Working Report 2002·21

Methods of sampling and analysis of dissolved gases in deep groundwaters

Mel Gascoyne

May 2002

POSIVA OY

T6616nkatu 4, FIN-00100 HELSINKI, FINLAND

Tel. +358-9-2280 30

Fax +358-9-2280 3719

Margit Snellman, POSIVAOY, Toolonkatu 4, FIN -001 00 Helsinki, Finland. Fax: 358-9-2280-3719

SUBMISSION OF REPORT

P.O. Box 141 6 Tupper Place

Pinawa, MB ROE 1 LO Canada

Phone 1-204-753-8879 Fax 1-204-753-2292

e-mail: gascoyne@granite.mb.ca

April 15, 2002

For Review on Methods for Sampling and Analysis of Dissolved Gases (P.O. Number 9677/01/MVS):

Dear Margit,

Please find enclosed the final copy of the report defined above. The report has been reviewed and approved according to the requirements of my company, Gascoyne GeoProjects Inc. and meets all quality assurance requirements of Posiva.

Yours sincerely,

·' ' . .,, i "·./, . • ._t,i " I ;,. · t i ,;t,..,.. "''·-~re...._-<

t

M. Gascoyne (President and CEO~ Gascoyne GeoProjects Inc.)

Working Report 2002-21

Methods of sampling and analysis of dissolved gases in deep groundwaters

Mel Gascoyne

Gascoyne GeoProjects Inc.

Pinavva, Manitoba, Canada

May 2002

Working Reports contain information on work in progress

or pending completion.

The conclusions and viewpoints presented in the report

are those of author(s) and do not necessarily

coincide with those of Posiva.

METHODS OF SAMPLING AND ANALYSIS OF DISSOLVED GASES IN DEEP GROUNDWATERS

ABSTRACT

Methods of sampling groundwaters for determination of dissolved gas content have been reviewed and examined for potential application to the Olkiluoto site, Finland, where high concentrations of C~ and H2 have been found in deep saline groundwaters. Problems of sampling include gas phase separation and possible gas loss or fractionation during the period that the groundwater is being pumped to the surface, caused by reduction in hydrostatic pressure. These problems were experienced by POSIV A during early attempts at dissolved gas analysis at Olkiluoto.

Several organizations, including AECL and the University of Waterloo (Canada), NAGRA, POSIV A, NIREX and the oil industry, have developed down-borehole groundwater samplers for dissolved gas analysis that maintain in-situ pressure on the sample during recovery. Examples of these methods are given. The results of sampling at the surface in the Canadian program are compared with those from down-borehole and from an underground facility. They show that where more than one method has been used, surface sampling gives good precision and accuracy, provided that back­pressure on the groundwaters is applied by restricting flow from the dissolved gas sample vessel during pumping.

A procedure for improving the speed and accuracy of dissolved gas sampling at Olkiluoto is proposed and involves replicate sampling of groundwater at the surface, periodic down-borehole sampling (using a simplified, non-electrical sampling system) and performing on-site gas extraction, and measurement of total gas concentration and composition, using a rugged quadrupole mass spectrometer in a mobile laboratory equipped with a stable electrical power supply.

Keywords: dissolved gases, noble gases, groundwater chemistry, groundwater sampling, salinity, isotopes

SYVIEN POHJAVESIEN LIUENNEIDEN KAASUJEN NAYTTEENOTTO JA ANAL YSOINTI

TIIVISTELMA

Tassa raportissa kasitellaan potentiaalisia liuenneiden kaasujen naytteenottomenetelmia kaasumaaran selvittamiseksi Suomesta Olkiluodon alueelta, jonka syvissa suolaisissa pohjavesissa on havaittu korkeita pitoisuuksia metaania ja vetya. Vesinaytteen pump­paaminen maan pinnalle alentaa veden hydrostaattista painetta, joka aiheuttaa kaasujen erottumista ( evakuoituminen), mahdollisesti kaasujen karkaamista tai fraktioitumista kaasunaytteenoton aikana. Kaasunaytteen oton ongelmat on havaittu Olkiluodossa aikaisemmin suoritettujen kaasunaytteenottojen aikana.

Useat organisaatiot, mm. AECL, Waterloon yliopisto (Kanada), NAGRA, POSIV A, NIREX ja oljyteollisuus, ovat kehittaneet kaasunaytteenottolaitteistoja, joilla on mah­dollista ottaa kaasunaytteita syvista kairanrei 'ista siten, etta naytteen paine sailyy kairanreiassa vallitsevassa paineessa. Raportissa esitellaan esimerkin omaisesti kaytossa olevat naytteenottomenetelmat. Kanadalaisten maanpaalta suorittamien kaasunaytteen­ottojen tuloksia verrataan kairanrei'ista ja maan alaisista tutkimustiloista suoritettujen kaasunaytteenottojen tuloksiin. Tuloksista havaitaan, etta maan paallisilla kaasunayt­teenotoilla saavutetaan hyva toistettavuus ja tarkkuus, kunhan pohjavedelle aiheutetaan pumppauksen aikana vastapaine rajoittamalla virtausta liuenneiden kaasujen kerays­sailiosta.

Olkiluodossa suoritettavien kaasunaytteenottojen nopeuden ja tarkkuuden paranta­miseksi esitetaan rinnakkaisten kaasunaytteenottojen suorittamista maan paalle pumpa­tusta vedesta ja jaksottaisia naytteenottokampanjoita kairanrei'ista (kayttamalla yksin­kertaistettuja, ilman sahkoa toimivia naytteenottomenetelmia). Lisaksi ehdotetaan kentalla tapahtuvaa kaasujen erottamista vedesta seka kaasumaaran ja koostumuksen analysointia valittomasti kenttalaboratoriossa kayttaen karkeaa kvadrupolimassa­spektrometria.

Avainsanat: liuenneet kaasut, jalokaasut, pohjavesikemia, naytteenotto, suolaisuus, kaasut

TABLE OF CONTENTS

ABSTRACT

TIIVISTELMA

1

Page

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

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

2 PROBLEMS IN DISSOLVED GAS SAMPLING ................................................ 7

2.1 Gas phase formation ............................................................................. 7 2.2 Gas solubility and fractionation ............................................................. 8 2.3 Previous sampling at Olkiluoto .............................................................. 9 2.4 Extraction, transfer and analysis of gases ........................................... 13

3 METHODS OF DISSOLVED GAS SAMPLING AND ANALYSIS .................... 15

3.1 Geological Survey of Canada .............................................................. 15 3.2 AECL .................................................................................................. 15

3.2.1 Sampling at the surface ........................................................... 17 3.2.2 Sampling down-borehole ......................................................... 18 3.2.3 Sampling from underground boreholes .................................... 20 3.2.4 Gas analysis ............................................................................ 20

3.3 University of Waterloo, Ontario .......................................................... 20 3.4 POSIVA .............................................................................................. 22

3.4.1 Early methods .......................................................................... 22 3.4.2 Recent methods ...................................................................... 23

3.5 European Community ......................................................................... 24 3.5.1 Mont Terri (Switzerland) ........................................................... 24 3.5.2 Mol (Belgium) .......................................................................... 24 3.5.3 Sellafield (UK) .......................................................................... 25 3.5.4 Stripa (Sweden) ....................................................................... 28 3.5.5 Aspo (Sweden) ........................................................................ 29

3.6 Other studies ...................................................................................... 29 3.6.1 Finland ..................................................................................... 29 3.6.2 The Westbay System ............................................................... 30 3.6.3 Oil industry ............................................................................... 32

4 CANADIAN EXPERIENCES AND DATA ........................................................ 35

4.1 Borehole completion systems ............................................................. 35 4.2 Sampling at the surface ...................................................................... 36 4.3 Down-borehole sampling .................................................................... 38 4.4 Underground sampling ........................................................................ 38 4.5 Discussion and summary .................................................................... 38

2

5 FEASIBILITY STUDY FROM SAMPLING AT OLKILUOTO ........................... 43

5.1 Methods for rapid sampling ................................................................. 43 5.2 Methods for on-site analysis ................................................................ 44 5.3 Methods development. ........................................................................ 45

6 RECOMMENDATIONS ................................................................................... 49

6.1 Rapid sampling and on-site analysis ................................................... 49 6.2 Analysis of limitations and potential errors .......................................... 50

REFERENCES ............................................................................................... 53

3

PREFACE

This study is part of the research program for disposal of spent nuclear fuel 1n crystalline bedrock in Finland.

The assistance of numerous researchers including Adrian Bath, George Darling, Lise Griffault, Brian Hitchon, Ian Hutcheon, Andreas Gautschi and Margit Snellman, is gratefully appreciated.

The work has been funded by Posiva Oy and I am grateful for the support of Margit Snellman in this study.

5

1 INTRODUCTION

The sampling and analysis of dissolved gases in groundwaters from bedrock boreholes at the Olkiluoto site, Eurajoki, has been performed as part of the site characterization activities of Posiva's nuclear fuel waste management program since 1991. Several methods have been used over the last 10 years to sample dissolved gases in groundwaters at depths of up to 1000 m. Early work (1991-1995) was based on sampling ground water from boreholes KR1-KR5 using surface-based methods. This work has been described by Lampen & Snellman (1993), Snellman et al. (1995), Ruotsalainen and Snellman (1996), and Pitkanen et al. (1994, 1996). Preliminary results of a more recent sampling, ofboreholes KR3-KR10, using the PAVE downhole geochemical sampler, have been reported by Pitkanen et al. (1999). Recently, Gascoyne (2000) has reviewed these various results in an attempt to determine the accuracy of the results and how they might impact on the long-term safety of an underground disposal facility.

This report extends the work described in Gascoyne (2000) and examines the problems in dissolved gas sampling, reviews current methods used in other programs and industry for dissolved gas sampling, analyses the problems associated with dissolved gas sampling at the surface, presents options for dissolved gas sampling at Olkiluoto and makes recommendations for future work to determine baseline conditions for dissolved gas concentrations at Olkiluoto.

7

2 PROBLEMS IN DISSOLVED GAS SAMPLING

2.1 Gas phase formation

Groundwaters that circulate to considerable depths tend to accumulate gases by dissolution of air entrained in bubbles during recharge ('excess air') and by dissolution of gases produced in the bedrock by radioactive decay (He, Ar, Rn), crusta! degassing (He, N2, Clit, H2) and thermogenic or biogenic decomposition (Clit, H2S). The amounts of these gases may be such that very high hydrostatic pressures may be needed to retain all gases in solution. If gas accumulation goes on for a prolonged time, then, at a stable hydrostatic pressure, gas phases could form at depth and these would tend to migrate rapidly upward, as microbubbles, mainly through fractures and fault zones.

Deep groundwaters brought to shallow depths or to the surface during sampling usually have dissolved gas contents higher than the solubility limits at one atmosphere pressure and will release the excess gas as they return to atmospheric pressure. The groundwaters will outgas in amounts according to the individual solubility of the gas and could become separated from the volumes of water that hosted them.

It has been a common experience in the Canadian program, when pumping groundwater from depths below ~ 100 m in boreholes in the Canadian Shield, to find a gas phase forming in the pump outlet tube at the surface. This is usually seen as spluttering of water discharging from the tube outlet or, as bubbles issuing from the inlet to a sealed (transparent) flow cell used for pH-Eh measurement. In these cases, the groundwater has traveled a considerable distance in the tube (because of the separation between the downhole pump and the surface) and gases have had the opportunity to coalesce to form discontinuous elongated bubbles of gas in the tube, separated by larger volumes of partially outgassed groundwater. When groundwater samples are taken close to the pump or, more usually, near to the fully pressurized borehole zone (as in the case of sampling from a borehole collared in an underground facility) exsolving gases form immediately as microbubbles and turn the groundwater an opaque, milky-white colour. As flow from the borehole zone continues, this colour disappears and larger gas bubbles are observed because the zone has been partly or fully depressurized and microbubbles are coalescing in the zone itself, before discharging through the tube.

Sampling from an underground environment makes it easier to obtain good quality dissolved gas samples because the high hydrostatic head obviates the need for pumping. In addition, outgassing is reduced because of the short distance to the pressurized zone and back-pressure can be easily applied by only partially opening the sampling valve. However, if the zone has low transmissivity, sampling will cause the zone pressure to decrease rapidly causing outgassing within the zone. If only a small groundwater sample is required, or if the zone is more transmissive so that only a slight reduction in pressure occurs during sampling, then partial opening of the sample valve will maintain some pressure and allow representative groundwater samples to be obtained.

Much of the early data from the Canadian and Finnish programs on dissolved gas sampling in groundwater were based on sampling at the surface. Typically, sample vessels were of low capacity (10-50 mL) and so gas phase formation in the sample

8

delivery tube or borehole zone could lead to a situation where gases included in the vessel would not be derived solely from the water in the vessel and thus dissolved gas concentrations would be too high. Conversely, loss of gas from the vessel during filling (as is likely to occur when the in-line vessel is stood vertically upwards and tapped to release air bubbles adhering to the vessel walls) will lead to the situation whereby the water in the vessel has lost more gas than is present in the vessel and so dissolved gas concentrations are too low. To some extent, correction for this phase separation can be made if one of the gases is known to be at saturation for the ambient pressure. However, this is unlikely in the case of dissolved gases typically found in a fractured crystalline rock since no one gas usually reaches these levels.

Modifications to the sampling method can minimize the problem of phase separation by either

1) Applying a restriction to the sample line so a back-pressure is developed, usually by partly closing the vessel outlet valve, thus inducing gases to remain in solution. However, this method reduces groundwater pumping rate and may cause leaks on the pump outlet line or rupture of the pump bladder although, generally, only a partial pressure need be exerted as most gases are not at saturation in the zone being sampled), and

2) Collection of a larger volume of sample; this tends to reduce the importance of excesses or deficiencies of gas as the differences are averaged over a larger volume of groundwater.

These approaches are considered in more detail in Section 4 of this report.

2.2 Gas solubility and fractionation

The relationship between the amount of gas dissolved and the hydrostatic pressure of the groundwater is described by Henry's Law:

n = p/k (1)

where n is the number of moles of a gas dissolved in one mole of water at partial pressure, p, and k is the Henry's Law constant (atm-1

) at that temperature.

Since most gases obey Henry's Law, especially if they are not very soluble, the maximum amount of gas that may be dissolved in a groundwater is directly proportional to the pressure. For example, the solubility of pure Ar in water is 56 mL/L. At a depth of 100 m (i.e. hydrostatic pressure of - 10 atmospheres or 1 MPa) the solubility increases to 560 mL/L if Ar can be assumed to behave as an ideal gas under these conditions.

When gases dissolve from a mixture, the solubility of each gas is proportional to its partial pressure and Henry's Law applies to each gas independent of the partial pressure of the other gases. However, at high pressures and high concentrations of other, more abundant gases, a 'salting out' effect can occur, thereby reducing the solubility of the

9

lower abundance gases. An example might be the reduced solubility of He in the presence of large amounts of Cllt. Also, it has been shown that gases are typically less soluble in saline water than freshwater (Hass 1978, see also summary in Gascoyne 2000). Degassing of the ground water results in depletion of the lighter (less soluble) gases because they tend to diffuse faster into the forming bubbles. The resulting fractionation of the gases can be approximated by a Rayleigh-type distillation equation.

Gases fractionate from each other as they exsolve because the lighter gases such as H2 and He tend to fractionate more than the heavier gases such as N2 and Ar. Therefore, a pattern of lower gas ratios (e.g. He/N2) should be seen for groundwaters that have undergone depressurization and gas loss. Evidence for the preferential loss of the lighter gases has been clearly seen by the low He/N2 ratios for samples collected at the surface in glass vessels compared with those collected at depth in Olkiluoto groundwater using the PAVE sampler (Figure 1, Gascoyne 2000). This information, plus the order-of-magnitude lower total gas concentrations renders the data for the surface-collected samples of minimal use.

2.3 Previous sampling at Olkiluoto

Groundwater sampling at the surface for dissolved gas analysis in the early part of Posiva's program was performed using I) Al-laminated bags (~ 150 mL capacity) for gas phase collection only and 2) glass vessels ( ~ 150 mL) fitted with ground glass taps. Larger sizes of the Al-bag were tested but the 150 mL size was believed to be optimal. Samples were taken in triplicate but the main problems with the Al-bags were that the seams were not always reliable (they were hand-made using a hot iron) and the rubber tube outlet connection from the bag was not always tight and so a number of samples were lost because of this. This was one of the main reasons why Posiva changed over to using glass vessels. A third problem was that only gas was collected into the bags during the overnight sampling and so and it was not possible to get absolute gas concentrations as no data were collected on how much water had passed through the line during sampling.

Using the glass vessel system, total gas concentrations of 10- 50 mL/L were obtained for groundwater samples from depths as much as 880 m. Subsequent use of the PAVE downhole system (Figure 2) sampled a larger volume ( ~260 mL) and gave an order of magnitude more gas (100- 2000 mL/L). The downhole samples were believed to be more accurate because 1) the PAVE samples were sealed and maintained at in situ pressure until analysis, whereas the glass samples stored the groundwater at atmospheric pressure, and 2) the glass samples would have experienced degassing in the pumping line to the surface and thus the collected groundwaters would have been stripped of much of their gas content. The difference in performance of the two gas sampling methods is shown in Figure 3 (from Gascoyne 2000). As a result of PAVE sampling, Olkiluoto groundwaters were recognized as being gas-rich, typically containing over 100 mL/L of total dissolved gas.

N z -G)

::1:

10

0.40

• 0.30

• Glass •PAVE

• 0.20 • •

• 0.10 • • • • • •

0.00 0 200 400 600 800 1000 1200

Depth (m)

Figure 1. Variation of He!N2 ratio for surface-collected (in glass vessels) and downhole (PAVE sampled) groundwaters showing how outgassing of the groundwaters results in loss of the lighter gases (e.g. He relative to N2), (from Gascoyne 2000).

Examination of the PAVE data, however, revealed that the volume of sampled water by PAVE varied somewhat (Gascoyne 2000). Although over 200 mL of water was obtained in most cases, two samples with high gas volumes, recovered less than 50 mL of water. The volume of water sampled by PAVE should be constant and equal to the internal volume of the sampler. It was argued that the anomalously low water volumes in the two deep samples was due to either 1) a limitation on the amount of water that can enter the sampler because of a gas phase or 2) the back-pressure of the inert filling gas, coupled with the low pumping rate used, was causing inadequate filling of the sampler. Leakage of the PAVE sampler was ruled out as the system was carefully checked each time during use and no evidence of leaking was found. The former explanation would indicate that there must be a gas phase existing at those depths and that the borehole was possibly venting gas. An additional problem was recognized in this case because the volumetric calculations of gas concentration (mL/L water) would not be accurate (even for PAVE samples) as the gases would have already fractionated in the borehole zone according to their relative solubilities.

Additional problems were experienced in both the glass and PAVE samplings, as shown by the presence of 02 in most gas samples. These concentrations were attributed to

11

contamination from the atmosphere as a result of not adequately purging the sample vessel before filling with groundwater (Gascoyne 2000) or, possibly, to Laboratory operations during analysis. The PAVE samples showed 0 2 to be present in all samples (70 to 35,000 J..LLIL) although, in most cases, the 0 2 content was low enough to be of little consequence.

.l

I I , I

-:nr1

i' _,

~

:i

P~essure

/<Jive

l •• :pJ;t.1~

GdC'<.@

lone to be ::.ampied

packec

Figure 2. The PAVE groundwater sampler (from Ruotsalainen et al. 1996).

If there has been no gas fractionation, the gas data may be corrected for air contamination (principally the N2 and Ar content) using the standard abundances of 0 2, N2 and Ar in air.

High N2 concentrations (up to 480 mL/L in borehole OL-KR4) were observed in a number of samples taken by PAVE. Resampling of this zone gave only 167 mL/L while maintaining the concentrations of other gases (H2, He and C~). Because a deeper zone (1030 m in OL-KR2) did not show high N2 concentrations it appeared that, pending more data from other deep zones, the high N2 concentration of OL-KR4 was a local feature or was due to contamination in some way.

10000

1 0 0 0

:J ::J -.s en CV --Cl 100 0 --Gl E ::J 0 > ... -L'" ...

•ft ...... ........ ... ... ... ... 1 0

0 200

---

400

12

- -.. - -

600

Depth (m)

--.A Glass

•PAVE

800 1000 1200

Figure 3. Relationship between volume of dissolved gas and depth of permeable zone for glass- and PAVE-sampled groundwaters (from Gascoyne 2000).

The principal observation in the dissolved gas sampling work performed at Olkiluoto was the high concentrations of C~ and H2 found in the most saline groundwaters. Significant amounts of the gases, particularly C~, were also found in some of the less saline (and shallower) groundwaters. In addition, an inverse relationship between C~ and concentration of S04 ion was observed and it was argued that bacterial reduction (and, therefore, removal of S04) controlled the concentration of each species. The lack of coexistence of S04 and C~ was clearly shown (Figure 4) where, below about 300 m, so4 concentration sharply decreased while c~ concentration strongly increased.

1200

1000

800

c 0 ;;

C'G """ .... c 600 G) (.) c • • 0 0 • •

400 • •

200 •• • •

0

0 200

•

• •

400

13

D El ~

D D

600

Depth (m)

D

D

+S04 (mg/L) DCH4 (ml/L)

D

800 1000 1200

Figure 4. Diagram showing the high concentrations of CH4 in deep groundwaters at Olkiluoto and the genera/lack of coexistence ofCH4 and S04 in groundwaters over the sampling depth range (from Gascoyne 2000 ).

2.4 Extraction, transfer and analysis of gases

Once a groundwater sample has been obtained in a sealed vessel (crimped Cu tube, steel 'bomb', or glass bulb), it is important to be able to transfer this water and its dissolved gas load quantitatively to a system that will separate the gas phase for analysis. This is usually done by clamping the vessel onto an evacuated gas-extraction rack which consists of a receiving glass bulb in which the groundwater can be degassed under vacuum, a series of cold traps to remove water vapour, and a pumping device (manual or electrical) that can concentrate the gas without loss or fractionation into a vessel that can be detached and taken to a gas chromatograph or mass spectrometer for gas analysis.

14

Problems that arise in this procedure include prior leaking of the groundwater sampling vessels because they contain groundwater and gas under pressure, inadequate degassing of the groundwater, incomplete transfer (with or without fractionation) of the gases through the cold traps, and poor inclusion of gas in the final gas vessel before analysis. These problems can be minimized by leak-testing the groundwater and gas vessels (e.g. pressurizing the vessel and looking for leaks when submerged in water), careful observation of pressure and vacuum gauges during gas transfer to prevent loss, and use of a transfer system that maximizes the ratio of gas vessel to transfer-tube volumes. The latter is particularly important in the final stage of gas extraction, when the gas is displaced into the tubing and pressure (= yield) monitoring devices that are also connected to the gas vessel. If this tubing has a relatively low volume then inadequate gas recovery or fractionation between gases should be negligible.

Additional problems in gas transfer and analysis can occur if gas volumes are too large (pressure gauges or yield measurement devices may be off-scale) or too small (contaminant gases and fractionation of gases may become significant). It may be necessary, therefore, in the case of unknown dissolved gas quantities, either to take two separate groundwater samples and use one for trial extraction or to add a section of line at some point on the transfer rack which allows splits of gas to be taken if volumes are too high. Further difficulties can arise, especially in the analysis for hydrocarbons such as CJL, C2a,, etc. if there is a machine blank or memory effect due to previous analysis of organic gases (e.g. acetone, benzene) because these gases are 'sticky' and dissolve in stop-cock grease or attack rubber seals.

An example of the gas transfer and analysis system used successfully for many years in the Canadian program is briefly described in section 3 .2.4.

15

3 METHODS OF DISSOLVED GAS SAMPLING AND ANALYSIS

A number of organizations involved in site screening and characterization work for nuclear waste disposal are attempting to determine dissolved gas concentrations in groundwaters in saturated fractured rock. The include AECL (Atomic Energy of Canada Limited), AND RA (France), NAGRA (Switzerland), NIREX (U.K), POSIV A (Finland) and the USDOE (in the saturated zone of Yucca Mountain tuff, Nevada). In support of these studies, scientists from several universities have made significant contributions in sampling and characterizing dissolved gases in mine waters in Shield areas (Professors S.K. Frape and P. Fritz, University of Waterloo, Canada) and in understanding of gas sources, evolution, and paleoclimatic significance, etc., (Professor J. N. Andrews, University ofBath, U.K.).

In addition, several other types of organization have interest in dissolved gases in deep fractured rock systems. They include oil companies (for hydrocarbons) and mining companies (for trace gases as indicators of ore deposits) and various research institutions studying geothermal sites, volcanic processes, deep-ocean gas contents, etc. The sampling and analytical methods used by various groups are summarized below and the types of sampler, materials used for sampling gases in groundwater are summarized in Table 1.

3.1 Geological Survey of Canada

As part of an early study of the usefulness of dissolved gases in groundwaters for mineral exploration, the Geological Survey of Canada (GSC) investigated commercially available downhole groundwater samplers but found that none of them were narrow enough to fit into slim diamond-drilled exploration boreholes. The GSC then designed and constructed a sampler and portable winch that could take water samplers from any desired depth in this type of borehole (Dyck et al. 1976). The sampler is illustrated in Figure 5.

The sampler was made of stainless steel and holds about 3 50 mL of water when full. A wire cable runs through the sampler tube and is fastened to the base. Initially, the spring-loaded jaws at the top are engaged in smooth holes so that, on lowering down a borehole, the sampler is open at top and bottom and water can pass freely through the tube. At the desired depth a 'messenger' (tripping weight) is sent down the cable to spread the jaws and release the tube onto a rubber stopper at the base. On retrival, the top of the tube is open but no significant mixing with shallower borehole waters was believed to occur. At the surface, the sample was poured into a sealed vessel to await analysis.

3.2 AECL

Three procedures have been used by AECL to sample dissolved gases in fractured crystalline rock of the Canadian Shield to depths of up to 1000 m. They include:

Table 1. Summary of sampling systems, materials and containers that have been used in sampling dissolved gases in groundwaters in fractured rock.

Material Container User Requirements Type of Seal Volume Reference Stainless steel 'bomb' type AECL, NIREX Open or cased Valve at each 50-100 mL Bottomley et al.

borehole end (1984), Ross & Gascoyne (1995), Bath et al. ( 1996)

Copper (soft) tube AECL Open or cased Crimp at each ~lOml Bottomley et al. borehole end (1984)

Aluminium flow-through POSIVA Open or cased Heat seam ~150 mL Lampen & Snellman cell/laminated bag borehole (1993)

Glass vessel with valves Open or cased Ground glass ~150 mL Lampen & Snellman borehole valves (1993)

Stainless steel PAVE sampler Open borehole Electronic ~250 mL Ruotsalainen et al. valves (1996)

Lead-glass vessel Open or cased High vacuum 50mL Sano et al. (1987) borehole stopcocks at

each end Stainless steel Westbay MOSDAX® W estbay-cased Electronic 1-2 L Westbay

borehole valves Instruments ( 1994) Stainless steel Waterloo sampler Open or cased Electrical 200mL Sherwood Lollar et

borehole valves al. (1994) _

1--'

0\

17

3.2.1 Sampling at the surface

Sampling is performed in duplicate or triplicate in stainless steel vessels from ground water pumped to the surface from ~ 100 m down the borehole. In this procedure, two or more vessels are connected in series to the water delivery tube, as close as possible to the borehole. Groundwater is used to purge the vessels of atmospheric gases for several minutes by allowing water to flow through the vessels when stood vertical and the walls of the vessels are tapped to displace bubbles of gas that accumulate on the walls. If the groundwater flow-rate is sufficient, the final valve at the outlet of the top vessel is held partly closed to maintain some back­pressure in the vessels and reduce outgassing and consequent gas loss. This is often problematic, however, if the downhole pump is a bladder or 'squeeze' pump because groundwater comes to the surface in pulses of typical duration 30-60 s, followed by a hiatus in water delivery of the same or greater duration. Therefore, applying a back­pressure can only work for the duration of the water pulse. Also, during the hiatus, the lack of pressure on the groundwater in tubing above the pump has chance to outgas at the reduced hydrostatic head.

I I ~ ~ I i

small diameter·

well

Figure 5. Schematic diagram showing the GSC dissolved gas sampler (from Dyck et al. 1976).

18

A further problem in surface sampling is the likelihood that the groundwater will pass through several tens of meters of tubing that is rolled onto a storage drum at the surface before discharge at the outlet (this roll allows the pump to be lowered to greater depths if the watertable is deep or if there is significant drawdown due to low permeability of the zone). The delay in discharge at the tube outlet results in the ground water warming up (in summer months) and out gassing even more than due to pressure change alone. To avoid this problem, the water delivery tubing used in sampling must be only as long as is necessary to lower the pump to the required depth in the borehole.

3.2.2 Sampling down-borehole

In early studies of the dissolved gas content of groundwaters in boreholes on the Canadian Shield, Bottomley et al. (1984) used soft copper tubes installed inside a large geochemical probe system that was lowered down the borehole. Groundwater was pumped through the tubes by a downhole bladder pump and then a positive pressure was applied to the groundwater delivery line at the surface to close a check valve on the pump intake and so preserve the in situ pressure while the entire probe was winched to the surface. The copper tubes were then crimped to isolate the sample. This method proved to be cumbersome and time-consuming because of the size of the geochemical probe, and in subsequent work, a more light-weight, mobile system was used. This method is described below in more detail.

In the current AECL program, to sample ground water in situ ( downhole ), two or more 50 mL stainless steel sampling vessels, each fitted with valves at the ends, are connected together using stainless steel Swagelok® fittings. The arrangement is shown in Figure 6. The vessels are then connected to a %-inch ( ~ 10 mm) thick-wall nylon tube of sufficient length to reach the permeable zones to be sampled. At the bottom of the sample vessel, a check valve is fitted to aid in sample retention and integrity and this is connected via a perforated stainless steel tube (for sample entry to the vessels) to a weight that assists in lowering the system to the depth required. The weight is typically at least 1 kg; the amount required is determined usually by trial and error, to offset the buoyancy of the sample string when initially full of gas but is not too large that removal from the borehole is difficult when the string is full of water.

In operation, the sample string is first purged and pressurized at the surface by N2 gas from a standard size cylinder fitted with a high-pressure regulator. The pressure applied is equal to the expected zone hydrostatic pressure plus ~ 10% (more, if the system is in saline groundwater). It is checked for leaks and lowered down the borehole to the required depth, while maintaining that pressure. The gas pressure is then released at the regulator and groundwater allowed to fill the sample string through the perforated stainless steel tube at the bottom. When gas ceases to come out of the release valve, the string is full of water (to the approximate level of the surficial water table) and the N2 pressure is re-applied to close the check valve and prevent sample loss and degassing. The string is removed from the borehole (a winch may be needed for this) and the valves turned by hand to isolate each of the sample vessels individually. The N2 pressure is released and the vessels disconnected for analysis.

S.S. SWAGELOK FITTINGS

S.S. SWAGELOK FITTINGS

S.S. SWAGELOK FITTINGS

DOWN HOLE DISSOLVED GAS SAMPLINGASSEMBLY

19

U 3/8 INCH DI.Allv1ETER NYLON TUBING ~ (SYNFLEX, GROUP 2/N 2000 PSI RATING)

~':/4 INCH NUPRO MINI S.S BALL VALVE (HANDLE CUT OFF TOALLOWPASSAGE INCASING)

~50 ML S.S. PRESSURE VESSEL

-~~r:::::r> './..INCH NUPRO S.S. BALL VALVE

<---------50 ML S.S. PRESSURE VESSEL

':/4 INCH NUPRO S.S.

':/4 INCH NUPRO CHECK VALVE (10 PSI)

'./..INCH S.S. TUBING, 5 INCHES LONG (PERFORATED WITH 118 INCH HOLES)

WEIGHT (SIZED FOR DESIRED SAMPLING DEPTH)

Figure 6. Schematic diagram showing the Canadian down-borehole dissolved gas sampling system.

The sampling string is best lowered immediately after the borehole zone has been pumped for groundwater sampling so that fresh groundwater is obtained (the pressure of the overlying water column will not allow that groundwater to degas). Ideally, the string should be enclosed by packers so that the permeable zone is completely isolated from the rest of the borehole and a membrane pump inserted in the string to ensure that fresh groundwater is constantly entering the interval between the packers. However, this increases the complexity and weight of the system. Provided that the zone being sampled is reasonably permeable (typically allowing a pumping rate of > 100 mL/min) and sufficiently removed from other permeable zones, the sample collected should be representative of the zone. Alternately the zone can be hydraulically stressed by installing a separate pump near the surface to draw water from the open borehole. However, if the borehole is well-fractured and contains freshwater overlying saline water, then the density effect will severely limit flow from the deeper zones.

20

3.2.3 Sampling from underground boreholes

As described in Section 2.1, this method of sampling dissolved gases may give more representative samples that those taken at the surface because the length of tubing over which gases may exsolve is short (typically 1-2 m) and a higher back-pressure can be exerted (up to the level of the ambient hydrostatic head) to keep gases in solution. Also, the pressure is continuous (not cyclic as in bladder pumped samples). The sampling procedure is similar to that used at the surface, however, and includes flushing the cylinders to remove atmospheric gases and maintaining a high back­pressure during sampling.

3.2.4 Gas analysis

In the Canadian program, gases are extracted from groundwater on a glass-tubing 'rack' on which are fixed a degassing vessel, cold traps, a mercury Toepler pump, pressure/vacuum gauges and a manometer for measurement of total gas concentration. The system is evacuated using a rotary forepump backing onto a mercury diffusion pump. Dissolved gases are stripped out of the water sample under vacuum, dried by one or more dry-ice/acetone cold traps and, by successively raising and lowering of mercury in a glass vessel fitted with one-way ground glass taps (the Toepler pump), are displaced into a section of the line containing a manometer (for yield measurement) and a small, valved gas vessel. This vessel is detached from the line and fitted to the inlet of a gas-source mass spectrometer for analysis of all gases.

3.3 University of Waterloo, Ontario

Several members of the Department of Earth Sciences at the University of Waterloo, Ontario, were involved in geoscience research for the Canadian program at an early stage (the late 1970's) and sampled dissolved gases in groundwaters issuing from boreholes in mines on the Canadian Shield. In the initial work (Fritz et al. 1987), gases were collected only from free-flowing boreholes in mines, using two methods: 1) for boreholes that discharged only gas (no water), gases were collected by an inverted bottle filled with ambient groundwater allowing gas to displace the water, and 2) in flowing boreholes, a gas stripping device was used (the early Waterloo sampler, Figure 7) to separate gases from the groundwater and the gas was collected in glass flow-through vessels or soft Cu tubing. Although the sampler did not permit sampling at formation pressures, it provided good samples of the free gas phase at the mine level. Much of the work reported in the period 1980- 1993 (Fritz et al. 1987, Sherwood-Lollar et al. 1988, 1993) was based on these sampling methods.

Analyses showed that some gas samples were contaminated with small amounts of02

and minor corrections to gas volumes were made. The lack of data on pressure and discharge and the high salinities of associated groundwaters made it difficult to calculate the solubility of the gases at formation pressures although estimates based on available solubility data (Duffy et al. 1961) indicated that all the gases were dissolved at the ambient pressures.

21

EVACUATED SAMPLE FLASK: FLOW-THROUGH FLASKS \ OR COPPER TUBING \

.--.==:;-;:::::::{ ~

[ o:A:o¥ I I c)o-o

oO 0 I O~o

'--""'" --~FLUSHING

DiSCHARGE

,..... i

BRINE DlSCHARGE

FROM BOREHOLE

Figure 7. Early Waterloo sampler for collecting gas discharge from boreholes in mines on the Canadian Shield (from Fritz et al. 1987)

Subsequently, a new sampler was constructed which could take down-borehole water samples. Its features included an overall narrow diameter (3 .2 cm), electrically operated by internal power units and triggering devices, capable of withstanding external pressures of up to 10 Mpa and could operate in both fresh and saline water or brine (Sherwood-Lollar et al. 1994). The design allowed the flexibility for also taking samples of environmental contaminants.

Two types of samplers were made, one for sampling groundwater for chemical analysis and the other for sampling groundwater for dissolved gas analysis. The latter collects a 200 mL water sample at formation pressure, from which the dissolved gases can be later released in a controlled laboratory environment. The sampler includes an internal pump designed to flush the sample chamber of atmospheric gases before sample collection and a dual valve system to ensure sampling-depth pressure is maintained inside the chamber during and after retrieval of the probe. The gas sampler is illustrated in Figure 8. It is similar to the water-only sampler except for minor differences in the sample chamber and operational sequence. The sampler consists of two sections, one containing a battery-operated pump, the other a mechanical valve, and the sections are connected by a 60-cm long tube which serves as a sample chamber. A removable hinged sheath covers the probe and sample chamber and acts as protection and structural support.

In operation, the mechanical valve is set open before lowering down the borehole and, half-way down the hole the pump activates and pushes water through a check valve to

22

TIMERS l \VATER WATER

BATIERY1 TR!~~R OUTPUT il'-IL'AKE RECHARGING~ ~WI 1 '-H PORTS -PORTS AITACHMENT

PORT l l CASING I t ELECTR"""NICS LiNKS I il l r Frr~I~GS l ¥£ r I

~~I \[f1tffi1tfitl (_~~j®!~ool~i[ ~~=~ ~~~~ 1~®1!!~ ~~~~ L.J~LJ~ 'Yt~.LJ'i

, ,. ,r VALVE , , j r PUMP , 1 1 BA'JTERY j MQTQR I l ~~CK, J l • •r

PACK ! ACCESS MOTORiZEDiVALVa:.1 PUMP I BATJERI ; " ,,~, ,rr: , _ MOTOR. rACK ; P v RTS .., .t'Y..I • c 1 ru .. TER '

ELECTROSICS SAMPLE i6g::s VESSEL

Figure 8. Schematic diagram of the Waterloo dissolved gas sampler (from Sherwood-Lollaretal 1994).

flush out remaining atmospheric gases. At the sampling depth, the pump stops causing the check valve to close and isolate a water sample under full hydrostatic pressure. The timing mechanism then closes the mechanical valve to further seal the sample chamber. The pump and the mechanical valve have individual power packs, electronics and timing systems. The timers are set by dip switches for predetermined intervals to coordinate the operation of the pump and valve sections during sample collection. At the surface, two manual valves at either end of the sample chamber are closed so that the chamber can be detached for analysis.

In the laboratory, the chamber is connected to a vacuum extraction line and the sample transferred to a vessel on the line where complete degassing is performed using ultrasonic methods. Gas yield is measured by a mercury manometer and a sample of the gas is taken by syringe for analysis. Stainless steel vessels are used in the downhole system if major gases are to be analyzed. For the analysis of rare gases (e.g. He, Ne, Kr), soft copper tubing is used.

This sampler was used successfully in sampling dissolved gases in groundwaters from boreholes at two sites in Finland (Pori and Outokumpu) and, in particular, for investigating the elevated concentrations of H2 and C~ at depths of~ 3 50 m in the Pori borehole.

3.4 POSIVA

3.4.1 Early methods

Two methods were used by Posiva to sample dissolved gases in groundwater that has been pumped to the surface from packer-isolated zones in boreholes at Olkiluoto.

23

Initially, gas and pumped groundwater were collected in an aluminum-laminated bag which was impermeable to gas diffusion. Subsequently, a glass vessel was used, fitted with ground glass taps to sample dissolved gases from most of the available borehole zones. These techniques suffered from two main problems: 1) groundwater was being sampled at surface pressures ( ~ 1 atmosphere) and so water would out gas and could be lost from the sampling equipment during travel to the surface, and 2) gases analyzed in the collection vessel (the Al or glass vessels) did not necessarily originate from the volume of water that was collected in the vessel because of exsolution and resulting phase separations in the sample tubing.

The latter problem prevented any quantitative assessment being made of dissolved gas concentrations in the fully pressurized groundwater at zone depths. Some estimate of relative abundances could be made but these may have been distorted because the different solubilities of each gas would cause them to fractionate as they outgas.

3.4.2 Recent methods

To address the problems of sampling at the surface, the PAVE sampler (Figure 2) was constructed to take groundwater samples at in-situ pressures, at depth in the borehole (Ruotsalainen et al. 1996). The sampler consists of a gas (N2) and water or solely water-inflatable membrane to pump groundwaters to the surface for sampling and monitoring, a chamber with a moving piston for sample collection and isolation, and two inflatable packers (normally 2 to 10 m apart although even 1 m has been used), to isolate the permeable zone from the rest of the borehole. Groundwater from the zone of interest is first pumped to the surface to remove contamination from the zone and allow the composition to stabilize. Valves on the PAVE sampler are then activated to allow ground water to enter the sample chamber and displace the piston. Argon (or, later, N2) gas is used in the chamber behind the piston to reduce the pressure drop when activating the sampler. The valves are then closed pneumatically, the packers deflated, and the sampler brought to surface.

Data from samples collected by the PAVE sampler were better than those from surface samples because the samples were taken down the borehole and sealed under the ambient hydrostatic pressure. High concentrations of N2, He, H2 and C!Lt ( 480, 154, 268 and 990 mLIL, respectively, Gascoyne 2000) were found in some deeper samples and concern was expressed that the gases, particularly CILt, may be near saturation at that pressure and potentially capable of forming a gas phase. Only a few samples from depths below 600 m were taken and no reliable replicate samples were available at the time. In addition, some difficulties were experienced in ensuring complete recovery of the gas samples during gas extraction prior to analysis.

During the operation of the PAVE sampler, it was observed that, the filling gas (Ar or N2) could diffuse past the piston in the sample vessel (Helenius et al. 1998). Also, on occasions, two sample cylinders in series (one above the other), were used in the gas sampling at Olkiluoto. The upper was back-filled with Ar and the lower with N2. Because the vessels were not separately isolated downhole, some gas transport might have occurred from the lower to the upper cylinder during the lift to the surface, before the manual valve separating the two was closed. Recently, tests were

24

performed without filling gas in the P A VB sampler (i.e. a vacuum was used). In addition, sampling by three cylinders in series has been performed.

3.5 European Community (EC)

The work of several organisations and multinational projects are summarised in this section. These include studies done by AND RA (France), NAGRA (Switzerland), NIREX (UK), and at the international study sites of Mont Terri (Switzerland), Stripa and Aspo (Sweden), and Mol (Belgium).

3.5.1 Mont Terri (Switzerland)

Most of the work on groundwater chemistry and dissolved gases in groundwater by AND RA has been done within the Mont Terri project to try to develop methodologies for collecting water samples and understand the water-rock interactions in clay formations. Groundwater has been collected at several locations in the Mont Terri tunnel from the Opalinus Clay. The rock matrix is highly impermeable (hydraulic conductivity~ 5 x 10 -B m/s) and even a fault zone in the formation contains no additional water and has a low permeability (3 x 10-13m/s), (Bath et al. 2001). Work in most other clay-type formations has not involved much dissolved gas sampling simply because of the low permeability of this rock type. Exceptions are the recent attempts in the Mont Terri tunnel to sample headspace gases above pore fluid seeping into upwards-inclined boreholes (Thury and Bossart 1999). ANDRA is now trying to develop this aspect within the experiments to be conducted in the underground research laboratory ofBure.

Instead of attempting to sample dissolved gases in groundwaters in boreholes in clay formations, some workers have used a technique developed by Osenbruck et al. (1998) to determine gases in pore fluids in freshly drilled core sections. The sections are trimmed to remove drilling contaminants and placed in an evacuated chamber for a four-week period, during which time gases in the pore fluids diffuse out of the core into the chamber (Ri.ibel and Sonntag 2001). The emanated gases are then directly transferred into the inlet system of a mass spectrometer for analysis.

3.5.2 Mol (Belgium)

In the area of the Belgian nuclear research centre at Mol, Belgium, groundwaters in two deep aquifers underlying the Boom clay were sampled for noble gases by CEA researchers using a stainless steel constant-pressure sampler manufactured by MetroMesure (Pitsch et al. 2001). When lowered to the required depth, a check valve is opened and a piston is displaced by the pressure of the incoming fluid, allowing the groundwater to fill the vessel. To ensure slow sampling (and, therefore, minimal pressure drop), a counter pressure is applied by introducing deionised water to a reservoir behind the piston. This water empties during sampling through a small check-valve. Dissolved gases were also determined in artesian well waters at the surface. Results of sampling both at the surface and downhole showed that gas concentrations were lower in the surface samples and gas loss was lower for the higher molecular weight gases.

25

3.5.3 Sellafield (UK)

Dissolved gas sampling has been performed in the NIREX program for nuclear waste disposal at the Sellafield site, northwest England, with assistance from the British Geological Survey (BGS). Considerable efforts were made to sample groundwaters from permeable zones that had been thoroughly flushed and cleaned of drilling fluid. This is because most of the boreholes were drilled with organic-based polymers to enhance viscosity and reduce drilling fluid losses to the formation, and decomposition of these polymers causes gas formation, which would interfere with analysis. Two main methods were used to obtain groundwater samples for dissolved gas analysis: pumping groundwater to the surface from the interval of interest using a back­pressure that was sufficiently high to prevent outgassing, and sampling groundwater down-borehole to prevent fractionation of gases due to outgassing (Bath et al. 1996). These are described in more detail below (NIREX 1997).

1) Downhole large volume samples (L VS) were collected in 1. 5 or 5 L stainless steel vessels after first evacuating or purging with N2. After a pre-set time, the sampler valve was opened and borehole water allowed to fill the vessel. The valve was then closed and the vessel recovered and gas and liquid phases transferred by N2

displacement. The L VS is shown in Figure 9.

2) Small volume samples ( 600 mL) were collected in a stainless steel Singlephase Reservoir Sampler (SRS), which contains an internal "power fluid" reservoir which maintains the sample in a single phase during recovery. The samples were then transferred to copper tubes or stainless steel vessels to await analysis. The sampler is shown in operation in Figure 10. It is interesting to note that some samples stored this way were not analysed for 3 years but they appeared to suffer no loss or contamination by interaction with vessel walls.

3) Groundwater samples were often taken directly from the drill rod on removal from the borehole. This was done either by air-lifting or draining of the rods.

4) Packer-chamber samples were taken from a chamber located just above the upper packer in the borehole (i.e. near the surface) and gas and liquid phases were transferred using N2 displacement.

5) A W estbay sampling probe was used as a means of collecting downhole groundwater samples in two Westbay cylinders suspended beneath the probe.

Of these five sampling methods, only the SRS ensures sample integrity by maintaining in situ pressure on the sample as it is brought to surface and stored.

In parallel work, the British Geological Survey is in the process of constructing a system that will fit onto a standard wireline. It works by pumping water through a steel 'bomb' at the required depth and then isolating it between two valves.

Transfer Valve

26

Sample Chamber

Sampling Port

Sampling Valve

Electronic Clock & Mechanism

Figure 9. Schematic diagram of the Large Volume Sampler used in the NIREX program (NIREX 1997).

FLOAllNG PIS TOll

CLOCK

REGULATOR VAll/E

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Figure 10. Principles of operation of the Singlephase Reservoir Sampler used in the NIREX program (from NIREX 199 7 ).

N .....,J

28

3.5.4 Stripa (Sweden)

As part of the detailed hydrogeochemical studies performed on ground waters from the Stripa mine, south-central Sweden, during the 1980's, the sampling and analysis of dissolved gases formed an important area of study. Two methods of sampling were used: large-volume (~ 15,000 L) collection and degassing in situ, in the mine, for analysis of 37 Ar, 39 Ar and 85Kr (Loosli et al. 1989), and a specially designed sampler for atmospheric and radiogenic gases (Andrews et al. 1989). Few details are available of the former sampler except that gases were stripped from the groundwater as it flowed into the tunnel and they were immediately compressed into evacuated cylinders.

More details were given describing the sampler designed by Andrews et al. (1989); a schematic diagram is shown in Figure 11. In this work, samples were obtained after pressurisation of the boreholes. Boreholes were shut in by packers and hydraulic pressure allowed to build in the sections to be sampled. For the depth of the tunnel(~ 360 m), pressures over 2 MP a were built up and, by careful adjustment of the well­head valve (valve A in Figure 11 ), the section including the sampler as far as the outflow valve (valve B) was pressurised to 1 MP a by allowing limited flow through the outflow valve. The sample was then isolated by pinching the copper tube as shown in Figure 11.

Swage Lock Connection

.s·wage Clamp

Copper Tube

* SwageCI-p

lj8fl Swage Lock Conriecll~n

~ Control Vatve A

l~p, ... , .... ~ ~.~

Flow from Borehole

Figure 11. Diagram showing the arrangement of equipment at the well-head for sampling dissolved gases under pressure in selected borehole zones at Stripa (from Andrews et al. 1989).

29

3.5.5 Aspo (Sweden)

Dissolved gases were routinely sampled and analysed by SKB in the 1990's as part of the characterisation of the Aspo Hard Rock Laboratory, south-west Sweden. Down­borehole groundwater samples were taken during the sampling and on-site analysis performed in the SKB field laboratory (Figure 12). However, despite the frequency of downhole sampling and availability of results in the Aspo database surprising! y little has been published in SKB' s program describing dissolved gas abundance, characteristics and their interpretations. However, a summary of the findings of this work given by Smellie and Laaksoharju (1992, p.227).

1. Flaw meter 2. Surface chemic:al

probe (pH. Eh. pS. p02 ccnd'.:cti\-ity)

3. Calibration unit 4. !n !ine filet 0.45""' 5. Carbcn-14 samPler 6. Sampling for;

A. Field analysis B. &.temal analysis c. Backup storage

FIELD LABORATORY

ANALYSED PARAMETERS

I DRILLING WATER CONTENT: I Uranine 1

DISSOLVED ELEMENTS; I Na K Ca Mg Si 0 2 Mn Fe (tot) Fe (11} HC03 Fa Br $04 S(·fl) N~ N03 NH.P041

DISSOLVEO ELEMENTS: NaKCaMgSi Mn Fe (tot) HC03 F Cl S(-11) N02 N02+N03 NH.P04 ~ SrUt AaTh URn AlTER RESIOUES: Ca AI Fe Mn S SI ,__ ___ .....,.......c-_-. ______ .__;~-t ISOTOPES:

tXlWNHOlJ: CHEMICAL PROBE: pH, Eh

2ti ~ 'to~ 14C ~u ORGANICS: TOC Fulvic and humic acids

I GASES:

GAS SAMPLER --------1 Na 0z H2 CO C02/v He ,CH.. ... C4H10

PUMP

Figure 12. The Swedish geochemical analytical system including a down-borehole dissolved gas sampler (from Smel/ie and Laaksoharju 199 2).

3.6 Other Studies

3.6.1 Finland

An alternative method for sampling dissolved gases in groundwater has been described by Nurmi and Kukkonen (1986). The method involves lowering a plastic tube fitted with check valves at 50 m intervals down the open borehole and allowing the standing water to fill the tube as it is lowered (Figure 13). The check valves are oriented such that they allow water to fill the tube but, on retrieval, they close and seal the overlying water segment in place and prevent loss of gas as the tubing comes to surface. The gas is sampled by connecting the tube to a water-filled glass vessel fitted with shut-off valves at each end and gas is allowed to flow from the tube to displace water from the vessel.

.--' ! ;

oi -· '"-"'I U'j.

I I

'

Overburden

"

\ \ \

Back. -pressure valve

30

Inflow of water

Figure 13. Schematic diagram showing the layout of the tube sampler (from Nurmi and Kukkonen 1986).

Although the method is simple and fast and the equipment robust, the main problems are the possibility of loss of gas through tubing walls during transfer and only water standing in the borehole can be sampled and, without prior pumping of the borehole, this water may have partially outgassed to the surface and may not be representative of groundwater in the permeable fractures at depth. In addition, without this pumping and monitoring of the pumped water, the ground waters sampled may contain residual drill water or groundwater from other permeable zones intersected by the borehole. Nurmi and Kukkonen (1986) recognised these shortcomings but suggest that the method is useful because of its ease and speed of use and its suitability for use in slim boreholes (<50 mm diameter).

3.6.2 The Westbay system

The Westbay multi-packer borehole completion system was first developed in Vancouver, Canada, about 20 years ago, and preliminary testing and design studies were performed in conjunction with AECL in boreholes at the Whiteshell Laboratories site, Manitoba. The system consists of sections of continuous plastic tubing that are placed into the borehole and, at desired intervals, packers and access

31

ports are inserted into the tubing string so that selected zones can be isolated (by inflation of the packers), measured for pressure, or sampled for chemical analysis (through the ports). The Westbay MP System® has since been used at many sites around the world, principally to monitor groundwater pressures but also to allow sampling for water quality measurements. Representative samples may be collected from each of the packer-isolated monitoring zones without repeated purging of the sampler because the sampler takes water from outside the casing (i.e. in contact with the rock) rather than inside the tubing string.

Several methods are available for groundwater sampling. Samples can be recovered through port valves if only a single 'grab' sample of groundwater in the zone is required (typical volume of sample is 1 L) or through the larger opening of a 'pumping port'. Sampling equipment includes a MOSDAX® sampling probe and non­vented containers for collecting discrete liquid or gas samples at formation pressure from depths of at least 1200 m. The sample volume is up to 1 L per trip for the MP 38 system and 2 L for the MP 55 system. The operation of the sampling probe is shown in Figure 14. Before lowering down the inside of the Westbay casing, the sample vessel is first evacuated, and the sampling valve closed. The probe and vessel are then lowered to below the selected port coupling, the location arm is released and the probe positioned in the port coupling (Figure 14a). The backing shoe is then activated to push the probe to the wall of the coupling so that the face seal on the probe seals around the port valve at the same time as the probe pushes the valve open (Figure 14b). On opening the sampling valve (by electronic signal from the surface) water from outside the port flows through the probe into the vessel (Figure 14c). When the vessel is full, the valve is closed, the backing shoe deactivated and the probe and vessel pulled to the surface.

For dissolved gas measurements, a non-vented sample vessel is used so that the formation pressure is maintained on the sample until gas separation is required in the laboratory. Advantages of this discrete sampling method are: 1) the sample is taken directly from groundwater of the permeable zone outside the port; therefore, there is no need to pump several well volumes prior to each sampling, 2) because there is no pumping, the sample is obtained with the minimum distortion of the natural groundwater flow regime, 3) samples can be obtained quickly because there is no need for pumping, even in relatively low permeability rock and 4) the groundwater only travels a short distance to the vessel. Disadvantages are, however, 1) the system can only be used if the Westbay MP® casing system is installed in the borehole, 2) it relies on electrical pulses sent down the borehole to activate valves, motors, etc. (this requires an umbilical line as well as the supporting wireline), 3) sampling of larger volumes through the pumping port requires replacing all of the casing water by the zone water above the level of the port; casing water below the port level cannot be replaced and may contaminate the pumped water slightly by mixing, and 4) the complete Westbay casing and sampling system is considerably more expensive than other methods described in this report.

MP Casing

Sampler Probe

Location Arm

Measurement Port Valve

Measurement Port Coupling

Sampling Valve

a) Probe located at measurement port

Pressure Transducer

32

Backing Shoe

b) Probe activated, sampling c) Sampling valve open, valve closed. collecting sample.

Figure 14. Schematic diagram showing details of the MOSD~ sampling probe (from Westbay Instruments 1994)

3.6.3 Oil industry

More than any other organization, the oil industry has long been interested in the composition of groundwaters and dissolved gases as an aid to exploration and understanding of oil reservoirs. A simple, early dissolved gas sampler, which taps water and dissolved gas from a flow line, is shown in Figure 15 (Collins 1975). Water is allowed to flow through the container, which is held above the flow line, until 10 or more container volumes have passed. The lower valve is then closed, the container removed and the upper valve closed. If any bubbles are present in the sampler, the sample is discarded and a new one obtained. This type of sampler, however, is not adequate to deal with groundwaters containing a lot of dissolved gas and which are actively outgassing in the supply line.

33

Figure 15. The flow-line dissolved gas sampler used in early sampling in the oil industry (from Col/ins 1975).

For some time now in the oil industry, it has often been necessary to obtain accurate compositional and pressure-volume-temperature (PVT) analyses of formation samples (usually oil-gas-water mixtures). This required the recovered sample to remain in downhole-formation conditions, which usually meant maintaining a monophasic sample. Many downhole samplers trap a fixed volume of single-phase fluid at reservoir conditions but, because of the great depths involved, as the sample returns to surface, temperature decreases, causing a pressure drop. This usually causes the sample to pass through the bubble point to give a gas and liquid mixture sometimes accompanied by precipitation of asphaltene.

To resolve this problem, the Schlumberger company has developed a down-borehole system to obtain pressures and/or samples from formations at different depths and maintain their formation conditions by imposition of excess pressure (Figure 16). The system is called MDT (Modular Dynamic Tester), and contains several different modules such as a Dual Packer module, an Optical Fluid analyzer (infra-red), a Pumpout module, etc. and three types of sampler (10 L, 4 L and six x 450 mL individual bottles). The system is controlled by hydraulics from the surface and has a 7 -line umbilical plus a steel cable support. The sampling system is known as the Oilphase Single Phase Multisample Chamber and is able to obtain accurate pressure readings at depth and take uncontaminated, fully pressurized samples. It has been used at considerable depths below surface; for instance, in the Hybernia oil-field,

34

samples have been obtained at depths of over 6000 m. The system is tested to 20,000 psi(~ 140 Mpa) and 360°F (~ 180°C).

The sample chamber allows for overpressuring the samples once they are taken using pistons operated by nitrogen gas. This compensates for the temperature-induced pressure-drop as the sample nears the surface. The samples obtained can be used for PVT analysis to determine conditions in the formation being tested. The minimum borehole size it can be used in is 6 inches (150 mm) because the tool diameters are about 4.75 inch (120 mm) diameter. This is an important limitation because most boreholes drilled for site investigations for nuclear waste disposal are smaller than this.

Electric power module

Hydraulic power module

Probe module

Sample module

Sample module

Pumpout module

Figure 16. The Schlumberger downhole sampler showing the various modules required for dissolved gas sampling.

35

4 CANADIAN EXPERIENCES AND DATA

Groundwaters have been sampled in the Canadian program for nuclear fuel waste management for almost 15 years in order to determine the dissolved gas content and its significance. Dissolved gas sampling was performed at three Research Areas during this time, the East Bull Lakes gabbro, the Eye-Dashwa Lakes granite and the Lac du Bonnet granite batholith. Most results have been obtained for the Lac du Bonnet batholith and, in particular, for the area of the Underground Research Laboratory (URL) in the southern part of the batholith. It is these latter results that are used here to identify problems in the sampling and determination of dissolved gas content of ground waters to a depth of 1000 m. It should be noted that these data have not been previously reported because they are not always believed to be consistent and, in some cases, appeared show evidence of contamination. However, they are ideally suited to the study being described here and, as will be shown, have important implications to the Olkiluoto situation.

4.1 Borehole completion systems

An important aspect of down-borehole dissolved gas sampling is whether or not casing has been installed in the borehole. This will determine both the maximum diameter possible for the sampling device and the method it uses to take the groundwater sample. In the Canadian program, three main types of borehole completion system have been used:

1) Open borehole without installed casing; sampling vessels may therefore be almost as large as the borehole (typically 75 or 150 mm in diameter) and they should be fitted with packers to straddle the permeable zone and a pump to ensure that fresh groundwater from the fracture is sampled, not just the standing water in the zone. Alternatively, stand-alone packers ("PIP's") can be placed in the borehole using a work-over rig and drill rod to set each packer and, ultimately, sample groundwater through the drill rod down to the next packer.

2) Multi-level packer assembly, typically isolating up to four zones; groundwater from each zone can be sampled by lowering a slim pump or peristaltic-pump tubing down each riser pipe that connects each zone to the surface (the pipes are generally 25 mm diameter). The bottom zone is easiest to sample as it is usually accessed through a drill rod (AQ size, ~ 35 mm diameter) and it and the lowermost packer hold the overlying packer assembly in place.

3) Plastic or plastic/stainless steel casing system, usually of the Westbay type; this system is modular and consists of lengths of plastic tubing ( ~ 3 5 mm internal diameter) connected together downhole with water-inflated packers on the outside placed at required intervals, and sampling ports and pressure measurement ports installed where needed (see Section 3.6.2).

Each type of installation allows groundwater sampling for dissolved gas analysis but with varying degrees of potential accuracy and reliability.

36

4.2 Sampling at the surface

At the URL lease area, dissolved gas data have been obtained for permeable zones in 25 boreholes and this is supplemented by data from zones in a further 13 boreholes in the surrounding area (the Whiteshell Research Area, WRA). Dissolved gas concentration data are shown in Table 2. B-, M- and URL-series boreholes are on the URL lease area whereas all W -series boreholes lie further away, in other outcropping areas of the granite.

In most cases, sampling for dissolved gases was performed at the surface in duplicate using stainless steel cylinders, as described in Section 3.2.1 (samples A, B and, occasionally, C, in Table 2). Whenever the flow of water was sufficient, back­pressure was applied to the sample vessels by partly closing the outlet valve. In some cases, down-borehole samples were also taken using the methods described in Section 3.2.2 (DHA, DHB).

It can be seen that, with some notable exceptions, most groundwaters have dissolved gas concentrations of 30 to 60 mL/L. Examination of the analytical data (not shown here) shows that most of this gas is N2 and He with significant Ar in the more saline groundwaters. Contamination by the atmosphere is minimal as most samples contain < 1 mL/L 02. Negligible amounts ofH2 and C~ (generally< 0.1 mL/L) were found in all groundwaters.

Comparison of the A and B duplicate samples is shown in Figure 17. Reasonably good agreement can be seen between most of the duplicates (mean deviation is ± 26%) although the large differences between some of the samples suggest that either one of the samples was contaminated (possibly with N2 leaking from packer systems) or outgassing had occurred and one sample vessel had accumulated more gas than the other. The latter is believed to be most likely because the ratio He/N2 was approximately the same even though total gas concentrations were up to a factor of five different for duplicate samples. Leakage of N2 from the packers might be expected to cause high N2 concentrations without comparable He concentrations but this cannot be distinguished for any given sample because of the lack of a reference concentration for either gas. In addition, it is difficult to determine which of the duplicates is the more accurate, because gas-phase separation could result in an excess of gas in one sample vessel and a deficiency in the other and, without data for a down­borehole sample, the more accurate surface sample cannot be identified.

It might be expected that the samples with higher dissolved gas concentrations would show a greater disparity between duplicates than those with lower concentrations because outgassing (and consequent gas-phase separation and fractionation) would be more likely to occur than in groundwaters with lower gas content (i.e. closer towards saturation at one atmosphere). On first impressions in Figure 15, this appears to be the case because samples with high gas concentrations stand out from the background level. However, closer inspection shows that a few of the low gas concentration duplicates are up to a factor of three different between duplicates and so there is no significant difference between the high and low gas concentration data.

37

Table 2. Dissolved gas concentrations in groundwaters sampled in duplicate from WRA boreholes.

8orehole Zone Sample Depth Interval Date A 8 c DHA DH8 No. m

834 -1 -4 0-32.5 24-0ct-S6 90.1 51.6 59.S 834 -2 -1 32.5-60 13-Jun-95 61.2 53.3 837 -1 -2 0-30 21-Aug-93 69.5 6S 62.3 62.3 837 -2 -1 30-60 23-Aug-93 49.3 47.6 65.1 65.1 M1B -2 -3 51-150 14-Aug-S7 33 31.7 M1B -2 -4 51-150 15-Feb-91 35.3 36.S M2A -3 -4 270-400 29-May-S6 1S1.S 121.4 M2B -2 4 111-152 OS-Dec-92 70.9 32 32.9 34 M2B -2 -1 130-160 8-Apr-S6 36.6 32.4 M2B -2 -9 111-152 3-Sep-S9 36.3 35.3 M3A -3 -5 351-400 26-Jun-91 60 33.6 M4A -4 -6 291-406 14-Jui-S6 1S7.7 1SS.S M4A -4 -9 2SS-403 2S-Jui-S9 95.5 6S.3 M4A -4 -6 291-406 2S-Aug-90 54.S 69 MSA -2 -10 2S7-35S 25-Apr-90 77.1 S0.5 MSA -2 -4 2S7-35S 26-Jul-91 61.2 61.2 M6 -2 -5 95-130 7-Aug-S6 61.2 24 M7 -4 -3 351-400 3-Nov-92 36.S 27.2 M7 -4 -11 351-400 5-Sep-S6 37.4 52.4 M7 -4 11 (B) 351-400 24-Apr-S9 53.4 42.3 M7 -4 -7 351-400 26-Jun-91 25.4 24.4 61.2 MS -3 -5 311-400 7-Jun-91 34.6 33.6 MS -3 -2 311-400 7-Sep-93 35.7 36.S 35.7

M10 -3 -2 400-430 15-Apr-S6 S0.1 50.9 M10 -3 -9 341-450 3-May-S9 71.4 7S.5 9S.6 M10 -3 -2 341-450 10-Sep-93 149 36.S M12 -159 11,12 159 24-Aug-S3 57.6 3S.6 M12 -171 -15 171 2S-Jui-S3 63.9 51.1 M13 -2 -3 226-443 16-Jun-S6 131.7 67.4 M13 -2 -10 226-446 26-Sep-S9 2SO 66.4 11S M14 -4 -4 300-3SO 3-Jun-S6 74 70.9 M14 -4 -5 301-3SO 12-Apr-S9 7S.2 65.1 M14 -4 -3 301-3SO 7-Jun-93 105 S3.7 ss

URL1 -S -16 316-347 20-Jui-S9 42.6 50.S URL3 -6 -9 142-167 20-Aug-SS 34.1 41.6 URL4 -5 -10 53-73 23-Jui-S6 37.6 44.2 URLS -7 -7 276-315 9-Feb-S9 4S.2 44.3 URL9 -6 -2 140-163 20-Sep-93 40.2 41.4 40.S 41.3 URL10 -3 -S 54-121 2-Aug-90 44 43.7 URL10 -6 -7 270-302 9-Jui-S6 45.6 57.2 URL11 -7 -7 1S3-202 30-Nov-SS 40.3 34.6 URL12 -10 -19 404-450 11-Apr-90 57.S 70.4 63.9 URL12 -11 -13 45S-502 11-Sep-S6 65.1 32.3 26.2 .URL12 -11 -11 45S-502 3-Nov-S9 54.S 61.S 47.1 URL12 -13 -1S 637-699 1S-Jui-S6 59.4 51.6 URL12 -13 -21 637-699 23-Jui-S6 66.7 50.2 URL12 -13 -13 637-699 2S-Mar-S9 79.4 62 S3 URL14 -S 0-397 23-Jui-S7 23.9 41.9 URL14 -270 -14 270-397 21-Jun-S9 5S.3 S5.6 URL15 -11 0-40S 23-Jui-S7 30.4 39.5 WA1 -2 -S 210-310 5-Jun-S7 37 39.5 WA1 -3 -6 310-3SO 3-Nov-S7 40.S 40 WA1 -5 -4 490-750 11-Mar-SS 110.7 97.3 WA1 -5 -7 490-750 18-Mar-88 89.9 142.8 WA1 -16 15 71S-740 22-Mar-91 107 114 WA2 -S -5 26S-2S3 16-Apr-91 35.7 50.7 WA3 -6 -S 110-134 3-May-91 50.7 50.7 WB1 -1 -5 0-210 5-Jun-S7 24.4 56.6 WB1 -2 -6 210-300 5-Jun-S7 26.3 3S.5 WB1 -5 -1S 6SO-S60 30-Sep-S7 35.S 27.2 WB1 -7 -9 S60-1120 27-May-SS 24.6 4S.5 WB1 -7 -S 1120-1203 23-Jan-91 35.S 45.7 WB2 -19 -5 92S-972 22-Dec-92 73.9 90.5 WB2 -20 -12 974-99S 5-Mar-91 49.9 70.3 WD2 -72 -5 63-301 1-Sep-SS 53 49.4 WD3 -S95 -S S00-1202 11-Sep-SS 43.4 53.3 WN1 -S -17 3S1-402 24-May-S7 43.2 44 WN4 -6 -S 356-402 S-May-S7 57.7 56 WNS -7 315-322 11-Jun-S6 57.3 57.7 WNS T2 315-322 1S-Feb-SS 45.4 46.3 WN10 -4 300-496 26-Mar-S7 42.S 62.1 WN11 -17 -15 1099-1201 9-Dec-S6 35.6 42.1 WN12 -17 320-349 27-Jun-S9 53.6 S4.9

38

4.3 Down-borehole sampling

Several of the ground waters listed in Table 2 were also sampled downhole at the same time as the surface sampling was performed. These results are listed under the DH columns in Table 2. These data are compared in Figure 18 for 14 samples .. For the five groundwaters where duplicate samples were taken down-borehole, there is excellent agreement between duplicates (< 3% variation). In addition, there is no indication that downhole samples contain any more or any less gas than surface samples. An examination of the He/N2 ratios of these samples also shows no significant trend that might indicate gas fractionation as the ratio values for surface samples range both higher and lower than downhole values.

4.4 Underground sampling

Dissolved gases have also been sampled from boreholes drilled into permeable zones from the URL at the 240 m depth level as described in Section 3.2.3. A total of 15 samples were obtained in duplicate (Table 3, Figure 19). Dissolved gas concentrations range from 40 to 60 mL/L, similar to surface and downhole samples, but, unlike the surface samples, they show much better agreement within duplicates (mean deviation is± 15%).

4.5 Discussion and summary

The analysis of dissolved gas data and sampling methods for groundwaters in the Lac du Bonnet granite batholith has shown the following:

1) Nitrogen and helium are the main gases dissolved in ground water in the URL area. Methane and hydrogen are generally at or below detection limit.

2) Dissolved gas concentrations are typically between 30 and 60 mL/L and are similar whether they are sampled at the surface, down-borehole or from an underground facility. Downhole samples do not seem to contain more gas than surface samples.

3) Results are more reproducible for groundwaters sampled down-borehole or from the underground, than from the surface.

4) Large concentration differences in some duplicate sets of surface-sampled data are probably due to differing amounts of gas-phase that collect in the sample vessels during sampling.

5) Leakage of inflation gas (N2) from packers may account for the high concentrations of N2 in some samples; however, this could also be explained by gas-phase enrichment in the sample as indicated in 4) above.

6) Significant fractionation of gases (due to different solubilities) has not occurred in most of the samples taken at the surface.

300

250

:::r 200 ::J E -f/) C'G C') 150

"C Cl) > 0 f/)

.!!! 100 c

50

- ~ B

-

11 B 11 1111 11 0 ~~~~ii~~~~~iiiii~s;s;s;s;!!iiiiiiiiiii~i;~;;~~~~~~~~~~~~~~~~~~~~~~ii~~illlii~ Surface Boreholes

Figure 17. Diagram showing dissolved gas concentrations of duplicate (A & B) samples from bore hole zones in the WRA.

w \0

300

250

::i 200 ::J E -en ftS 0) 150 "C Cl)

> 0 en en c 100

50

0

-

-

,.... --

837

..... -- - ~ --

r-

~~

I T

837 M2B M3A M7

~

DA

• a ODHA

ODHB

.--

~

- ,.... ~

--~

~

-~

r- ~ .--

Il I I I I I

M7 MS M13 M14 URL9 URL10 URL12 .URL12 URL12

Surface Boreholes

Figure 18. Diagram showing dissolved gas concentrations of duplicate surface (A & B) and downhole (DHA & DHB) samples in the WRA.

,.J:::.. 0

350 ~----------------------------------------------------------------------------------,

300

-250 ...J :J E -;; 200 cu C) ,., CD ~ 150 0 Cl)

.!!! c

100

50

0

HC6 HC7 HC7 HC8 HC11 HC14 HC15 HC16 HC16 HC18 HC19 HC26 HGW1 101- 205-0C1 PH2

Underground Boreholes

Figure 19. Diagram showing dissolved gas concentrations of duplicate (A & B) samples from underground boreholes in the URL facility.

~ ~

42

Table 3. Dissolved gas concentrations for duplicate underground samplings of groundwater from boreholes in the URL.

Bore hole Sample Date A 8 HC6 -7 01-Sep-87 45.4 49.8 HC7 -1 08-Aug-86 33.2 56.6 HC7 -1 05-Sep-86 68.2 56.2 HC8 -10 27-Jan-88 49.4 54 HC11 -10 27-Jan-88 56.1 52 HC14 -2 11-Sep-95 32.3 30 HC15 -3 03-Feb-87 88.5 107 HC16 Z2-10 27-Jan-88 48.9 40.2 HC16 Z3-10 27-Jan-88 49.2 60 HC18 513,17 09-Jun-93 48.4 53.6 HC19 -18 21-Nov-89 47 43.6 HC26 -4 20-Jun-95 202 312 HGW1 -5 03-Jun-96 57.8 56.1

101-0C1 -5 13-Jun-96 54.4 54.4 205-PH2 -8 08-Nov-95 40.8 42.5

These results indicate that, with care, sampling of groundwaters at the surface for dissolved gas analysis can be performed with confidence. Separation of a gas phase in the sampling tube appears to be the main concern and can give rise to poor reproducibility of duplicates. The fact that most of the surface samples obtained in the Canadian program show good reproducibility is probably because a back-pressure was applied whenever possible to the sampling vessels (by partial closure of the outlet valve) thus keeping most of the dissolved gas in solution.

43

5 FEASIBILITY STUDY FOR SAMPLING AT OLKILUOTO

In a site evaluation program, where the subsurface is principally characterized by surface-collared boreholes, it is important that substantial opportunity and time is made available for hydrogeochemical characterization activities such as pumping and chemical monitoring of permeable zones, flow cell measurements (for pH, Eh), large volume sampling for specific isotopes (e.g. 14C, 36Cl, 129I) and dissolved gas sampling for both atmospheric and noble gases. These measurements are best made on groundwater pumped from individual fractures or fracture zones so that the chemical changes with depth and type of permeable zone can be identified. In particular, doing these measurements down-borehole (such as for pH, Eh and sampling for dissolved gases) is often claimed as essential to ensure the accuracy of the measurements. However, this type of characterization is time-consuming and expensive, particularly when there are many permeable zones and a number of boreholes to test and may impede other activities of equal importance (e.g. hydraulic testing, tracer tests, downhole geophysical measurements).

In the Canadian program for site characterization over the period 1980 - 1995, geoscience activities generally took place in the sequence: drilling, cleaning and flushing, downhole geophysics, isolation of permeable zones by well-spaced temporary packers, preliminary hydrogeochemical testing (between the temporary packers), permanent casing installation (e. g. Westbay casing), hydraulic head measurements, detailed hydrogeochemical testing. It was common, therefore, that preliminary hydrogeochemical results were not obtained until at least a year after the borehole was drilled, and full hydrogeochemical characterization not until 2 - 5 years after. These delays were partly due to the limited number of personnel and availability of equipment for these tasks but also to the number of new boreholes to be characterized (an indication of this is shown in Table 2 and Figure 17 where 51 zones in 37 boreholes were sampled (often more than once) over a 12-year period; this was in addition to characterization of a further ~40 zones in underground boreholes in the URL).

The methods used in groundwater sampling and analysis in the Canadian program have been summarized in Ross and Gascoyne (1995) and procedures manuals have been written to describe methods of groundwater sampling, sample treatment, on-site chemical analysis, and laboratory analysis. (Ross et al. 1995, Ross 1995, Watson 1996).

5.1 Methods for rapid sampling

To improve the efficiency of groundwater sampling in the Canadian program, in later years, several changes were made including reducing or eliminating the time spent in obtaining downhole electrochemical measurements (mainly pH and Eh), using a simplified downhole dissolved gas sampler (see Figure 6) in place of the cumbersome downhole probe sampler (Section 3 .2.2), and less flushing of boreholes prior to sampling (this was made possible because the boreholes had been well-flushed by previous sampling efforts).

44

At Olkiluoto, it is important to determine the concentration and composition of dissolved gases at all depths, but, in particular, at proposed repository depths (500- 1000 m). Previous work (Lampen and Snellman 1993, Pitkanen et al. 1994, Gascoyne 2000) has already shown that high concentrations of C~ and H2 may exist in groundwater at these depths and there is potential for gas phase formation. Determination of dissolved gas concentration and composition should be performed for all groundwaters in the Olkiluoto boreholes so that a comprehensive data set can be established to define the baseline conditions of the site to a depth of at least 1 km. Because downhole sampling using sophisticated samplers such as PAVE is time consuming and potentially complex, it is possible that downhole dissolved gas sampling would be omitted during actual field work in order to meet a pre-determined schedule of borehole activities. Consideration should therefore be given to simplifying dissolved gas sampling so that it becomes a rapid and routine activity.

Several of the methods described in previous pages could be used here but re­instituting sampling at the surface with supporting in situ results from rapid downhole 'grab' sampling seems to be the best combination to employ at Olkiluoto. One method of sampling at the surface involves collecting water and exsolved gas at atmospheric pressure but from a large volume of groundwater (perhaps 1 L or more) so that the effect of outgassing and loss or gain of gas is minimized.

Alternatively, the results obtained for dissolved gas sampling in AECL's WRA study (Section 4) showed that, in general, sampling at the surface using duplicate stainless steel cylinders as sample vessels gave good reproducibility providing that a back­pressure was applied during sampling. Comparisons of surface and downhole sampling should be made whenever possible and, if it appears that good agreement is consistently found, even for the CHJH2-rich waters, then downhole sampling can be removed from the procedures.

When time and resources permit, more labour-intensive methods of downhole sampling, such as the PAVE system, can still be used, once it has been demonstrated to be able to consistently take a full-volume water sample, at depth, without contamination with back-pressure gas and to be able to maintain downhole pressure when brought to the surface. If a multilevel packer system is to be installed in the boreholes, then any of the sampling devices discussed in Section 3 could be used providing the sampler is sufficiently slim to go down the access tubing and is compatible with the access ports of the packer system. In most cases, even if a packer system is installed, it is still possible to perform the dissolved gas sampling downhole using the same techniques as proposed for open borehole sampling, above.

5.2 Methods for on-site analysis

Of the organizations and their sampling procedures described in Section 3, none of them attempt to analyze dissolved gases on site. All of them store the sample vessels until analysis can be done by off-site laboratories that specialize in this type of work. This is not surprising because the extraction procedures required are quite complex and demand fragile equipment and sophisticated methods. For instance, in the Canadian program, all samples obtained by both AECL and the University of Waterloo from Canadian Shield plutons and mines were submitted to AECL's

45

Analytical Science Branch at Whiteshell Laboratories for gas extraction and analysis. As noted in Section 3.2.4, gas extraction from the groundwater sample requires expansion into an evacuated line, cold-trapping of water vapour, and repetitive pumping and gas transfer from the sample vessel to a glass break-seal or valved flask to ensure 1) complete gas extraction and 2) no fractionation of gases or their isotopes during transfer.

Analysis of head-space gas in a sample vessel will not normally give an accurate measure of total gas concentration and composition although this method was used in early work by some groups (e.g. Posiva, using the Al-bags (Section 2.3), the oil­industry (Figure 15) and the University of Waterloo using the gas-trapping method (Figure 7)). However, if it can be designed so that the water sample is released into a large-volume evacuated container connected to drying traps, it should be possible to abstract an aliquot of this gas for immediate analysis with minimal gas fractionation. The possibility arises, therefore, of performing on-site analysis using this technique for gas extraction and either a gas chromatograph (GC) or mass spectrometer (MS) on-site analysis.

A number of companies make rugged GC or MS instruments that would be suitable for on-site dissolved gas analysis and can be installed in a mobile field laboratory serviced with a stable power supply. For instance, ThermoFinnigan make a compact mass spectrometer, 'Trace MS'® which would be suitable for rapidly measuring the abundance of gases, especially the lighter ones. Similar instruments are made by Varian (1200 Quadrupole MS), MicroMass (MM GCT), etc. Application of mass spectrometry to analysis of noble gases in ground water has been described by Poole et al. (1997).

5.3 Methods development

In the sections above, two options for dissolved gas sampling have been described which involved sampling at the surface using simple but effective techniques: 1) sampling at atmospheric pressure but from a large volume of groundwater so that the effect of outgassing is minimized, and 2) sampling under a back-pressure to prevent outgassing. Most organizations involved in dissolved gas analysis of groundwater have chosen to develop downhole sampling procedures and equipment and so the first method is largely untested.

As described in Section 2.1, gas bubbles will form in the water delivery tube as the groundwater is pumped to surface and hydrostatic pressure reduces. These bubbles may become separated from the water that initially contained them and so a water sample and its associated gases may contain more or less gas than it did at formation pressure. This will have a large effect when groundwater sample vessels are small (e.g. at typical dissolved gas concentrations, a 10 mL water sample would contain only 0.5 mL gas and this is readily taken up in adjacent gas bubbles or supplemented by gas from other ground water aliquots ). However, taking a larger volume of water (e.g. 1 L) would reduce this 'edge' effect and give a more accurate result. Testing of the reproducibility and accuracy of dissolved gas analyses is needed. Gas vessels such Al-laminated bags, glass or, preferably, steel cylinders, of volume about 1 L, fitted with a good quality ball valve at each end, should be used in triplicate for

46

various dissolved gas contents. It is especially important to test the sampling of deep groundwaters containing high C~ and H2 concentrations using this method.

The second surface sampling technique is to use leak-proof vessels (e.g. stainless steel cylinders fitted with Nu pro® -type valves), in triplicate, arranged in series and connected to a water-delivery line. Back-pressure is applied during sampling by partial closure of the final outlet valve. If a cyclical pumping method is used (e.g. a downhole bladder pump) then the outlet valve should be closed near the end of every discharge cycle to maintain pressure in the vessels until the next cycle commences. The vessels should be stood upright and tapped to displace any gas bubbles adhering to the inner walls of the vessels.

In parallel with the development and testing of a sampler, testing of downhole sampling methods may also be performed to help verify that surface sampling is acceptable, particularly for high gas concentration groundwaters.

To fully transform dissolved gas sampling into a rapid and routine technique, development of a simple gas extraction procedure is required. A system that may meet requirements involves constructing a large-volume degassing chamber (approximately 10 times the volume of the water sample), a line leading from the chamber to a dry-cold trap (dry ice and methanol, for instance) to remove water vapour, and a vacuum bellows (similarly to those used on a mass spectrometer) to draw in the gas and then compress it into a detachable gas sample vessel for analysis. A possible design for this system is shown in Figure 20.

In operation, the system is first evacuated up to the valve on the sampling vessel with the bellows compressed, using a turbo-molecular pump. Stable vacuum conditions should be apparent by monitoring the vacuum gauge when the system is isolated from the pump. The inlet valve is opened to allow water sample and dissolved gas into the large degassing chamber. The gases are drawn through the drying trap by opening the bellows. The valve after the drying trap is then closed and the valve to the gas sample vessel is opened. The bellows is then closed to drive as much gas as possible into the gas vessel, which can then be detached for gas analysis. The total concentration of dissolved gases is determined either by reading the vacuum gauge or, if large amounts of gas are present, a pressure gauge (the gauges are pre-calibrated using known amounts ofN2 gas).

This system would be inexpensive and rugged, and can be constructed using steel tubing and minimal glass. It requires only a vacuum pump and a supply of dry ice (readily obtained using a C02 cylinder). Developmental work would be needed to optimize the relative volumes of the water vessel, the degassing chamber, and the bellows, with the sensitivity of the analyzing equipment (see Section 5.2) so that a good signal could be attained from the gas available. Some comparison with analyses performed by laboratory-based gas extraction systems should be made initially to confirm accuracy and lack of gas fractionation.

Two recent developments have importance for dissolved gas sampling in the Finnish program:

47

1) A preliminary study and evaluation is underway at VTT, Helsinki, on the potential use of techniques where the gas composition from both the liquid and gas phases can be determined. This study uses an on-line mass spectrometer.

2) Gas sampling would be a more applicable method for boreholes drilled from ONKALO, the proposed underground test facility at Olkiluoto, where continous flow of ground water could be ensured (as used successfully at the URL).

Detachahle Gas Vessel VacuUlll Gauge

Sieel Bellows

Pressure Gauge

Degass:i.ng Ch.amher Coldtnp

Figure 20. Proposed system for use in field laboratory for gas extraction from groundwaters.

49

6 RECOMMENDATIONS

The foregoing sections have reviewed a number of methods used by different groups for the sampling of dissolved gases in groundwaters both in open boreholes and in boreholes installed with a multi-packer system. Many of the downhole sampling methods appear to give good results but often require complex electronic circuitry to operate valves and ports in the sampler. In addition, some of the samplers are too large to fit inside the 76 mm diamond-drilled boreholes that are commonly used in site characterization programs. In all of the methods, there is the uncertainty of whether the sample vessel has opened at the correct depth and whether it may have leaked on return to the surface. The recommendations below attempt to avoid these uncertainties and make dissolved gas sampling a rapid and routine procedure, performed at the surface, and with the level of accuracy that is required for understanding dissolved gas concentrations, sources and their implications.

6.1 Rapid sampling and on-site analysis

Based on the discussion in Section 5 and supported by the general consistency of results of AECL's dissolved gas sampling work (described in Section 4), it is recommended that sampling of dissolved gases in groundwater be performed routinely at the surface rather than down-borehole. Two methods may be used, either sampling a large volume of water and associated gas from a pump discharge outlet at atmospheric pressure or, preferably, using steel vessels and applying back-pressure during sampling.

Work to improve and test PAVE should continue so that down-borehole groundwater samples can be taken for dissolved gas analysis with confidence that outgassing of the sample and contamination by other gases do not occur. The results can be used for comparison with those of surface samplings. It is also worthwhile, however, constructing and testing a more simple downhole sampler such as that shown in Figure 6, where in-situ pressure is maintained by exerting pressure on the column of water in the tubing above the sampler to close a check valve below the sample vessels. This technique can be used either in open boreholes or in boreholes containing a multi-packer casing system; the diameter of the sample cylinders can be readily adjusted to fit inside the casing.

Although the sampling of groundwaters for dissolved gases may be simplified and become routine practice, as described above, often the greatest delay in hydrogeochemical characterization of a site is the time taken for analysis, because the samples have to be sent to an external laboratory. Three steps are involved in the analysis: 1) extracting the gas from the water sample, 2) measuring the total gas concentration, and 3) analyzing the gas composition. It is recommended that the methods proposed in Sections 5.2 and 5.3 are developed and tested to permit rapid on­site extraction and analysis to remove this problem.

A simple but effective system for gas extraction and measuring of total gas concentration was described in Section 5.3. It involves transferring the dissolved gases from the sampling vessel into a large evacuated degassing chamber, drying the gases, and compressing them into a gas vessel ready for analysis, without significant

50

gas fractionation. In Section 5 .2, the possibility of making accurate, on-site analysis of dissolved gas composition was described, using a modern, compact quadrupole mass spectrometer, installed in a mobile laboratory that is equipped with a stable electrical power supply.

6.2 Analysis of limitations and potential errors

Any downhole dissolved gas sampling method is subject to the following limitations and potential errors:

1) Sample equipment diameter may be too large to fit in the borehole, or may have insufficient clearance to go down the hole without difficulty.

2) Weights may need to be added to help 'sink' the sampler; once it is full of water the system may be too heavy for easy recovery and a winch may be needed.

3) Electrical connections or electronic signals, if fitted to the sampler, may fail due to moisture ingress, corrosion, etc.

4) The sampler may open to the groundwater at the incorrect time due to electrical problems, delays in getting the sampler down the borehole (for pre-timed actuators), sticking valves, etc.

5) The sampler may leak during retrieval; this could be due to a poor seal on the sampling port, partial blockage of the port by sediment, failure of electrical signals and components, etc.

6) The sample may become contaminated with back-pressure gas (in a piston assembly), with packer-inflation gas (if there is a packer leakage), or with ground water from shallower depths in the borehole (due to inadequate sealing of ports).

7) Unless sample over-pressurization is used, cooling or warming of the water sample during recovery and storage may cause gas-phase formation and the gas could be separated from the sampled water before analysis (e.g. by bubbles adhering to vessel walls).

In addition, a down-borehole probe or multipurpose sensing/sampling system is inevitably complex and awkward to use. It is generally heavy and becomes difficult to handle at depth (> 500 m) and if the borehole inclination shallows at depth, as is common in inclined holes. The sampler may fail for a variety of reasons, particularly if it is electrically operated, and this failure may not be apparent until the results of analysis are received sometime later. Recognition of failure may not be easy; obvious indications to look for are: 1) the presence of 02 in the gases indicating atmospheric contamination (either due to incomplete flushing before sampling or leakage prior to analysis), 2) the presence of high concentrations ofN2 in a multilevel packer system containing packers inflated by N2, 3) presence of high concentrations of piston­backing gas (e.g. Ar) in the sample, 4) low water sample volumes indicating incomplete filling of the vessel, due either to poor operation of valves and ports at

51

depth or leakage at the surface (usually associated with presence of 0 2), and 5) poor agreement between downhole duplicates or with surface-sampled replicates.

The limitations and potential errors associated with downhole samples for dissolved gases, as described above, argue strongly for an alternative method of sampling dissolved gases. Sampling at the surface is proposed here with parallel sampling, when possible, downhole using a simple pressurized device similar to that shown in Figure 6. The advantages of these methods are:

1) Sampling at the surface is simple, efficient and accurate in most situations (errors are detected by poor agreement between replicates, 0 2 contamination, etc.).

2) The downhole sampler can be made to fit in any size ofborehole.

3) The system is relatively lightweight and, in most cases, can be retrieved by hand.

4) There are no electrical or electronic connections, signals, or motorized valves and ports to give problems (gas pressure is used to close one check valve at the bottom of the assemb 1 y during retrieval and to maintain in -situ pressure).

5) The potential for leakage is minimized because only one check valve needs to be closed before retrieval.

Similar concerns regarding contamination by residual atmospheric gases, leakage of the vessels during storage, and gas-phase formation during retrieval still apply, however, and additional problems may exist including:

1) Inability to flush the sample vessels completely with groundwater because there is no downhole pump and groundwater sampling is 'passive' (i.e. groundwater slowly fills the vessels and the connecting tube to surface after the system is in place.

2) Only standing water in the open borehole or casing is sampled; any pumping or flushing must be done first using a downhole pump (this is true for several other types of sampler, however).

For the downhole sampler proposed here, the same indications for contamination or performance apply as described above. Sampling at the surface together with the rapid downhole method proposed here should give consistent and accurate results if sufficient replicate samples are obtained. The potential errors noted above can be minimized if all sample vessels (surface and downhole) are evacuated and checked for air leaks prior to use and the borehole zone is well-flushed by pumping groundwater until a stable chemistry is attained. Surface samples should be taken at the end of the pumping period, before the downhole sampler is raised.

The potential for introducing 02 contamination into the samples during gas transfer in the laboratory is a key problem which could be resolved by development of an on-line rapid analytical procedure such as proposed here and recently initiated by VTT.

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53

REFERENCES

Andrews, J.N., Hussain, N. & Youngman, M.J. 1989. Atmospheric and radiogenic gases in groundwaters from the Stripa granite. Geochimica et Cosmochimica Acta, 53, 1831-1841.

Bath, A.H., McCartney, R.A., Richards, H.G., Metcalfe, R. & Crawford, M.B. 1996. Groundwater chemistry in the Sellafield area: a preliminary interpretation. Quarterly Journal ofEngineering Geology, 29, S39-S57.

Bath, A.H., Pearson, F.J., Gautschi, A. & Waber, H.N. 2001. Water-rock interactions in mud-rocks and similar low-permeability material. Proceedings of the 1Oth International Symposium on Water-Rock Interaction, WRI-10, Villasimius, Italy, 10-15 July 2001, A. A. Balkema Publishers, Lisse, Netherlands, 3-12.

Bottomley, D.J., Ross, J.D. & Clarke, W.B. 1984. Helium and neon isotope geochemistry of some ground waters from the Canadian Precambrian Shield. Geochimica et Cosmochimica Acta, 48, 1973-1985.

Collins, A. G. 197 5. Geochemistry of Oilfield Waters. Developments in Petroleum Science, 1. Elsevier Scientific Pub. Co.

Duffy, J.R., Smith, N.O. & Nagy, B. 1961. Solubility of natural gases in aqueous salt solutions. Geochimica et Cosmochimica Acta, 24, 23-31.

Dyck, W., Pelchat, J.C. & Meilleur, G.A. 1976. Equipment and procedures for the collection and determination of dissolved gases in natural waters. Geological Survey of Canada Paper 75-34.

Fritz, P., Frape, S.K. & Miles, M. 1987. Methane in the crystalline rocks of the Canadian Shield. In: Saline Waters and Gases in Crystalline Rocks, eds. Fritz, P. & Frape, S.K., Geological Association of Canada Special Paper 33, 211-224.

Gascoyne, M. 2000. Dissolved gases in groundwaters at Olkiluoto. POSIVA OY Working Report 2001-49.

Haas, J.L. Jr. 1978 An empirical equation with tables of smoothed solubilities of methane in water and aqueous sodium chloride solutions up to 25 weight%, 360 oc and 138 MPa. USGS Open File Report No. 78-1004.

Helenius, J., Karttunen, V., Hatanpaa, E. & Makinen, R. 1998. Groundwater sampling from deep boreholes OL-KR2, OL-KR3, OL-KR4, OL-KR5, OL-KR8, OL-KR9 and OL-KR19 at Olkiluoto, Eurajoki in 1997. Posiva Working Report 98-23 (In Finnish with an English abstract).

Lampen, P. & Snellman, M. 1993 Summary report on groundwater chemistry. Nuclear Waste Commission ofFinnish Power Companies, Report YJT-03-14.

54

Loosli, H.H., Lehmann, B.E. & Balderer, W. 1989. Argon-39, argon-37 and krypton-85 isotopes in Stripa groundwaters. Geochimica et Cosmochimica Acta, 53, 1825-1829.

NIREX 1997. Acquisition of deep borehole groundwater compositions, Sellafield Geological Investigations for Deep Radioactive Waste Repository, Nirex Report No. SA/97/072.

Nurmi, P.A. & Kukkonen, I.T. 1986. A new technique for sampling water and gas from deep drill holes. Canadian Journal ofEarth Sciences, 23, 1450-1454.

Osenbriick, K., Lippmann, J. & Sonntag, C. 1998. Dating very old porewaters in impermeable rocks by noble gas isotopes. Geochimica et Cosmochimica Acta, 62, 3041-3045.

Pitkanen, P., Snellman, M. & Leino-Forsman, H. 1994 Geochemical modelling of the groundwater at the Olkiluoto site. Nuclear Waste Commission of Finnish Power Companies, Report YJT -94-10

Pitkanen, P., Snellman, M. & Vuorinen, U 1996 On the ong1n and chemical evolution of groundwater at the Olkiluoto site. Posiva Oy, Report POSIVA-96-04.

Pitkanen, P., Luukkonen, A., Ruotsalainen, P., Leino-Forsman, H. & Vuorinen, U. 1999 Geochemical modelling of groundwater evolution and residense time at the Olkiiluoto site. Posiva Oy, Report POSIV A-98-1 0.

Pitsch, H., Beaucaire, C., Meier, P. & Grappin, S. 2001. Sampling techniques and pH measurement methods for geochemical analysis of deep groundwaters. Proceedings of the lOth International Symposium on Water-Rock Interaction, WRI-10, Villasimius, Italy, 10-15 July 2001, A.A. Balkema Publishers, Lisse, Netherlands, 325-328.

Poole, J.C., McNeill, G.W., Langman, S.R. & Denis, F. 1997. Analysis of noble gases in water using a quadrupole mass spectrometer in static mode. Applied Geochemistry, 12, 707-714.

Ross, J.D. 1995. Chemical analysis of groundwater in the field. Site Screening and Site Evaluation Procedures Manual, Atomic Energy of Canada Limited, Doe. No. SI002.004, Whiteshell Laboratories, Pinawa, Manitoba.

Ross, J.D. & Gascoyne, M. 1995. Methods for sampling and analysis of ground waters in the Canadian Nuclear Fuel Waste Management Program. Atomic Energy of Canada Technical Record TR-588/COG-93-36.

Ross, J.D., Gascoyne, M. & Watson, R.L. 1995. Field methods for groundwater sampling. Site Screening and Site Evaluation Procedures Manual, Atomic Energy of Canada Limited, Doe. No. SI002.003, Whiteshell Laboratories, Pinawa, Manitoba.

55

Rubel, A. & Sonntag, C. 2001. Profiles of noble gases and stable isotopes across the Opalinus Clay formation at Mont Terri, Switzerland. Proceedings of the lOth International Symposium on Water-Rock Interaction, WRI-10, Villasimius, Italy, 10-15 July 2001, A. A. Balkema Publishers, Lisse, Netherlands, 1367-1370.

Ruotsalinen, P. & Snellman, M. 1996. Hydrogeochemical baseline characterisation at Romuvarra, Kivetty and Olkiluoto. Posiva Oy, Work Report PATU-96-0le.

Ruotsalainen, P., Alhonmaki-Aalonen, A., Aalto, E., Helenius, J. & Selge, R. 1996 Development of the pressurised ground water sampling equipment (in Finnish with an English abstract). Posiva Oy, Work Report PATU-96-82.

Sano, Y., Wakita, H. & Giggenbach, W.F. 1987. Island arc tectonics ofNew Zealand manifested in helium isotope ratios. Geochimica et Cosmochimica Acta, 51, 1855-1860.

Sherwood-Lollar, B., Fritz, P., Frape, S.K., Macko, S.A., Weise, S.M. & Welhan, J.A. 1988. Methane occurences in the Canadian Shield. Chemical Geology, 71, 223-236.

Sherwood-Lollar, B., Frape, S.K, Weise, S.M., Fritz, P., Macko, S.A., & Welhan, J.A. 1993. Abiogenic methanogenesis in crystalline rocks. Geochimica et Cosmochimica Acta, 57, 5087-5097.

Sherwood-Lollar, B., Frape, S.K. & Weise, S.M. 1994. New sampling devices for environmental characterization of groundwater and dissolved gas chemistry. Environmental Science & Technology, 28, 2423-2427.

Smellie, J. A. & Laaksoharju, M. 1992. The Aspo Hard Rock Laboratory: Final evaluation of the hydrogeochemical pre-investigations in relation to existing geologic and hydraulic conditions. Swedish Nuclear Fuel and Waste Management Company, Technical Report SKB TR 92-31.

Snellman, M., Helenius, J., Makinen, R. & Rajala, R. 1995 Groundwater sampling of the multipackered boreholes during 1993-1995 at Olkiluoto, Eurajoki (in Finnish with an English abstract), Teollisuuden Voima Oy, Site Investigations, Work Report PATU 95-57.

Thury, M. & Bossart, P. 1999. Mont Terri Rock Laboratory. Results of the hydrogeological, geochemical and geotechnical experiments performed in 1996 and 1997. Geological Report 23, Bern, Swiss National Hydrological and Geological Survey.

Watson, R.L. 1996. Chemical analysis of groundwaters in the laboratory. Site Screening and Site Evaluation Procedures Manual, Atomic Energy of Canada Limited, Doe. No. SI002.005, Whiteshell Laboratories, Pinawa, Manitoba.

Westba~ Instruments. 1994. Multi-level ground water monitoring with the MP System . Internal report by Westbay Instruments Inc., 1992-1994, Vancouver, B.C., Canada.