Interlaboratory comparison of methods to determine the stable isotope composition of soil water

Chemical Geology (Isotope Geoscience Section), 111 (1994) 297-306

297

Elsevier Science B.V., Amsterdam [PD]

Interlaboratory comparison of methods to determine the stable isotope composition of soil water Glen R. Walker ~, Peter H. Woods a'" and Graham B. Allisonb aC.S.LR. O. Division of Water Resources Centre for Groundwater Studies, PMB 2, Glen Osmond, S.A. 5064, Australia bC.S.LR. O. Division of Water Resources Private Bag, Wembley, W.A. 6014, Australia (Received August 5, 1992; revised and accepted April 6, 1993)

ABSTRACT This paper presents results of an interlaboratory comparison of the effects of different techniques for extracting soil water on its measured 2H and ~sO composition. In the comparison, four soils (a sand, a gypseous sand, and a clay soil at high and low water contents) were prepared and distributed to fourteen laboratories. Water was then extracted from these samples and analysed using each laboratory's standard method. A number of verification procedures was used to ensure that the experiment was truly a comparison of extraction techniques and that reported variations were not due to sample preparation, transport or measurement. The extraction techniques used included azeotropic, vacuum and microdistillation methods. The results show a large variation between laboratories in the isotopic composition of the water extracted (of up to 300/00for 2H and 3.40/00for 180). The variation increased as the water content of the soil decreased and was greater for clays than sand at comparable soil matric suctions. The ~-value obtained was correlated with the final extraction temperature, with incomplete extraction being the most likely cause for the variation. The study highlights the need to develop standard protocols for the extraction of water from soils for isotopic analysis.

1. Introduction

The concentration of stable isotopes of water in soil water are increasingly being used in studies of groundwater discharge (Fontes et al., 1986; Christmann and Sonntag, 1987; Barnes and Allison, 1988), recharge (Dinner et al., 1974; Hendry, 1983; Saxena and Dressie, 1984; Darling and Bath, 1988), and plant water use (Bariac et al., 1983; Brunel et al., 1990). The ability to extract isotopically unaltered water from soils is essential to these studies. Unfortunately, it is difficult to validate the methods used for extracting water. Socalled "doping" experiments, in which water of a known isotopic content is added to oven(2H, 180)

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dried or vacuum-dried soils can suffer in that some tightly bound water may not be removed during the drying process. The water extracted for isotopic analyses is then a mixture of the water added and residual soil water, the isotopic content of which is generally unknown. This residual water becomes more important as the quantity of added water decreases, i.e. for dryer soils. This was also observed by Turner and Gailitis ( 1988 ). Not only does this dry range represent the area of interest of many of the isotope studies, it also represents the range where relative errors in the extraction process are likely to be greatest. There are several soil water extraction techniques in use and each has several variations. It is rare for two laboratories to use exactly the same method. It has not been demonstrated that the different methods yield the same results. Some early experimental results (Woods,

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298

1990) suggested that different methods do not yield consistent results for relatively dry soils. To enable this problem to be addressed, an interlaboratory comparison was conducted in which several methods were represented. Fourteen laboratories participated. Unlike many other interlaboratory comparisons which compare similar methods used in different laboratories, this study compares different techniques. If differences in isotopic composition can be correlated with differences in technique, the interlaboratory comparison would not only be a check of consistency, it would also enable some critical analysis of bias associated with the different methods of extraction.

G.R. WALKER ET AL.

2.2. Vacuum distillation A temperature gradient under vacuum between the soil sample and the collection vessel is created (Jusserand, 1980; Saxena and Dressie, 1984). Usually, the collection vessel is cooled with liquid nitrogen and the soil sample is heated either by a heated water bath or electrical heating mantle. Differences in technique between laboratories include heating temperature, time taken for the distillation, use of heating coils to avoid condensation and method of evacuation.

2.3. Azeotropic distillation 2. Experimental methods

2.1. Overall design

Four different samples were used: a sand, a gypseous sand and a clay at two different water contents. The soils were treated as described in the following section and then sent to each of the participating laboratories. Those laboratories were requested to process the samples in their usual way and if possible send an aliquot of the water extracted to the our laboratory after analysis. Finally, duplicate samples were processed by our laboratory in order to assess the degree of sample-to-sample variability. Laboratories are identified by letter and method only. They are listed in arbitrary order in Table 1. All results in this paper are reported relative to V-SMOW. The three main techniques in the comparison were vacuum distillation, azeotropic distillation and microdistillation with zinc. One laboratory (D) centrifuged the samples at 13,000 rpm using Arklone ® as the displacing liquid; however, this technique only gave sufficient sample for analysis from the gypseous sand.

In this method (e.g., Revesz and Woods, 1990), an immiscible hydrocarbon is added to the soil sample and the mixture heated. A hydrocarbon-water azeotrope forms, the boiling point of which is less than that of either of its constituents. Upon cooling, the components separate with the less dense hydrocarbon returning to the distillation vessel. The main difference in the techniques used by the participating laboratories using azeotropic distillation is in the choice of the hydrocarbon. The hydrocarbon influences both the temperature at which the azeotrope is formed and the final temperature to which the sample is heated, the latter being the boiling point of the hydrocarbon. The azeotropic temperatures for hexane, toluene and kerosene are 61 °, 81 ° and 96°C, respectively, and the boiling temperatures 65 °, I10 ° and 185°C, respectively. The boiling point for petroleum ether used by laboratory G was 120 ° C. Most of the soil water is removed at the azeotropic temperature but some water is usually removed at temperatures between this and the boiling point of the hydrocarbon. The other main difference between the laboratories is the time allowed for distillation.

METHODSTO DETERMINETHE STABLEISOTOPECOMPOSITIONOF SOILWATER

299

TABLE1 List of laboratories participating in the interlaboratory comparison of methods to determine the isotope composition of soil water Group and/or organization

City

Country

Isotope Division, Bhabha Atomic Research Centre Institute of Hydrology, British Geological Survey CSIRO Division of Water Resources CSIRO Division of Water Resources CSIRO Division of Water Resources Institute of Nuclear Sciences, DSIR Institute of Hydrology, GSF Miinchen Division of Research and Laboratories, IAEA U.S. Geological Survey Department of Dynamic Geology, Pierre et Marie Curie University Laboratory of Isotope Hydrology and Geochemistry, University of South Paris Division of Hydrology, Uppsala University Department of Earth Sciences, Waterloo University Weizmann Institute of Science

Bombay Wallingford Adelaide, S.A. Perth, W.A. Canberra, A.C.T. Lower Hutt Neuherberg Vienna Reston, Va. Paris

India U.K. Australia Australia Australia New Zealand Germany Austria U.S.A. France

Orsay

France

Uppsala Waterloo, Ont. Rehovot

Sweden Canada Israel

Laboratories are listed in an arbitrary order.

2.4. Microdistillation with zinc (for deuterium only) A small soil sample is placed in a side-arm of a reaction vessel containing zinc. The zinc is heated to 450°C while the soil sample is heated to between 100 ° and 200°C (e.g., Turner and Gailitis, 1988 ). Water vapour thus formed is then reduced by the zinc to hydrogen ready for mass spectrometry. The main difference between the laboratories is the temperature of the soil sample.

2. 5. Errors associated with each of the major techniques The main error for all of the techniques except centrifugation is incomplete extraction of the soil water. The vacuum, azeotropic and microdistillations can all be modelled as a Rayleigh distillation process. Therefore, if the amount of water remaining in the soil is greater than 1-2% of that originally present, large errors can result. For a Rayleigh distillation, the relative errors in j2H to those in fi180 de-

creases slightly with temperature from a ratio of 7.8 at 35°C to 6.1 at 80°C. For high-temperature vacuum distillations and microdistillations, the distillation is unlikely to proceed at constant temperature, and non-equilibrium effects could become important. Temperature will have other effects. Firstly, the use of higher temperatures will generally speed the rate of distillation. Also, there may be decomposition of organic matter or extraction of some water of crystallisation. Other sources of error besides incomplete extraction include for vacuum distillation, leaks in the vacuum line and condensation (and hence incomplete collection of the sampie); for azeotropic distillation, loss of water through the condenser and condensation on the glassware; and for microdistiUation, a bias for clays (Turner and Gailitis, 1988).

2. 6. Sample preparation Bulk samples of soil were crushed and passed through a 2-mm sieve. With the exception ot the gypseous sand, they were then oven dried

300

G.R. WALKERET AL.

TABLE 2 Reproducibility of the isotopic composition of pore water in prepared samples by a single method-azeotropic distillation Sample

Hydrocarbon

Number of replicates

s.d. (%ovs. SMOW) ~2H

~tsO

Sand

kerosene

10

-+ 1.0

+ 0.30

Gypseous sand Gypseous sand

hexane kerosene after hexane.

10 6

± 1.2 ± 1.1

_+0.20 ± 0.24

Clay (high water content) Clay (low water content)

kerosene kerosene

10 10

-+0.5 ±0.8

_+0.13 +_0.18

s.d. = standard deviation. TABLE 3 Some physical properties of the test soils. Soil type

08 air

(g g- ~) after addition of water

CI(mg kg-~ )

Gypsum (g g-~ )

CaCO3 (g g- ~)

Organic carbon (mg g-~ )

Matric suction (kPa)

(1) Sand (2)Gypseous (3) Clay (4) Clay

0.001 0.002 0.043 0.043

0.051 0.102 0.147 0.252

900 160 100 100

0.00 0.35 0.00 0.00

0.000 0.037 0.000 0.000

1.3 2.2 2.9 2.9

160 47 5,000 150

Calcium carbonate and organic carbon were determined by measurement of C O 2 gas evolved on ignition of plain and acid-washed aliquots on a Leco ® carbon analyser. Chloride concentration was determined by colorimetric methods on a dilution extract (Taras et al., 1974), matric suction by the filter paper method (Greacen et al., 1989) and gypsum content by the difference between vacuum-dry and oven-dry weights (Nelson et al., 1978 ). The isotope content of water extracted from the air-dry clay sample was analysed and is also given in Table 4.

at 105 ° C. The gypseous soil was dried at room temperature in a desiccator under vacuum. The soils were then allowed to equilibrate with atmospheric water vapour. Bulk samples were homogenised and then divided for use with a geological sample splitter. The gypseous soil was prepared as a 50:50 mixture of a low-clay, quartzose sand and soil from a gypseous dune (lunette) deposit. The average total "dry" gravimetric water content, 0g (including any pore water and water o f crystallisation o f gypsum) was 0.0997+_0.0012 g g-~ ( +_ 1.2%, eight subsamples). For the air-dry sand and clay, the average q was 0.0014+_0.004 g g - i (four replicates ) and 0.0434 +_0.0002 g g - ~ (four replicates), respectively. A pre-determined amount o f water was

added to the air-dry soil and the containers shaken. The average error in the amount of water added was less than +_0.05 g in 10-30 g total ( +_0.2-0.5%). The water used to prepare the samples was Adelaide (South Australia) tap water, with fi2H = - 16.2°/oo and 18O = _ 2.73o/oo. Sample to sample variability for each soil type was tested at our laboratory using only one mode of extraction. The standard deviations (shown in Table 2) are less than +_ 1.2%o for fi2H and less than +_0.3%o for ~180. Some general physical properties of the soils are listed in Table 3. Soil water in the samples ranged between freely available in the gypseous soil (matric suction ~ 47 kPa) to tightly bound in one of the clays (matric suction

METHODS TO DETERMINE THE STABLE ISOTOPE COMPOSITION OF SOIL WATER

301

~ 5000 kPa). The percentage of organic carbon was low in all of the samples.

added to the dry soils. A more detailed description of these results is given below.

3. Results and discussion

3.2. ~2H-81sO relationships

3.1. Errors associated with analyses

~2H-~180 plots for the four soils are shown in Fig. 1a-d. All show a large spread of results which is most pronounced for the dry clay and the gypseous soil. The spread of results is not random, and lies close to the Rayleigh line for distillation at 35°C. (Note that the reference is the volume of water added plus that in the air-dry sample. ) The difference in the isotopic composition (3.7%o for t~2H and 0.5%o for ~80) of the water sample extracted by centrifugation of the gypseous sample and the mass balance suggest that there may be some error in using this as a reference.

Where possible, participating laboratories returned part of their water sample for analysis to our laboratory to provide a comparison of the analyses done by the our laboratory with those in the participating laboratories. The RMS difference in analyses is 2.3%o for t~EH and 0.360/oo for 81sO. The results, showing a large spread of results are given in Table 4. Good correlation is observed between 82H and 8lSO. Almost all of the analyses are significantly lighter than the water TABLE 4 Results s u m m a r y Laboratory

A B D D' D" E F G H I J K L M N N' N"

Method ~

a a m v c v v a v v m m m v a a a

Temp. 2 ( °C)

185 110 175 150 room 50 80 120 35 100 200 110 200 185 65 185

(1) Sand

(2) Gypseous

( 3 ) Clay, dry

( 4 ) Clay, wet

52H

51s O

52H

5180

52H

5180

52H

5180

-27.8 -24.7 -31.0 -30.8

-4.72 -4.23

-1.46 -2.31

-30.3 -33.1 -43.1 -41.6

-4.42 -4.84

-3.81 -4.30

-4.93

-25.0 -27.2 -34.5 -30.2

-32.7 -26.8 -28.3 -27.4 -29.1 -41.9 - 26.1 - 32.0

-4.51 -4.58 -4.76 -5.03 -4.41

-31.0 -29.1 -39.5 -30.2 -20.0 -29.2 -32.1 -26.3 -32.1 -29.9

-48.6 -38.1 -34.3 -57.1 -39.0

-6.30 -6.00 -5.21 -8.02 -4.87

-38.3 -36.6 -30.0 -44.2 -31.3

-4.90 -5.76 -4.68 -6.57 -4.21

-24.6

-3.81

-4.30

-1.42 -3.27 -3.73 -3.20 -3.72 -4.88 -1.28

- 34.7 - 28.0 -28.7 -23.2 -35.5

broken

- 36.4 broken -0.78 -3.75 -2.92

-28.1

-3.55

-3.61

- 40.0 - 27.0 -24.0

-3.43

All isotope values are in %0 relative to V - S M O W . Dope water composition 5 2 H = - 16.2%0, 51sO = - 2 . 7 3 % 0 . Multiple methods by a laboratory: D, D ' , D" = microdistillation, v a c u u m distillation and centrifugation, respectively; N, N', N" = azeotropic distillation with kerosene, hexane and kerosene after hexane, respectively. Air-dry isotope composition 5 2 H = - - 37.0%o, 5~sO = - 3.82%o. Where analyses have been done by b o t h laboratories, the analysis by the laboratory who did the distillation is given. 1Methods: a = azeotropic; v = v a c u u m distillation; c = centrifugation; m = microdistillation with zinc. 2Temp. denotes final boiling temperature. For laboratories using azeotopic m e t h o d 185 °C implies kerosene, 1 2 0 ° C petroleum ether, 110 ° C toluene, 65 ° C hexane. For some of the 1 laboratories using microdistillation or h i g h - t e m p e r a t u r e v a c u u m distillation, the final boiling temperature m a y only be approximate.

302

G.R.WALKERETAL. -10

-10'

DRY CLAY (a) -20 azeotropic d i s t i l l a t i o n s 2

WET CLAY

1

-20

~

/iZH -40

i g h temperature //6~ ~ ' ) -Wacuumdistillati°n

/

-50

/ J [h~i

-60

azeotropic distillat~n

~

--S

-

--3

~-

-50 i

/~ldow temperature "=vacuumdistillation

-;/

-9

2 H-40,

,, ~ / , 2

~

hi~tem~p rature vacuumdistillation I

~

low temperature -60 ' vacuum distillation -

-1

-70

-7

-9

-

-s

-10

-10~ GYPSEOUS SAND -15, : I -20 ~ _/~l-water -25

i

(c)

SAND

-15

-20 -25

g/.t/

-3

-1

(d) from centrifugation

I

/i2H -30

~2H-30'-35' h ~

ef

-40, -45, -6

-

/il8 0

618 0

-35 -40 -45 -50

1

-30~

-30

-70

(b)

-4

5z

0

]

~ils O

-so -61

b din

water of erystalli tion "JV" - ' 4 n''

-4

-z

-

blSo

Fig. 1. d2H-d~sO plots for: (a) dry clay; (b) wet clay; (c) sand; and (d) gypseous soil. d-values are in %o relative to VSMOW. Each data point associated with a distillation is denoted by the code of the participating laboratory. The numbers 1 and 2 denote the dope water value and a mass balance of dope plus residual water (from air-dry sample), respectively. The line is that associated with a Rayleigh distillation at 35 °C using the mass balance of the air-dry sample plus dope water as a reference. The scale for the clay samples is different from that used for the sands. Where extra analyses have been done, a dash has been used to distinguish the extra sample. In particular, for the sand, h' denotes the 70°C vacuum distillation. For the gypseous sands, d' denotes the centrifuged water, n' the hexane distillation, n" the hexane-kerosene distillation and e' the vacuum distillation which removed more water.

3.3. Low-temperature distillations The low-temperature vacuum distillations (laboratories E, F, H) display the most negative values for both 82H and j t 8 0 . The &values are close to the Rayleigh distillation line. Two laboratories (E, F) both reported yields close to 100% for the sand, but 85% and 80% for the dry and wet clays, respectively. Assuming a Rayleigh distillation process, this would represent differences in analyses of the order of ( 11-23°/oo for 82H, 1.9-3.0%o for 81so), i.e. in the same range as the observed differences. Gypsum releases some or all of its water of crystallisation on heating past ~ 45 ° C (Har-

die, 1967). Distillations that are conducted at a lower temperature than this should not remove any water of crystallisation, while above this temperature, some or all water will be removed. As shown later (p.304), incomplete extraction should result in the final isotope values being to the right of the Rayleigh distillation line. This is consistent with the experimental results in that the distillation at 35°C (laboratory H) yields values for the gypseous sand that are similar to that of the sand; duplicate distillations at 50°C (E) for the gypseous sand yielded one value similar to that of the sand (the yield was the same as that expected for the pore water only) and one value to the

METHODS TO DETERMINETHE STABLEISOTOPECOMPOSITIONOF SOILWATER

slope. ) It is difficult for laboratories using the azeotropic distillations to weigh the water extracted due to the presence of solvent. Laboratory A was the only laboratory to send recorded yields. Their yields are similar or slightly greater than that for the low-temperature extractions. For the gypseous soil, our laboratory performed two additional azeotropic distillations. The first used hexane, the azeotropic temperature for which is 61 °C and hence it will remove water of crystallisation of gypsum, albeit slowly. Experimental work (Woods, 1990) showed that if the distillation time is restricted to 2-3 hr, the contribution of water of crystallisation of the gypsum is relatively small. This

right of the Rayleigh line (yield corresponding to a 0g of 1.6% greater than pore-water content); and the distillation at 80 ° C (F) also gave a result to the right of the Rayleigh line (the yield was 4% higher than the pore-water content).

3.4. Azeotropic distillations Water extracted by azeotropic distillation (laboratories A, B, G, N) generally lie not far from the Rayleigh distillation line at 35 ° C, although a better fit is obtained with a higher temperature line. (The petroleum ether distillations consistently lie along a line of lower

-10,

-10, DRY

-20,

303

CLAY

(a)

-20, -,11-added water a

-30,

a

a a

-30,

V

~2 H

m v

-40,

v a

a

~i2H

(b)

WET CLAY

-.91-- added water

v

m

-40, m

v -50,

-50 '

-61] , - 50 70 90 110 130 150 170 190 30 FINAL EXTRACTION TEMPERATURE e C) -10,

-60

added

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

50 70 90 110 130 150 170 190 FINAL EXTRACTION TEMPERATURE [°C)

-10

(c)

SAND

i.

30

G Y P S E O U S SAND

(d)

water

-20

-20

~2 H

a m

a

c~

water from centrifugation a-h

~2 H

a

a

a v

-30

v v

v

-30

m

¥

v

a

a V

v

a

v

m

-40

.

20

. . . . . . . . . . . . . . . 40 60 80 100 120 140 160 180 200 FINAL EXTRACTION TEMPERATURE e C)

.

.

___~_t~a-hk water of crystallisation of gypsum 40 . . . . . . . . . . . . . . . m _ _ 20 40 60 80 100 120 140 160 180 20C FINAL EXTRACTION TEMPERATURE (oC)

Fig. 2. Plots o f 62H (in %0 relative to V - S M O W ) against final extraction temperature for: (a) dry clay; ( b ) wet clay; (cl sand; and ( d ) gypseous sand. Each data p o i n t is d e n o t e d by the letter o f the technique by which the water was extracted a = a z e o t r o p i c distillation ( a - h = d i s t i U a t i o n by hexane, a-hk--hexane-kerosene distillation); v = v a c u u m distillation m = microdistillation; c = centrifugation..

304

G.R. WALKER ET AL.

3. 5. High-temperature vacuum distillations

is consistent with its lying near the Rayleigh distillation line. A subsequent distillation on the same sample used kerosene. Thus, most of the water removed in this step should be water of crystallisation rather than water held in the soil pores. As expected, the water so obtained lies far from the Rayleigh distillation line. The results from the low-temperature vacuum distillations show a mixing line with endmembers being water extracted from sand and the water of crystallisation of the gypsum. The same is true of the azeotropic distillations. The kerosene distillations lie closest to the water of crystallisation of the gypsum, followed by the extraction with toluene while the extraction with petroleum ether (laboratory G) lies close to the Rayleigh distillation line.

The high-temperature distillations (laboratories D, I) lie below the Rayleigh distillation line at higher values of 82H and 8180 than for low-temperature distillations. The result for the gypseous soils are similar to those of the other extractions in that the sample appears to contain water of crystallisation.

3.6. Temperature dependence Figs. 2a-d and 3a-d show the individual 2H and t so results, respectively, plotted against final temperature of extraction. For t80, there is a strong correlation with temperature for all soils. For 2H, this correlation is weaker. In fact,

-1

-2 -3

-1,

DRY CLAY ~added

(a)

water

-4 ~180 5

WET CLAY

-2,

-3 J ~ a d d e d

a av

~18 0

v

a

v

-6'

-7

-7, v v

-9

190

v

30

FINAL EXTRACTION TEMPERATURE (°C) i 1 SAND

50

70

90

110

130

150

170

(c)

23

~8 0

5180 -

GYPSEOUSSAND

(d)

a-hk

water of crystallisation of gypsum

o -1

!t

-2

~

v

a

-2 added water

a

a

-3

c -.ql---w----------v-water from centrifugation

-4

v

6t . . . . 20 40 60

v v

a

v

v

a . . . . . . . . . 80 100 120 140 160

a . . . . 180 200

FINAL EXTRACTION TEMPERATURE (°C)

190

FINAL EXTRACTION TEMPERATURE (°C)

1

.

a a

v

-8,

-9 . . . . . . . . . . . . . . . . . 30 50 70 90 110 130 150 170

-

water v

-4, -5,

-6

-8

(b)

a-h

-5 .

v

v

a a

6 20

. . . . . . . . . . . . . . . . . . 40 60 80 100 120 140 160 180

200

FINAL EXTRACTION TEMPERATURE (°C)

Fig. 3. P l o t s o f S t s O (in %0 relative to V - S M O W ) against final e x t r a c t i o n t e m p e r a t u r e for: ( a ) dry clay; ( b ) wet clay; ( c ) sand; a n d ( d ) g y p s e o u s sand. E a c h d a t a p o i n t is d e n o t e d b y t h e letter o f t h e t e c h n i q u e b y w h i c h t h e w a t e r was extracted: a = a z e o t r o p i c d i s t i l l a t i o n (a-h=distillation b y h e x a n e , a-hk=hexane-kerosene d i s t i l l a t i o n ) ; v = v a c u u m distillation; m = microdistillation; c = centrifugation..

M E T H O D S T O D E T E R M I N E T H E STABLE ISOTOPE COMPOSITION O F SOIL W A T E R

if the microdistiUation is treated as a high-temperature vacuum distillation, there appears to be a maximum in the 2H results from vacuum distillations in the 80-105 ° C range for the dry clay and sand. No explanation is offerred for this. Given that the probable cause of the variation is incomplete extraction, which will tend to produce lower values, the correlation with temperature suggests that heating may be a necessary component for acceptable distillation results. 3. 7. Microdistillations

The microdistillation results are shown in Fig. 2a-d. The results are consistent with the high-temperature vacuum distillations for all soils but the gypseous sand. The J2H-value for the gypseous sand lies close to the value of the water of crystallisation of gypsum.

4. Conclusions This paper presents the results of the first interlaboratory comparison of techniques of extracting water from soil for analysis of 2H and 1sO. The aim of the comparison was to test the consistency of the extraction methods. Extraction of water from soils of low water content by different techniques or variations of the same techniques gives unacceptably large variations of analyses (up to 30%o for t~EH and 3.5%o for j lSO). We show that this variation is due to extraction technique rather than an artifact of the experimental design: ( 1 ) the sample to sample variation using one technique is comparable with measurement error; (2) a comparison of measurements by the participating laboratories with our laboratory show that the variation cannot be explained by measurement errors associated with mass spectrometry; and (3) the errors are systematic in that they lie near a Rayleigh distillation line and are correlated with temperature of extraction. The suggested cause of the variation in iso-

305

topic analyses is incomplete extraction of the soil water. The data are generally consistent with this notion in that they lie along a Rayleigh distillation line. Also, this was supported by the yield of water extracted often being less than 100%. The study suggests strongly that higher temperatures (100-150°C) may be necessary for both vacuum and azeotropic distillations, particularly when extracting water from clays. However, the higher temperatures will remove the water of crystallisation of gypsum. This study shows that some extraction techniques may lead to a significant but reasonably reproducible bias in t~EH and Jl80. In some applications such as the estimation of evaporation from dry soils (e.g., Fontes et al., 1986 ) such a bias may not impact on the interpretation of the isotope data. However, in other applications, where soil data are compared with the isotopic composition of the atmosphere, vegetation, rainfall or groundwater it is essential to remove any bias from the data for soil water.

Acknowledgements Thanks are expressed to all the laboratories participating in the experiment. Much of this work was carried out as part of the Ph.D. studies of P.H.W., supported by the CSIRO Division of Water Resources, the Centre for Groundwater Studies in Adelaide, and the Hinders University of South Australia. Thanks are expressed to John Dighton, Ashleigh Kennett-Smith and Kerryn McEwen for additional laboratory work, and with Fred Leaney and Professor Jean-Charles Fontes for useful discussions regarding the project.

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