Tritiogenic3He in groundwater in Takaoka

Earth and Planetary Science lz, tters, 85 (1987) 74-78 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

74

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Tritiogenic 3He in groundwater in Takaoka Nobuo Takaoka

~ and Yoshihiko Mizutani

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I Department of Earth Sciences, FaculO"ofSctence. Yamagata Uniters'tO', Yamagata 990 (Japan) ' Department of Earth Sciences, Facul(v of Sctence, Toyama Unwerstty, Tovama 930 (Japan)

Received March 20, 1987: revised version received June 19, 1987 ;l-le/':He ratios up to 3.5 times the ratio of atmospheric He were found in groundwater samples. The ~I'te enrichment can be attributed to radiogenic 3He produced by in-situ beta-decay of 3H. This shows that tritiogenic ~He is accumulating in confined waters. From tritiogenic 3He and 3H concentrations, ages of groundwaters can be calculated. Detection of tritiogenic 3He gives a tool to trace a tritium contamination which occurred in the past and cannot be assessed only by the 3H counting method.

I. Introduction T r i t i u m is a radioactive isotope of h y d r o g e n which decays to 3He with the half-life of 12.33 years. The a n t h r o p o g e n i c 3H released by nuclear tests in the a t m o s p h e r e as well as injected 3H has been used for studies in hydrology, l i m n o l o g y a n d o c e a n o g r a p h y . In addition, the m e a s u r e m e n t of ~He, the stable d a u g h t e r p r o d u c t of 3H, can give m o r e useful i n f o r m a t i o n : Torgersen et al. [1] have reviewed the 3H-3He m e t h o d a n d potential of its a p p l i c a t i o n to hydrology. It has been a p p l i e d to limnology to calculate effective water mass ages, to estimate gas exchange rates, gas renewal at turn-over, a n d vertical diffusivity in the e p i l i m n o n [2]. These studies showed e n r i c h m e n t s of 3H a n d tritiogenic 3He in lakes, but no e n r i c h m e n t of tritiogenic 3He was r e p o r t e d in g r o u n d w a t e r . However, 3H c o n c e n t r a t i o n s in p r e c i p i t a t i o n in excess of 1000 T U were o b s e r v e d in the 1960's at the peak of a t m o s p h e r i c nuclear w e a p o n s testing [3]. As a result, high-3H water can be found in g r o u n d w a t e r [4]. This m e a n s a possible a c c u m u l a tion of tritiogenic 3He in confined water. 2. Experimental W a t e r samples (No. 1 - 7 ) were collected from flowing wells in T a k a o k a and its environs, T o y a m a Perfecture, Japan. The s a m p l i n g sites are shown in Fig. l a . T r i t i u m c o n c e n t r a t i o n s of this area have 0012-821X/87/$03.50

:.," 1987 Elsevier Science Publishers B.V.

been surveyed by M i z u t a n i et al. [4], A borosilicare glass vessel (ca. 680 cm 3) with a stopcock was used as a water container. It was e v a c u a t e d to 1 0 3 Torr. A plastic tube a t t a c h e d to the stopcock was filled with the flowing water and was inserted into a c o n d u c t o r p i p e from the well to a d e p t h of a b o u t 1 m to collect s a m p l e water without a t m o spheric c o n t a m i n a t i o n . Care was taken of air cont a m i n a t i o n by air b u b b l e s a t t a c h i n g on the tube wall. The water was sucked into the glass container by o p e n i n g the stopcock. T h e n the stopcock was closed. Dissolved gas of a b o u t 5 cm 3 was isolated from water by this v a c u u m suction. It is s u p p o s e d that most of H e and N e moved to the gas phase. T h e r e f o r e the gas was a d d e d to the s a m p l e gas for He and N e analysis. He and N e were s e p a r a t e d from water in a b o r o s i l i c a t e glass e x t r a c t i o n line which was p u m p e d by a mercury diffusion p u m p . U l t r a s o n i c v i b r a t i o n was a p p l i e d to the s a m p l e for ten m i n u t e s to extract the r e m a i n i n g part of noble gases from the s a m p l e water. W a t e r v a p o r and other cond e n s a b l e gases evolved were r e m o v e d by a liquid nitrogen cold trap. H e a v y noble gases were also removed. T h e residual gases including He a n d N e were collected in a b r e a k a b l e - s e a l glass b u l b (ca. 100 cm 3) using a T o e p l e r p u m p . The gas c o n t e n t was corrected for the collection efficiency of the T o e p l e r p u m p . He and N e were a n a l y s e d by conventional techniques of noble gas mass s p e c t r o m e try [5]. The mass analysis was carried out in two

75 Since total He in the sample is 26 × 10 - 6 cm 3, there is no change in the 3H e / 4 He ratio caused by permeation of atmospheric He through the glass wall. Tritium concentrations were determined by a liquid scintillation method on the same samples as He for samples 1, 4 and 7. For other samples, He and 3H were measured on the different ones. In both cases, 3H was corrected for time lag between 3H and He measurements. The correction is less than 1% for the former and about 15% for the latter case. 3. Results and discussion

ES] Fig. 1. (a) A map of sampling sites. The zone of flowing wells (shown by dots) in Takaoka is located at the end of Shogawa Fan. The principal flow directions are shown by arrows [10]. (b) A geologic cross section of the region reproduced after Fujii (unpublished). 1 = permeable layers of gravel; 2 = impermeable layers of clay, silt and sand; 3 ~ basement of Tertiary sediments; 4 = boundary between cobble gravel (upper) and gravel and sand (below).

weeks after sample collection. He blank was about 0.05 x 10 6 cm 3 for He extraction and mass analysis, and less than 0.2 × 10 - 6 c m 3 for 8 days storage of the sample water in the glass container.

The result of He isotope analysis is given in Table 1. Errors cited for 3He/4 He are statistical ones (one sigma). Uncertainties in 4He and 4He/Z° Ne are about 20%. Definitely high 3He/4 He ratios ( R ) are found for three samples. Among them, a sample (No. 1) collected at the Kanebo-cho well shows an excellently high value: R = (4.77 50.14) x 10 - 6 . This ratio is as high as found in samples that have mantle He [6,7]. However, it is shown below that the present samples do not contain mantle He. Samples with high 3 H e / 4 H e ratios have high 3H (Fig. 2a). If He and Ne in the samples are mixtures between groundwater (A) saturated by the atmospheric gases and groundwater ( M ) enriched in the mantle-derived gases, a linear array of data points is to be observed along a mixing line between two end points (A and M ) in a plot of 3H e / 4 He versus 20N e / 4 He. Such correlation is not found in Fig. 2b. No correlation is found between 3 H e / 4 H e and 4He (Fig. 2c). The He concentration for the present sample is systematically lower than a solubility (46 × 10 -9 cm3/g) of atmospheric He into water of 0.1%~ salinity at an average temperature of 12.3 ° C [8-10]. Especially samples 3, 4 and 5 are undersaturated by more than 20%. This may be ascribed to incomplete He extraction. The good correlation between 3He/4 He and 3H, and no correlations between 3 H e / 4 H e and 2 ° N e / 4 H e nor 3 H e / 4 H e and 4He indicate that the enrichment of 3He found in the present samples is not attributed to mantle-origin 3He, but to radiogenic 3He produced by in-situ decay of 3H in groundwater. No addition of mantle He is also

76 FABLE 1 Helium isotopic ratios, tritiogenic ]He and water ages for groundwater samples collected in lakaoka and its environs. Toyoma Prefecture, Japan (collection date: October 10, 1986). Data for groundwater sample (No. 8) enriched in mantle gas are given for comparison Sample No. T 2 3 4 5 6 7 8

Sampling site (depth)

"He (10-9cm3/g)

Kanebo-cho (?) Sano (?) Beniya (29 m) Wakamizu-cho (32 m) Sanga (35 m) Hirata (54 m) Ayako (72 m) Nogyo-shikenjo Yamagata (?)

3He/4 He ( x l 0 ~')

42 43 34 37 36 41 41

4.77 ± (I.14 2.18+0.10 1.8(1_+.11.13 1.47 +-0.11 1.41 _+0.11 1.33 + 0.10 1.21 +-0.12

12,200

8.70 ± U.45

131tc1, (10 15cm3/g) 1h

2b

140 34 14 3 1

195 46 24 5 1.4

4Hc/m~Ne

0.23 0.22 0.21 0.22 0.22 (/.23 (I.22

3tl (TU)

Age (y)

32.7 + 0.5 24.8_,+(I.5 " 21.4+_0.5 " 13.1 ~ 0.5 15.5+-0.5 " 1.8 + 05 " 1.5 +_0.5

20 + 2.0 8.8 + 1.0 5.4 + 1.3 2.1 ,'-0.5 0.6 + t).l

98.1

~t1 was calculated from Mizutani et al. [4] by correcting for time lag of measurement. Errors cited are statistical ones (one sigma) of counting. b Tritiogenic 31te is calculated with two assumptions. See the text. Errors for ages show limits given by the assumptions used to calculate tritiogenic aHc.

s u g g e s t e d by the low H e c o n c e n t r a t i o n b e c a u s e g r o u n d w a t e r c o n t a i n i n g m a n t l e H e is u s u a l l y in s u p e r s a t u r a t i o n [11] as s h o w n by s a m p l e 8. T h e 3 H e / 4 H e ratio in g e o l o g i c a l s a m p l e s such as v o l c a n i c gases, g e o t h e r m a l fluids a n d v o l c a n i c rocks is o f t e n used to i d e n t i f y the m a n t l e c o m p o nent. T h i s s t u d y s h o w s that t r i t i o g e n i c ~He is a c c u m u l a t i n g in c o n f i n e d water, s u g g e s t i n g that o n e s h o u l d take the t r i t i o g e n i c 3 H e i n t o a c c o u n t in the H e i s o t o p e s t u d y o f g e o l o g i c a l s a m p l e s . F o r

instance, N a g a o ct al. h a v e r e p o r t e d R = 15.4 × 10 -~' for a g e o t h e r m a l p r o s p e c t i n g well gas at O n u m a [12]. T h i s is an e x t r a o r d i n a r i l y high ratio for i s l a n d s - a r c m a n t l e He. N a g a o et al. d i d not give a r e a s o n a b l e i n t e r p r e t a t i o n to this high ratio. It c o u l d be i n t e r p r e t e d by m i x i n g b e t w e e n i s l a n d s arc m a n t l e H e a n d t r i t i o g e n i c H e e v o l v e d f r o m a i r - s a t u r a t e d g r o u n d w a t e r c o n t a m i n a t e d by s o m e 2000 T U 3H. S i n c e 3H c o n c e n t r a t i o n s up to 1500 T U h a v e b e e n o f t e n o b s e r v e d in T o k y o f r o m 1962

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(b)

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i

Atm L

3H (TU)

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He

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A

2ONel4He

5

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I00

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4

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He (x ll-gcctg)

Fig. 2. (a) A correlation plot between 3He/4He and 3He. The atmospheric 311e/4tle ratio is given by a dashed line. (b) A correlation plot between 3He/a He and 2oNe/4 He. No samples fit the mixing line between the atmospheric gas (A) and the mantle gas ( M ). (c) A correlation plot between 3He/'tHe and 4 He showing the atmospheric (A) and mantle (M) components.

77

to 1964 [3], 2000 TU 3H might be possible in precipitation at Onuma which locates at the northern part of Japan. Determination of 3H and tritiogenic 3He concentrations in groundwater enables us to calculate the water age. In calculation, we use the same formulae as given by Torgersen et al. [1]. To calculate tritiogenic 3He, we assume two cases: (1) The samples are in saturation of atmospheric He, and (2) the apparent low solubility resulted from incomplete He extraction from groundwater which was supersaturated, say by 25%, as found in groundwater samples [13]. With the first assumption, tritiogenic 3He is given by: (3He)t = ( R , , -

R~)(4He)m

where subscripts t, s and m mean tritiogenic, saturated and measured, respectively. For the saturation ratio of He, we use R s = 1.38 × 10 -6 [14]. With the second assumption of 25% supersaturation, tritiogenic 3He is given by: (3He), = [1.25(R m - Ra) + (R a - Rs)] (4He).~ where R~ = 1.399 × 10 -6 for the atmospheric ratio [15] and (4He)~ = 46 × 10 -9 cm3/g for airsaturated water ( 0 . 1 ~ salinity) at 12.3 ° C. The result is given in Table 1. Mean values are listed for ages calculated with two assumptions. Errors cited show limits given by those assumptions. The distribution of age shows a progressive increase from south to north. This correlates with the principal flow direction of shallow confined water [10], as shown in Fig. la. From the tritium survey of this area, Mizutani et al. [4] argue mixing between shallow confined-water of low 3H and deep water of high 3H which is outflow from a deeper aquifer (Fig. lb). Variation of tritiogenic 3He can be explained by different mixing ratios between both waters. The age progression suggests that the main outflow from the deeper aquifer occurs the most downstream near Kanebo-cho. The age (i.e. 20 years) for sample 1 is therefore a lower limit of the residence time for confined water in this region, taking inflow of young water into account. For samples 6 and 7, the ~ H e / 4 H e ratio is lower than the atmospheric ratio, indicating that they contain crustal He enriched in radiogenic 4He. As mentioned earlier, the apparent concentrations of He in these samples are in under-

saturation as a result of incomplete extraction. The ratios slightly lower than R , suggest that these samples were not in great supersaturation by crustal He, however. They are recharged mainly by a tributary of the Oyabe River, while samples 1-5 are recharged by the Sho River. Therefore samples 6 and 7 are quite different in origin. This is shown by difference in the chemical composition [10]. The low 3H concentration indicates that they resided underground at most for 32 years, assuming a 3H concentration in precipitation in the pre-nuclear era to be less than 10 T U [2]. In conclusion, tritiogenic 3He in groundwater is a new tool to assess groundwater hydrology. It can be used to calculate the age of groundwater since recharge. It can also be used to trace an environmental 3H contamination which occurred in the past and can not be assessed only by the 3H counting method. For example, the initial 3H concentration can be calculated to be 90 TU on the basis of the present 3H and the age of water.

Acknowledgements The authors express their thanks to Dr. M. Sakanoue. This work is greatly indebted to his continuing interest and suggestion for determination of tritiogenic 3He in water. They also express their thanks to Dr. H. Craig and two anonymous referees for useful comments are to Dr. S. Fujii for use of an unpublished geological map. They are indebted to one of the referees for polishing English style. This work is supported by the Grantin-Aid for Fusion Research (No. 60050024 and 61050046) of the Ministry of Education, Science and Culture.

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7~ 5 N. "lakaoka, A low-blank metal system for rare gas analysis, Mass Spectrom. 24, 73-86, 1976. 6 W.B. Clarke. M.A. Beg and H. Craig, Excess 3He in the sea: evidence for terrestrial primordial helium, Earth Planet. Sci. l,ett. 6, 213-220, 1969. 7 B.A. Mamyrin, I.N. Tolstikhin, G.S. Anufriev and I.L. Kamenskii, Anomalous isotopic composition of helium in volcanic gases, Dokl. Akad. Nauk SSSR 184, 1197-1199, 1969. 8 RF. Weiss, Solubility of helium and neon in water and seawater, J. Chem. Eng. Data 16. 235-241, 1971. 9 Y. Mizutani and M. Oda, Stable isotope study of groundwater recharge and movement in the Shogawa Fan, Toyama, Chikyu Kagaku (Geochemistry) 17, 1-9. 1983 Cin Japanesel. 10 S. Kato. Y. Mizutani, T. Uchida and C. lida, Geochemical study of ground water systems in the Shogawa Fan, Toyama, Chikyu Kagaku (Geochemistry) 18, 29-35. 1984 (in Japanese).

11 T. Torgersen and W.J. Jenkins, Helium isotopes in geothermal system: Iceland, the Geysers, Raft River. and Steamboat Spring, Geochim. Cosmochim. Acta 46,739-748. 1982. 12 K. Nagao, N. Takaoka and O, Matsubayashi, Rare gas isotopic compositions in natural gases of Japan, Earth Planet. Sci. Left. 53, 175-188, 1981. 13 T. Torgersen, Controls on pore-fluid concentration of al'le and 22"Rn and the calculation of 4He/222Rn ages, J. Geochem. Explor. 13, 57-75, 1980. 14 RF. Weiss, Helium isotope effect in solution in water and seawater, Science 168, 247-248, 1970. 15 B.A. Mamyrin, G.S. Anufriyev, I.L, Kamenskiy and I.N. Tolstikhin, Determination of the isotopic composition of atmospheric helium, Ge~x:hem. Int. 7, 498-505, 1970.