Appl. Radial. ht. Vol. 43. No. 7, pp. 939-942, 1992 ht. J. Radiat. Appt. Instrum. Part A Q Pergamon Press Ltd 1992. Printed in Great Britain 0883-2889/92 $5.00 + 0.00
we attempted to make it easier to use the 85Kr technique on a broader scale in field investigations.
Krypton-85 Occurrence and Origin Noble gases like krypton which are dissolved in water by contact with the atmosphere, are nearly ideal tracers for hydrological investigations because they are not subject to chemical or biological reactions in the aquifer. The radioactive isotope “Kr (half-life 10.76 years, maximum B-energy 670 keV) is practically entirely man-made. The main contribution is released by nuclear fuel reprocessing plants. The remaining fraction is released from nuclear reactors during the annual fuel changing. Contrary to tritium the contribution of nuclear weapon tests to the r5Kr inventory is less than 1% and nowadays negligible. Due to the growth of the nuclear industry since the early 1950s a steady increase of the s5Kr content of the lower atmosphere has been observed [e.g. Weiss (1986)J. The only important sink of the 85Kr content is through radioactive decay. Uptake by the oceans, less than 3%, is small (Schroder and Roether, 1975).
A Simplified Method of 85Kr Measurement for Dating Young Groundwaters JOSEF HELD*, SIEGMUND SCHUHBECK WERNER RAUERTt GSF-Institut
and
fur Hydrologie. 8042 Neuherberg, Germany (Receitled
3
October
1991)
The technique of gas extraction from water and gas sample preparation for “Kr low-level measurements was further developed to improve its applicability to isotope-hydrological “age” determinations of young groundwaters. A water sample of about 200 L is flushed with helium to extract the dissolved gases. Krypton is separated from the extracted gas mixture by repeated adsorption on charcoal, fractionated desorption and subsequent gas chromatography. The detection limit for a 10,000 min counting time in a 10 mL counter Icontaining the krypton (15 pL) and methane as counting gas corresponds to 25 mBq R5Krper mL (STP) krypton or 2.5% Iof the mean atmospheric s5Kr activity concentration in the year 1990.
General Remarks on Sampling Under
normal sampling conditions (T,,,,, = lO”C, hPa1 cn 80 UL krvoton are dissolved in 1000 L of water in contact with’air. For dating of groundwater the krypton dissolved in a water sample of ca 200 L volume has to be extracted most effectively. Oeschger et al. (1974) describe a degassing method by heating of the water up to Z95’C. Salvamoser (1982) extracted the gases from water in a container under lowered pressure in the field, whilst Schroder (1975) and Rozanski and Florkowski (1979) combined the two methods. We use the technique of gas extraction by flushing the water sample with helium. This method enables a degassing in the laboratory as well as in the field. In the latter case it is possible to take samples from several sampling points in the field, because only the extracted gases and not heavy masses of water have to be transported to the laboratory. The technique of extraction with a flushing gas provides a high, reproducible efficiency of degassing connected with a minor risk of air contamination, as the degassing of the sample is carried out at a permanent slight over-pressure (contrary to degassing by means of lowered pressure). P... = 1013 “L.
Introduction t n isotope hydrology the age classification of young groundwater is mainly based on low-level measurements of environmental tritium. Due to its predominant origin from the nuclear weapon tests in the years between 1952 and 1963, the tritium concentrations in precipitation increased to maximum values in 1963 and decreased since then. This i,tput pattern gives often ambiguous results when interpreti ig tritium concentrations of groundwater with respect t J their “age” or mean residence time. The additional measurement of s5Kr was suggested in order to support tile age determination of groundwater (e.g. Oeschger and Siegenthaler, 1972; Oeschger et al., 1974). s5Kr displays a monotonously increasing concentration in the surface-near air and should therefore, at least for simple groundwater systems, give unequivocal interpretation results within n-arly the same age range like tritium. Whilst s5Kr concentrations in sea water were determined for the first time by Schroder (1975) in the frame of oceanographic studies, first results of groundwater dating by S5Kr were reported by Rozanski and Florkowski (1979) and by Salvamoser (1981, 1982). Up to the present only little use has been made of ss Kr measurements in hydrology [mainly in connection with 39,\r dating of groundwater, e.g. Forster et al. (1984, 1991); Loosli et al. (1991)]. This is due to the expenditure of work connected with the r5Kr technique. On the other hand there is an increasing demand, for example, in contaminant hydrology to estimate the age of young groundwaters and to recognize components of greatly different ages. Therefore *Present address: 7861 Sallneck, Am Buchenacker Germany. TAuthor for correspondence.
64,
Flush-Gas Extraction Method The flush-gas extraction method used for the preparation of the gas is shown in Fig. 1. At the sampling point in the field four polyethylene (4 x 56 L) or stainless steel (4 x 50 L) containers, pretilled with nitrogen to avoid air contact, are filled with sample water. Then the sample is flushed with helium in the container itself for 2 h at a flow of 1.1 L Heimin. The helium together with the flushed eases is Dassed in a closed cycle through a cold trap and a charcoal trap-both cooled with liquid nitrogen. In the first trap water vapour and CO, of the sample are frozen. Inorganic carbon, as CO, can be obtained for ‘jC and “‘C measurement from this trap by previously acidifying the water sample with H,PO,. In the charcoal trap the remaining gases which were dissolved in the water sample (N,, 02, Ai and trace gases) are frozen except for H, and He. After flushing for 2 h 95% of the krypton from the water sample is frozen in the charcoal trap. The first separation of the krypton from the other gases is done by fractionated desorption. The charcoal trap is slowly heated to 140°C and
939
940
Technical Note
h
heligm membrane
3
sample
water
sample
container
Fig. 1. Extraction of krypton from water: FM, flow meter; P, manometer; C, cock; V, valve. only gases desorbing above -60°C are filled into a 1 L pressure bottle after the trap has been flushed with tank He. This desorption procedure effects a first enrichment of the krypton relative to the other gases extracted from the of adsorption and water sample. The technique desorption on charcoal has been developed by Stockburger and Sittkus (1975) for the continuous separation of krypton from the atmosphere in order to measure the *‘Kr content of air. Further Enrichment of the Krypton The preparation setup for the second and third krypton enrichment step and filling of the counter tube is shown in Fig. 2. The enrichment is done using H, as flushing gas by performing the adsorption and desorption process first in a
a
sample
8r V
II i
primary trap 1
Fig. 2. Preparation
charcoal trap with a volume of 1 mL and finally in a 30 JI L charcoal trap. The residual gases of 2-3 mL volume in the last trap contain ca 15 PL krypton. This corresponds to an enrichment by a factor of 1300-1900 referring to the gases solved in water in equilibrium with the atmosphere. In the last step the total amount of krypton is separated from the residual gases by gas-chromatographic determination and separation and filled together with the methane carrier gas to atmospheric pressure in a lOmL-volume proportional counter. The column of the gas chromatograph consists of a stainless steel tube (length 800 mm, i.d. 3 mm) filled with charcoal (35-50 mesh) and is operated at a temperature of 45°C. The quantity of ISpL krypton can be determined with an accuracy of 0.8% (significance level 95%) by integration of the krypton peak in the gas-chromatogram obtained with a thermal conductivity detector.
charcoal trap lml
\ prlmary trap 2
-Alcharcoal trap 3Opl
---portional !LZJ
prfItLwr
GC column
and filling setup for krypton: GC, gas chromatograph; TCD, thermal conductivity detector; FM, flow meter; FR, flow regulator; C, cock; V, valve.
941
Technical Note
i$3
proportional
counter
with
preamplifier
compute
high vbltage
anticoincidence counter
prehmplifier
SuPPlY main
dmplifier
Fig. 3. Measurement setup for 8SKr with three proportional
1
2
3
4
counters (1,2, 3).
5
6
Fig. 4. Cross-section of a low-level proportional counter (volume 10 mL): 1, anode wire (stainless steel, diameter 40nm); 2, isolation (PE); 3, gas inlet; 4, cathode (electrolytic copper); 5, equipotential ring (brass), isolated by a glass capillary against the anode wire, which itself is mounted in a stainless steel capillary (not shown in figure); 6, nut.
Measurement The *5Kr activity is measured, as shown in Fig. 3, with one of three 10 mL proportional counters shielded by lead and an anticoincidence ring counter. With this setup three samples can be measured simultaneously. The counter tubes were constructed especially for measuring low activities. The cathode is made of chemically pure copper, which contains low radioactivity. The anode is a stainless steel wire of 10 pm diameter. A cross-section of the counter is shown in Fig 4. The background counting rate n, is between 0.14 and x).20 counts/min depending on the tube. The background t.:ounting rate is in our case equal to the blank counting rate, ,is the small krypton fraction in the counting gas (methane) does not affect the counting efficiency. This has been c,xperimentally confirmed by the measurement of two ramples of tritium-free water. The counting efficiency n is “0%. The usual measuring time I is 10,OOOmin.Assuming furthermore no = 0.20 counts/min, the minimum detectable *‘Kr activity concentration c,,, is 20 mBq/mL (STP) krypt.)n (significance level 95%), referring to:
1i, is the average krypton volume of 15 p L extracted from the water sample. Due to long-term fluctuations of the background counting rate and uncertainties introduced by the preparation, c,,,~”increases in reality up to ca 25 mBq/mL Kr. This corresponds to 2.5% of the mean atmospheric @Kr activity concentration of about 1.0 Bq/mL Kr in the year 1990. Referring to groundwater the detection limit of 25 mBq/mL Kr is equivalent to 2.0 nBq sSKr/L water, assuming that 80 y L Kr is dissolved in 1000 L of water.
As the practice has shown, extending the measuring time beyond 10,000 min will not considerably reduce the detection limit. On the other hand it is possible to achieve a lower detection limit by degassing larger amounts of water and, as a consequence, measuring larger amounts of krypton. In general the accuracy and detection limit achieved with the amount of krypton from 200 L water samples is good enough for many hydrological applications in view of the uncertainties introduced by the model age interpretation. In addition there are practical limits of sampling and transporting, and the extraction of very large amounts of water, if possible at all, can disturb the groundwater system to be investigated. Concluding Remarks Up to the present, 33 samples of groundwater and surface water have been processed using the technique described. The measured ‘jKr activity concentrations range from detection limit to the present atmospheric content as determined in river water. The hydrological implications of the results of these 85Kr measurements will be discussed together with estimates of groundwater residence times based on tritium analyses. References Forster M., Moser H. and Loosli H. H. (1984) Isotope hydrological study with carbon-14 and argon-39 in the bunter sandstones of the Saar region. In Isotope Hydrology 1983, p. 515. IAEA, Vienna. Forster M., Loosli H. H. and Weise S. (1991) 39Ar-, s5Kr-, )He- and ‘H-isotope measurements for dating of groundwaters out from aquifers in “Bocholt” and
942
Technical
“Segeberger Forest”. In Hydrochemische Vorgdnge im Wasserkreislauf in der ungesiittigten und gestittigten Zone (Deutsche Forschungsgemeinschaft Ed.). Springer, Berlin-Heidelberg (in press). Loosli H. H., Lehmann B. E. and DCppen G. (1991) Dating by radionuchdes. In Applied Isotope Hydrology-A Case Study in Northern Switzerland (Pearson F. J., et al., eds). Nagra Technical Report 88-01, p. 153. Elsevier, Amsterdam. Oeschger H. and Siegenthaler U. (1972) Umgebungsisotope im Dienste der Hydrologie und Ausblick auf neue Methoden. gu+Das Gas- und Wasserfach, Wasser, Abwasser, 113, Heft 11, 501. Oeschger H., Gugelmann A., Loosli H. H., Schotterer U., Siegenthaler U. and Wiest W. (1974) ‘9Ar dating of groundwater. In Isotope Techniques in Groundwater Hydrology 1974, Vol. 11;~. 179. IAEA, Vienna. Rozanski K. and Florkoswki T. (1979) *‘Kr dating of groundwater. In Isotope Hydrology 1978, p. 949. IAEA, Vienna. Salvamoser J. (1981) Vergleich der Altersbestimmung von
Note Grundwasser mit Hilfe des Tritiumund Krypton-85Gehalts. Naturwissenschaften 68, 328. Salvamoser J. (1982) 85Kr im Grundwasser-MeDmethodik, Modelliiberlegungen und Anwendung auf natiirliche Grundwassersysteme (85Kr in groundwater-measurement, models and application to natural groundwater systems). Thesis, University of Munich, Germany. Schriider J. (1975) Krypton-85 in the Ocean. 2. Naturforsch. 30a, 962. Schroder J. and Roether W. (1975) The releases of s5Kr and tritium to the environment and tritium to 85Kr ratios as source indicators. In Isotope Ratios as Pollutant Source and Behaviour indicators, p. 231. IAEA, Vienna. Stockburger H. and Sittkus A. (1975) Messung der Krypton-85-Aktivitat der atmosphlrischen Luft. Z. Naturforsch. 30a, 959. Weiss W., Stockburger H., Sartorius H., Rozanski K., Heras C. and Gstlund H. G. (1986) Mesoscale Transport *‘Kr Originating from European Sources. of Nuclear Instruments and Methods in Physics Research B17, 571.
we attempted to make it easier to use the 85Kr technique on a broader scale in field investigations.
Krypton-85 Occurrence and Origin Noble gases like krypton which are dissolved in water by contact with the atmosphere, are nearly ideal tracers for hydrological investigations because they are not subject to chemical or biological reactions in the aquifer. The radioactive isotope “Kr (half-life 10.76 years, maximum B-energy 670 keV) is practically entirely man-made. The main contribution is released by nuclear fuel reprocessing plants. The remaining fraction is released from nuclear reactors during the annual fuel changing. Contrary to tritium the contribution of nuclear weapon tests to the r5Kr inventory is less than 1% and nowadays negligible. Due to the growth of the nuclear industry since the early 1950s a steady increase of the s5Kr content of the lower atmosphere has been observed [e.g. Weiss (1986)J. The only important sink of the 85Kr content is through radioactive decay. Uptake by the oceans, less than 3%, is small (Schroder and Roether, 1975).
A Simplified Method of 85Kr Measurement for Dating Young Groundwaters JOSEF HELD*, SIEGMUND SCHUHBECK WERNER RAUERTt GSF-Institut
and
fur Hydrologie. 8042 Neuherberg, Germany (Receitled
3
October
1991)
The technique of gas extraction from water and gas sample preparation for “Kr low-level measurements was further developed to improve its applicability to isotope-hydrological “age” determinations of young groundwaters. A water sample of about 200 L is flushed with helium to extract the dissolved gases. Krypton is separated from the extracted gas mixture by repeated adsorption on charcoal, fractionated desorption and subsequent gas chromatography. The detection limit for a 10,000 min counting time in a 10 mL counter Icontaining the krypton (15 pL) and methane as counting gas corresponds to 25 mBq R5Krper mL (STP) krypton or 2.5% Iof the mean atmospheric s5Kr activity concentration in the year 1990.
General Remarks on Sampling Under
normal sampling conditions (T,,,,, = lO”C, hPa1 cn 80 UL krvoton are dissolved in 1000 L of water in contact with’air. For dating of groundwater the krypton dissolved in a water sample of ca 200 L volume has to be extracted most effectively. Oeschger et al. (1974) describe a degassing method by heating of the water up to Z95’C. Salvamoser (1982) extracted the gases from water in a container under lowered pressure in the field, whilst Schroder (1975) and Rozanski and Florkowski (1979) combined the two methods. We use the technique of gas extraction by flushing the water sample with helium. This method enables a degassing in the laboratory as well as in the field. In the latter case it is possible to take samples from several sampling points in the field, because only the extracted gases and not heavy masses of water have to be transported to the laboratory. The technique of extraction with a flushing gas provides a high, reproducible efficiency of degassing connected with a minor risk of air contamination, as the degassing of the sample is carried out at a permanent slight over-pressure (contrary to degassing by means of lowered pressure). P... = 1013 “L.
Introduction t n isotope hydrology the age classification of young groundwater is mainly based on low-level measurements of environmental tritium. Due to its predominant origin from the nuclear weapon tests in the years between 1952 and 1963, the tritium concentrations in precipitation increased to maximum values in 1963 and decreased since then. This i,tput pattern gives often ambiguous results when interpreti ig tritium concentrations of groundwater with respect t J their “age” or mean residence time. The additional measurement of s5Kr was suggested in order to support tile age determination of groundwater (e.g. Oeschger and Siegenthaler, 1972; Oeschger et al., 1974). s5Kr displays a monotonously increasing concentration in the surface-near air and should therefore, at least for simple groundwater systems, give unequivocal interpretation results within n-arly the same age range like tritium. Whilst s5Kr concentrations in sea water were determined for the first time by Schroder (1975) in the frame of oceanographic studies, first results of groundwater dating by S5Kr were reported by Rozanski and Florkowski (1979) and by Salvamoser (1981, 1982). Up to the present only little use has been made of ss Kr measurements in hydrology [mainly in connection with 39,\r dating of groundwater, e.g. Forster et al. (1984, 1991); Loosli et al. (1991)]. This is due to the expenditure of work connected with the r5Kr technique. On the other hand there is an increasing demand, for example, in contaminant hydrology to estimate the age of young groundwaters and to recognize components of greatly different ages. Therefore *Present address: 7861 Sallneck, Am Buchenacker Germany. TAuthor for correspondence.
64,
Flush-Gas Extraction Method The flush-gas extraction method used for the preparation of the gas is shown in Fig. 1. At the sampling point in the field four polyethylene (4 x 56 L) or stainless steel (4 x 50 L) containers, pretilled with nitrogen to avoid air contact, are filled with sample water. Then the sample is flushed with helium in the container itself for 2 h at a flow of 1.1 L Heimin. The helium together with the flushed eases is Dassed in a closed cycle through a cold trap and a charcoal trap-both cooled with liquid nitrogen. In the first trap water vapour and CO, of the sample are frozen. Inorganic carbon, as CO, can be obtained for ‘jC and “‘C measurement from this trap by previously acidifying the water sample with H,PO,. In the charcoal trap the remaining gases which were dissolved in the water sample (N,, 02, Ai and trace gases) are frozen except for H, and He. After flushing for 2 h 95% of the krypton from the water sample is frozen in the charcoal trap. The first separation of the krypton from the other gases is done by fractionated desorption. The charcoal trap is slowly heated to 140°C and
939
940
Technical Note
h
heligm membrane
3
sample
water
sample
container
Fig. 1. Extraction of krypton from water: FM, flow meter; P, manometer; C, cock; V, valve. only gases desorbing above -60°C are filled into a 1 L pressure bottle after the trap has been flushed with tank He. This desorption procedure effects a first enrichment of the krypton relative to the other gases extracted from the of adsorption and water sample. The technique desorption on charcoal has been developed by Stockburger and Sittkus (1975) for the continuous separation of krypton from the atmosphere in order to measure the *‘Kr content of air. Further Enrichment of the Krypton The preparation setup for the second and third krypton enrichment step and filling of the counter tube is shown in Fig. 2. The enrichment is done using H, as flushing gas by performing the adsorption and desorption process first in a
a
sample
8r V
II i
primary trap 1
Fig. 2. Preparation
charcoal trap with a volume of 1 mL and finally in a 30 JI L charcoal trap. The residual gases of 2-3 mL volume in the last trap contain ca 15 PL krypton. This corresponds to an enrichment by a factor of 1300-1900 referring to the gases solved in water in equilibrium with the atmosphere. In the last step the total amount of krypton is separated from the residual gases by gas-chromatographic determination and separation and filled together with the methane carrier gas to atmospheric pressure in a lOmL-volume proportional counter. The column of the gas chromatograph consists of a stainless steel tube (length 800 mm, i.d. 3 mm) filled with charcoal (35-50 mesh) and is operated at a temperature of 45°C. The quantity of ISpL krypton can be determined with an accuracy of 0.8% (significance level 95%) by integration of the krypton peak in the gas-chromatogram obtained with a thermal conductivity detector.
charcoal trap lml
\ prlmary trap 2
-Alcharcoal trap 3Opl
---portional !LZJ
prfItLwr
GC column
and filling setup for krypton: GC, gas chromatograph; TCD, thermal conductivity detector; FM, flow meter; FR, flow regulator; C, cock; V, valve.
941
Technical Note
i$3
proportional
counter
with
preamplifier
compute
high vbltage
anticoincidence counter
prehmplifier
SuPPlY main
dmplifier
Fig. 3. Measurement setup for 8SKr with three proportional
1
2
3
4
counters (1,2, 3).
5
6
Fig. 4. Cross-section of a low-level proportional counter (volume 10 mL): 1, anode wire (stainless steel, diameter 40nm); 2, isolation (PE); 3, gas inlet; 4, cathode (electrolytic copper); 5, equipotential ring (brass), isolated by a glass capillary against the anode wire, which itself is mounted in a stainless steel capillary (not shown in figure); 6, nut.
Measurement The *5Kr activity is measured, as shown in Fig. 3, with one of three 10 mL proportional counters shielded by lead and an anticoincidence ring counter. With this setup three samples can be measured simultaneously. The counter tubes were constructed especially for measuring low activities. The cathode is made of chemically pure copper, which contains low radioactivity. The anode is a stainless steel wire of 10 pm diameter. A cross-section of the counter is shown in Fig 4. The background counting rate n, is between 0.14 and x).20 counts/min depending on the tube. The background t.:ounting rate is in our case equal to the blank counting rate, ,is the small krypton fraction in the counting gas (methane) does not affect the counting efficiency. This has been c,xperimentally confirmed by the measurement of two ramples of tritium-free water. The counting efficiency n is “0%. The usual measuring time I is 10,OOOmin.Assuming furthermore no = 0.20 counts/min, the minimum detectable *‘Kr activity concentration c,,, is 20 mBq/mL (STP) krypt.)n (significance level 95%), referring to:
1i, is the average krypton volume of 15 p L extracted from the water sample. Due to long-term fluctuations of the background counting rate and uncertainties introduced by the preparation, c,,,~”increases in reality up to ca 25 mBq/mL Kr. This corresponds to 2.5% of the mean atmospheric @Kr activity concentration of about 1.0 Bq/mL Kr in the year 1990. Referring to groundwater the detection limit of 25 mBq/mL Kr is equivalent to 2.0 nBq sSKr/L water, assuming that 80 y L Kr is dissolved in 1000 L of water.
As the practice has shown, extending the measuring time beyond 10,000 min will not considerably reduce the detection limit. On the other hand it is possible to achieve a lower detection limit by degassing larger amounts of water and, as a consequence, measuring larger amounts of krypton. In general the accuracy and detection limit achieved with the amount of krypton from 200 L water samples is good enough for many hydrological applications in view of the uncertainties introduced by the model age interpretation. In addition there are practical limits of sampling and transporting, and the extraction of very large amounts of water, if possible at all, can disturb the groundwater system to be investigated. Concluding Remarks Up to the present, 33 samples of groundwater and surface water have been processed using the technique described. The measured ‘jKr activity concentrations range from detection limit to the present atmospheric content as determined in river water. The hydrological implications of the results of these 85Kr measurements will be discussed together with estimates of groundwater residence times based on tritium analyses. References Forster M., Moser H. and Loosli H. H. (1984) Isotope hydrological study with carbon-14 and argon-39 in the bunter sandstones of the Saar region. In Isotope Hydrology 1983, p. 515. IAEA, Vienna. Forster M., Loosli H. H. and Weise S. (1991) 39Ar-, s5Kr-, )He- and ‘H-isotope measurements for dating of groundwaters out from aquifers in “Bocholt” and
942
Technical
“Segeberger Forest”. In Hydrochemische Vorgdnge im Wasserkreislauf in der ungesiittigten und gestittigten Zone (Deutsche Forschungsgemeinschaft Ed.). Springer, Berlin-Heidelberg (in press). Loosli H. H., Lehmann B. E. and DCppen G. (1991) Dating by radionuchdes. In Applied Isotope Hydrology-A Case Study in Northern Switzerland (Pearson F. J., et al., eds). Nagra Technical Report 88-01, p. 153. Elsevier, Amsterdam. Oeschger H. and Siegenthaler U. (1972) Umgebungsisotope im Dienste der Hydrologie und Ausblick auf neue Methoden. gu+Das Gas- und Wasserfach, Wasser, Abwasser, 113, Heft 11, 501. Oeschger H., Gugelmann A., Loosli H. H., Schotterer U., Siegenthaler U. and Wiest W. (1974) ‘9Ar dating of groundwater. In Isotope Techniques in Groundwater Hydrology 1974, Vol. 11;~. 179. IAEA, Vienna. Rozanski K. and Florkoswki T. (1979) *‘Kr dating of groundwater. In Isotope Hydrology 1978, p. 949. IAEA, Vienna. Salvamoser J. (1981) Vergleich der Altersbestimmung von
Note Grundwasser mit Hilfe des Tritiumund Krypton-85Gehalts. Naturwissenschaften 68, 328. Salvamoser J. (1982) 85Kr im Grundwasser-MeDmethodik, Modelliiberlegungen und Anwendung auf natiirliche Grundwassersysteme (85Kr in groundwater-measurement, models and application to natural groundwater systems). Thesis, University of Munich, Germany. Schriider J. (1975) Krypton-85 in the Ocean. 2. Naturforsch. 30a, 962. Schroder J. and Roether W. (1975) The releases of s5Kr and tritium to the environment and tritium to 85Kr ratios as source indicators. In Isotope Ratios as Pollutant Source and Behaviour indicators, p. 231. IAEA, Vienna. Stockburger H. and Sittkus A. (1975) Messung der Krypton-85-Aktivitat der atmosphlrischen Luft. Z. Naturforsch. 30a, 959. Weiss W., Stockburger H., Sartorius H., Rozanski K., Heras C. and Gstlund H. G. (1986) Mesoscale Transport *‘Kr Originating from European Sources. of Nuclear Instruments and Methods in Physics Research B17, 571.
