measurement of tritium in tree rings: relationship between tritium concentrations in pine tree rings...

Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 132, No. 1 (1989) 59-64

MEASUREMENT OF TRITIUM IN TREE RINGS: RELATIONSHIP BETWEEN TRITIUM CONCENTRATIONS IN PINE TREE RINGS AND ENVIRONMENTAL SAMPLES

Y. YAMADA,* M. ITOH,** N. KIRIYAMA,* K. KOMURA,** K. UENO**

*School of Pharmacy, Hokuriku University, Kanagawa-machi, Kanazawa 920-11 (Japan} **Low level Radioactivity Laboratory, Kanazawa University,

Tatsunokuchi-machi, Ishikawa 923-12 (Japan}

(Received December 21, 1988)

All of the combustion water samples extracted from cellulose in pine tree rings corresponding to the 1983-1987 period showed elevated tritium concentrations of approximately 65 pCi/dm 3 , which were 30 to 35 pCi/dm 3 higher than those for precipitation and atmospheric vapor in recent years. In addition, other environmental samples, viz. the tissue-free water in tree rings, and of combustion and tissue-free water in pine needles and spring water near the pine tree site also showed concentrations similar to the combustion water of cellulose. These findings suggest that most of the tritium in tree rings was supplied from underground water containing a high tritium concentration in the root zone of the pine tree.

Introduction

Tritium distribution in nature is mainly the result of continuous production of tri-

tium by cosmic rays in the upper atmosphere 1 and prolonged exposure from ther-

monuclear bomb tests carried out during the 1950's and early 1960's. 2

Most tritium released into the environment, whether it be cosmogenic or anthro-

pogenic, is eventually converted into tritiated water and mixed with the water cir-

culating in the hydrological cycle. Since water is incorporated into living things,

tritium can thus be found in organisms as tritiated water or organically bound tritium

in a variety of chemical compounds at concentrations related to environmental levels 3,4

The measurement of tritium in one particular tree can provide us with useful

information about the behaviour characteristics of environmental tritium for a given

site.S -8 The recent level of t r i t ium at the growth site of the tree is demonstrated in

the tritium concentration of tissue-free water contained in the outermost layer of

annual growth rings, whereas its variation in relation to time is preserved in the tri-

tium content of organically bound hydrogen in the series of annual growth rings.

Elsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, Budapest

Y. YAMADA et al.: MEASUREMENT OF TRITIUM IN TREE RINGS

In order to elucidate changes occurring in the tritium content in successive layers of tree rings without the contamination problem from subsequent years, an isolated and chemically unaltered organic component, such as cellulose, should be analyzed, and the probability for isotopic exchange of hydrogen between the organic component and surrounding water should be considered.4, 6,7,9 -11

In a previous paper, we investigated a promising process utilizing explosive depressurization for the isolation of fibrous cellulose from wood chips resulting in a rela- tively pure, high yield product in a short period of t ime) 2

Here, the same process is applied to a pine tree grown in Tatsunokuchi-machi, Ishikawa, Japan, to determine the past distribution of tritium in the ecosystem. The results of tritium measurement in the tree rings of this pine tree are compared with the tritium concentrations of various environmental samples.

Experimental

Samples of pine tree rings

A pine tree, approximately 80 years old, was used. The tree was situated halfway

down a small hill, located in Tatsunokuchi-machi, Ishikawa, Japan, and cut down on April 4, 1987. The individual annual growth rings with a ring width of 0.5-3.5 mm were separated manually with a chisel from a section of trunk, 0.5 m in diameter and 2.5 m long. More than 600 g of wood chips were obtained from each tree layer after drying. The outer tree ring samples, which corresponded to the 1983=1987 period, were used to measure tritium in this study.

Several environmental water samples, together with pine needles, were also obtained for the comparison with tree ring samples in a period from April to June in 1987.

Isolation of a-cellulose from wood chips

Approximately 150 g of thin wood chips were steam-exploded in a process which combined a short pre-hydrolysis (2 min.), at a pressure of 3.55 MPa, with rapid decom- pression.12 The insoluble residue was then extracted with 1.5 dm a of wanned EtOH

for an hour after washing with distilled water. A large portion of the noncellulosic

substances, including lignin and hemicellu]ose, could be removed from cellulose b y this explosive depressurization operation and successive EtOH extraction.

Further treatments were required to remove any adherent impurities. The cellulose residue was heated to approximately 80 ~ with 1.5 dma of a solution containing 95 g of NaC102 and 15 cm a of acetic acid for two hours, the mixture being stirred at intervals. Fresh portions of NaC102 (10 g) and acetic acid (3 cm 3) were added

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occasionally to the solution, sufficient for the completion of the reaction process. After filtration and washing, the precipitate was submitted to boiling in a 17.5% NaOH solution for approximately one hour and successive washings with hot distilled water until free of sodium hydroxide.

In order to substitute original exchangeable hydrogens of hydroxyl groups in cellulose molecule with tritium free hydrogens, the cellulose residue was allowed to stand in a boiled solution of 0.4M (mol/dm a )HC1 prepared with background water for 30 min and subsequently washed with this water. (The background water had been obtained from a deep well, about 150 m in depth, in Kanazawa city and confirmed to have no

detectable amount of tritium. 1 a )

Approximately 40 g of a-cellulose was isolated from 150 g of wood chips using

the above procedures.

Combustion water

The combustion apparatus is shown in Fig. 1. A weighed amount of compressed t~-cellulose (100 g) was introduced into a quartz tube with CuO and Cu20 pellets, the back part of which was maintained at 600 ~ to allow complete oxidation. After

complete drying at 110 ~ the sample was burned up gradually around a tempera-

,Compressed sample

rtow I- Quortz tube (150cm~Imin) I / / ~ I I / ~ I /'(r

I I . . . . . . I - - " ' i~ '5 /" )--I -~ / / / /A I I I I I ;CuO ar~ CuzO petlets;'- l 's I ~ ~ r419 , - t

Io IIN, I Electric furnace I I I I I I I I I J J J ] (-330~ (-700~ ~ C~l~d trQps ~ J

(02: N2= 1:1) Dry-ice EtOH.liq. Nz (--78oC) (--110~

Fig. 1. Combust ion aoDaratus for cellulose and pine needles

ture of 330 ~ in 150 cma/min flow of mixed gas containing oxygen and nitrogen (1:1). The water thus produced was condensed in dry-ice traps connected in series. More than 40 c m 3 of combustion water was recovered for each sample.

Dried pine needles (I50 g) were also burned and approximately 10 cm a of combus-

tion water was obtained. The resulting carbon dioxide gas was collected in an EtOH trap cooled with liquid

nitrogen behind dry-ice traps for the following study.

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Tissue-free water

TiSsue-free water was collected on a trap by drying the wood chips which corresponded

to the outermost tree ring or raw pine needles in vacuum.

Tritium measurement

Tritium concentration was measured using a liquid scintillation counting method.

All samples containing acidic contaminants were distilled after the addition of

KMnO4 and Na202 prior to measurement. This treatment was repeated at least two times for the samples of combustion water and the water extracted from soils.

The sample for measurement was prepared as follows. 40 cm 3 of distillate or water

which had an appropriate composition of distillate and background water (in the

case of the sample of pine needles) was mixed with 60 cm 3 of sol-gel emulsifier- type scintillating cocktail, Instagel (United Technologies Packard) in a 100 cm 3

Teflon vial. It was allowed to stand in the counter under cooled and dark conditions

for three days before counting, to permit the decay of chemiluminescence.

Measurement was conducted under temperature-stabilized conditions at 12 ~ for

50 min X 20 times or 50 min • 40 times using a low background liquid scintillation counter, Aloka LB-1. The counting background of the 100 cm 3 Teflon vial was

2.85+0.03 cpm at a rate of 15.3% efficiency. The rate of efficiency for counting was determined by the external standard channels ratio method.

Results and discussion

The pine tree is a common species found in most areas of Japan and has the

advantage that its growth rings can be easily separated owing to significant differ-

ences in hardness between the regions of early and late wood. An 80-year-old pine

tree, therefore, located halfway down a small hill in Tatsunokuchi-machi was analyzed

to study the past distribution of environmental tritium at this site.

The changes in tritium concentration of the combustion water extracted from cellulose in the s~quence of pine tree rings, corresponding to the 1983-1987 period,

were tabulated (see Table 1) together with the values of other environmental samples. The results are corrected for radioactive disintegration of tritium back to the growth

year of tree rings or the sampling date. It should be noted that rio attempt was made

to adjust for exchangeable hydroxy hydrogens of cellulose, which had been considered

by several other authors.6, 7 One of the most important studies concerned with elim-

inating the possible problem of exchangeable hydrogens associated with the measure-

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Table 1 Tritium concentrations in combustion water samples of cellulose isolated from pine tree rings and those of several envircnmental samples collected in Tatsunokuehi-maehi, Ishikawa, Japan

Tritium concentration, Sample pCi/dm s

Cellulose from 1987's ring Cellulose from 1986's ring Cellulose from 1985 's ring Cellulose from 1984's ring Cellulose from 1983's ring Tissue-free water in the trunk Pine needles Tissue-free water in pine needles Spring water near the pine tree Surface soil moisture around the pine tree

68.2• 4.9 63.3- + 9.3 64.6-+ 7.0 65.1-+ 6.5 64.7-+ 4.6 63.3-+ 5.0 63.4-+10.5 61.9• 5.1 59.3 + - 6.5 47.1-+ 7.0

ment of tritium is that of BROWN. 6 He utilized the equilibration of cellulose with a

boiled solution of diluted hydrochloric acid prepared with tritium-free water and evaluated

the original tri t ium content in cellulose by multiplying the measured value by a factor

of 10/7, since a 28 to 30% replacement of cellulose hydrogens was found to have

occurred. 9

However, our preliminary experiments on the hydrogen exchange between cellulose

and tritiated water have shown that the ratio of readily exchangeable hydrogens to

total hydrogen atoms in cellulose was 16% in the vacuum-dried cellulose which had

been pre-equilibrated with tracer levels of tr i t iated water (50 tzCi/cm 3) at 92-+I ~

for a week. Moreover, more than 99% of the substi tuted tritium in the vacuum-dried

cellulose was removed when the cellulose was dried in a vacuum at 150 ~ or in a

flow of oxygen and nitrogen (1:1) at 110 ~ for 6 hours. These results suggest that most of the labile hydrogens in the product are el iminated

by treatment at elevated temperatures, which may be attr ibutable to the conformational

changes of cellulose structure, accompanied by dehydrat ion at elevated temperatures.

Therefore, the contamination problem of labile hydroxy hydrogens which arose

during the isolation process of cellulose can be disregarded since this process allowed

for absolute dryness at elevated temperatures prior to the combustion o f cellulose.

The correction of measured values for hydrogen exchange is also unnecessary for the

hypothesis that the remaining hydroxy hydrogens in cellulose have very low abili ty

to exchange with ambient water at the temperatures at which trees grow in the

natural environment and their tri t ium content may reflect the tri t ium concentrat ion

of water utilized during the time of cellulose synthesis as well as the carbon-bound

hydrogens.

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An interesting result is that the combustion water samples extracted from cellulose

in tree rings corresponding to the 1983-1987 period demonstrated elevated tritium

levels of approximately 65 pCi/dm 3, which were 30 to 35 pCi/dm 3 higher than the

monthly mean tritium concentrations of precipitations or the concentrations of peri-

lnnln i . . . .

,~ -6

~- 20 ~-

[3_ >,,

0 1983 1984 1985 1986 1987

YeQr

Fig. 2. Variations in tritium concentration of precipitation and atmospheric vapour collected during the last five years in Tatsunokuchi-maehi, Ishikawa, Japan: [], tritium concentration of atmospheric vapour, [7 tritium concentration of precipitation,1 monthly precipitation

odically sampled atmospheric vapours in recent years (see Fig. 2). More remarka01e is the finding that the concentrations in combustion water samples of cellulose agreed

with those of the following water samples: tissue-free water in the outermost tree

ring; both combustion and tissue-free water in the needles; and spring water flowing at the foot of the hill where the pine tree had grown.

Our results suggest an answer to the question, from which source the excess tritium detected in the tree rings for the 1983-1987 period and the needles was supplied. The excess tritium in the tree is considered to have come from the soil water absorbed

by the tree roots, which was significantly influenced by the water stored in the under-

ground reservoir at this site. In addition, the contribution of uptake or loss of tri-

tium through pine needles from atmospheric vapour was small, although it was revealed

that appreciable amounts of tritium could move directly from the atmosphere into the leaf tissue by diffusion besides transpiration from soils.4,14

The underground water contained in the root zone of the tree appears to have orig-

inated from precipitations of 20 to 30 years ago containing high tritium concentrations due to the series of past thermonuclear bomb tests. The spring water near the tree also seems to have been derived from the Same origin.

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It is apparent from these considerations that the distribution of tritium content in

a sequence of pine tree rings demonstrates the change of annual tritium concentration

in underground water available for the tree, but not those in past precipitations, as

revealed by several authors. 6- s The relationship between underground water and past

precipitation, therefore, should be taken into consideration for elucidating the varia-

tion of tritium concentration in terms of past precipitation.

In contrast to these water samples, the moisture of surface soils around the tree

was found to demonstrate an intermediate value between the spring water sample and

those of precipitation in recent years. It may be ascribed to the mixing of a sequence

of recent rainfalls with the underground water contained in the root zone of the tree

having a high tritium content.

The authors wish to express their thanks to Professor Emeritus Masanobu SAKANOUE, Kanazawa University, for his kind encouragement and discussions. We Would also like to thank Professor Tatsuro SAWADA and Mr. Yoshitoshi NAKAMURA, Department of Chemical Engineering, Kanazawa University, for their suggestions and demonstration of the explosive depressurization operation of wood chips.

References

1. S. KAUFMAN, W. F. LIBBY, Phy. Rev., 93 (1954) 1337. 2. T. TAKAHASHI, M. NISHIDA, S. OHNO, T. HAMADA, Radioisotopes (Tokyo), 18 (1969)

560. 3. B. G. BLAYLOCK, F. O. HOFFMAN, M. L. FRANK, Radiat. Protection Dosimetry, 16 (1986)

65. 4. Y. BELOT, Radiat. Protection Dosimetry, lb (1986) 101. 5. K. KIGOSHI, Y. TOMIKURA, Bull. Chem. Soe. Japan, 34 (1961) 1739. 6. R. M. BROWN, Behaviour of Tritium in the Environment, IAEA, Vienna, 1979, p. 405. 7. K. KOZAK, Acta. Phys. Hung., 52 (1982) 429. 8. K. KOZJ~K, T. BfRO, F. GOLDER, V. RAJNER, D. RANK, F. STAUDNER, Acta. Phys.

Hung., 59 (1986) 59. 9. A. R. G. LANG, S. G. MASON, Can. J. Chem., 38 (1960) 373.

10. J. MANN, Cellulose and Cellulose Derivatives, Part 4, N. M. BIKALES, L. SEGAL (Eds), Wiley-Interscience, New York, 1971. p. 89.

11. M. J. GRINSTED, A. T. WILSON, New Zealand J. Sci., 22 (1979) 281. 12. Y. YAMADA, M. ITOH, N. KIRIYAMA, C. NISHIMOTO, K. KOMURA, K. UENO, J.

Radioanal. Nucl. Chem., 130 (1989) 169. 13. Y. YAMADA, M. ITOH, I. KATO, M. SAKANOUE, Chikyu Kanku, 20 (1986) No. 1 (in

Japanese). 14. Y. TAKASHIMA, N. MOMOSHIMA, M. INOUE, Y. NAKAMURA, Intern. J. Appl. Radiation.

Isotopes, 38 (I987) 255.

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measurement of tritium in tree rings: relationship between tritium concentrations in pine tree rings...

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