samples was cause for concern. Since all pots were watered regularly
it was presumed that repeated and thorough mixing would have resulted
in the HTO concentration being rather uniform. The fact that this
large variation occurred is an indication that the samples were
stratified with respect to tritium and that a profile sampling

should have been performed. Nevertheless, it is clear that the
rooting media HTO was higher than the extractable root HTO (Figure 2).
Thus the overall order of tritium contamination at equilibrium was
soil > leaves > roots. Since the concentration in the roots was
lower than in the soil, root uptake of tritiated water seems im-
probable, The tritium gradient from leaves to roots indicates that
foliar absorption was the predominant plant contamination pathway.
This apparent inconsistency is thought to have resulted from sampling
the soils randomly and not taking a soil profile. If the soil sur-
face was a reaction site for the conversion of HT to HTO, then

a highly contaminated layer may have existed at the surface. Mix-
ing this layer would have blended it with the layers lower in HTO
concentration. This could account for the high and variable values
in Table 2 and would offer an explanation for the apparent contra-
diction. Subsequent studies regarding soil contamination and
stratification have proven this to be true and will be reported
elsewhere (McFarlane and Batterman, 1975).

Six-inch petri dishes were placed in the chamber and 25 ml of
uncontaminated water was placed within each dish. Samples were
periodically removed, and the transfer resistance was calculated to
be 3.4 cm/sec. Higher values would have been expected had the
site of conversion been at the water surface. This additional bit
of evidence supports the concept of a catalytically active surface

perhaps on the leaves, or more probably at the surface of the soil.

14


4—QUTER LEAVES
e—INNER LEAVES
=—ROOTS

HTO nCi/mi

5
DAYS AFTER LEAVING HT ENVIRONMENT

Figure 3. Tritium concentration {uto] in the extractable plant water
after removing lettuce plants from HT-contaminated environment.


Recommendations of the International Commission on Radiological
Protestion, Report of Committee II on Permissable Dose for
Internal Radiation, 1959, ICRP Publication 2, Pergamon Press
Oxford, London.

Lang, A. R. G. and Mason, S. G., "Tritium Exchange Between
Cellulose and Water: Accessibility Measurements and Effects
of Cyclic Drying," Can, J. Chem., 38, pp 373-387 (1960).

Lieberman, R. and Moghissi, A. A., "Low-Level Counting by Liquid
Scintillation II Applications of Emulsions in Tritium Counting,"
Journal of Applied Radiation and Isotopes, 21, pp 319-327 (1970).

McFarlane, J. C., "Tritium Fractionation in Plants," EPA-NERC-LV,
(1975).

McFarlane, J. C. and Batterman, A., "Tritiated Water in Soils
Exposed to Elemental Tritium," unpublished data, (1975).

McFarlane, J. C., Beckert, W. F., and Brown, K. W., "Tritium
in Plants and Soil," EPA-NERC-LV, (1975).

Moghissi, A. A., Bretthauer, E. W., Plott, W. F., McNelis, D. N.,
and Whittaker, E, L., "Oxygen Bomb Combustion of Environmental
and Biological Samples for Tritium Analysis," accepted for
publication in Analytical Chemistry (1975).

Moghissi, A. A., Bretthauer, E. W. and Compton, E. H., "Separation
of Water From Biological and Environmental Samples for Tritium
Analysis," Analytical Chemistry, 45, pp 1565-1566 (1973).

Pinson, E. A,, "The Body Absorption,.Distribution, ard Excretion
of Tritium in Man and Animals," USAEC Report LA-1218, Los Alamos
Scientific Laboratory (1951).

Radiological Science Laboratory, “Private Communication Exchange
Characteristics of Gaseous Tritium in the Environment," need
statement, O2AFO, USEPA (1972).

Weaver, C. L., Howard, D,, and Peterson, H. T., Jr., "Tritium

in the Environment from Nuclear Power Plants," Public Health
Report 84 (4), pp 363-371 (1969).

18


INTRODUCTION

METHODS

RESULTS AND DISCUSSION
CONCLUSIONS

LITERATURE CITED

CONTENTS

ili

17

18


Table 2. TRITIATED WATER EXTRACTED FROM POTTING MIXTURE AFTER

EXPOSURE TO 5 nCi/1 OF ELEMENTAL TRITIUM FOR 6 DAYS

A B
HTO extracted from mix HTO extracted from mix
(aCi/m1) (nCi/m1)
2.05 0.87
1.28 1.11
1.94 1,22
2.77 0.99
1.22 1.99
x = 1.84 + 0.64* = = 1.246 +.0,44*

Pots contained a potting mix of peat and vermiculite 1:1
(no plants).

Pots contained mix and one 4-week old lettuce plant growing in
each pot.

Standard deviation.

15


LITERATURE CITED

Berry, W. L., "Evaluation of Phosphorous Nutrient Status in
Seedling Lettuce," J. Amer. Soc. Hort. Sci. 96 (3), pp 341-344
(1971).

Cline, J. F., "Absorption and Metabolism of Tritium Oxide and
Tritium Gas by Bean Plants," Plant Physiol. 28, pp 717-723 (1953).

Cochran, J. A., Griffin, W. R., Troianello, E. J., "Observation
of Airborne Tritium Waste Discharge From a Nuclear Fuel Reproces-
sing Plant," Office of Radiation Programs, U.S. Environmental
Protection Agency EPA/ORP 73-1 (1973).

Cowser, K. E., Boegly, W. J., Jr., Jacobs, D. G., "®5kr and
Tritium in the Expanding World Nuclear Power Industry," Health
Physics Division, Annual Progress Report for Period Ending July
31, 1966, USAEC Report ORNL~4007, Oak Ridge National Laboratory,
Oak Ridge, IN (1966).

Eakins, J. D. and Hutchinson, W.P., "The Radiological Hazard
From the Conversion of Tritium to Tritiated Water in Air by
Metal Catalysts," Tritium, edited by A. A. Moghissi and M. W.
Carter, pp 392-399, Messenger Graphics, Phoenix, AZ and Las
Vegas, NV (1973).

Ehhalt, D. H., Heidt, L. E., Lueb, R. H., and Roper, N., "Vertical
Profiles of CHy, H2, CO, NOj, and C02 in the Stratosphere," Third
Conference on CIAP, U.S. Dept. of Transportation, pp 153-160
(1974).

Gest, H., "Oxidation and Evolution of Molecular Hydrogen by
Microorganisms," Bacteriol, Rev., 18, pp 43-73 (1954).

Griffin, W. R., Cochran, J. A., and Bertuccio, A. A., "A Sampler
for Nonaqueous Tritium Gases," Tritium, edited by A. A. Moghissi
and M. W. Carter, pp 533-541, Messenger Graphics, Phoenix, AZ
and Las Vegas, NV (1973).

Hill, A. C., "A Special Purpose Plant Environmental Chamber for
Air Pollution Studies," J. Air Pollution Control Association, 17,
pp 743-748 (Nov. 1967).

17


gradient of elemental tritium to tritium oxide which is evident on
a volume and atomic basis, and should favor chemical reactions, the
rapid reaction rates observed only in the presence of potted plants
suggest a facilitated oxidation or exchange. Whether the conversion
reaction is a surface catalysis or an enzymatically facilitated
reaction is unknown. The reverse reaction (production of Hj from
H20) is known to operate in lower life forms at rapid rates (Gest,
1954) and this suggests that a reaction occurring at the soil sur-
face involving the soil microbiota is possible. Detailed studies
regarding conversion rates and reaction sites are currently in
progress.

For comparison, an experiment was conducted where plants were
exposed for one week to tritiated water vapor. The concentration
of tritium in the vapor was monitored and controlled by evaporating
water of a known concentration. At the end of one week of exposure,
leaves and roots were sampled and the water was extracted and analyzed
for tritium. A comparison of these two experiments is best accom-
plished by examining the ratios of tritium concentrations in various
segments of each experiment. Equation 2 describes the ratio of
HTO concentrations found in plants and air when the plants were ex-

posed to HT.

Equation 2: Plants exposed to 5 nCi/1 of HT.

HTO in leaves (nCi/ml) = 0.84
HTO in air (nCi /mi)

Equation 3: Plants exposed to 115 nCi/ml of HTO in water
vapor.

HTO in leaves (nCi/ml)

Hf0 in-air (nCi/ml) ~

A similar comparison is shown in Equation 3 when plants were exposed
to HTO vapor. The higher ratio in Equation 2 probably indicates
amore direct transfer of tritium when plants were exposed to HT.

This may have been caused by a more available source (i.é. higher

12


between the two samples, thus indicating the absence of HTO as an
impurity. Tritium concentration was determined by a flow-through
internal proportional tritium analyzer (Johnson Laboratories Inc.,
Tritium Monitor Model 112 B). The air to be analyzed was drawn at
the rate of 5 liters per hour through a column of solid desiccant
was subsequently removed and shown to have a tritium concentration
(HTO) equivalent to that found in the chamber atmospheric moisture
collected by alternate means.

The output from the tritium analyzer was connected to a limit

switch which activated a valve when the concentration decreased be-
low the desired level. Upon demand, a timed, metered pulse of tritium
from the pressurized bottle was injected into the chamber. The flow
rate and duration of this pulse were recorded on a strip chart re-
corder and used to calculate the amount of elemental tritium re-
quired to maintain the prescribed concentration (5 nCi/1). Since
the chamber was a closed system, any decrease in tritium gas con-
centration must have been caused by a conversion of HT/T2 to HTO.
By integrating the injection rate over time, the HT to HTO conver-
sion rate was calculated. This automatic system allowed the simu-
lation of a uniform HT contaminated environment throughout the test
period.

Five plants were randomly selected from the plants remaining
in the chamber on days 7, 12, 18 and 24. On day 18, five young
plants and five unplanted pots were placed in the chamber and were
sampled on day 24. The outer 16 leaves of each plant were separated
from the stem and constituted the "outer" or old leaf tissue. Most
of these leaves were completely expanded and none were smaller than
2 cm long. The remaining leaves constituted the inner or new tissue,
Roots were separated from the potting mix by shaking. .

Samples collected for water analysis were placed immediately
into a l1-liter round-bottom flask containing benzene. The criticality
of appropriate plant sampling is discussed by McFarlane et al. (1975)


INTRODUCTION

In the past, tritium was a pollutant associated primarily with
the testing of nuclear weapons. In the decade prior to the Test
Ban Treaty of 1963, approximately 1,700 megacuries of tritium were
injected into the environment through such activities (Weaver et al.
1969). This release of large quantities of tritium into the world
environment increased the natural equilibrium concentrations by a .
factor of 25. Due to radioactive decay, the present concentration
will decrease until about 1995 when a new increase is predicted to
occur because of increasing releases of tritium from the nuclear
power industry (Cowser, et al., 1966). In the production of elec-
tricity by nuclear reactors, approximately 50 mCi of tritium are
produced per day for each megawatt of electrical energy generated.
Most of the tritium produced is retained within the fuel element
and released during reprocessing. It has been estimated that about
25% of the tritium produced is released by the stack, 65% as liquid,
and only 10% is retained in waste storage (Weaver, et al., 1969).
Evaluations by Cochran et al. (1973) indicate that approximately
25% of the tritium released through the stack was in a gaseous state
(Ty) and 75% as water vapor. The Radiological Science Laboratory
in the New York State Department of Health (USEPA, 1972) has pre-
liminary information which they report indicates that possibly 90%
of the tritium released from nuclear fuel reprocessing is elemental
and has not been previously accounted for by routine monitoring of
HTO. Recent advances in the sampling of non-aqueous tritium, allow
a more accurate determination of gaseous tritium releases from various
phases of the nuclear power industry (Griffin et al. 1973).

The importance of gaseous tritium (HT or Tj) as an aerial pol~
lutant has largely been discounted because of the relative stability
of hydrogen gas. It was shown by Pinson (1951) that although oxida-
tion or exchange of HT occurred during inhalation, less than 0.0042

1


ACKNOWLEDGEMENTS

Grateful appreciation is expressed to Mr. Harry Hop for his
assistance in the accomplishment of this research. His professional
attitude and technical expertise have added greatly to the success of

this endeavor.

iv


HTO concentrations near the leaves than in the bulk air) or it may
indicate that HT penetrated the plant and was converted in the sub-
stomatal cavity to HTO and subsequently absorbed from that side.

A portion of the tritium absorbed by the leaves was trans-
located in the phloem to the roots. The tritium concentration in
the roots is dependent on its concentration in the leaves, on the
transport rate in the phloem, and on the dilution rate caused by
water transport through the xylem (McFarlane 1975). Equation 4
shows the ratio of tritium in the leaves and roots of plants ex-

posed to HT, and Equation 5 shows a similar ratio of plants exposed
to HTO vapor.

Equation 4: Plants exposed to 5 nCi/1 air of HT
HTO in leaves (nCi/m1)

HTO in roots (nCi/m1)~ 2.4

Equation 5: Plants exposed to 115 nCi/ml of HTO

HTO in leaves (nCi/ml)

Ho in tests acta) 75.6
The lower value in Equation 4 is thought to indicate a higher
basipetal movement of tritiated water and labeled substrate. However,
it should be remembered that soil contamination was not possible
in the plants exposed to HTO vapor since a septum was placed between
the foliar and subterranean portions of the plant.

After the third harvest, 10 pots previously unexposed were
placed in the exposure chamber. Five contained 4-week-old lettuce
plants and the others were filled with the potting mix (no plants).
The automatic watering system was connected to all pots and after
exposure to HT, the potting medium in each pot was mixed and a sam-
ple taken from each pot for analysis. The water was extracted and
analyzed for tritium concentration.

On the basis of these data no significant differences were
evident between the pots with and without plants. The large varia-
tion in tritium concentration in what were supposed to be replicate

13


Plants were grown from seeds in 5-inch pots and 25 plants of uniform
size were transferred from the glasshouse to the growth chamber after
12 days. Treatment was started 24 days after seeding at which time
the foliar fresh weight was about 12 g per plant and the llth node was
just developing.

Studies were conducted in plant growth chambers similar to those
designed by Hill (1967). Light was produced by cool white/fluorescent
1500-milliamp tubes and 200-watt incandescent lamps (input wattage:
73% and 27%, respectively). The radiant flux was measured between 480
and 950 nanometers in the center of the chamber at pot height; it
decreased from 220 to 200 watts per square meter from the beginning
until the termination of the experiment, Light exposure was 16 hours
with an abrupt light/dark change. The air temperature at the top of
the plants was 25 + 0.5°c during the light period and 20 + 0.5°C
during the dark, The relative humidity was maintained at 75 + 5% and
the air flow moved horizontally across the chamber at approximately
50 m/minute. The growth chamber is an air tight system and therefore
required the injection of carbon dioxide to maintain photosynthesis.
The injection rate of co, required to maintain a constant concentration
(350 parts per million) was done automatically. The recorded flow
rate gave a measure of the CO, assimilation rate, Humidity in the
chamber was controlled by the temperature of a cold-water condensation
coil. The condensate from this coil was collected in a fraction
collector. The times of collecting each fraction were recorded and
used to calculate the rate of transpiration. Leachate from the pots
was collected through a common drain in the bottom of the chamber.

Elemental tritium was received in a stainless steel bottle
pressurized with No. The absence of HTO was verified by sampling a
dilute standard gas directly as well as through a column of anhydrous

Caso, (the resultant dew point was < -5°C). No variation occurred


TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)

J. C. McFarlane

iT. REPORT NO, 2 [3. RECIPIENT'S ACCESSION NO.
EPA~600/3-76-006
4, TITLE AND SUBTITLE 5. REPORT DATE
TRITIUM ACCUMULATION IN LETTUCE FUMIGATED WITH January 1976
ELEMENTAL TRITIUM 6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S) ls. PERFORMING ORGANIZATION REPORT NO.

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT NO.
Environmental Monitoring and Support Laboratory 1FA083
Office of Research and Development TT. CONTRAt TANT NO.

U.S. Environmental Protection Agency
Las Vegas, Nevada 89114

same as above

12, SPONSORING AGENCY NAME AND ADDRESS 13. TYPE OF REPORT AND PERIOD COVERED

14, SPONSORING AGENCY CODE

Ecological Effects

EPA-ORD Office of Health &

15. SUPPLEMENTARY NOTES

76. ABSTRACT

Lettuce plants fumigated with elemental tritium accumulated tritium in the
plant water as well as in the organic constituents. The conversion rate of
elemental tritium to tritiated water varied from 0.5 nanocuries per minute per
pot at the start to 0.8 nanocuries per minute per pot at the termination of the
24-day exposure to an air concentration of 5 nanocuries per liter. Based on
the concentration of tritium in various plant tissues, foliar absorption was
postulated as the route of plant contamination. The data indicated that an
enzymatically facilitated conversion on the leaf or soil surface was the
probable mechanism.

plant chemistry

2. KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS lb. 1DENTIFIERS/OPEN ENDED TERMS |c. COSATI Field/Group
tritium 06C.
nuclear fuel reprocessing 18B
radioactive contaminants 183

18. DISTRIBUTION STATEMENT 19. SECURITY GLASS (This Report] 21, NO. OF PAGES
RELEASE TO PUBLIC UNCLASSIFIED 24
20. SECURITY CLASS (This page) (22. PRICE
UNCLASSIFIED

EPA Form 2220-1 (9-73)



EPA-600/3-76-006 ; j
January 1976 Ecological Research Series



X—FREE WATER EXTRACED FROM PLANT TISSUES

O—WATER OF COMBUSTION (ORGANICALLY
INCORPORATED TRITIUM)

T

_—O.
l

x 1

0 2 4 6 8 0 2

DAYS

Figure 1. ‘Tritium concentration in lettuce leaves exposed to
5 nCf/1 of HT

6 8 20 22 24

X— FREE WATER EXTRACTED FROM PLANT TISSUE

3. O— WATER OF COMBUSTION (ORGANICALLY
INCORPORATED TRITIUM)

nCi/ml

HTO

2 4 #6 %8 2 22 24
DAYS

2 4 6 8 10

Figure 2, Tritium concentration in lettuce roots of plants
exposed to 5 nCi/1 of HT


and all their precautions were used. Plant and soil water was ex-
tracted via azeotropic distillation as reported by Moghissi et al.
(1973).

The tritium concentrations associated with the organic constitu-
ents of the plants were determined by oxidizing the dry plant material
in a pressurized oxygen bomb and subsequently analyzing the water of
combustion (Moghissi, et al., 1975). Tritium concentrations in leach-
ate, transpired water, water extracted from the plants, and the water
of combustion, were analyzed using liquid scintillation techniques as
described by Lieberman and Moghissi (1970).

RESULTS AND DISCUSSION

Tritium concentrations in leaf water and organic plant material
are shown in Figure 1. Equilibrium in the plant free water occurred
at about day 12. The tritium concentration in the organic fraction
increased throughout the exposure. The organic tritium concentration
in the outer or older leaves was slightly lower throughout the experi-
ment. However, organic tritium increases in the older, middle and
younger tissues were similar; this indicates that all tissues were
growing and actively producing and utilizing metabolites. By day 24,
approximate equilibrium was obtained. The different slopes of tri-
tiated free water and tritium in the organic phase can be explained
on the basis of the organic plant material already in existence be-
fore the tritium fumigation started. During the last 2 weeks of
fumigation, plant growth had reached its maximum rate. In this lat-
ter stage, the dilution caused by uncontaminated organic material
was insignificant and the concentration found is thought to represent
the equilibrium level.

Tritium concentration in the lettuce roots is shown in Figure 2.
The organic tritium concentration was constant after day 12. This

value was approximately 25% of the organic tritium concentration


of the inhaled dose was retained by man. When bean leaves were ex-
posed to HT in air at 200 mCi/cc, the extractable tritium (HTO)
reached levels up to 11,000 mCi/g of plant tissue (Cline, 1953).

The guidelines for occupational exposure to elemental tritium
(ICRP publication No. 2, 1959) are based on a decreased absorption
in man and consequently lower doses than when exposed to similar
concentrations of HTO. The maximum permissible concentrations of
tritium gas (HT) and tritiated water (HTO) in air, are 400 pCi/cc
and 5 pCi/cc, respectively. Eakins and Hutchinson (1973) reported
that the conversion of HT to HTO was a very slow reactions. They
calculated the conversion times for these reactions in the presence
of various. catalyzing surfaces. Reaction half-times varied from
four years in the presence of platinum to 1151 years for glass. Based
on these findings they suggest that elemental tritium was of no
consequence as a local contaminant.

The uncertainty of gaseous tritium release and the lack of a
eritical evaluation regarding the influence of plants and soils
on the oxidation or exchange of HT to HTO make it important that
additional information be obtained. If large amounts of tritium
are released in the reprocessing of nuclear fuels, it is important
to know the environmental impact of this pollutant. The research
discussed in this report was aimed at determining the importance
of tritium as a gas in the contamination of plants and soils.

METHODS

Lettuce plants (Lactuca sativa var. Grand Rapids) were grown
in a mixture of peat and vermiculite (1:1) and watered with a
modified Hoaglands nutrient solution (Berry 1971). Water was applied
at 4-hour intervals and sufficiently over-watered (100 ml per appli-
cation) at each period to cause drainage through the pots. This

ensures optimum water availability and constant nutrient availability.


CONCLUSIONS

The presence of potted plants in the growth chamber facilitated
the conversion of HT to HTO. Lettuce plants appeared to have been
contaminated by the foliar absorption of tritiated water vapor.
Although the site of the conversion reaction was not identified,
high concentrations of HTO in the soil suggest that the soil or
soil microorganisms may have been involved. Conversion half-times
calculated from the injection rates in Table 1 range from 19 to 23
hours. It is clear from these values and from the observed elevated
HTO concentration in the chamber water vapor only a short period
after the start of the treatment that the release of elemental tri-
tium into the environment may present a local contamination threat.
If lettuce is representative of all plants, then guidelines for
the release of elemental tritium should take into account the rapid

conversion to HTO which occurs in the plant environment.

16


DISCLAIMER

This report has been reviewed by the Environmental Monitoring
and Support Laboratory-Las Vegas, U.S. Environmental Protection
Agency, and approved for publication. Mention of trade names or

commercial products does not constitute endorsement or recommendation
for use.

di


HTO nCi/ml

e—INNER LEAVES
3.0 4—OUTER LEAVES

2.0

05:

1 2 3 4 5 6 7 8
DAYS AFTER LEAVING HT ENVIRONMENT

Figure 4, Organically incorporated tritium (combustion water) in
lettuce plants after removal from HT contaminated
environment.


Table 1, RATES OF TRITIUM CONVERSION IN THE PRESENCE OF POTTED PLANTS

aT prcatuent HT ——HTO Conversion Rate:
° per minute per pot per g fresh wt.
(days) : (nCi/min) (nCi/min/pot) (nCi/min/g)
1-6 13.3 0.53 4.43 x 107?
6-11 13.7 0.72 0.38 x 1072
ll - 17 11.3 0.81 0.28 x 107?

11


EPA-600/3-76-006
January 1976

TRITIUM ACCUMULATION IN LETTUCE FUMIGATED
WITH ELEMENTAL TRITIUM

by

J. C. McFarlane ’
Monitoring Systems Research and Development Division
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114

Program Element 1FA083

U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
LAS VEGAS, NEVADA 89114


RESEARCH REPORTING SERIES

Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:

1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research

4. Environmental Monitoring

5. Socioeconomic Environmental Studies

This report has been assigned to the ECOLOGICAL RESEARCH series. This series
describes research on the effects of pollution on humans, plant and animal
species, and materials. Problems are assessed for their long- and short-term
influences. Investigations include formation, transport, and pathway studies to
determine the fate of pollutants and their effects. This work provides the technical
basis for setting standards to minimize undesirable changes in living organisms
in the aquatic, terrestrial, and atmospheric environments.

This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.


in the aerial plant organs. Free water tritium reached a maxi-

mum of about 68% of the concentration in the leaves. The de-
crease in extractable HTO after day 12 is thought to be caused by
higher transpiration rates and the development of the rooting system
deeper in the pot. The concentration gradient of tritium in both
free water and organic material from leaves to roots suggests that
the route of contamination was, foliar. The gradient between tri-
tium in the organic fractions of leaves and roots could indicate
that organic material synthesized in the leaves and translocated

to the roots was subject to exchange with hydrogen or some hydrolytic
activity.

At the termination of the tritium fumigation, a set of samples
was transferred to another growth chamber to determine the rate of
tritium loss. It was found that the tritium concentration in the
free water fraction decreased exponentially and that the loss rate
was independent of the age of the leaf sample (Figure 3). By day
3, the extractable water from the leaves contained less than 10%
of the original concentration, An initial decrease of approximately
30% occurred in all organic tritium (Figure 4). This was probably
the result of protium exchange with tritium on the hydroxyl units
of the cellulose (Lang and Mason, 1960). After this.initial_decrease,
the tritium concentration in the organic fraction of the inner leaves
decreased more rapidly than the concentration in the outer leaves.
This can be explained by the dilution created by the synthesis of
new untritiated plant material.

Throughout the study, the environment in the chamber was
maintained at 25°C and 75% relative humidity. Using these values,
the concentration of tritium determined by the HT analyzer, and the
concentration of tritium collected on the condensation coil, the
ratio of HT to HTO in the gas phase was calculated to be:

Equation 1: HT (nCi/1)

ETO (ncz/i) ~ °9


That means there were approximately 83 times as many tritium atoms
in the gaseous (HT) form than in the water vapor (HTO) form. By
applying the gas law and using the specific activity of tritium
(9.64 x 103 Ci/g), and assuming that the hydrogen concentration in
the growth chamber air was 0.5 parts per million (Ehhalt et al. 1974)
and the tritium concentration (HT) was 5 nCi/l, it was calculated
that 2.58 x 105 atoms of hydrogen were present per atom of tritium.
Similar calculations show that there were 6.46 x 10!! atoms of
hydrogen per atom of tritium in the water vapor. Thus, on an atomic
level there were approximately a million times as many tritium atoms
per hydrogen atoms in the form of tritium gas than in the form of water
vapor,

One hour after the initiation of HT fumigation, the first
sample of transpired water was collected and analyzed for tritium.
The elevated concentration of tritium in this sample indicated a
rapid conversion of HT to HTO. Data were collected to compare the
amount of HT injected into the chamber with the HTO lost through
transpiration, leaching, and through incorporation into plant materials.
In this budget, more than 95% of the tritium injected was accounted
for in the form of HTO. The rest was presumably either lost when
samples were removed, through leaks,or was not accounted for be—
cause of inaccurate measurement of water loss volumes. In control
experiments in the absence of plants or soil, HT to HTO conversion
rates were extremely slow and corresponded to the values reported
by Eakins and Hutchinson (1973). No special effort was made in
these experiments to correlate the tritium conversion rate with
plant mass, area, metabolism, or soil microorganism activity, but
from harvest data the fresh weight of the entire plant material was
estimated, In Table 1, the HT injection rates and the HT to HTO
conversion rates are listed, No valid correlation of conversion
rate with leaf mass seems to exist, but a trend is apparent for con-

version based on the number of pots. Also, despite the large

10