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
P.O. Box 141
6 Tupper Place
Pinawa, MB ROE 1LO
Canada
Phone 1-204-753-8879
Fax 1-204-753-2292
e-mail: [email protected]
Margit Snellman,
POSIVAOY,
Toolonkatu 4,
FIN -001 00 Helsinki,
Finland.
Fax: 358-9-2280-3719
April 15, 2002
SUBMISSION OF REPORT
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 "·./,
'
.
•
I ;,. · t
.
.,,
._t,i "
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
POSIVA during early attempts at dissolved gas analysis at Olkiluoto.
Several organizations, including AECL and the University of Waterloo (Canada),
NAGRA, POSIVA, 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
ANALYSOINTI
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, POSIVA,
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
1
TABLE OF CONTENTS
Page
ABSTRACT
TIIVISTELMA
PREFACE
............................................................................................................. 3
1
INTRODUCTION .............................................................................................. 5
2
PROBLEMS IN DISSOLVED GAS SAMPLING ................................................ 7
2.1
2.2
2.3
2.4
3
METHODS OF DISSOLVED GAS SAMPLING AND ANALYSIS .................... 15
3.1
3.2
3.3
3.4
3.5
3.6
4
Gas phase formation ............................................................................. 7
Gas solubility and fractionation ............................................................. 8
Previous sampling at Olkiluoto .............................................................. 9
Extraction, transfer and analysis of gases ........................................... 13
Geological Survey of Canada .............................................................. 15
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
University of Waterloo, Ontario .......................................................... 20
POSIVA .............................................................................................. 22
3.4.1 Early methods .......................................................................... 22
3.4.2 Recent methods ...................................................................... 23
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
Other studies ...................................................................................... 29
3.6.1 Finland ..................................................................................... 29
3.6.2 The Westbay System ............................................................... 30
3.6.3 Oil industry............................................................................... 32
CANADIAN EXPERIENCES AND DATA ........................................................ 35
4.1
4.2
4.3
4.4
4.5
Borehole completion systems ............................................................. 35
Sampling at the surface ...................................................................... 36
Down-borehole sampling .................................................................... 38
Underground sampling ........................................................................ 38
Discussion and summary .................................................................... 38
2
5
FEASIBILITY STUDY FROM SAMPLING AT OLKILUOTO ........................... 43
5.1
5.2
5.3
6
Methods for rapid sampling ................................................................. 43
Methods for on-site analysis ................................................................ 44
Methods development. ........................................................................ 45
RECOMMENDATIONS ................................................................................... 49
6.1
6.2
Rapid sampling and on-site analysis ................................................... 49
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 groundwater 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, H 2S). 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 groundwater 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.
10
0.40
•
0.30
• Glass
•PAVE
N
z
G)
•
•
0.20
::1:
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 N 2), (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
P~essure
/<Jive
l ••:pJ;t.1~
GdC'<.@
_,i'
lone to
be ::.ampied
~
:i
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 N 2 and Ar content) using the standard abundances of 0 2,
N2 and Ar in air.
High N 2 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.
12
10000
--- ---...
:J
::J
.sen
CV
Cl
0
--
-
1000
100
Gl
E
::J
0
..
.A Glass
•PAVE
... L'" ...
•ft ......
........
......
...
>
10
0
200
400
600
800
1000
1200
Depth (m)
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.
13
1200
1000
D
800
D
c
0
;;
....c"""C'G
G)
(.)
+S04 (mg/L)
DCH4 (ml/L)
600
•
•
• • •
c
0
0
D
•
400
• •
••• • •
200
D
El
D
~
D
0
0
200
400
600
800
1000
1200
Depth (m)
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), ANDRA (France), NAGRA (Switzerland), NIREX (U.K), POSIVA
(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
borehole
Valve at each
end
50-100 mL
Copper (soft)
tube
AECL
flow-through
cell/laminated bag
POSIVA
Crimp at each
end
Heat seam
~lOml
Aluminium
Open or cased
borehole
Open or cased
borehole
~150
mL
Glass
vessel with valves
Open or cased
borehole
Ground glass
valves
~150
mL
Lampen & Snellman
(1993)
Stainless steel
PAVE sampler
Open borehole
~250
mL
Lead-glass
vessel
Open or cased
borehole
Ruotsalainen et al.
(1996)
Sano et al. (1987)
Stainless steel
Westbay MOSDAX®
Stainless steel
Waterloo sampler
W estbay-cased
borehole
Open or cased
borehole
Electronic
valves
High vacuum
stopcocks at
each end
Electronic
valves
Electrical
valves
50mL
1-2 L
200mL
Bottomley et al.
(1984),
Ross & Gascoyne
(1995),
Bath et al. ( 1996)
Bottomley et al.
(1984)
Lampen & Snellman
(1993)
Westbay
Instruments ( 1994)
Sherwood Lollar et
al. (1994) _
1--'
0\
17
3.2.1 Sampling at the surface
Sampling is performed in duplicate or triplicate in stainless steel vessels from
groundwater 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
groundwater warming up (in summer months) and outgassing 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 groundwater 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 N 2 pressure is released and the vessels disconnected for
analysis.
19
U
S.S. SWAGELOK
FITTINGS
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
S.S. SWAGELOK
FITTINGS
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)
S.S. SWAGELOK
FITTINGS
DOWN HOLE DISSOLVED
GAS SAMPLINGASSEMBLY
'./..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
\
.--.==:;-;:::::::{
~
'--""'"
--~FLUSHING
[ o:A:o¥
II
,.....
c)o-o
I
DiSCHARGE
oO
0
O~o
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
BATIERY 1TR!~~R OUTPUT
RECHARGING~ ~WI 1 '-H PORTS
PORT l l CASING I
I il
l
WATER
il'-IL'AKE
-PORTS
AITACHMENT
t ELECTR"""NICS LiNKS
l
¥£
r
I
Frr~I~GS
r
~~I \[f1tffi1tfitl
(_~~j®!~ool~i[ ~~=~ ~~~~ 1~®1!!~
~~~~
L.J~LJ~
'Yt~.LJ'i
,
BA'JTERY
PACK
,.
,r
VALVE
,
I
,
j
r
MQTQR
l ~~CK,
! ACCESS MOTORiZEDiVALVa:.1
; "
,,~, ,rr: ,
_
j
; Pv RTS
ELECTROSICS
PUMP ,
.., .t'Y..I • c 1
ru. TER
SAMPLE
VESSEL
J
1
l
I
MOTOR.
PUMP
'
1•
•r
BATJERI
rACK
i6g::s
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 H 2 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 outgas 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 groundwater 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
N 2) 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 AVB 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 ANDRA (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 10-13 m/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 (LVS) were collected in 1. 5 or 5 L stainless steel
vessels after first evacuating or purging with N 2 . 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 N 2
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.
26
Transfer
Valve
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).
CLOCK
AIR CHAMBER
REGULATOR
VAll/E
CLOSURE
DEVICE
FLDAllNG
PISTON
N
FLDATIIO
.....,J
PISTON
FLOAllNG
PIS TOll
.IAMPLIIIG
l'ORTS
·>:<·fiXED
SAMPLING ~~PISTON
PORTS
KEY
SPOOL VALVE
DIEifRYDIIFILIO
(iJ IUI'f!RRUID
•
POWERR.UIO
.MI!IIOG£11
1 RllliiiiiiG POSinON
'M111DGBIQWIIII!CIIIUIIfACl
_.,.PDW!IIFWII
"WWPIJIIIPCII11CLDIED
._a.DCJimiii'DIIIG
nMEOfftiWIATOIIVAI.VI
2 SlMT SAMPUNO
"RE&UWiliiVMftiii'DEDIYI:UICI
'IUfRIIFWIIPUIDIOAIRC•R.OAIUIGI'IIION-.JITIIIOIIliiDF
REI!ImllllflUID
3 COMPLETE SAMPUNO
4 PRESSURE COMPENSAllON
"UWI.I~FIIUDFIEifii'IOIRflUIO
"AilDDIIIIImiii'VmTIII!IIAMIEDROPI
•flOAnMD -ACTI ell ClOIIJIE DMC!
1111110¥! maii'IITIHIIIDLAniiG LIMPI.IIIG
I'OIITI
"II'GDIV.IlVIDI'OIInRlliiGMlOiniiiW.
(IIJIIFACfPR!IfllPRlSSilllfDIWII'I.!
NIIUIIII'UIIIUNIUI
"AI'IIOETPIIfiiUIIEIIIMIITAIIIIIJON
111EIAIIPI.EIY111EI'WDI'UIIl
"PRERTI'IIEIIIIIIIIIDEID•IEIIf
llllii1CIIIICIWI;ii'IIE-PIIIDII
ID RUNNING
Figure 10. Principles of operation of the Singlephase Reservoir Sampler used in the NIREX program (from NIREX 199 7).
28
3.5.4 Stripa (Sweden)
As part of the detailed hydrogeochemical studies performed on groundwaters 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 37Ar, 39Ar 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 MPa 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).
ANALYSED PARAMETERS
I
DRILLING WATER CONTENT:
Uranine
DISSOLVED ELEMENTS;
Na K Ca Mg Si 0 2 Mn
Fe (tot) Fe (11} HC03 Fa
Br $04 S(·fl) N~ N03
NH.P041
FIELD LABORATORY
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
Backup storage
c.
I
I
1
DISSOLVEO ELEMENTS:
NaKCaMgSi Mn
Fe (tot) HC03 F Cl S(-11)
N0 2 N02 +N03 NH.P0 4 ~
SrUt AaTh URn
AlTER RESIOUES:
,_____.....,.......c-_-.______
.__;~-t
Ca
AI Fe Mn S SI
ISOTOPES:
2ti
tXlWNHOlJ: CHEMICAL
PROBE: pH, Eh
~ 'to~ 14C ~u
ORGANICS:
TOC Fulvic and humic acids
GASES:
GAS SAMPLER
I,CH.....
--------1 Na 0z H2 CO C02/v He
C4 H10
PUMP
Figure 12. The Swedish geochemical analytical system including a down-borehole
dissolved gas sampler (from Smel/ie and Laaksoharju 1992).
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.
30
Overburden
"
.--'
! ;
-·
oi
'"-"'I
U'j.
I
I
'
\
\
\
Back. -pressure
valve
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 groundwaters 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.
32
MP Casing
Sampler
Probe
Location Arm
Pressure
Transducer
Measurement
Port Valve
Backing Shoe
Measurement
Port Coupling
Sampling
Valve
a)
Probe located at
measurement port
b) Probe activated, sampling
valve closed.
c)
Sampling valve open,
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.
20,000 psi(~ 140 Mpa) and 360°F (~ 180°C).
The system is tested to
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 groundwaters 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 N 2 from the packers might be
expected to cause high N 2 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
834
834
837
837
M1B
M1B
M2A
M2B
M2B
M2B
M3A
M4A
M4A
M4A
MSA
MSA
M6
M7
M7
M7
M7
MS
MS
M10
M10
M10
M12
M12
M13
M13
M14
M14
M14
URL1
URL3
URL4
URLS
URL9
URL10
URL10
URL11
URL12
URL12
.URL12
URL12
URL12
URL12
URL14
URL14
URL15
WA1
WA1
WA1
WA1
WA1
WA2
WA3
WB1
WB1
WB1
WB1
WB1
WB2
WB2
WD2
WD3
WN1
WN4
WNS
WNS
WN10
WN11
WN12
-1
-2
-1
-2
-2
-2
-3
-2
-2
-2
-3
-4
-4
-4
-2
-2
-2
-4
-4
-4
-4
-3
-3
-3
-3
-3
-159
-171
-2
-2
-4
-4
-4
-S
-6
-5
-7
-6
-3
-6
-7
-10
-11
-11
-13
-13
-13
-270
-2
-3
-5
-5
-16
-S
-6
-1
-2
-5
-7
-7
-19
-20
-72
-S95
-S
-6
-17
Sample
No.
-4
-1
-2
-1
-3
-4
-4
4
-1
-9
-5
-6
-9
-6
-10
-4
-5
-3
-11
11 (B)
-7
-5
-2
-2
-9
-2
11,12
-15
-3
-10
-4
-5
-3
-16
-9
-10
-7
-2
-S
-7
-7
-19
-13
-11
-1S
-21
-13
-S
-14
-11
-S
-6
-4
-7
15
-5
-S
-5
-6
-1S
-9
-S
-5
-12
-5
-S
-17
-S
-7
T2
-4
-15
-17
Depth Interval
m
0-32.5
32.5-60
0-30
30-60
51-150
51-150
270-400
111-152
130-160
111-152
351-400
291-406
2SS-403
291-406
2S7-35S
2S7-35S
95-130
351-400
351-400
351-400
351-400
311-400
311-400
400-430
341-450
341-450
159
171
226-443
226-446
300-3SO
301-3SO
301-3SO
316-347
142-167
53-73
276-315
140-163
54-121
270-302
1S3-202
404-450
45S-502
45S-502
637-699
637-699
637-699
0-397
270-397
0-40S
210-310
310-3SO
490-750
490-750
71S-740
26S-2S3
110-134
0-210
210-300
6SO-S60
S60-1120
1120-1203
92S-972
974-99S
63-301
S00-1202
3S1-402
356-402
315-322
315-322
300-496
1099-1201
320-349
Date
A
8
c
24-0ct-S6
13-Jun-95
21-Aug-93
23-Aug-93
14-Aug-S7
15-Feb-91
29-May-S6
OS-Dec-92
8-Apr-S6
3-Sep-S9
26-Jun-91
14-Jui-S6
2S-Jui-S9
2S-Aug-90
25-Apr-90
26-Jul-91
7-Aug-S6
3-Nov-92
5-Sep-S6
24-Apr-S9
26-Jun-91
7-Jun-91
7-Sep-93
15-Apr-S6
3-May-S9
10-Sep-93
24-Aug-S3
2S-Jui-S3
16-Jun-S6
26-Sep-S9
3-Jun-S6
12-Apr-S9
7-Jun-93
20-Jui-S9
20-Aug-SS
23-Jui-S6
9-Feb-S9
20-Sep-93
2-Aug-90
9-Jui-S6
30-Nov-SS
11-Apr-90
11-Sep-S6
3-Nov-S9
1S-Jui-S6
23-Jui-S6
2S-Mar-S9
23-Jui-S7
21-Jun-S9
23-Jui-S7
5-Jun-S7
3-Nov-S7
11-Mar-SS
18-Mar-88
22-Mar-91
16-Apr-91
3-May-91
5-Jun-S7
5-Jun-S7
30-Sep-S7
27-May-SS
23-Jan-91
22-Dec-92
5-Mar-91
1-Sep-SS
11-Sep-SS
24-May-S7
S-May-S7
11-Jun-S6
1S-Feb-SS
26-Mar-S7
9-Dec-S6
27-Jun-S9
90.1
61.2
69.5
49.3
33
35.3
1S1.S
70.9
36.6
36.3
60
1S7.7
95.5
54.S
77.1
61.2
61.2
36.S
37.4
53.4
25.4
34.6
35.7
S0.1
71.4
149
57.6
63.9
131.7
2SO
74
7S.2
105
42.6
34.1
37.6
4S.2
40.2
44
45.6
40.3
57.S
65.1
54.S
59.4
66.7
79.4
23.9
5S.3
30.4
37
40.S
110.7
89.9
107
35.7
50.7
24.4
26.3
35.S
24.6
35.S
73.9
49.9
53
43.4
43.2
57.7
57.3
45.4
42.S
35.6
53.6
51.6
53.3
6S
47.6
31.7
36.S
121.4
32
32.4
35.3
33.6
1SS.S
6S.3
69
S0.5
61.2
24
27.2
52.4
42.3
24.4
33.6
36.S
50.9
7S.5
36.S
3S.6
51.1
67.4
66.4
70.9
65.1
S3.7
50.S
41.6
44.2
44.3
41.4
43.7
57.2
34.6
70.4
32.3
61.S
51.6
50.2
62
41.9
S5.6
39.5
39.5
40
97.3
142.8
114
50.7
50.7
56.6
3S.5
27.2
4S.5
45.7
90.5
70.3
49.4
53.3
44
56
57.7
46.3
62.1
42.1
S4.9
59.S
DHA
DH8
62.3
65.1
62.3
65.1
32.9
34
61.2
35.7
9S.6
11S
ss
40.S
63.9
26.2
47.1
S3
41.3
38
4.3
Down-borehole sampling
Several of the groundwaters 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 groundwater 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 N 2 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
::J
200
~
-
B
-
E
f /)
C'G
C')
150
"C
Cl)
>
0f/)
w
.!!! 100
\0
c
50
0
-
11
B
11
1111 11
~~~~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 borehole zones in the WRA.
300
~
250
DA
•a
::i
::J
E
eftSn
200
0)
150
-
ODHA
-
ODHB
"C
-
Cl)
>
0
en
en 100
.--
c
.....
,....
50
--
--
-
-
,.J:::..
~
0
~
,....
-
~
~
rr-
~~
0
837
Il
T
I
837
M2B
M3A
M7
M7
~
~
I
M13
M14
URL9
-
.--
I
I
MS
~
I
I
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.
350 ~----------------------------------------------------------------------------------,
300
-250
...J
:J
E
-;; 200
cu
,.,
C)
CD
~
0
150
Cl)
.!!!
c
~
100
~
50
0
HC6
HC7
HC7
HC8
HC11
HC14
HC15
HC16
HC16
HC18
HC19
HC26
HGW1
101-
0C1
205PH2
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
HC6
HC7
HC7
HC8
HC11
HC14
HC15
HC16
HC16
HC18
HC19
HC26
HGW1
101-0C1
205-PH2
-7
-1
-1
-10
-10
-2
-3
Z2-10
Z3-10
513,17
-18
-4
01-Sep-87
08-Aug-86
05-Sep-86
27-Jan-88
27-Jan-88
11-Sep-95
03-Feb-87
27-Jan-88
27-Jan-88
09-Jun-93
21-Nov-89
20-Jun-95
03-Jun-96
13-Jun-96
08-Nov-95
-5
-5
-8
A
45.4
33.2
68.2
49.4
56.1
32.3
88.5
48.9
49.2
48.4
47
202
57.8
54.4
40.8
8
49.8
56.6
56.2
54
52
30
107
40.2
60
53.6
43.6
312
56.1
54.4
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 H 2 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
Consideration should therefore be given to
schedule of borehole activities.
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 groundwater 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 groundwater 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 Nupro®-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 groundwater 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
groundwater 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 1y 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.
52
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, 1015 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 krypton85 isotopes in Stripa groundwaters. Geochimica et Cosmochimica Acta, 53, 18251829.
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 POSIVA-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, WRI10, 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
groundwaters 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, 1015 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 groundwater 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, 18551860.
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 groundwater monitoring with the MP
System . Internal report by Westbay Instruments Inc., 1992-1994, Vancouver, B.C.,
Canada.