Environment International, Vol. 17, pp. 23-29, 1991 0160-4120/91 $3.00 +.00 Printed in the U.S.A. All fights reserved. Copyright ©1991 Pergamon Press pie

UPTAKE OF TRITIUM BY PLANTS FROM ATMOSPHERE AND SOIL

Hikaru Amano Japan Atomic Energy Research Institute, Department of Environmental Safety Research, Tokai-mura, Naka-gun, Ibaraki-ken 319-11, Japan

C.T. Garten, Jr. Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

EI 88-121 (Received 26 September 1988; accepted 4 April 1990)

Uptake of tritiated water (HTO) by plants was examined under field conditions when tritium was available to leaves from only the atmosphere and when tritium was available from both the soil (root uptake) and the atmosphere. Maple, oak, and elm trees, planted in clean soil, were trans- ported to a tritium-contaminated forest, where the atmospheric tritium concentration was ele- vated, to examine HTO uptake by tree leaves when the source was only in the atmosphere. The results partially agreed with a diffusion model of tritium uptake by plants. Discrepancies found between predicted and measured leaf HTO/air HTO ratios should be attributed to the existence of some isolated water, which is isolated from the transpiration stream in the leaves, that was not available for rapid turnover. The uptake of tritium by trees, when the source was both in the soil and atmosphere, was also examined using deciduous trees (maple and elm) resident to the tritium-contaminated forest. The results were in agreement with a prediction model.

INTRODUCTION

Because plants can absorb tr i t ium (3H) f rom the soil and the a tmosphere , some addi t ional radia t ion above natural background may be rece ived f rom the ingest ion of plant foods grown in the immedia te vi- cini ty of nuclear faci l i t ies that re lease small amounts of 3H into the surrounding env i ronment .When triti- ated water (HTO) in a tmospher ic mois ture is the only source to the plant , then plant leaves absorb tr i t ium by di f fus ional p rocesses descr ibed mathemat ica l ly by Belot et al. (1979). I f the only source of t r i t ium to the plant is soil water , then plants will absorb tr i t ium

and t ranspire it into the a tmosphere , a process that is a f fec ted by many env i ronmenta l pa ramete r s (Raney and Vaadia 1965). Murphy (1984) has deve loped a general t rea tment of the equat ions for evapora t ion and diffusion of HTO to the a tmosphere f rom plant leaves by adding the t ime-dependen t s torage term. He presented a general t rea tment of the equat ions not only when the source of tr i t ium was in the a tmo- sphere, but also when the source of tr i t ium was in the soil. But, in the real envi ronment , even when the or iginal source of t r i t ium is only in the a tmosphere , the surface soil absorbs t r i t ium and will contain tri-

23

24 H. Amano and C.T. Garten, Jr.

tium after some period of exposure (Amano and Kasai 1988). Likewise, the atmosphere above tri- tium contaminated soil will contain tritium due to evapo-transpiration of HTO with soil moisture (Amano et al. 1987).

The purpose of this paper was to examine the validity of the predictive model of Belot et al. (1979) when plants absorb tritium only from the atmosphere. Also considered here is a modified model proposed by Raney and Vaadia (1965) for tritium in plants when the source of tritium is both the atmosphere and the soil. Both of these models are simple but have some drawbacks. Leaf and atmospheric temperature should be the same. Steady-state conditions should prevail. In the experiments, the difference between the leaf and atmospheric temperatures was proved to be within I°C from the result of their measurement using a porometer. The specific activity of atmo- spheric moisture increased gradually in the course of this experiment, but could be considered as a steady-state condition. Under these conditions, two s imple models could be appl ied to the real en- vi ronment instead of the more general but more c o m p l i c a t e d model which was deve loped by Murphy (1984).

THEORETICAL CONSIDERATIONS

Source of tritium (HTO) is only in the atmosphere

Belot et al. (1979) describe tritium uptake by plants, when the tritium source is only in the atmosphere, as a diffusion process via the plant leave between atmo- spheric moisture and leaf water. Their formula is based on a diffusion equation:

F = ( X - Xi) / r (1)

(5) the exchange resistances for HTO and for water are equal, and (6) steady-state conditions prevail. So, tritiated water in the sub-stomatal cavities is in equi- librium with tissue water.

If tritiated water in the sub-stomatal cavities is in equilibrium with tissue water, then

I x ( d C / d t ) = ( l / r ) * ( X - ( ¢ / a ) C) (3)

where Ix is the amount of water per unit area of leaf (g/cm2), C is the tritium concentration in tissue water (Bq/g), r is the foliar resistance (s/cm), t is elapsed time (s), ¢ is the density of water vapor in saturated air (g/mL), a is the isotopic ratio (T/H) in liquid and vapor (=1.1), and X is the atmospheric tritium con- centration (Bq/mL air).

When the boundary conditions are such that C = 0 at t = 0, then the relative concentration of tritium in leaf water ( C / X) tends to a value (a / ¢) which is a function of leaf temperature, or

C / X = (a / ~) * (1 - e " i t ) (4)

and k = ~ / (a* Ix* r) (5)

where k is the elimination rate constant (s 1) for tritium from the leaf.

The equilibration rate between tritium in leaf water and tritium in the atmosphere is governed by the elimination constant, k.

As an alternative of equation (4)

C / C a = a * E * (1 - e "kt) (6)

because E = H a / ¢ (7)

r = r, + r, (2)

where F is the net flux of vapor per unit area of leaf, X is the tritium concentration in the turbulent air, X i is the tritium concentration in the sub-stomatal cav- ities, r is the foliar resistance, r a is the boundary layer resistance, and r, is the stomatal resistance.

The many basic assumptions that are part of the model described by Belot et al. (1979) are: (1) stag- nant air of the sub-stomatal cavities is saturated by water vapor, (2) a major part of the tissue water is accessible for rapid turnover, (3) liquid diffusion of HTO in the accessible tissue water is much more rapid than its gaseous diffusion through stomata, (4) little water is translocated from the exposed leaves,

X = H a * C a (8)

where C is the tritium concentration in leaf water (Bq/g), C a is the tritium concentration in the atmo- spheric water (Bq/g), E is the atmospheric relative humidity, and H a is the absolute humidity (g/mL air).

So, (C / C a) tends to a value a * E.

Belot et al. (1979) proposed that a major part of the leaf water is exchangeable with atmospheric mois- ture. Even though some part of the water in the leaf is not exchangeable, their treatment is applicable if at least part of the water in the leaf is assumed to be

Trit ium uptake by plants 25

accessible for exchange with the atmospheric mois- ture.

Source of HTO in the soft and atmosphere

In contaminated environments, when the initial source of tritium is only the atmosphere, surface soil absorbs tritium from the atmosphere. When the orig- inal source of tritium is soil water, tritium in soil water will evaporate into the atmosphere. Plants ab- sorb tritium from both the atmosphere and the soil. Plants take up tritium in soil water through the roots in transpiration via the leaf stomata, but some water also enters plant leaves from the atmosphere via diffusion. The following formulation, to mathemati- cally describe the uptake of tritium by plant leaves when the source of tritium is both the soil and the atmosphere, was originally proposed by Raney and Vaadia (1965) and was somewhat modified by Amano (Amano and Kasai 1988).

Transpiration is proportional to the difference in vapor pressure between the leaf and the atmosphere. The net transpiration rate per unit area of leaf (F) represents the difference between outward diffusion from the leaf (dl) and inward diffusion to the leaf (d2). Each component is proportional to the vapor pressure at its source. Therefore,

F = d 1 - d 2 (9)

and F = (Pl - Pa) / R (10)

where Pl is the vapor pressure in the leaf, Pa is the vapor pressure in the atmosphere, and R is the foliar resistance.

Under semi-steady state conditions, when transpi- ration almost equals absorption by the leaf, the fol- lowing relation holds:

(d, - d2) * C s = (d 1 / A~" * C L - d 2 * C a (11)

where C, is the tritium concentration in soil water, C L is the tritium concentration in the leaf water, C a is the tritium concentration in atmospheric moisture, E is the relative humidity (Pa / Pl), and A is the ratio of the isotopic proportion T/H in water and in vapor at equilibrium (= 1.1).

Since d 1 and d 2 are proportional to Pl and Pa, respectively, the following relation holds if leaf and air temperatures are equal:

C L = A * E * C a + ( 1 - E ) * C , * A (12)

If the soil tritium is constant, the leaf tritium is a function of relative humidity and atmospheric tritium concentration. The concentration of tritium in leaf water (CL) can be predicted from the relative humid- ity, the tritium concentration of soil water and the tritium concentration of the atmospheric moisture.

MATERIALS AND METHODS

Study area

Field studies were undertaken in a contaminated floodplain forest (occupied mostly by deciduous trees), adjacent to a low-level radioactive waste storage area (Solid Waste Storage Area No. 5) at Oak Ridge Na- tional Laboratory (ORNL) near Oak Ridge, Tennes- see, USA. Tritium concentrations at the study area have been described in detail by Amano et al. (1987). Briefly, elevated atmospheric tritium (HTO concen- trations in the floodplain forest (approximately 37 to 740 kBq/L air moisture) made the site ideal for ex- periments on the uptake of tritium by plants from the atmosphere.

Experiments on the uptake of tritium from the atmo- sphere

Small maple, oak, and elm trees that were potted in clean, covered soil were carried into the forest and left to examine tritium uptake by tree leaves when the source of tritium was only in the atmosphere. These experiments were done in late September for maple and in early October for elm and oak trees. Leaves were sampled from each tree immediately upon their arrival at the contaminated site and after various time periods of exposure. Samples were sealed in plastic bottles and frozen prior to analysis.

Source of tritium in the atmosphere and soil

Deciduous trees at the study site included syca- more, red maple, elm, yellow poplar, hickory, and dogwood. Tritium concentrations in soil water at this site range from approximately 370 to 14 000 kBq/L during the growing season. Monthly changes in groundwater levels and soil water tritium concen- trations indicate that 3 H a t this site migrates up- ward to the surface soil from subterranean flows with the upward movement of groundwater during the wet season (Amano et al. 1987). Tritium in the surface soil gives rise to tritium concentrations in the atmosphere by evaporation of surface soil water. Leaves were sampled from various species of trees at the site, sealed in plastic bottles, and frozen prior to analysis.

26 H. A m a n o and C.T. Garten, Yr.

Measurement of environmental parameters and radioactivity

At each sampling time, environmental parameters such as temperature, leaf temperature, humidity, and leaf resistance were measured using a porometer (Type: L1-COR-LI-1600; Steady-State Porometer; L1-COR, Lincoln, Nebraska, USA). Atmospheric moisture was collected cryogenically, above the soil, at various time intervals by suspending clean plastic bottles, containing dry ice. Air moisture that condensed on the outside surface of the bottles was collected, mixed with liquid scintillator (Aquasol ®, New England Nu- clear), and analyzed for 3H using a liquid scintilla- tion counter (LSC).

Leaves, in sealed plastic bottles, were extracted with a known volume of distilled water or 0.01 M HC1 to remove exchangeable tritium. Some leaves were extracted after being torn in about 1 cm 2 squares. The efficiency of this extraction procedure was supposed to be large. Aliquots of the extracted tritiated water were analyzed by the same procedures as atmospheric moisture. The effect of quenching from organic mat- ter in leaf extracts was checked by dividing some extracts into two parts: one part was directly mixed with liquid scintillator and counted, the second part of the extract was distilled to recover only HTO for analysis.

The water and HCI extraction method for determi- nation of exchangeable HTO was also compared to the measurement of leaf HTO by vacuum distillation. To compare the water extraction method and vacuum extraction method, samples of leaves and stems which contained tritium were homogeneously mixed and divided into two parts. Tritiated water in the first part was extracted with distilled water and that in the second part was extracted by vacuum distillation. Vacuum distillation was done under room tempera- ture for a few days to reach complete dryness of each environmental sample and extracted water was col- lected in tubes submerged in liquid nitrogen. Water from each method was analyzed for tritium. In addi- tion, some tissues were extracted with distilled water after treatment by the vacuum distillation method to determine if additional tritium could be removed after complete vacuum distillation.

RESULTS AND DISCUSSION

Comparison between water extraction method and vacuum distillation

Figure 1 shows the time required to extract tritium from maple and hickory leaves using distilled water and 0.01 M HC1. Leaf tritium was extracted com- pletely in 24 h with distilled water and there was no

difference between extractions using distilled water or 0.01 M HC1.

Some organic matter in leaves might be extracted during this extraction procedure and disturb the mea- surement of tritium by LSC. But no difference was found in the determinations of leaf HTO between directly extracted water and its distilled water by extraction methods (Fig. 2), indicating that there was no effect of quenching on the measurement of tritium in the directly extracted water.

Extractions of leaf HTO with water gave consis- tently higher tritium levels than the vacuum distilla- tion method (Fig. 3). In Fig. 3, "H20-EXTRACT" means total tritium extracted with the water extrac- tion method. "Vat-Distillation" means tritium with complete vacuum distillation, and "Residue" means tritium removed with water after treatment by the vacuum distillation method. The "Total" means tri- tium from "Vac-Distillation" and "Residue". The "total" tritium agreed with tritium from "H20-EXTRACT". This indicated the presence of weakly bound organic tritium in the leaves and stems that was extractable with water, but not extracted using vacuum distilla- tion. The efficiency of the water extraction procedure was proved to be high because the extracted tritium from this method exceeds tritium from vacuum dis- tillation. Extracted tissue was not analyzed for non-

4000

A

I

2000 E

. _

~ 10O0 0

I , e

I I I 0 I00 2O0

EI0psed time (h )

-o- MAP-H=O -4- MAP-HCL

T.MAP-H=O .o- T. MAP- HCI. 4- HIK-HaO -o- HIK-HCI.

Fig. 1. Extracted tritium from leaves with distilled water or 0.01 M HCI.

MAP-HffiO MAP-HCL T.MAP

HIK

:Maple leaves with distilled water :Maple leaves with 0.01 M HCI :Maple leaves torn into pieces of about I cm + with scissors :Hickory leaves.

Tritium uptake by plants 27

0 " nr~

0

"10

. m .i..- f¢1

I

"6 4) __I

40

20

I I

0 20 40

Leaf-extracted w0ter (Bq/g)

Fig. 2. Comparison of the measurement of tritium by LSC from directly extracted water from leaves using distilled water and its

distilled water.

extractable tritium. Non-extractable tritium is supposed to be left in the tissue following this extraction.

Source of tritium is in the atmosphere

When the source of tritium (HTO) was only in the atmosphere, the leaves of potted maple (Fig. 4), elm (Fig. 5), and oak (Fig. 6) trees absorbed atmospheric tritiated water. In this experiment, the difference between the leaf and atmospheric temperatures was proved to be within I°C from the result of their measurement using a porometer. Specific activity of the atmospheric moisture increased gradually in the course of this experiment, and it could be considered a steady-state condition. Specific activity of the atmospheric moisture ranged from 37 Bq/gH20 from start of exposure to 83 Bq/gH20 at termina- tion time about 5 h later, for the experiment using maple (24 September 1986). The correlation between

Hickory -Stem

¢, Maple- Leaf:

E ¢' Maple-Leaf: 0'3

Mople-Leof : C:Fe--/-r-e~

[ ] HzO-EXTRACT [ ] Voc-Distillation [] Residue [ ] Total (Bq/mL)

I I I I

50O0 I I z I

0 I0000

Extrocted tritium (Bq/mL)

Fig. 3. Comparison of tritium extracted from tree leaves by distilled water and vacuum distillation.

,.o F ] ,.o - -o- Leaf/Air

• e ~ 0.8[- • 0.8* Predicted / !F- ~ 0.8

o.6 .~ E 0.6 .~= ~

~ ' ~ 0.4 .'=- c 0.4 -

0 . 2 . _ 0 . 2

0 100 200 3oo 0 Elapsed time (rain)

Fig. 4. Measured and predicted values of the leaf HTO/air HTO ratio in a maple tree with the elapsed time from atmospheric ex-

posure of HTO.

E l m - -o- L e a f / A i r - 0.6 ~Predicted

| oo 20o 3o0 Elopsed time (rain)

400

Fig. 5. Measured and predicted values of the leaf HTO/air HTO ratio in an elm tree with the elapsed time from atmospheric e x -

p o s u r e of HTO.

28 H. Amano and C.T. Garten, Jr.

e--

G.)

• ~ 1.0

E = ~ 0.8

._ 0.6

~ m 0.4

0 0

Ook -o- Leaf/Air 0.5 * Predicted

0.2 0 I00 200 300 400

EI0psed time (rain)

Fig. 6. Measured and predicted values of the leaf HTO/air HTO ratio in an oak tree with the elapsed time from atmospheric ex-

posure of HTO.

the spec i f i c ac t iv i ty of a tmospher ic mois ture (Coi, Bq/g H20 ) and elapsed time (T; min) was as follows:

Cair = 36.9 + 0.16 T ; (R 2 = 1.0, N = 12).

Relative humidity of the atmosphere (RH %) was decreased from 84% to 56% in the course of the experiment using maple trees.

Transpiration (TR lxg*cm2/s) was increased from 0.21 lxg*cm2/s to 0.44 l.tg*cm2/s in the course of the experiment and was related to the relative humidity (RH %) as follows:

TR = 0.91 - 0.0087 RH ; (R 2 = 0.92, N = 7).

For the experiment using elm and oak trees, the specific activity of atmospheric moisture ranged from 8.8 Bq/gH20 from start of exposure to 52 Bq/gH20 at termination about 5 h later, for the experiment using elm and oak trees (16 October 1986). The correlation between the specific activity of atmospheric mois- ture (Cair; Bq/gH20 ) and elapsed time (T; min) was as follows:

C,i r = 8.0 + 0.15 T ; (R 2 = 0.98, N = 9).

Relative humidity of the atmosphere (RH%) de- creased from 65% to 31% in the course of the exper- iment using elm and oak trees.

Transpira t ion (TR Ixg*cm2/s) increased from 0.87 I.tg*cm2/s to 1.16 ~tg*cmZ/s for the elm tree in

the course of the experiment and was related to the relative humidity (RH%) as follows for the elm tree:

TR= 1 . 4 2 - 0 . 0 0 8 4 R H ; ( R 2= 1.0, N = 4 ) .

The time trends in the ratio HTO leaf/HTO air was in agreement with the values predicted from Belot's model. However, some discrepancies existed between measured and predicted values that required the application of a correction factor to the predicted leaf/air ratios (Figs. 4-6). These discrepancies might come about as a result of some isolated water in tree leaves which is not accessible for rapid turnover. The existence of the isolated water in tree leaves which is isolated from the transpiration stream is recog- nized in the field of plant physiology. The ratio of isolated water to total water seems to be about 20% for maple leaves, 40% for elm leaves, and 50% for oak leaves as shown, respectively, in Figs. 4-6. Couchat et al. (1983) showed that about 84% of the total leaf water was exchanged with atmospheric tritiated water vapor for sunflowers. It is likely that trees have more isolated cell water than vegetations like sunflowers.

Source of tritium in the atmosphere and soil

The leaves of maple and elm trees in the study area were sampled at various times during a day and tri- tium concentrations were measured. At the same time, soil tritium (depth of 0-10 cm, 10-20 cm) and atmo- spheric tritium concentrations (at the height of the leaves) were also sampled and measured. The distri- bution of soil tritium was very heterogeneous and so, the tritium concentration in the water of the stem bearing the leaf was used as a measure of the soil tritium concentration detected in the tree roots. The measured tritium in the tree leaves was compared with the predicted concentrations according to the model equation (12) and the results are shown in Table 1. The agreement of the values between measured and predicted leaf tritium concentrations was good (N = 10, R 2 -- 0.70), showing the validity of the model presented. According to the equation (12), the abso- lute concentration of leaf tritium strongly depends on the relative humidity and the soil tritium concentra- tion, because the specific activity of the atmospheric tritium was rather lower than that of the stem (soil) tritium in the present case. The largest discrepancy of almost 40% between measured and predicted leaf tritium concentrations might be due to the variation of the soil tritium concentration. The specific activ- ity of HTO in some isolated water in these tree leaves, which are isolated from the transpiration stream, should be equilibrated with that of soil water because

l'ritium uptake by plants 29

Table 1. Comparison between measured and predicted leaf tritium concentrations (N=10, R==0.70).

Dale & Time Tree Measured RH Air Stem Predicted (kBq/L-H=O} (%} (kBq/L-HzO) (kBq/L-HzO} (kBq/L-HzO)

17 Sap 1986 1030 Maple-A 188 71 6.2 371 143 17 Sep 1986 1130 Maple-8 398 70 27.8 1740 613 17 Sep 1986 1130 Mop~e-B 345 70 37.0 t740 620 17 Sep 1986 1420 Maple-B 527 66 I l l 1740 751 17 Sep 1986 1420 Maple-8 466 66 148 1740 777 17 Sep 1986 1430 Maple-C 728 66 t09 720 418 17 Sep 1986 1450 iople-A 319 66 83.7 371 223 17 Sep 1986 1030 EIm-A 816 71 16.2 2708 915 17 Sap 1986 1300EIm-A 1166 66 45.7 2708 1128 17 Sep 1985 t420 Elm- A 1406 66 109 2708 It74

the change of the specific activity of the soil tritium was rather slow in this study area (Amano et al. 1987).

CONCLUSION

Potted maple, oak and elm trees were transported to a floodplain forest, where the atmospheric tritium concentrations were elevated, to examine the uptake of tritium by tree leaves from the atmosphere. Ob- served concentrations were in partial agreement with predictions made using a mathematical model (Belot et al. 1979) of tritium uptake by plants when the only source of tritium is the atmosphere. Some discrep- ancy was found between predicted and measured val- ues. The existence of some isolated water, which is isolated from the transpiration stream in the tree leaves, may account for this discrepancy. To examine tritium uptake by plants, when the tritium source is both the soil and the atmosphere, maple and elm trees growing in the tritium contaminated forest soil were examined. Measured leaf tritium concentrations were in agreement with a model originally proposed by Raney and Vaadia (1965) and slightly modified by Amano (Amano and Kasai 1988).

A c k n o w l e d g m e n t - - The authors gratefully acknowledge G.E. Taylor, Jr. and E.A. Bondietti, Environmental Sciences Division, Oak Ridge National Laboratory for their help and valuable discus- sions about this work. Thanks are also due to R.D. Lomax, ORNL, for his assistance with the field work. This research was sponsored jointly by the Japan Atomic Energy Research Institute, Tokai Research Establishment, Japan, and the U.S. Department of En- ergy under Contract No. DE-AC05-840R21400 with Martin Mari- etta Energy Systems, Inc.

REFERENCES

Amano, H.; Garten, Jr., C.T.; Lomax, R.D. A field survey of environmental tritium in areas adjacent to ORNL solid-waste storage areas. ORNL-TM/10438. Oak Ridge, TN, USA: Oak Ridge National Laboratory; 1987.

Amano, H.; Kasai, A. The transfer of atmospheric HTO released from nuclear facilities under normal operation. L Environ. Radioact. 8:239-253; 1988.

Belot, Y.; Gauthier, D.; Camus, H.; Caput, C. Prediction of the flux of tritiated water from air to plant leaves. Health Phys. 37:575- 583; 1979.

Couchat, P.; Puard, M.; Lasceve, G. Tritiated water vapor ex- change in sunflowers. Health Phys. 45:757-764; 1983.

Murphy, Jr., C.E. The relationship between tritiated water activi- ties in air, vegetation and soil under steady-state conditions. Health Phys. 47:635-639; 1984.

Raney F.; Vaadia, Y. Movement and distribution of HTO in tissue water and vapor transpired by shoots of h e l i a n t h u s and nico t i -

ana . Plant Phys. 40:383-388; 1965.