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Uptake of tritium by plants from atmosphere and soil
Environment International, Vol. 17, pp. 23-29, 1991 Printed in the U.S.A. All fights reserved.
0160-4120/91 $3.00 +.00 Copyright ©1991 Pergamon Press pie
UPTAKE OF TRITIUM BY PLANTS FROM ATMOSPHERE AND SOIL
Hikaru Amano Japan Atomic Energy Research Institute, Departmentof EnvironmentalSafety 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 transported to a tritium-contaminated forest, where the atmospheric tritium concentration was elevated, 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
and transpire it into the a t m o s p h e r e , a p r o c e s s that is a f f e c t e d by m a n y e n v i r o n m e n t a l p a r a m e t e r s (Raney and Vaadia 1965). M u r p h y (1984) has d e v e l o p e d a general t r e a t m e n t of the equations for e v a p o r a t i o n and diffusion o f H T O to the a t m o s p h e r e f r o m plant leaves by adding the t i m e - d e p e n d e n t storage term. He p r e s e n t e d a general t r e a t m e n t of the equations not only w h e n the source of tritium was in the a t m o sphere, but also when the source of tritium was in the soil. But, in the real e n v i r o n m e n t , even when the original source o f tritium is only in the a t m o s p h e r e , the surface soil a b s o r b s tritium and will contain tri-
B e c a u s e plants can a b s o r b tritium (3H) f r o m the soil and the a t m o s p h e r e , s o m e additional radiation a b o v e natural b a c k g r o u n d m a y b e r e c e i v e d f r o m the ingestion of plant f o o d s grown in the i m m e d i a t e vicinity o f nuclear facilities that release small a m o u n t s of 3H into the surrounding e n v i r o n m e n t . W h e n tritiated water ( H T O ) in a t m o s p h e r i c m o i s t u r e is the only source to the plant, then plant leaves absorb tritium by diffusional p r o c e s s e s d e s c r i b e d m a t h e m a t i c a l l y by B e l o t et al. (1979). I f the only source o f tritium to the plant is soil water, then plants will absorb tritium 23
24
H. A m a n o and C.T. Garten, Jr.
tium after some period of exposure (Amano and Kasai 1988). Likewise, the atmosphere above tritium 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 atmospheric moisture increased gradually in the course of this experiment, but could be considered as a steady-state condition. Under these conditions, two s i m p l e m o d e l s c o u l d be a p p l i e d to the real env i r o n m e n t instead of the more general but more c o m p l i c a t e d m o d e l w h i c h was d e v e l o p e d by Murphy (1984).
(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 equilibrium with tissue water. If tritiated water in the sub-stomatal cavities is in equilibrium with tissue water, then
THEORETICAL CONSIDERATIONS
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)
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 atmospheric moisture and leaf water. Their formula is based on a diffusion equation: F = ( X - Xi) / r
r = r, + r,
(1)
Ix(dC/dt)=(l/r)*
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 concentration (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)
C / C a = a * E * (1 - e "kt)
(5)
(6)
because
(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 cavities, 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) stagnant 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-(¢/a)
E = Ha / ¢
(7)
X = Ha * Ca
(8)
where C is the tritium concentration in leaf water (Bq/g), C a is the tritium concentration in the atmospheric 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 moisture. 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
Tritium uptake by plants
25
accessible for exchange with the atmospheric moisture. 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 original source of tritium is soil water, tritium in soil water will evaporate into the atmosphere. Plants absorb 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 mathematically 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 - d2
(9)
F = (Pl - Pa) / R
(10)
and
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 transpiration almost equals absorption by the leaf, the following relation holds: (d,
-
d2)
* C s =
(d 1 / A~" *
CL -
d2 * Ca
(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: CL = A * E *
Ca + ( 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 humidity, 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 National Laboratory (ORNL) near Oak Ridge, Tennessee, USA. Tritium concentrations at the study area have been described in detail by Amano et al. (1987). Briefly, elevated atmospheric tritium (HTO concentrations in the floodplain forest (approximately 37 to 740 kBq/L air moisture) made the site ideal for experiments on the uptake of tritium by plants from the atmosphere. Experiments on the uptake of tritium from the atmosphere
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 sycamore, 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 concentrations indicate that 3 H a t this site migrates upward 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
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 Nuclear), and analyzed for 3H using a liquid scintillation 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 matter 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 determination 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 temperature for a few days to reach complete dryness of each environmental sample and extracted water was collected in tubes submerged in liquid nitrogen. Water from each method was analyzed for tritium. In addition, 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.
H. A m a n o and C.T. Garten, Yr.
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 measurement 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 consistently higher tritium levels than the vacuum distillation method (Fig. 3). In Fig. 3, "H20-EXTRACT" means total tritium extracted with the water extraction 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 tritium 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 distillation. The efficiency of the water extraction procedure was proved to be high because the extracted tritium from this method exceeds tritium from vacuum distillation. Extracted tissue was not analyzed for non-
4000 A
-o- MAP-H=O -4- MAP-HCL
I
T.MAP-H=O .o- T. MAP- HCI. 4- HIK-HaO -o- HIK-HCI.
2000
E ._
~
10O0
0
I,e
I
I
I00
0
I
2O0
EI0psed time ( h ) 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 completely in 24 h with distilled water and there was no
Fig. 1. Extracted tritium from leaves with distilled w a t e r o r 0.01 M HCI. MAP-HffiO MAP-HCL T.MAP
:Maple leaves with distilledwater :Maple leaves with 0.01 M HCI :Maple leaves torn into pieces of about I c m + with
HIK
:Hickory leaves.
scissors
Tritium uptake by plants
27
extractable tritium. Non-extractable tritium is supposed to be left in the tissue following this extraction.
40 0"
nr~
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 termination time about 5 h later, for the experiment using maple (24 September 1986). The correlation between
0
"10
20
.m .i..f¢1 I
"6 4)
__I I
0
I
20 Leaf-extracted w0ter (Bq/g)
40
Fig. 2. Comparison of the measurement of tritium by LSC from directly extracted water from leaves using distilled water and its distilled water.
Hickory -Stem ¢, Maple- Leaf: E
[] [] [] []
¢' Maple-Leaf: 0'3 Mople-Leof : C:Fe--/-r-e~ I
I
z
HzO-EXTRACT Voc-Distillation Residue Total (Bq/mL)
I
I
I
I
I
50O0
0
I0000
Extrocted tritium
(Bq/mL)
Fig. 3. Comparison of tritium extracted from tree leaves by distilled water and vacuum distillation.
,.o F
--o- Leaf/Air • 0.8* Predicted
] !F- ~
0.8
0.6
.~= ~
o.6
0.4
.'=- c
0.4
0.2
._
0.2
•e ~
0.8[-
.~ E
~'~
/
,.o
Elm
--o- L e a f / A i r - 0.6 ~Predicted
-
0
100 200 Elapsed time (rain)
3oo
Fig. 4. Measured and predicted values of the leaf HTO/air HTO ratio in a maple tree with the elapsed time from atmospheric exposure of HTO.
0
| 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.
H. A m a n o and C.T. Garten, Jr.
28
the course of the experiment and was related to the relative humidity (RH%) as follows for the elm tree:
e-G.)
•~
1.0 Ook
E
= ~
0.8
._
0.6
~m
0.4
0
0
-o- Leaf/Air 0.5* Predicted
T R = 1 . 4 2 - 0 . 0 0 8 4 R H ; ( R 2= 1.0, N = 4 ) .
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 exposure of HTO.
the s p e c i f i c a c t i v i t y of a t m o s p h e r i c m o i s t u r e (Coi, Bq/g H20 ) and elapsed time (T; min) was as follows:
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 recognized 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
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 moisture (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%) decreased from 65% to 31% in the course of the experiment using elm and oak trees. T r a n s p i r a t i o n (TR Ixg*cm2/s) i n c r e a s e d from 0.87 I.tg*cm2/s to 1.16 ~tg*cmZ/s for the elm tree in
The leaves of maple and elm trees in the study area
were sampled at various times during a day and tritium concentrations were measured. At the same time, soil tritium (depth of 0-10 cm, 10-20 cm) and atmospheric tritium concentrations (at the height of the leaves) were also sampled and measured. The distribution 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 absolute concentration of leaf tritium strongly depends on the relative humidity and the soil tritium concentration, 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 activity 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 17 17 17 17 17 17 17 17 17 17
Sap Sep Sep Sep Sep Sep Sep Sep Sap Sep
1986 1986 1986 1986 1986 1986 1986 1986 1986 1985
Tree
1030 Maple-A 1130 Maple-8 1130 Mop~e-B 1420 Maple-B 1420 Maple-8 1430 Maple-C 1450 iople-A 1030 EIm-A 1300EIm-A t420 Elm- A
Measured (kBq/L-H=O}
RH (%}
Air Stem Predicted (kBq/L-HzO) (kBq/L-HzO} (kBq/L-HzO)
188 398 345 527 466 728 319 816 1166 1406
71 70 70 66 66 66 66 71 66 66
6.2 27.8 37.0 Ill 148 t09 83.7 16.2 45.7 109
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. Observed 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 discrepancy was found between predicted and measured values. 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).
371 1740 t740 1740 1740 720 371 2708 2708 2708
143 613 620 751 777 418 223 915 1128 It74
-The authors gratefully acknowledge G.E. Taylor, Jr. and E.A. Bondietti, Environmental Sciences Division, Oak Ridge National Laboratory for their help and valuable discussions 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 Energy under Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. Acknowledgment
REFERENCES Amano, H.; Garten, Jr., C.T.; Lomax, R.D. A field survey of environmental tritium in areas adjacent to O R N L solid-waste storage areas. ORNL-TM/10438. Oak Ridge, TN, USA: Oak Ridge National Laboratory; 1987. Amano, H.; Kasai, A. The transferof atmospheric H T O 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 tritiatedwater from air to plant leaves. Health Phys. 37:575583; 1979. Couchat, P.; Puard, M.; Lasceve, G. Tritiated water vapor exchange in sunflowers. Health Phys. 45:757-764; 1983. Murphy, Jr., C.E. The relationship between tritiated water activities 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 n i c o t i a n a . Plant Phys. 40:383-388; 1965.
0160-4120/91 $3.00 +.00 Copyright ©1991 Pergamon Press pie
UPTAKE OF TRITIUM BY PLANTS FROM ATMOSPHERE AND SOIL
Hikaru Amano Japan Atomic Energy Research Institute, Departmentof EnvironmentalSafety 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 transported to a tritium-contaminated forest, where the atmospheric tritium concentration was elevated, 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
and transpire it into the a t m o s p h e r e , a p r o c e s s that is a f f e c t e d by m a n y e n v i r o n m e n t a l p a r a m e t e r s (Raney and Vaadia 1965). M u r p h y (1984) has d e v e l o p e d a general t r e a t m e n t of the equations for e v a p o r a t i o n and diffusion o f H T O to the a t m o s p h e r e f r o m plant leaves by adding the t i m e - d e p e n d e n t storage term. He p r e s e n t e d a general t r e a t m e n t of the equations not only w h e n the source of tritium was in the a t m o sphere, but also when the source of tritium was in the soil. But, in the real e n v i r o n m e n t , even when the original source o f tritium is only in the a t m o s p h e r e , the surface soil a b s o r b s tritium and will contain tri-
B e c a u s e plants can a b s o r b tritium (3H) f r o m the soil and the a t m o s p h e r e , s o m e additional radiation a b o v e natural b a c k g r o u n d m a y b e r e c e i v e d f r o m the ingestion of plant f o o d s grown in the i m m e d i a t e vicinity o f nuclear facilities that release small a m o u n t s of 3H into the surrounding e n v i r o n m e n t . W h e n tritiated water ( H T O ) in a t m o s p h e r i c m o i s t u r e is the only source to the plant, then plant leaves absorb tritium by diffusional p r o c e s s e s d e s c r i b e d m a t h e m a t i c a l l y by B e l o t et al. (1979). I f the only source o f tritium to the plant is soil water, then plants will absorb tritium 23
24
H. A m a n o and C.T. Garten, Jr.
tium after some period of exposure (Amano and Kasai 1988). Likewise, the atmosphere above tritium 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 atmospheric moisture increased gradually in the course of this experiment, but could be considered as a steady-state condition. Under these conditions, two s i m p l e m o d e l s c o u l d be a p p l i e d to the real env i r o n m e n t instead of the more general but more c o m p l i c a t e d m o d e l w h i c h was d e v e l o p e d by Murphy (1984).
(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 equilibrium with tissue water. If tritiated water in the sub-stomatal cavities is in equilibrium with tissue water, then
THEORETICAL CONSIDERATIONS
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)
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 atmospheric moisture and leaf water. Their formula is based on a diffusion equation: F = ( X - Xi) / r
r = r, + r,
(1)
Ix(dC/dt)=(l/r)*
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 concentration (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)
C / C a = a * E * (1 - e "kt)
(5)
(6)
because
(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 cavities, 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) stagnant 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-(¢/a)
E = Ha / ¢
(7)
X = Ha * Ca
(8)
where C is the tritium concentration in leaf water (Bq/g), C a is the tritium concentration in the atmospheric 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 moisture. 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
Tritium uptake by plants
25
accessible for exchange with the atmospheric moisture. 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 original source of tritium is soil water, tritium in soil water will evaporate into the atmosphere. Plants absorb 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 mathematically 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 - d2
(9)
F = (Pl - Pa) / R
(10)
and
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 transpiration almost equals absorption by the leaf, the following relation holds: (d,
-
d2)
* C s =
(d 1 / A~" *
CL -
d2 * Ca
(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: CL = A * E *
Ca + ( 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 humidity, 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 National Laboratory (ORNL) near Oak Ridge, Tennessee, USA. Tritium concentrations at the study area have been described in detail by Amano et al. (1987). Briefly, elevated atmospheric tritium (HTO concentrations in the floodplain forest (approximately 37 to 740 kBq/L air moisture) made the site ideal for experiments on the uptake of tritium by plants from the atmosphere. Experiments on the uptake of tritium from the atmosphere
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 sycamore, 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 concentrations indicate that 3 H a t this site migrates upward 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
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 Nuclear), and analyzed for 3H using a liquid scintillation 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 matter 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 determination 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 temperature for a few days to reach complete dryness of each environmental sample and extracted water was collected in tubes submerged in liquid nitrogen. Water from each method was analyzed for tritium. In addition, 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.
H. A m a n o and C.T. Garten, Yr.
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 measurement 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 consistently higher tritium levels than the vacuum distillation method (Fig. 3). In Fig. 3, "H20-EXTRACT" means total tritium extracted with the water extraction 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 tritium 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 distillation. The efficiency of the water extraction procedure was proved to be high because the extracted tritium from this method exceeds tritium from vacuum distillation. Extracted tissue was not analyzed for non-
4000 A
-o- MAP-H=O -4- MAP-HCL
I
T.MAP-H=O .o- T. MAP- HCI. 4- HIK-HaO -o- HIK-HCI.
2000
E ._
~
10O0
0
I,e
I
I
I00
0
I
2O0
EI0psed time ( h ) 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 completely in 24 h with distilled water and there was no
Fig. 1. Extracted tritium from leaves with distilled w a t e r o r 0.01 M HCI. MAP-HffiO MAP-HCL T.MAP
:Maple leaves with distilledwater :Maple leaves with 0.01 M HCI :Maple leaves torn into pieces of about I c m + with
HIK
:Hickory leaves.
scissors
Tritium uptake by plants
27
extractable tritium. Non-extractable tritium is supposed to be left in the tissue following this extraction.
40 0"
nr~
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 termination time about 5 h later, for the experiment using maple (24 September 1986). The correlation between
0
"10
20
.m .i..f¢1 I
"6 4)
__I I
0
I
20 Leaf-extracted w0ter (Bq/g)
40
Fig. 2. Comparison of the measurement of tritium by LSC from directly extracted water from leaves using distilled water and its distilled water.
Hickory -Stem ¢, Maple- Leaf: E
[] [] [] []
¢' Maple-Leaf: 0'3 Mople-Leof : C:Fe--/-r-e~ I
I
z
HzO-EXTRACT Voc-Distillation Residue Total (Bq/mL)
I
I
I
I
I
50O0
0
I0000
Extrocted tritium
(Bq/mL)
Fig. 3. Comparison of tritium extracted from tree leaves by distilled water and vacuum distillation.
,.o F
--o- Leaf/Air • 0.8* Predicted
] !F- ~
0.8
0.6
.~= ~
o.6
0.4
.'=- c
0.4
0.2
._
0.2
•e ~
0.8[-
.~ E
~'~
/
,.o
Elm
--o- L e a f / A i r - 0.6 ~Predicted
-
0
100 200 Elapsed time (rain)
3oo
Fig. 4. Measured and predicted values of the leaf HTO/air HTO ratio in a maple tree with the elapsed time from atmospheric exposure of HTO.
0
| 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.
H. A m a n o and C.T. Garten, Jr.
28
the course of the experiment and was related to the relative humidity (RH%) as follows for the elm tree:
e-G.)
•~
1.0 Ook
E
= ~
0.8
._
0.6
~m
0.4
0
0
-o- Leaf/Air 0.5* Predicted
T R = 1 . 4 2 - 0 . 0 0 8 4 R H ; ( R 2= 1.0, N = 4 ) .
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 exposure of HTO.
the s p e c i f i c a c t i v i t y of a t m o s p h e r i c m o i s t u r e (Coi, Bq/g H20 ) and elapsed time (T; min) was as follows:
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 recognized 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
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 moisture (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%) decreased from 65% to 31% in the course of the experiment using elm and oak trees. T r a n s p i r a t i o n (TR Ixg*cm2/s) i n c r e a s e d from 0.87 I.tg*cm2/s to 1.16 ~tg*cmZ/s for the elm tree in
The leaves of maple and elm trees in the study area
were sampled at various times during a day and tritium concentrations were measured. At the same time, soil tritium (depth of 0-10 cm, 10-20 cm) and atmospheric tritium concentrations (at the height of the leaves) were also sampled and measured. The distribution 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 absolute concentration of leaf tritium strongly depends on the relative humidity and the soil tritium concentration, 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 activity 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 17 17 17 17 17 17 17 17 17 17
Sap Sep Sep Sep Sep Sep Sep Sep Sap Sep
1986 1986 1986 1986 1986 1986 1986 1986 1986 1985
Tree
1030 Maple-A 1130 Maple-8 1130 Mop~e-B 1420 Maple-B 1420 Maple-8 1430 Maple-C 1450 iople-A 1030 EIm-A 1300EIm-A t420 Elm- A
Measured (kBq/L-H=O}
RH (%}
Air Stem Predicted (kBq/L-HzO) (kBq/L-HzO} (kBq/L-HzO)
188 398 345 527 466 728 319 816 1166 1406
71 70 70 66 66 66 66 71 66 66
6.2 27.8 37.0 Ill 148 t09 83.7 16.2 45.7 109
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. Observed 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 discrepancy was found between predicted and measured values. 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).
371 1740 t740 1740 1740 720 371 2708 2708 2708
143 613 620 751 777 418 223 915 1128 It74
-The authors gratefully acknowledge G.E. Taylor, Jr. and E.A. Bondietti, Environmental Sciences Division, Oak Ridge National Laboratory for their help and valuable discussions 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 Energy under Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. Acknowledgment
REFERENCES Amano, H.; Garten, Jr., C.T.; Lomax, R.D. A field survey of environmental tritium in areas adjacent to O R N L solid-waste storage areas. ORNL-TM/10438. Oak Ridge, TN, USA: Oak Ridge National Laboratory; 1987. Amano, H.; Kasai, A. The transferof atmospheric H T O 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 tritiatedwater from air to plant leaves. Health Phys. 37:575583; 1979. Couchat, P.; Puard, M.; Lasceve, G. Tritiated water vapor exchange in sunflowers. Health Phys. 45:757-764; 1983. Murphy, Jr., C.E. The relationship between tritiated water activities 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 n i c o t i a n a . Plant Phys. 40:383-388; 1965.