ja lisännyt pohjaveden metaanipitoisuutta. Rikastuminen olisi kuitenkin erittäin hidas prosessi ja mm. suolaisen pohjaveden vähittäin tapahtunut laimeneminen, jonka on arvioitu kestäneen vähintään glasiaalisyklin verran, olisi nopeampaa.

Mahdollisella kaasufaasin muodostumisella on kuitenkin huomattava turvallisuusmerkitys (vaikuttaa mm. pohjaveden kiertoon). Siksi on tärkeää kerätä lisää kaasuaineistoa (H2, CH4 ja muut hiilivedyt sekä niiden isotoopit) syvistä suolaisista pohjavesistä, täydentää vastaavaa isotooppiaineistoa liuenneen karbonaatin ja rakokalsiittien osalta sekä laajentaa myös mikrobinäytteenottoa. Metaani- ja sulfaattivyöhykkeiden rajapinnassa tapahtuva sulfidin muodostus kaipaa myös lisäarviointia mm. sulfaatin pelkistysnopeuden osalta.

Jalokaasu- ja typpitulokset puolestaan tukevat aiempaa käsitystä pohjavesityyppien alkuperästä ja niiden sekoittumisesta, paleohydrogeologisesta kehityksestä sekä suhteellisista viipymäajoista. Erityisesti He, jolla on laaja vaihteluväli pohjavesissä, ja sen isotooppit (ei ole tehty) voisivat tuottaa yksityiskohtaista tietoa hydrogeologisista olosuhteista koskienmyös pohjaveden suotautumista ja purkautumista.

Avainsanat - Keywords

hydrogeokemia, syvät kaasut, metaani, hiilivedyt, metanogeneesi, abiogeneesi, sulfaatin pelkistyminen, paleohydrogeologia

ISBN

ISBN 978-951-652-152-0 ISSN

ISSN 1239-3096 Sivumäärä – Number of pages

57 Kieli – Language

Englanti

Posiva-raportti – Posiva Report

Posiva Oy FI-27160 OLKILUOTO, FINLAND Puh. 02-8372 (31) – Int. Tel. +358 2 8372 (31)

Raportin tunnus – Report code

POSIVA 2007-04

Julkaisuaika – Date

Maaliskuu 2007

1

TABLE OF CONTENTS

ABSTRACT

TIIVISTELMÄ

PREFACE....................................................................................................................... 3

1 INTRODUCTION .................................................................................................... 5

2 SAMPLING METHODS AND ANALYSES.............................................................. 7

3 GEOLOGICAL AND HYDROGEOCHEMICAL SETTING....................................... 9

4 RESULTS OF GASES AND DISCUSSION.......................................................... 19 4.1 Range of gas contents and uncertainties in the data.................................... 19 4.2 Origin of hydrocarbons.................................................................................. 23

4.2.1 Isotopic evidence of hydrocarbons........................................................ 25 4.3 Helium........................................................................................................... 36 4.4 Argon ............................................................................................................ 37 4.5 Nitrogen ........................................................................................................ 39

5 SUMMARY AND CONCLUSIONS........................................................................ 45

REFERENCES ............................................................................................................. 49

APPENDICES............................................................................................................... 53

2

3

PREFACE

This study is a part of description of the baseline conditions at Olkiluoto site and belongs to nuclear waste management of the Olkiluoto and Loviisa power plants programme for research, development and technical design for 2004-2006 (TKS-2003).

The study is performed in VTT. The contact persons were Mia Ylä-Mella from Posiva Oy and Petteri Pitkänen from VTT. The authors wish to thank Margit Snellman (Saanio & Riekkola Oy) and Mia Ylä-Mella for their comments on the draft. We are particularly grateful to Dr Mel Gascoyne (Gascoyne GeoProjects Inc.) for his review and comprehensive suggestions to improve the manuscript.

4

5

1 INTRODUCTION

The Olkiluoto site in SW Finland (Fig 1-1) at Eurajoki is the selected candidate for final disposal of spent nuclear fuel deep into the bedrock. The geochemical and hydrogeologic characteristics are among important factors to be considered while assessing the suitability and safety of the site. Groundwater studies within the Olkiluoto site also involving sampling and analysis of dissolved gases. The significance of dissolved gases varies for the safety of geological disposal of spent nuclear fuel. Noble gases and nitrogen are chemically inert and do not pertain to any particular safety issue but they can be useful in characterisation of a repository site. Oxygen, carbon dioxide, nitrogen species, hydrogen sulphide, hydrocarbons and hydrogen instead are chemically active and therefore their occurrence is important for safety studies. Dissolved oxygen hydrogen sulphide and nitrogen species are primary safety issues, e.g. increasing canister corrosion, whereas the main concern of the others is linked to chemical processes which they may favour, e.g. sulphide production and calcite dissolution. High contents of radioactive radon and methane released in tunnels should be avoided during the operating phase of the repository.

Substantial quantities of dissolved gases have been observed at Olkiluoto particularly in deep groundwaters (e.g. Pitkänen et al. 1999, 2004, Gascoyne 2005, Hatanpää et al. 2005) as well as frequently in other Precambrian rock sites (e.g. Sherwood Lollar et al. 1993a,b, Ward et al. 2004). The major sources of dissolved gases vary and single gas phases can generally have multiple origins. Some gases have surficial sources and dissolve in infiltrating water in contact with atmosphere or air bubbles (equilibration) or in soil layers due to respiration and decay of organic debris, e.g. CO2 and nitrogen oxides. Others such as hydrocarbons, noble gases, nitrogen, and hydrogen are formed below the groundwater table by microbial, radiogenic and thermogenic processes or even derived from the mantle and their contents may increase notably due to the higher solubility of gases in higher pressures of subsurface water. Temperature does not play as significant role as pressure, because the temperature gradient with depth in shield areas is typically low.

Gas concentrations, potential source processes, input and consumption of chemically active gases should be evaluated in order to assess their significance for chemical evolution and safety. In addition to observed contents of gases their isotopic compositions are important in interpreting the origin and transformation of each species. Isotopic composition may depend directly on formation process or it may be changed by fractionation in chemical reactions, which are both useful in interpreting the origin or consumption of gas species.

This report describes the variation of gas composition with depth and with hydrogeochemical conditions at Olkiluoto, evaluates the origin of different gases, their coupling to chemical reactions with other dissolved species and also gives a summary potential safety issues related to various processes originated from gases. Gas sampling from deep boreholes is a complicated task and results may contain significant uncertainties particularly quantitatively (Gascoyne 2005, Hatanpää et al. 2005).

6

Reliability of gas results is also evaluated based from mostly on the variation of different gas contents integrated with observations done in the reports above.

Figure 1-1. The map of the investigation area of Olkiluoto, Finland, shows the location

of the deep boreholes KR1 – KR40. The boreholes taken gas samples (PAVE –sampled)

are marked in red. The depth of sampled boreholes varies from 300m to 1000m. The

scale of grid on the map is 500m.

7

2 SAMPLING METHODS AND ANALYSES

Dissolved gas samples from Olkiluoto groundwater have been collected using three different techniques over the years. The earlier two methods have been used to collect samples from the packered borehole sections either into aluminium-laminated bags or glass vessels on the surface. Samples taken utilizing these techniques had problems in representiveness because when transporting the samples to the surface degassing of dissolved gases could take place and some gas could be lost. The potential phase separation in the sampling equipment may have also changed gas composition and content collected in the sampling vessel, thus not representing the water volume of the original sampling depth. The exsolution complicates the quantitative analysis of the gas samples. The different gases with different solubilities tend to fractionate during exsolution causing the less soluble gases to diffuse more easily into the forming bubbles than the ones with higher solubility (Gascoyne, 2005).

The third sampling method, PAVE, was developed to avoid these problems and to improve quantitativity of results by taking the water samples in a pressurised vessels at in situ depth in the borehole. The samples are maintained at in-situ –pressure until dissolved gases are released in the laboratory for the gas chromatography analysis. The PAVE method has been utilized regularly since 1997 in sampling campaigns. The precise description of the development history, sampling procedure and experiences from the usage of the PAVE sampling method are found in Hatanpää et al. (2005). The first observations showed that gas concentrations were an order of magnitude higher when collected by the PAVE system than by previous methods thus supporting the expectations of increased accuracy of gas sampling at in situ pressure with the PAVE system (Gascoyne 2000). Further studies of the representativity of gas samples collected by PAVE system have been performed by Gascoyne (2005) and Hatanpää et al. (2005).

One problem detected in using the PAVE method is related to the use of N2 and Ar as back-pressure gases. They have been observed to diffuse trough the pressure vessel containing the water sample past the piston, most probably during emptying in the laboratory. The leakage could be detected in the large range of Ar contents due to its assumed small quantities in the samples. The N2 is also expected to diffuse to the sampling vessel but the observation is more difficult due to the high natural concentration of nitrogen in the samples. Gascoyne (2005) argued that the gas transport could exist between the upper and lower cylinder during the lift to the surface, because the vessels are not isolated. Oxygen is found in most of the PAVE samples and has no importance in most cases, but greater contamination occurred in sample treatment either in the field or more probably during the laboratory analyses (Hatanpää et al. 2005).

Gascoyne (2005) discussed the importance to the gas results of occasionally observed incomplete filling by water in the pressure vessels. Previously three reasons were considered for the inadequate filling. They are leakage of the PAVE sampler, confined entering of water into a pressure vessel caused by presence of potential gas phase in deep groundwaters and back-pressure of the inert gas coupled with the pumping rate. However, the risk of these is probably small or even unsubstantial. Gascoyne emphasized instead that pumping groundwater to the surface through the PAVE system

8

may cause a pressure drop in the zone of sampling and formation of gas phase due to the high dissolved gas content in deep groundwaters at Olkiluoto. These exsolved gases may cause incomplete filling of the pressure vessels and distort both gas composition and contents.

The main criteria for evaluating the reliability of collected gas samples are low oxygen content, good water filling of the vessels and low argon concentration. The criteria of representativity estimations for the gas sampling have been considered by Hatanpää et al. (2005). In the present study, the degree of filling of the pressure vessels as a reliability control is considered by calculating the filling factor (water amount in the vessel divided with the vessel volume) (Appendix 1). However calculated values are not quantitative because volumes of different vessels vary slightly (therefore the filling factor can exceed 1) and so they are used just as indicators of potential uncertainties.

Altogether, 71 samples (Appendix 1) collected from 21 boreholes (Fig 1-1) between 40 and 956.5 m depth from Olkiluoto groundwaters are employed in this study with 14 different gas species analysed. The analysed parameters during the gas sampling campaigns, in addition to the total amount of dissolved gases are N2, O2, CO, CO2, H2,Ar, He, CH4, C2H2, C2H4, C2H6, C3H4, C3H6 and C3H8. The analysed isotopes from the dissolved gasses are 13C(CH4),

13C(C2H6),13C(C3H8),

13C(CO2),18O(CO2),

2H(CH4),2H(C2H6) and 15N(N2). The H2S contents of the samples were analysed during the ordinary sampling and analysis campaign for dissolved solid concentrations in the Olkiluoto groundwaters (Pitkänen e al. 2004).

9

3 GEOLOGICAL AND HYDROGEOCHEMICAL SETTING

The bedrock of Olkiluoto consists of Precambrian-age Svecofennian schists and gneisses deformed and metamorphosed during the Svecofennian orogeny 1900–1800 Ma ago. Large areas to the east and south-east of Olkiluoto Island are covered with roughly 1570 Ma old rapakivi granites (Vaasjoki 1977, Lehtinen et al. 2005). The unmetamorphosed Satakunta sandstone formation north-east of the Olkiluoto area accumulated over 1400–1300 Ma (Simonen 1980, Lehtinen et al. 2005) and has been protected from erosion in a graben-like depression of the crust and on the bottom of Gulf of Bothnia off the Olkiluoto island. Obviously related to block movements, the youngest rocks in the surroundings of the Olkiluoto site are olivine diabases. The two main lithological units of the Olkiluoto site are 1) high-grade metamorphic rocks including migmatitic gneisses, granitic gneisses, mica gneisses, mafic gneisses and quartzitic gneisses, and 2) igneous rocks consisting granite pegmatites and diabase dykes (Posiva 2005).

Calcites, sulphides, clay minerals and graphite are generally found in gneisses as fracture minerals Kärki & Lahdenperä (2002), Gehör (2007). Graphite and iron sulphides are usually found in fractures and they occur simultaneously in host rock as well. Migmatised mica-rich gneisses with abundant graphite and sulphide minerals represent a strongly reducing environment which has been able to buffer the system against any oxidising events.

Gas contents and compositions are dependent on hydrogeochemical conditions, which in turn reflect geological and hydrogeological conditions. In strongly reducing groundwater environments such as Olkiluoto (Pitkänen et al. 2004) dissolved gas species representing surficial high redox conditions, (e.g. oxygen and nitrogen oxides including nitrate and nitrite) are presumably reduced at shallow depths due to microbial metabolism. The distribution of reducing gases also depends on the hydrogeochemical environment. For example, hydrogen and methane are unstable in the presence of sulphate-rich water (Appelo & Postma 1993, Plummer et al. 1994, Whiticar 1999, Megonical et al. 2005). Therefore it is important to understand the evolution of hydrogeochemical system when evaluating gas generation. In addition to chemical conditions, gas composition is significantly affected by microbial activity, which generally catalyses redox processes by consuming or producing gases. The amounts of noble gases reflect radiogenic activity, the amount of source nuclides in bedrock and the residence time of the water ( Andrews et al. 1989).

The solubility of gases depends on pressure, temperature and salinity of groundwater (Matthess 1982, Gascoyne 2005). The solubility of gases increases with increasing hydrostatic pressure (i.e. depth), but decreases with increasing temperature and salinity. Hydrostatic pressure is the most important factor in Olkiluoto because it is directly proportional to pressure whereas the effect of temperature (the gradient is low in Shield areas) and salinity can be a few tens of per cent. The high solubility of gases with long residence time in deep groundwater conditions favours accumulation of gases generated in subsurface conditions.

10

The interpretations of hydrochemical data from Olkiluoto have revealed the complex nature of the sources of salinity and groundwater evolution (Pitkänen et al. 1996, 1999, 2004, Posiva 2005). Changes in climate and geological environment have had a significant effect on local palaeohydrogeological conditions, and have left clear imprints on chemical and isotopic signatures. As a consequence of these changes, the current groundwater compositions and the chemical data show a great variability with depth, notably in salinity, indicating differences in infiltrated water sources.

The groundwater chemistry over the depth range 0-1000 m at Olkiluoto is characterised by a significant range in salinity. Fresh groundwater with low total dissolved solids (TDS < 1 g/L; Davis 1964) is found only at shallow depths, in the uppermost tens of metres (Fig. 3-1). Brackish groundwater, with TDS up to 10 g/L dominates at depths, varying from 30 m to 450 m. Fresh and brackish groundwaters are classified into three groups on the basis of characteristic anions (Fig. 3-1), which also reflect the origin of salinity in each groundwater type. Chloride is normally the dominant anion in all bedrock groundwaters, but the near-surface groundwaters are also rich in dissolved carbonate (high DIC in Fresh/Brackish HCO3 type), the intermediate layer (100-300 m) is characterised by high SO4 concentrations (Brackish SO4 type) and the deepest layer solely by Cl (Brackish Cl type), where SO4 is almost absent. In crystalline rocks high DIC contents are typical of meteoric groundwaters which have infiltrated through organic soil layers, whereas high SO4 contents indicate to a marine origin in crystalline rocks without SO4 mineral phases. Saline groundwater (TDS > 10 g/L) dominates below 400 m depth. The highest salinity observed so far is 84 g/L, which is actually below the limit generally used for brine (TDS > 100 g/L). Sodium and calcium are the dominant cations in all groundwaters and Mg is notably enriched in SO4-rich groundwaters, supporting their marine origin.

The position of stable isotope compositions relative to the global meteoric waterline (GMWL) indicates the presence of potential chemical and physical conditions and processes (e.g. Clark & Fritz 1997, Frape et al. 2004), which are indicated in the Figure 3-2.

The stable isotopic composition of groundwater is in most cases controlled by meteorological processes and the shift along the GMWL reflects climatic changes in precipitation. Cold climate precipitation has a lighter isotopic composition (more negative values), whereas warmer climate shows a heavier composition. A shift to the right or below the GMWL typically indicates evaporation in surficial waters, which are enriched due to fractionation in heavier isotopes, particularly 18O, relative to the vapour phase. Therefore, seawater composition is, for example, below the GMWL. The shift above the GMWL is unusual and observed mainly in shield brines. In order to produce such strong fractionation in oxygen and hydrogen isotopes, it has been proposed that this is due to effective primary silicate hydration under a low water-rock ratio (Clark & Fritz 1997, Gascoyne 2004, Frape et al. 2005).

11

Figure 3-1. TDS, Cl, DIC and SO4 concentrations as a function of depth of Olkiluoto

groundwaters.

12

Figure 3-2. Relationship between 18

O and 2H in Olkiluoto water samples. Arrows

depict to compositional changes caused by informed conditions. Global meteoric water

line (GMWL) after Craig (1961).

The interpretation of chemical and isotopic data (Pitkänen et al. 1996, 1999, 2004, Posiva 2005 and Figs. 3-1 and 3-2) indicates however, that there are water types from at least six different sources influencing current groundwater compositions at the site. They originate from different periods ranging from modern times, through former Baltic stages, to pre-glacial times:

13

modern - Meteoric water, which infiltrated during terrestrial recharge (during the last 0 – 2500 a), plots over a limited area along the GMWL in Figure 3-2. Shallow groundwaters from overburden and bedrock (Fresh HCO3-type)are the best representatives of this source, but clear imprints of meteoric water mixed with former groundwaters can be seen down to 100 m to 150 m depth (Brackish HCO3-type).

- Seawater from the Gulf of Bothnia (0 – 2,500 a ago) has a signature showing evaporative effects. The influence of current seawater can be seen as a slight increase in salinity in fresh HCO3-type samples.

- Water in the Korvensuo reservoir (domestic water used in drilling - originally river water) shows an even stronger evaporative tendency than seawater in Figure 3-2. The signature of the reservoir is observed from a few shallow groundwater samples in the vicinity of the reservoir.

relic - The influence of Littorina seawater (infiltrated 2500 - 8500 a ago) dominates in brackish SO4-type groundwater, as well being observable in brackish HCO3-type samples. Both groundwaters tend to shift to the right from the GMWL towards the current seawater composition (Fig. 3-2). The marine origin of dissolved solids is indicated by the marine signatures of Br/Cl, SO4/Cl and Mg/Cl ratios, and the Littorina origin by a higher salinity than in modern Baltic water and a clearly lower radiocarbon content than in HCO3-rich groundwaters.

- The colder climate meteoric water signature in the groundwater data (Fig. 3-2) probably results from the inclusion of glacial meltwater (more than 10 000 years ago). The influence is observable both in brackish SO4-type groundwater (does not even reach the stable isotope composition of current seawater) and particularly in brackish Cl-type groundwaters. The latter type shows similar non-marine chemical signatures to saline groundwater (e.g. Br/Cl ratio is twice the marine signature in the brackish SO4-type) and a further depleted radiocarbon content, which indicates a longer residence time than brackish SO4-type groundwater.

- The stable isotopic signature in saline groundwater (Fig. 3-2) above the GMWL indicates strong hydration of silicates. There are several indications for assuming an extremely long residence time for saline water. In particular, the observations from fracture infillings and fluid inclusion studies indicate elevated temperatures for hydration and a saline source water (see also Pitkänen et al. (2004) pp. 84-86 or Posiva (2005) pp. 180-182). Therefore, the saline water source (probably brine) intruded and/or formed under the influence of hydrothermal fluids, which, according to present geological knowledge, prevailed possibly during the early Phanerozoic under thick sedimentary cover (Kohonen & Rämö 2005) or during the Precambrian. The original brine end-member has later been diluted with meteoric water from precipitation in a colder climate than at present. Brackish Cl-type groundwaters represent the end-product of this dilution.

Table 3-1 shows the general hydrogeochemical features of the baseline conditions with depth. Groundwater types are mixtures of infiltrated source waters and former

14

groundwaters, for example, SO4-rich groundwater containing glacial meltwater and older groundwater diluted from brine, in addition to Littorina seawater (e.g. Pitkänen et al. 2004, Posiva 2005, Luukkonen et al. 2005).

Table 3-1. Vertical variation of the main hydrochemical parameters and microbes at Olkiluoto shown against indicative depth ranges (updated since Pitkänen et al. 2004). Variation in pH corresponds with calcite equilibrium in deep groundwaters and follows the variation in carbonate (alkalinity) and increasing contents of Ca (not shown in table). Vertical lines in the redox column depict steady state conditions. Microbe populations according to observations by Haveman et al. (1998, 2000) and Pedersen (2006). MOB, MRB, IRB, SRB are methane oxidation and manganese, iron, sulphate reducing bacteria, respectively.

The interpretations of chemical processes are based on the chemical, isotopic and microbiological data of water samples and minerals. Water-rock interaction (Fig. 3-3), such as carbon and sulphur cycling and silicate reactions, buffer the pH and redox conditions and stabilise water chemistry at Olkiluoto (Pitkänen et al. 2004).

The baseline hydrogeochemical system at Olkiluoto seems to include two natural metastable interfaces where the majority of chemical processes are concentrated (Pitkänen et al. 2004, Posiva 2005). The upper is an infiltration zone mainly in overburden and the lower lies between two brackish groundwater types at 200-300m depth where SO4-rich groundwater changes to SO4-poor, but becomes methane-rich groundwater (see Ch. 4). The disequilibrium at both interfaces is caused by differences in redox states (Figs 3-3, 3-4). At the upper interface oxic waters from surface (meteoric or sea water) infiltrate in the anoxic organic rich layer. Respiration of organic matter releases CO2 in the groundwater (carbonic acid) that activates weathering processes, e.g. calcite and silicate dissolution that buffer groundwater pH to neutral levels. At the lower interface between sulphidic and methanic redox environments, SO4-rich marine-derived groundwater is mixed with methane to result, in places (250 – 350m), in exceptionally high levels of dissolved sulphide (Fig. 3-5) and carbonate as microbially mediated reaction products, which may precipitate as pyrite and calcite.

15

Figure 3-3. Illustrated hydrogeochemical site model of baseline groundwater

conditions (redrawn after Pitkänen et al. 2004) with main water-rock interactions

processes at Olkiluoto. Blue arrows represent flow direction. Enhanced chemical

reactions dominate in infiltration zone in shallow depths, and in interface between Na-

Cl-SO4 and Na-Cl groundwater types. Hydrogeologically most dominating zones (HZ)

are also presented (Posiva 2007). Note that the illustration depicts hydrogeochemical

conditions in water conductive fracture system, not diffusion dominated pore space

inside rock blocks.

Figure 3-4. Sequential redox system in groundwaters with depth at Olkiluoto (updated

from Pitkänen et al. 2004). Relative concentrations of redox sensitive dissolved species

and correspodning redox couples correlating with Berner’s (1981) classification of redox environments are indicated. Specific status of hydrogen in the groundwater

system is currently unclear (see following discussion of gases).

16

Figure 3-5. Dissolved a) sulphate, b) methane and c) sulphide contents in groundwaters with depth at Olkiluoto.

Certain gases play a particularly active role in controlling redox processes in the groundwater system and some of these (such as O2, H2S, CH4 and H2) may also be of primary concern to repository safety, and some, like CO2, indirectly in buffering pH conditions.

Earlier studies of hydrogeochemical features have shown high amounts of reducing gases in deep groundwaters at Olkiluoto (e.g. Gascoyne 2005, Pitkänen et al. 1999, 2004, Posiva 2003). Therefore, an understanding of the sequence of vertical reduction processes and zones is important for interpretation of the origin of gas distribution at Olkiluoto. The sequence of dominating redox couples with characteristic redox species in Olkiluoto groundwaters is shown in Figure 3-4. In each couple, oxidisable species (on the left) are thermodynamically unstable in the presence of oxidative species of redox couples representing higher redox conditions, which should cause aerobic or anaerobic oxidation of the reduced couple, for example:

CH4 + 2O2 CO2 + 2H2O (Eq. 3-1)

CH4 + SO42-

HCO3- + HS

- + H2O (Eq. 3-2)

2H2 + O2 2H2O (Eq. 3-3)

4H2 + SO42-

S2-

+ 4H2O (Eq. 3-4)

4H2 + CO2 CH4 + 2H2O (Eq. 3-5)

However, in prevailing temperatures (< 20 C), these reactions are not spontaneous, but are mediated by microbes using hydrogen or methane as a substrate in reducing carbonate, sulphate or oxygen.

17

Generally microbes use organic carbon as substrate in reducing an oxidative redox couple, (e.g. even small amounts of organic carbon can easily consume O2 in soil layer to produce anaerobic conditions or reduce SO4 to sulphide). Methanotrophs oxidise methane as well, which is generally formed in shallow depths in anaerobic soil (Eq. 3-1).

Deeper in the bedrock the amount of dissolved organic carbon is, however, minor and as in Olkiluoto, dominating SO4 contents derived from infiltrated Littorina sea water may have buffered and forced microbial activity to a minimum level due to deficiency of organic substrates.

Below the brackish SO4-type groundwater, microbes can use CH4 or H2 as an energy source (Eq. 3-2 and 3-4) and hydrogen sulphide concentrations are locally elevated in the interface between brackish SO4-type and brackish Cl-type groundwaters due to mixing of these groundwater types (Fig. 3-5). It is also possible that deep in the bedrock a third metastable redox interface may exist comparable to the zones of O2 consumption and SO4 reduction, where microbes use hydrogen to reduce dissolved carbonate (Eq. 3-5), further increasing the CH4 pool in saline groundwater. The dissolved carbonate content is very low in saline groundwaters, but probably a combination of prevailing calcite equilibrium and very high calcium contents is the dominat reason for limiting the DIC content. It is unclear, therefore, the role which is played by active carbonate reduction on lowering the DIC content at the current investigations depths. Fracture calcites form a significant carbonate source for microbial reduction, if they dissolve as a consequence of microbial consumption of DIC. Detailed investigations of fracture calcites in transmissive fractures at great depth may give more information on this subject.

Geomicrobial studies of groundwater samples (Haveman et al. 1998, 2000, Pedersen 2006), though limited in number, have shown suitable microbes in each redox zone, for supporting ongoing redox processes (Table 3-1). Sulphate reducers (SRB) have been observed to be the most abundant species and tend to be particularly associated with groundwaters at an intermediate depth range (~ 250 – 330m). The deeper, saline groundwaters contain very low numbers of SRB and may even be lacking in them completely. Methanogenic and acetogenic groups seem also to be active in deep saline groundwater, representing hydrogen-driven biosphere. Autotrophic acetogens may use hydrogen as energy source in reducing carbonate to produce organic carbon (Eq. 3-6), which in turn may be consumed by heterotrophic methanogens (fermentation, Eq 3-7). Autotrophic methanogens use primarily hydrogen as substrate to reduce carbonate in producing methane (Eq. 3-5).

4H2 + 2CO2 CH3COOH +2H2O (Eq. 3-6)

CH3COOH CH4 + CO2 (Eq. 3-7)

Recent geomicrobial studies and interpretations by Pedersen (2006) in shallow groundwaters from overburden and bedrock (down to 20m depth) have shown also active populations of cultivable heterotrophic aerobic bacteria and methane-oxidizing

18

species (methanotrophs); however, significant populations of anaerobic groups, such as methanogens, acetogens and SRB, have also been observed at those depths.

19

4 RESULTS OF GASES AND DISCUSSION

4.1 Range of gas contents and uncertainties in the data

Gas compositions and total dissolved gases (measured at NTP) are presented in Figure 4-1 and in Appendix 1. Hydrogeochemical and microbial results indicated prevailing anaerobic conditions in bedrock groundwaters. Notable amounts of reducing species (CH4, HS- and H2, see Pitkänen et al. 2004) do not support occurrence of dissolved oxygen in deep groundwaters. Hatanpää et al. (2005) have also noted the potential of air contamination during sampling or laboratory treatment. Therefore results are corrected for air contamination assuming that all O2 measured in a single sample is due to contamination during sampling and laboratory measurements. Corresponding amounts of N2, Ar and CO2 are subtracted from each species.

Oxygen concentrations are mainly below 1 mL/L corresponding to generally less than 1 vol. % of total gases (see App. 1). Nine samples have oxygen more than 2 vol. % representing at least 10 % air contamination in those samples. This should be taken into account when evaluating variables and their contents which can react with oxygen, such as H2 and CH4. In particular, oxidation may increase 13C value in CH4 due to microbial fractionation if the original methane content was very small.

Total dissolved gases show a fairly coherent increasing trend with depth indicating relatively good reliability of the gas sampling system (Fig. 4-1). Large variations are however, observable in single samples from the same borehole section, for example, in the results of the deep groundwater samples from KR4_860_1 & 2 and KR4_861_1 (tot. gas vol. vary from 870 to 1,900 mL/L) reflecting significant uncertainty in quantitative results. This large variation may result from incomplete filling of the gas vessel. Gascoyne (2005) suggested that incomplete filling of gas vessels may be created by pumping of the groundwater during sampling causing a pressure drop in the zone being sampled, particularly in low permeability zones. This in turn allows degassing from water phase partly filling the vessels thereby increasing total gas content and, in particular, lighter and more insoluble gases such as H2, He and N2 because they tend to diffuse faster into the forming bubbles, i.e. degassing fractionates gas composition, heavier gases enrich in the water phase and lighter gases in the gas phase. The degree of filling (calculated in App. 1) of sample KR4_861_1 was very low (<30%), which clearly indicates uncertainties in measured values and calculated volumetric contents. It also shows extraordinary high contents of H2 (268 mL/L) and He (154 mL/L) which are one order of magnitude higher than other measured values from similar depths and groundwaters. Nitrogen content (480 ml/l) is also more than double. Methane content (990 mL/L) is the highest of all samples though a few other samples in the data from similar depths also have above 900 mL/L CH4. As a conclusion, the lower level of total dissolved gases in this sample is considered more plausible and generally samples with incomplete filling of thee gas vessel by water is an indicator of uncertainties in gas contents and composition. The following discussion does not contain data with filling factor lower than 0.7.

20

Figure 4-1. The content of dissolved gases (mL gas/one L water at NTP) with depth in

Olkiluoto groundwater samples (all PAVE samples), a) N2, He and O2, total dissolved

gases, and b) CO2, H2, CH4 and the sum of higher hydrocarbons (CnHn).

21

Nitrogen and methane form the major portion of total gas contents. Nitrogen completely dominates the dissolved gas inventory in the upper 300 m, whereas below 300m CH4 isdominant. Methane contents vary considerably by about three orders of magnitude around 300m depth. This reflects the variation in groundwater types and redox conditions (Pitkänen et al. 2004, Gascoyne 2005), and correspond well with the concept of instability between SO4 and CH4 (Fig. 4-2) due to the bacterial reduction of SO4 and the oxidation of CH4 (Niewöhner et al. 1998, Whiticar 1999), and that simultaneous presence of high contents of both dissolved agents indicates a mixing of different waters (Plummer et al. 1994). The contents of higher hydrocarbons (actually the sum: C2H6+C3H8, expressed as C2+ later in the text) show a similar trend as CH4, though the levels are much lower. Carbon dioxide has a decreasing trend with depth, whereas the level of H2 increases with depth, which may reflect the instability between these gases and the increasing gradual bacterial reduction of CO2 with depth in saline groundwater. The argon content is nearly constant, whereas He increases with depth.

The amount of dissolved gases deep in the bedrock of Olkiluoto are fairly high and correspond with gas contents taken from similar depths in other locations in Finland (Sherwood Lollar et al. 1993a) with similar lithologies (migmatitic mica and graphite-bearing gneisses and schists at Enonkoski and Ylistaro and with serpentinized ultramafic and gabbroic rocks at Ylivieska). Mineralised sulphides are common for all of these sites reflecting the strongly reducing nature of bedrock system. Nitrogen, methane and hydrogen also show similar ranges of gas contents to Olkiluoto. At sites with other rock types reflecting an oxidising geological history, with abundant iron oxyhydroxides in fractures (hematite and goethite), such as Hästholmen (rapakivi granite) and Pori (Jothnian sandstone) total gas contents are smaller particularly at great depth due to only trace amounts of CH4 (Pitkänen et al. 2001). However, N2 contents at these sites as well as He at Hästholmen (not analysed from Pori) correspond with Olkiluoto. These latter sites have also high SO4 concentrations in deep groundwaters whereas SO4 is typically very low or missing in samples with high CH4 contents at the former sites.

22

Figure 4-2. Relationship between CH4 and SO4 in groundwater samples from Olkiluoto.

The potential origin of different gas components deviates. Some of them, such as N2, He and Ar may have both atmospheric and crustal origins in which case detailed studies of them may give information of evolution and residence time of groundwaters. This kind of interpretation also needs isotopic data for these gases which are limited in our studies. Hydrocarbons and hydrogen appear to have a crustal origin; however, several sources are possible. Shallow formation of minor methane in overburden layers is observed (cf. geomicrobial results in Ch.3), but the gas is probably isolated from deep groundwaters. In the next chapters, the origin of these gases as well as their status for understanding the hydrogeochemical system are discussed.

23

4.2 Origin of hydrocarbons

Hydrocarbon (HC) gases are most commonly formed by biological processes either directly by microbial processes in low temperatures (bacterial gas) or by thermal breakdown of more complex organic precursors (thermogenic gas) (e.g. Schoell 1988, Whiticar 1990, 1999). Bacterial methanogenesis may proceed either with fermentation of methylated substrates such as acetate (Eq. 4-1) by heterotrophic bacteria or through a carbonate reduction pathway using hydrogen gas to reduce CO2 (Eq. 4-2) by autotrophic bacteria. Formation of higher HC is limited to trace levels, which is one typical indicator for identifying bacterial methane. The molecular ratio (C1/C2+) between CH4

and the sum of higher HC in bacterial HC is generally very high (more than 1000). Thermogenesis occurs in deeply buried organic sediments in sedimentary basins and the molecular C1/C2+ ratio is low sometimes less than 10 (depending on precursor material; humic source has the lowest, whereas algae and plankton source can produce higher ratios). Bacterial oxidation of CH4 (e.g. during SO4 reduction) can decrease the molecular ratio significantly and lead to difficulties in interpretation of gas sources.

CH3COOH CH4 + CO2 (Eq. 4-1)

CO2 + 4H2 CH4 + 2H2O (Eq. 4-2)

The fermentation pathway is normally restricted in freshwater environments to shallow depths and features rich in organic substrates, such as in bogs and lake-bottom sediments. Microbial studies (Haveman et al. 2000, Pedersen 2006) cannot completely solve the methanogenic pathway deeper, because both autotrophic and heterotrophic methanogens and acetogens are commonly observed in groundwaters. Autotrophic groups can metabolise carbonate and hydrogen representing the carbonate reduction pathway, whereas heterotrophic groups are metabolising dissolved organic substances such as acetate therefore representing the fermentation pathway. However fermentation may be based on organics produced by autotrophic acetogens, i.e. the primary catalysing process would be carbonate reduction. The carbonate reduction pathway is considered to be a more realistic bacterial formation process for CH4 in deep groundwater at Olkiluoto. Evident lack of organic substrates, which are probably consumed by SRB and IRB in the SO4-rich groundwater zone, and slight accumulation of hydrogen in SO4-free deep groundwater zone, support the carbonate reduction pathway.

During the last decade, an abiogenic source for crustal HC has also been suggested (e.g. Sherwood Lollar et al. 1993b, 2002; Ward et al. 2004) at a number of Precambrian Shield sites. The abiogenic HC gases can be formed from crustal inorganic carbon sources (graphite, CO2 or DIC), for example, in hydrothermal systems during water-rock interactions involving the Fisher-Tropsch synthesis reaction (bulk reaction is same as in autotophic methanogenesis, Eq. 4-2) or low grade metamorphism of graphite-carbonate bearing rocks with hydrogen (e.g. Eq. 4-3 and see discussion in Sherwood Lollar et al. 1993b). Abiogenic pathway is totally based on inorganic compounds and processes in contrast to bacterial methanogenesis or thermogenesis. Higher

24

hydrocarbons are produced by polymerisation of methane and molecular ratio is considered to be on similar level to that of thermogenic HC. Later in the report, the term ‘thermal HC’ is used generally for both abiogenic and thermogenic HC for cases where actual specification of the origin of precursor is not essential for HC formed in elevated temperatures and the term only distinguishes their origin from bacterial HC.

C + 2H2 CH4 (Eq. 4-3)

Molecular ratios between CH4 and higher HC in gas samples from Olkiluoto support an origin of bacterial methanogenesis as well as methane and higher HC formed at elevated temperatures (Figure 4-3). A few samples with low CH4 content show very high molecular ratios and in general the ratio increases in brackish groundwater types at intermediate depths. However, even these high molecular values do not represent pure bacterial gas but seems to include either some mixing with high temperature HC or possible bacterial oxidation of CH4. Correspondingly, molecular ratios which maintain above 40 in deep saline groundwater, support both thermal and bacterial origin production for the high CH4 and HC contents

Figure 4-3. Relationship between CH4 and molecular ratio of methane and higher

alkanes (C1/C2+) for hydrocarbon gases in Olkiluoto groundwaters.

Low molecular ratios and minor HC contents in part of SO4-rich groundwater samples may be residues of old HC due to mixing groundwater types or their CH4 content is

25

decreased by anaerobic oxidation of CH4 by SRB in which case the origin may evidently be bacterial in these waters. Hydrocarbon contents in these samples are small and so even minor microbial oxidation of CH4 would have a strong influence on molecular number.

4.2.1 Isotopic evidence of hydrocarbons

General isotopic systematics

The differences in origin of HC are also reflected by their isotopic compositions, which is also observed systematically in empirical data (e.g. fields in Fig. 4-4). Single isotope ratio is not alone very useful (e.g. 2H(CH4)) due to considerable overlap between gases from different origins. However, using isotopes in combination with additional HC data can distinguish effectively CH4 produced by different source processes, providing the isotopic values have not suffered secondary processes such as fractionation by CH4

oxidation or mixing from different sources (Schoell 1988, Clark & Fritz 1997, Whiticar 1990, 1999). The dissimilarity in isotope distributions of bacterial, thermogenic and abiogenic CH4 (Fig. 4-4) is related to source processes including differences in formation temperatures, isotope effects (type of fractionation) and precursor compounds.

The 13C(CH4) value of thermogenic gases progressively increases with the maturity of decomposition, finally approaching the carbon isotopic ratio of the original organic matter. Typically, the carbon isotope separation between thermogenic CH4 and organic matter ( 13C(org. matter) - 13C(CH4)) can vary from 30‰ to 0‰. Organic matter has

13C values above -30‰ (typically around -25‰), therefore 13C(CH4) in thermogenic gas are seldom below -50‰ (Whiticar 1999). Experimental results indicate that production of abiogenic CH4 from crustal carbon sources by water-rock interactions can result in a similar range in 13C(CH4) values as in thermogenic gas. The hydrogen isotope ratios differ between these gases and particularly depletion in 2H(CH4) values (e.g. Kidd Creek in Fig. 4-4) compared to typical thermogenic gases (-275 – -100‰) has been suggested to be characteristic of abiogenic CH4 (Sherwood Lollar et et 1993b, 2002). Hydrogen isotope ratios in abiogenic CH4 may range largely from -500‰ to -100‰, however, the values are typically lower than in thermogenic gases with similar

13C values.

26

Figure 4-4. The variation of 2H and

13C of methane at Olkiluoto compared to

empirically determined fields for thermogenic and bacterial methane produced either by

fermentation or carbonate reduction pathway after Whiticar (1999). The arrows depict

potential mixing of bacterial methane in potential thermal CH4 source at Olkiluoto, and

the influence of bacterial oxidation on isotopic composition. The field of Kidd Creek

represents isotopic compositions of methane sampled from Kidd Creek mine in

Canadian Shield and postulated as typical abiogenic HC gases by Sherwood Lollar et

al. (2002).

Isotopic composition of 13C and 2H isotopes in CH4 derived from bacterial methanogenesis tend to be depleted due to the fact that methanogenic bacteria prefer to metabolize lighter isotopes because the lower energy needed to break bonds between lighter isotopes (Clark & Fritz 1997). This becomes evident in compositions between bacterial fermentation and thermogenic CH4 (Fig. 4-4), which both processes use organic precursors. Different methanogenic pathways, i.e. bacterial methyl type fermentation and carbonate reduction produce divergent compositions as well. The 13Cvalues in bacterial methanes are typically lower than -50‰ and significantly lower in CH4 derived by carbonate reduction. The separation of carbon isotopic compositions between bacterial pathways is caused by a higher difference between CH4 and

27

coexisting CO2 during carbonate reduction (50‰ – 95‰) than the fermentation (40‰ - 60‰) process. The 2H(CH4) values of the carbonate reduction pathway are generally higher than -250‰ whereas they are significantly lower in fermentative methane. The depletion in 2H(CH4) values in various carbonate reduction environments appears to be around 160‰ to 180‰ relative to formation water, whereas the isotope effect in the fermentation pathway tends to be from 300‰ to 400‰ (e.g. Sugimoto & Wada 1995, Waldron et al. 1999, Whiticar 1999)

The isotopic composition of CH4 also depends on precursors. The carbon source may vary from different organic compounds to inorganic such as carbonate or graphite (abiogenic source) depending on CH4 production pathway. Similarly, the hydrogen source varies from organics to formation water or hydrogen gas. For example, during carbonate reduction, hydrogen for CH4 formation is ultimately derived from water (100% hydrogen source) though it is possibly influenced by isotopic exchange with hydrogen gas, whereas in the fermentation pathway three methyl group hydrogens (75%) is on average used to build up CH4 molecule (Whiticar 1999).

Isotopic compositions of HC gases at Olkiluoto

Isotopic compositions of CH4 (Fig. 4-4) also indicate gas mixtures of thermally and bacterially formed CH4 dissolved in groundwater at Olkiluoto (cf. Fig. 4-3). The isotopic compositions of CH4 from saline groundwaters with the highest CH4 contents plot below the thermogenic field and show, therefore, similar tendencies as abiogenic gases demonstrated by Sherwood Lollar et al. (1993b, 2002) and Ward et al. (2004) in Canadian and Fennoscandian Shields and the Witwaterstrand basin in South Africa, i.e. relative depletion in 2H compared to thermogenic gases. Generally, isotopic composition at Olkiluoto seems to change from an abiogenic end-member towards the bacterial carbonate reduction field as CH4 contents and salinity decrease (from saline groundwaters to the brackish Cl type) and the groundwater type becomes younger. However, this trend does not prove that CH4 and HC gases in general have only two end-members at Olkiluoto. Thermogenic admixture is also possible in the system. The scattering in the samples with the lightest carbon isotope ratio plots partly in the field between the two bacterial pathways. Unfortunately deuterium measurements of CH4

samples from Olkiluoto have partly failed, in particular for the samples with low CH4

content and the most depleted 13C values. The slightly low 2H levesl for carbonate reduction may also result from temperature effect on the fractionation. Generally fractionation is higher in lower temperatures (e.g. Whiticar 1999) though hydrogen isotope effects are poorly understood. The temperature gradient in the Olkiluoto bedrock is low and temperature is less than 10 C at the depths of the samples with the lowest 13C(CH4) values. This could lead to slightly lower deuterium values in CH4 than is assumed in general.

Isotopic compositions of CH4 in a few SO4–rich groundwater gas samples (Fig 4-4) show a trend towards significantly heavier isotope ratios relative to the other data. This kind of shift is typical of bacterial oxidation of CH4 that enriches the residual CH4 in heavier isotopes. The trend supports bacterial anaerobic oxidation of CH4 together with SO4 reduction at these depths, as well as depleted 13C values in CO2 in the interface between SO4-rich and –poor groundwater types, as already stated above. Observations

28

of dissolved sulphide and the SRB metabolic group in this depth zone (Haveman et al. 2000) strongly indicate ongoing anaerobic consumption of CH4 with SO4 reduction. A corresponding shift is missing in two of the gas samples from brackish Cl-type samples with the lightest isotopic composition, which argues against significant CH4 oxidationand clearly indicates mixed composition of bacterial and thermal HC in those samples. Isotopic results from fracture calcites from KR1 borehole at Olkiluoto (Blyth et al. 2000) show dominantly 13C values higher than -20‰ particularly those potentially representing lower temperature. Gehör et al. (2002) concentrated their study on late stage calcites which showed 13C values higher than -10‰. These values also do not support significant oxidation of CH4 in a SO4-free groundwater system at Olkiluoto, whereas high positive 13C values in calcites have been observed in fairly shallow depths (Gehör et al. 2002), currently dominated by SO4-rich groundwater types. This indicates bacterial methanogenesis has also prevailed formerly in the upper parts of the bedrock.

Carbon isotope measurements of CH4 have been successful in general, contrary to deuterium, and its relation to CH4 content helps in understanding the HC issue at Olkiluoto. The main CH4 and HC pool deep in the bedrock (Fig. 4-5) at Olkiluoto is a thermal, non-bacterial end-member, which has higher 13C(CH4) and significantly depleted 2H(CH4), indicating a major abiogenic origin for HC. The 13C value decreases with decreasing CH4 upwards in the groundwater system suggesting formation and relative enrichment of bacterial CH4 in the HC pool (cf. a mixing curve). Bacterial enrichment also seems to be potential in the deepest aquifer with the highest CH4 contents. Haveman et al. (2000) observed cultivable autotrophic methanogens from the groundwater sample with the highest CH4 content. Its 13C(DIC) value is extremely high +16.8‰ (Pitkänen et al. 2004), strongly supporting ongoing bacterial methanogenesis deep below brackish SO4 groundwaters.

The 13C(CH4) values seem to increase in brackish SO4-type groundwaters, which have low CH4 contents (Fig. 4-5). The main source of methane in SO4-rich groundwater may result from mixing with brackish Cl-type groundwater. The enrichment of 13C in SO4-rich groundwaters probably reflects anaerobic oxidation of CH4 also indicated by isotopic compositions of CH4 in Figure 4-4 and some very low 13C(DIC) values in groundwaters sampled from the interface between brackish SO4-type and Cl-type groundwaters (Pitkänen et al. 2004). Depleted 13C values in two HCO3 -rich samples may represent a totally different methanogenetic system (surficial fresh groundwater system with fermentative pathway) than other gas samples.

29

Figure 4-5.13

C(CH4) vs. methane in groundwaters at Olkiluoto. Arrows depict interpreted processes caused the compositional changes observable in CH4 gas data.

The isotopic results have also long-term hydrogeological implications. The depletion in saline and brackish Cl-type groundwaters is surprisingly steady as 13C values approach -50‰, after which the drop is abrupt. The trend resembles the irregular behaviour of stable isotopes of water in saline and brackish Cl-type groundwaters (Fig. 3-2) as well as the relation between 18O and Cl (Luukkonen et al. 2005, Posiva 2005, Andersson et al. 2007). As the stable isotope composition approaches the GMWL salinity decreases and the trend turns along the GMWL in corresponding groundwaters as the relation between CH4 content and 13C values have a fold in Figure 4-5. The strongly depleted

13C samples correlates with the light stable isotopes of groundwater (Fig.4-6). The fold in these trends may be linked to the hydrogeological system indicating that the deep stable groundwater system has not been disturbed by deglacial and postglacial transients. Isotopic results also suggest that no oxidation of methane, anaerobic or aerobic, has occurred in the deeper groundwater system below the SO4-richgroundwater (i.e. -300m and deeper). This may indicate that neither oxidising glacial melt nor marine water, which can cause aerobic or anaerobic oxidation of CH4,respectively, have mixed in this deeper system, although it could have taken place on several occasions due to the periodic glacial cycles during the Quaternary.

30

Figure 4-6. The variation between 13

C(CH4) and 18

O in saline and brackish Cl-type

groundwaters.

Figure 4-7 provides further insight for mixing between bacterially or thermally originated CH4 end-members. The fields of typical bacterial and thermal HC gas are shown in the figure as well as two mixing lines between potential end-member compositions. These compositions are selected so that mixing lines represent the HC data and 13C(CH4) particularly well. A rather high molecular ratio of HC gases for the thermal end-member has to be selected as the upper curve in order that the major part of the samples can be described. Bacterial CH4 oxidation, as well as migration of CH4 in the groundwater system, can lead to difficulties in the interpretation of natural gas sources (Whiticar 1990, 1999). However, oxidation of CH4 in HC-rich groundwater types (brackish Cl and saline) seems to have been insignificant.

31

Figure 4-7. C1/C2+ vs. 13

C(CH4) of brackish-Cl and saline groundwater samples at Olkiluoto. Mixing lines are calculated between the potential bacterial and thermal end-

members and provide a conservative estimate of bacterial CH4 contributing to the

samples (contours). The relative compositional effects of migration (with either release

or dissolution) and oxidation of CH4 compositions are also indicated.

Sample distribution in the diagram (Fig. 4-7) indicates bimodal divergent compositions for both end-members in the system. According to Gascoyne (2005), CH4 solubility is near saturation in the deepest groundwater samples. Therefore, ebullition and the vertical migration of CH4 due, for example, to the continuous formation of bacterial CH4

et great depths, and the re-dissolution at shallower depths that were less saturated with CH4, could have caused a subvertical shift in the diagram. This shift in the re-dissolution case should be towards the upper right-hand corner of the figure, because CH4 released from greater depths has a heavier carbon isotopic composition.

Vertical migration and dissolution may be a possible reason why the wide zone is needed to explain the mixing between the end-members in the Figure 4-7. Variation in

32

carbon isotopic composition of source carbonate may also scatter the end-member composition for bacterial methanogenesis. The range of 13C both in DIC and fracture calcites exceeds 10‰ in methanic groundwater system at Olkiluoto (Blyth et al. 2000, Pitkänen et al. 2004). However, the vertical range particularly observable in saline groundwater data suggests the influence of secondary processes such as methane migration on variable compositions.

Based on calculated mixing lines (Fig. 4-7), CH4 in saline groundwaters may be as high as 50% bacterial in origin and, therefore, in brackish Cl-type waters significantly more. These estimates are insensitive as to whether the molecular ratio is 1000 or more for the bacterial end-members. However, they depend on selected 13C(CH4) values for the end-members, and thus give only an indication of the relative importance of bacterial CH4 gas in the Olkiluoto groundwater system. In addition mixing lines cannot fully represent the distribution of gas samples as discussed above.

The isotopic composition of CH4 gas at Olkiluoto tends to support an abiogenic origin for thermal CH4 rather than thermogenic decay of organic sediments. Sherwood Lollar et al. (2002) noted that the 13C value in CH4 was higher than in ethane and propane, because the lighter carbon isotope (12C) reacts faster than the heavy isotope (13C) in an abiogenic polymerisation reaction (kinetically controlled), whereas carbon isotopic composition becomes heavier as C number increases in thermogenic HC. Hydrocarbon gases at Olkiluoto do not show such an abiogenic trend (Fig. 4-8), rather, the 13C value in CH4 is generally lower than 13C in higher HC. However, CH4 formed by bacterial methanogenesis in groundwater at Olkiluoto can decrease significantly the 13C value in CH4 originated from thermal processes. The tendency in Fig 4-8 is that 13C in methane approaches 13C in higher HC as carbon isotopic composition becomes heavier (fraction of thermal CH4 increases, Fig. 4-7) and finally equals 13C of coexisting ethane in a couple of samples. Therefore, it seems evident that 13C of methane has been heavier than in ethane or propane in the original end-member, thus supporting a similar origin for the thermal end-member as described by Sherwood Lollar et al. (2002) and Ward et al. (2004) for abiogenic HC gases in Kidd Creek and Witwaterstrand.

33

Figure 4-8. Plot of 13

C values of methane, ethane and propane against carbon number for dissolved gas samples at Olkiluoto. Note: input of bacterial methanogenesis

decreases 13

C in CH4 species which is particularly reflected on the trends of brackish

groundwaters.

What is then the source carbon and process of forming abiogenic HC in the system? Potential mechanisms for abiogenic gas synthesis include the Fischer-Tropsch synthesis, hydrothermal or low grade metamorphism of graphite-carbonate-bearing rocks and formation of CH4 from graphite and hydrogen gas. Polyphase, hydrothermal water-rock interaction is frequent in the bedrock (Gehör 2007). The lack of reasonable temperature estimates of thermal CH4 and carbon isotopic results of graphite makes it impossible to indicate any particular mechanism. Formation temperature can be estimated by deuterium geothermometers if 2H results of H2 gas were available or could be measured in future. The 13C of graphite may give more specific information of the potential carbon source of abiogenic CH4 complementing isotopic data of fracture calcites.

As observed the origin of HC at Olkiluoto is a very complex question. Pitkänen et al. (2004) considered all three sources: bacterial, thermogenic and abiogenic potential. However, current extended data can well be explained by the mixing of two end-members, abiogenic and bacterial gas, though we cannot completely exclude thermogenic admixture, because isotopic data also show partly thermogenic features.

34

Favourable conditions for abiogenic HC may have prevailed during Precambrian and early Phanerozoic times, which may connect thermal HC to the formation of the brine end-member at Olkiluoto. Abiogenic HC may have formed in situ or at a much greater depth and has migrated to shallower depths more recently. Such slow migration may cause the near saturation state of CH4 in the deep samples (Gascoyne 2000, 2005). Carbon isotopic data with molecular ratios of HC suggest potential CH4 migration (gas ebullition) from great depths. Bacterial methanogenesis may also increase CH4 content to the near saturation state. On the other hand, why CH4 is not throughout saturated below SO4-rich groundwater, under 300m depth as we can assume steady CH4

production in the groundwater system, and no significant CH4 oxidation if any takes place in these depths. Hydrogen gas contents are low in general (Fig. 4-9) and the availability of H2 may limit bacterial carbonate reduction. Also, mass-balance calculations testing bacterial methane production (based on groundwater samples obtained from Olkiluoto) and utilising carbon isotopic data in CH4 and DIC, indicated only minor bacterial CH4 production (Pitkänen et al. 2004).

Methane saturation is approached only in the deepest samples taken below 800 m (Gascoyne 2005), which may reflect the relative residence time of groundwaters. The dilution of saline and brackish Cl-type groundwaters above 800 m has been faster than the increase of methane caused by migration and bacterial production in these groundwaters. However, the dilution seems to be an older process than the last deglaciation and melt water infiltration in saline groundwaters as indicated by the discussion of Figure 4-5, indicating that CH4 enrichment is a very slow process, corresponding to say at least 104 to 105 years time scales.

High concentrations of H2 in the bedrock of Olkiluoto would be unexpected because soluble fracture calcites form a significant pool for bacterial carbonate reduction, although the DIC precursor is low. Low concentrations are likely similarly restricted as are CH4 contents in SO4-rich groundwaters above -300m depth (cf. Fig. 3-5). If hydrogen is migrating from great depths upwards it is probably mainly metabolised by methanogens deeper than the currently studied bedrock volume. The produced bacterial methane may degas, migrate upwards, and increase the fraction of bacterial methane at depths, where CH4 is clearly undersaturated by re-dissolution. High, nearly saturated concentrations of CH4 at the bottom of the characterized rock volume and a few elevated H2 contents may indicate that main H2 consumption zone is not significantly deeper than the current observation level at about -1000 m. Deuterium measurements of hydrogen gas (unsuccessful so far) may give further information for methanogenesis and methane accumulation in deep groundwater system at Olkiluoto.

35

Figure 4-9. Depth distribution of dissolved H2. Note that hydrogen was below detection

limit in numerous samples particularly in brackish groundwaters.

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4.3 Helium

Helium is considered to be a primordial element as well as argon. Helium remains rather rare in the systems of the Earth because of its efficient degassing and escape from the atmosphere. Therefore, He concentration stays low in the atmosphere as well as its capability to dissolve in infiltrating water. During groundwater migration, the radiogenic gases He and Ar (particularly isotopes 4He and 40Ar due to their principal production in rocks) may dissolve from rock matrix, thus increasing the concentrations of these gases dissolved during recharge (Andrews et al. 1989). Helium concentrations and radiogenic 4He have been observed to increase considerably along a flow path, therefore they are a competent tracer tools to give information on hydrogeological conditions e.g. recharge and discharge areas in addition to flow directions in groundwater system. The main source of excess He compared to atmospheric equilibration is the generation of 4He by radioactive decay of U and Th and their alpha-emitting daughters in rock minerals.

The helium results in Figure 4-10 clearly show an increasing trend as a function of depth and salinity. During the first 200 m helium contents remain rather constant, staying below 1 mL/L. The helium content begins to increase steadily in waters below 200 m. The saline waters have the most scattered results, especially between the 450 and 650 m with a range from 6 mL/L to 18 mL/L. The maximum helium concentration 22 mL/L is reached below 800 m.

Helium contents seem to separate groundwater samples fairly distinctly relative to their mean residence times (cf. Pitkänen et al. 2004, Posiva 2005). The youngest brackish SO4 and HCO3 waters in the upper 200 m show contents below 1 mL/L (Fig. 4-10). Sulphate-rich samples with about 1 mL/L He have the highest fraction of Littorina sea water and they also contain notable proportion of older groundwater types (Pitkänen et al. 2004, Posiva 2005, Luukkonen et al. 2005). The uppermost brackish Cl-type samples do not contain post-glacial water components, but show the strongest glacial signal (Fig. 3-2) and this dilution effect of glacial melt is also seen in dissolved He contents. The samples classified as brackish SO4-type, plotting in the same group with brackish Cl-type samples at 300-350 m depth are known to be mixtures from the transition zone between these two water types. Pre-glacial groundwater, which is the main component of the brackish Cl and saline types also represent a majority in these samples. The increasing trend with depth and salinity may represent increasing age of groundwater and a slow dilution of brine.

Consequently, dissolved helium in groundwaters at Olkiluoto is considered to originate mainly from the bedrock either by in situ production and diffusion in shallow crust or by deep crustal degassing. Helium concentrations increase from recently recharged (post glacial) HCO3- and SO4-rich groundwaters to much older SO4-poor brackish and saline groundwaters thus supporting previous interpretations (Pitkänen et al. 2004, Posiva 2005) of relative residence times of groundwater types and the concept of paleohydrogeological evolution and mixing at the site. The rather high variance may reflect uncertainties in the He gas data which may result from problems during sampling and analysis. Isotopic measurements of He may give more detailed information of

37

relative residence time and flow directions in the groundwater system, but this work has not been done in the current study.

Figure 4-10. Depth distribution of dissolved helium in a) logarithmic and b) linear

scale in Olkiluoto groundwaters.

4.4 Argon

Argon (as 40Ar) in the bedrock matrix is mainly produced by decay of 40K contained in feldspars and micas (biotites and muscovite) of rocks. Andrews et al. (1989) calculates that the 40Ar content of a mineral, which contains 5% K and is 650 Ma old is 4.0 * 10-4

m3 STP m-3 (standard temperature and pressure). This is very close to the Ar-content equilibrated with the atmosphere at 10°C during recharge, and so provides no reason for Ar to diffuse from rock into fracture fluids due to a concentration gradient. However, K-rich host rock that is older than 650 Ma could induce diffusion.

In Olkiluoto the K –bearing migmatitic gneisses, granites and mica gneisses are Precambrian in age (1,900-1,800 Ma) and the average K-content in the most K-rich pegmatite granites is ~ 6 % (Pitkänen et al. 1999). Therefore, the maximum theoretical 40Ar content is about 11.7 * 10-4 m3 STP m-3, which would cause diffusion between the rock matrix and fracture fluids. The excess of air-equilibrated Ar may be detected by total Ar measurements but measuring of the 40Ar/36Ar-ratios in groundwaters gives more precise information and estimates of recharge temperatures (Andrews et al. 1989).

38

Gascoyne (2005) proposed that some of the argon samples have been contaminated with Ar used as a back-pressure gas. In this study, all the Ar results from vessels where argon was used as back-filling gas, are replaced with results from N2-filled vessels or discarded if only Ar-filling was used. However, two results of Ar-filling are presented, for the samples KR12_365_2 and KR27_503_1 (App. 1), because the results from N2-filled vessel are too high (suggesting possible mistake in the database). The result of KR15_449_1 is considered abnormally high and is therefore discarded.

In Figure 4-11 the depth distribution of the Ar results of different water types show no clearly distinguished behaviour of argon concentrations. Most Ar results lie between 0.5 and 1.5 mL/L indicating that radiogenic 40Ar has been added to dissolved Ar in groundwaters. During the first 300 m, the argon contents plot mainly between 0.5 and 1.3 mL/L. From 300 m to 400 m, Ar appears to increase slightly in the brackish Cl-type water (from 1 to 1.5 mL/L) whereas Ar contents in saline-type groundwaters represent again same level as in the upper 300 m despite of a couple anomalously high results (KR11_621_1, KR29_800_1). The slightly higher content in brackish Cl-type groundwaters (which also show glacial signature in stable isotopic composition Fig. 3-2) may result from recharge conditions during deglaciation. The solubility of gases increases in colder water, as well as at higher pressures, which both could have influence during recharge under a melting ice sheet. Air bubbles in ice may be the excess gas dissolved in melting ice.

39

Figure 4-10. Depth distribution of Ar in Olkiluoto groundwaters.

4.5 Nitrogen

Nitrogen is the most abundant (~78 %) gas in the atmosphere and participates in most of the biological processes. Nitrogen has two stable isotopes 14N and 15N. The amount of 15N in air is very constant (0.366%) (Junk and Svec, 1958) enabling the use of air as a standard in studying 15N results.

The three main processes in controlling the nitrogen cycle in the biosphere are fixation, nitrification and denitrification. In nitrogen fixation the relatively inert N2 of the atmosphere is converted to a reactive form such as NH3. The process is performed by bacteria and results in organic compounds with slightly fractionated (between -3 and +1 ‰) nitrogen-isotope values (Fogel and Cifuentes, 1993).

Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of nitrites into nitrates. The oxidation of ammonia into nitrite, and the following oxidation to nitrate are carried out by two different bacteria. Nitrification can be expressed as a two-oxidation reaction as follows:

40

NH3 + O2 NO2 + 3H+ + 2e (Eq. 4-4)

NO2 + H2O NO3 + 2H+ + 2e (Eq. 4-5)

The fractionation occurs mainly during the first reaction due to the rapid reaction oxidation of nitrite.

Denitrification is the step of a nitrogen cycle, where nitrate is reduced back into gaseous nitrogen (Eq. 4-6). The process is performed by heterotrophic bacteria. Denitrification mostly takes place under anaerobic conditions (Megonigal et al. 2005) such as stratified water bodies. The 15N values are strongly depleted in reduced nitrogen gas whereas the residual nitrate enriches highly due to the denitrification. Final 15N values in nitrogen gas depend on the extent of denitrification process:

NO3- + 5/4CH2O = 1/2N2 +5/4HCO3

- +1/4H+ +1/2H2O (Eq.4-6)

The 15N -values usually varies between -20‰ and +30‰ in normal terrestrial systems (Fig 4-12). The main nitrogen source in atmospheric systems is close to 0‰ (N fixed into by plants and circulated into a soil by organisms), artificial nitrogen sources such as fertilizers manufactured from atmospheric nitrogen have slightly larger range of 15N -values (0 ± 3 ‰). Nitrogen sources originated higher in the food chain have highly enriched (from animal manure) 15N -values from 10 to 25‰.

Nitrogen in the shallow crust and of sedimentary origin has positive 15N values, averaging 6‰. The fractionation of nitrogen occurs in the metamorphism of sedimentary rocks due to the loss of ammonium during devolatilization, causing the enrichment of 15N values. Therefore the metamorphic rocks and granites (the main rock types in Olkiluoto) have generally positive 15N -values. The nitrogen of mantle origin extracted from geologically young MORB (Mid Ocean Ridge Basalts) (Marty and Zimmermann, 1999) or from old diamonds (from Archean to Mesozoic) gives a negative 15N value about -5‰ (Mohapatra and Murty, 2004), which seems to be independent of the age of mantle release.

41

Figure 4-12. The range for 15

N in different natural materials (modified after Clark and

Fritz, 1997).

The N2 results (Fig 4-13a) a have relatively coherent, slightly increasing trend with depth as an indication of the reliable data. The highest values have been measured from the saline waters. Local maxima exist in part of brackish-SO4 and -Cl samples between 100 and 400m depth. These higher values, seen also in Ar results may relate to colder climate recharge as indicated in the Figure 4-13b. The N2 correlates with decreasing 18Oresults thus supporting the concept of elevated atmospheric gas equilibration with recharging water during deglaciation.

Increased values in saline groundwater indicate excess nitrogen gas dissolution deep in the bedrock. Gascoyne (2005) examined the origin of nitrogen using information obtained from N2-Ar ratio. He noted wide variability (uncertainty) in gas results and ratios, which hampered the interpretation. However, it seemed probable based on majority of samples that N2 was generally slightly enriched compared to the N2-Ar ratio (37.7) for dissolved gases in water which has been equilibrated with air at 10 C. The ratios were mainly below the value of atmospheric ratio (83.5), which suggested that the excess of N2 may be caused by entrainment of air during recharge (Gascoyne 2005). Denitrification may also be added as a significant source for N2 during infiltration.

The current data and quality evaluation in this study clarify slightly the information that could be obtained from the N2-Ar ratio (Fig. 4-14). It seems evident that particularly in

42

saline groundwater with a N2-Ar ratio above atmospheric values, excess of N2 occurs which would suggest the presence of deep crustal source or degradation of organic substances.

Figure 4-13. Nitrogen abundances of different water types as a function of a) depth

(note the logarithmic scale for N2) and b) 18

O isotope results (the saline type samples has extracted from the figure b for clarification).

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Figure 4-14. Variation of N2 –Ar ratio with depth in dissolved gases at Olkiluoto in

comparison to values of the ratio in air and in water equilibrated with air at 10 C.

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Few 15N analyses of dissolved nitrogen in Olkiluoto groundwaters (Fig. 4-15a) have been made during the recent years. The range of them, from -1.7 to 1.5‰, is rather narrow possibly due to the small number of the 15N analyses. However, the results seem to show a trend from slightly positive values towards lighter negative values with increasing depth suggesting that thereare two other sources for nitrogen in addition to an atmospheric source. Nitrogen-15 is the most enriched (1.5‰) in brackish SO4-type waters above 300 meters, which indicates biogenic input in dissolved nitrogen gas. The negative signature in deeper groundwaters suggests input of nitrogen from a deep crustal source or derivation from the mantle. This interpretation is supported in the Figure 4-15b, where 15N of nitrogen is plotted against 13C of methane. Thermal abiogenic methane values correlate with the negative 15N values indicating a non-surficial source for the deep gases whereas biogenic input is supported for shallower samples. However, Hoefs (2004) has pointed out that nitrogen-rich and HC-rich gases in a common occurence can be genetically unrelated. Excess nitrogen in the deep saline groundwater system at Olkiluoto can therefore be, for instance mantle derived. The origin of this excess nitrogen is unclear at this point; however, active production can probably be excluded.

Fig 4-15.15

N from dissolved nitrogen gas as a function of a) depth and b) 13

C(CH4)in Olkiluoto groundwaters.

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5 SUMMARY AND CONCLUSIONS

Dissolved gases are an important factor in the groundwater system and they also constitute a notable mass of dissolved species at Olkiluoto. They have significance in controlling hydrogeochemical evolution and in evaluating palaeohydrogeology or recent groundwater flow and therefore in assessing the safety of a geological repository for spent nuclear fuel.

Chemically active gases like O2, H2S (with its anionic dissociation products HS- and S2-

), CO2 (HCO3- and CO3

2-), CH4 and H2 are fundamental species of redox couples controlling and/or buffering redox conditions sequentially with depth in groundwaters at Olkiluoto (cf. Fig. 3-4). Oxygen and H2S are primary safety issues due to their ability to increase copper corrosion, whereas methane as an energy source for bacteria may activate further SO4 reduction producing H2S, which is probably an active process in the interface between SO4–rich and CH4-rich brackish groundwaters (250 – 350 m) at Olkiluoto. Similar thermodynamic instability as between dissolved SO4 and CH4 also prevails between DIC and H2, which used by methanogenic bacteria in producing CH4.Potential production increases methane concentration, which exceeds CH4 saturation in groundwater, and which may in turn lead to a gas phase, as suggested by Gascoyne (2005), for the deepest parts of saline groundwater. This could evidently complicate groundwater flow in a fracture network and its analyses in the safety case.

Chemically inactive gases (noble gases and nitrogen) are comparable to other frequently used conservative tracers in evaluating a groundwater system (source waters, residence time and mixing) such as Cl, Br and stable isotopes of water. The amounts of radiogenic noble gases (He and Ar measured at Olkiluoto) as well as their isotopic compositions, reflect the amount of their source nuclides in bedrock which deviate from atmospheric values dissolved at recharge. Helium in particular is useful because its underground production and diffusion in groundwater is considerable, compared with the amount in water equilibrated with the atmosphere.

The data of dissolved gas samples (71 in total) examined in this report have been collected during 1997 to 2005 with the PAVE sampler to take groundwater samples at in-situ pressure. A summary of conclusions by interpreting the data, partly noted already before (e.g. Gascoyne 2005, Hatanpää et al. 2005, Pitkänen et al. 2004), and also by comparing with results reported previously, are as follows:

Most gas samples seem to be representative and the data are considered reliable (see App. 1). However, incomplete filling of the sample vessel causes fractionation of gases and distorts both composition and contents of gases, particularly of the light, less soluble gases (H2, He, N2) in CH4 –rich samples. A filling factor higher than 0.7 is recommended for the data to be used in interpretation. Examination of the N2 and Ar contents indicates that some contamination of back-filling gases in the sampler seems evident. Argon is particularly sensitive to contamination, because its contents are relatively small in groundwaters. The results of Ar are uncertain if nitrogen is not used as back-

46

filling gas in any sampling vessel. Recommended values are shown in Appendix 1.

Dissolved gas contents are notable in deep groundwaters at Olkiluoto, however, they correspond well with other lithologically similar sites in Shield areas. Hydrocarbons are typically high for sites with rock types of low oxygen fugacity during crystallisation, typically containing graphite and sulphides.

Varying amounts of O2 found in samples are probably caused by contamination during sample treatment. Reducing species and gases in groundwater and fracture minerals, carbon isotopic results of calcites, and geomicrobial results do not indicate O2 infiltration during recent geological history into the deep groundwater system.

The anomalous high H2 content (over 200ml/l) which was observed in earlier studies (e.g. Pitkänen et al. 2004) resulted probably from incomplete filling of sampling vessels. Significant amounts of hydrogen are unstable in depths with abundant carbonate (DIC is low in saline groundwater, but fracture calcites form a considerable precursor) when autotrophic methanogens and acetogens are present. Recent analyses and quality evaluation of gas samples indicate a much lower level (about few tens of mL/L) in the very deepest samples below 800 m depth though they are certainly enriched compared to upper depth levels. It seems that the role of H2/H2O redox couple will increase in these depths and may even dominate at greater depths. However, hydrogen contents show a large range in saline groundwaters indicating uncertainties in quantitative results due to the susceptibility of hydrogen to be disturbed during sampling.

Methane, the most abundant gas in saline groundwater at Olkiluoto seems to have two primary sources: thermal abiogenic hydrocarbons dominate in the deepest parts of sampled groundwaters whereas the fraction of bacterial methane increases steadily with decreasing depth and CH4 content. In SO4-rich groundwater above 300 m depth only traces of hydrocarbons are observed, which corresponds well with the theory of instability between dissolved CH4 and SO4 in a common system when, for instance SRB are present.

Carbonate reduction is the most probable primary process for bacterial methane. Hydrogen required in the process seems to increase below 800 m at Olkiluoto suggesting, however, that the main CH4 production zone is at the deepest parts of current investigations, although they are dominated by abiogenic CH4 or deeper, where CH4 is also close to saturation level as has been noted before.

Abundant hydrogen may be metabolised by methanogens producing CH4

saturated system and degasing excess CH4 which may migrate upwards and dissolve in depths where CH4 is clearly undersaturated (above 800m). Migration of CH4 is in addition to solubility calculations suggested by isotopic and molecular ratios of hydrocarbons. Bacterial CH4 production seems also evident in the upper part but it is probably curtailed due to trace contents of H2.

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Dissolved He contents closely support previous interpretations of the relative residence times and the concept of mixing of groundwater samples and types at Olkiluoto. Helium and its isotopes could have particular characteristics to give detailed information of hydrogeological conditions (recharge, discharge, flow direction, mixing, residence time) due to the fairly large concentration range of He and preferential crustal origin of 4He. It is worth to consider more detailed He isotopic studies at the site.

The slight enrichment of dissolved Ar and N2 support the status of glacial melt in decreasing 18O and 2H compositions in the isotopically most depleted brackish Cl-type groundwaters.

Extraordinary analogy is observable in relations between 2H - 18O, 18O - Cl and 13C(CH4) - CH4 parameter couples in groundwater samples below the SO4-richgroundwater layer (beneath 300 m depth). Each relation shows very steady trends from the deepest samples up to samples in which have been observed clear influence of glacial or younger waters (sampled above 300m) and also a drastic change in these steady trends. This may be more an indication of lack of disturbance of this deep groundwater system during glacial and postglacial times. On the other hand the dilution of saline and brackish Cl-type groundwaters above 800 m seems to have been faster than the increase of methane caused by migration and bacterial production. However, the dilution in saline groundwaters seems to be an older process than the last deglaciation and melt water infiltration, indicating that CH4 enrichment is a very slow process.

Gas phase formation has, however, substantial importance for safety and it is therefore essential to obtain more data of hydrogen, methane, higher HC, DIC, fracture calcites (all with isotopic compositions, also deuterium analysis from higher HC) and microbes from the deep groundwater system in order to evaluate CH4 production and potential gas phase ebullition. Sampling and isotopic measurements should be particularly performed in the most saline groundwater samples. Corresponding, further studies should be directed to the interface between methanic and sulphidic systems in order to evaluate potential SO4

reduction.

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49

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APPENDICES

APPENDIX 1. Gas data utilized in this study collected from Olkiluoto groundwaters. Uncertain result are marked with italics digits, results below detection limit with “<” and discarded samples with grey colour.

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APPENDIX 1 (1/3)

56APPENDIX 1 (2/3)

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APPENDIX 1 (3/3)