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Journal of Environmental Radioactivity 118 (2013) 113e120

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Journal of Environmental Radioactivity

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Relation between the tritium in continuous atmospheric release andthe tritium contents of fruits and tubers

V.Y. Korolevych*, S.B. KimEnvironmental Technologies Branch, Nuclear Sciences Division, CRL, AECL, Stn. 51A, Chalk River, ON K0J 1P0, Canada

a r t i c l e i n f o

Article history:Received 2 June 2012Received in revised form16 November 2012Accepted 6 December 2012Available online 19 January 2013

Keywords:HTOOBTSpecific activity model

* Corresponding author.E-mail address: korolevv@aecl.ca (V.Y. Korolevych

0265-931X/$ e see front matter Crown Copyright �http://dx.doi.org/10.1016/j.jenvrad.2012.12.004

a b s t r a c t

Concentrations of organically bound tritium (OBT) and tissue-free water tritium (TFWT, also referred toas HTO) in fruits and tubers were measured at a garden plot in the vicinity of the source of chronicairborne tritium emissions during the 2008, 2010, and 2011 growing seasons. A continuous record of HTOconcentration in the air moisture was reconstructed from the continuous record of Ar-41 ambientgamma radiation, as well as from frequent measurements of air HTO by active samplers at the gardenplot and Ar-41 and air HTO monitoring data from the same sector. Performed measurements were usedfor testing the modified Specific Activity (SA) model based on the assumption that the average air HTOduring the pod-filling period provides an appropriate basis for estimating the levels of OBT present inpods, fruits and tubers. It is established that the relationship between the OBT of fruits and tubers and theaverage air HTO from a 15e20 day wide window centred at the peak of the pod-filling period isconsistent throughout the three analysed years, and could be expressed by the fruit or tuber’s OBT to air-HTO ratio of 0.93 � 0.21. For all three years, the concentration of HTO in fruits and tubers was found to berelated to levels of HTO in the air, as averaged within a 3-day pre-harvest window. The variability in theratio of plant HTO to air HTO appears to be three times greater than that for the OBT of the fruits andtubers. It is concluded that the OBT of fruits and tubers adequately follows an empirical relationshipbased on the average level of air HTO from the pod-filling window, and therefore is clearly in line withthe modified SA approach.

Crown Copyright � 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Tritium (3H) is a radioactive isotope of hydrogen. The tritiumnucleus containsoneprotonand twoneutrons,whereas theprotium(1H) nucleus contains one proton andno neutrons. Tritiumdecays to3He via beta emission with an average energy of 5.7 keV, and hasa half-life of 12.35 years (ICRP,1983; Okada andMomoshima,1993).

Hydrogen is ubiquitous in the environment and is part of manycompounds, including water and most organic materials. Tritium,as an isotope of hydrogen, freely enters into these compounds,forming tritiated hydrogen gas (HT) and, combining with oxygen,tritiated water (HTO). Tritium can also replace hydrogen in organiccompounds, forming exchangeable organically bound tritium (OBT)if the bonds involve sulphur, oxygen, phosphorus, or nitrogen, andnon-exchangeable OBT if the bonding is with carbon.

In CANDU reactors, tritium is primarily produced by the2H(n,g)3H reaction on the deuterium (2H) of the heavy water in themoderator and the primary heat transport system. Most tritium

).

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released from CANDU reactors is in the form of HTO, which is alsothe form that is most commonly found in the environment. WhenHTO is released into the atmosphere, it mixes with air moisture andexchanges with water in soil and plants.

Tritium can be bound to organic compounds either by exchangereactions or by enzymatically-catalysed reactions (Diabate andStrack, 1993; Belot, 1986). In exchange reactions, tritium bonds tooxygen, sulphur, phosphorous, or nitrogen atoms as hydroxides,thiols, phosphides, or amines, respectively. Conventionally, theseresulting compounds are termed exchangeable (or labile) OBT.ExchangeableOBT is considered to be in equilibriumwith theHTO inplants. In enzymatically catalysed reactions, the tritiumbonds to thecarbon chain of an organic molecule as fixed or non-exchangeableOBT. Such bonds are strong and can be dissolved only during cata-bolic reactions, leading to a longer retention time than forexchangeable OBT.

The literature contains differing views on whether the generaldefinition of OBT should include exchangeable OBT. Diabate andStrack (1993) state that OBT should refer only to non-exchangeableOBT. More recently, Baumgärtner and Donhärl (2004) andBaumgärtner (2005) have identified another form of OBT, termed

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V.Y. Korolevych, S.B. Kim / Journal of Environmental Radioactivity 118 (2013) 113e120114

buried tritium,which is defined as tritium in exchangeable positionsthat is trapped in large biomolecules. Buried tritium present in drymatter cannot be removed by rinsing with tritium-free water.Baumgärtner andDonhärl (2004) suggest that buried tritiummakesup at least half of what is traditionally measured as OBT. The defi-nition of OBT by the Environmental Agency (2001) and the CanadianStandards Association, or CSA (2008) is wider, and includes anyorganicmatter containing tritium, either exchangeable orfixed. Thisdefinition ensures that all forms of OBT are taken into account inconservative dose assessments.

Some of the tritium that enters plants asHTO can be incorporatedintoorganiccompounds to formOBT(Diabate andStrack,1993).Non-exchangeable OBT formed by photosynthesis appears initially incarbohydrates. Subsequent polymerization results in the incorpora-tionofOBT in suchcomplexmolecules aspolysaccharides (starchandcellulose), proteins, lipids, and nucleic acids. The amount of OBTproduced depends on a large number of environmental factors andplant parameters, including light levels, oxygen and carbon dioxideconcentrations, temperature, air circulation, andwater supplyeall ofwhich show considerable diurnal and seasonal variations. OBTconcentrations are reduced by conversion back to HTO by mainte-nance respiration, but this process is slow, and the biological half-lifeofnon-exchangeableOBT inplants is about25days (NCRP,1979).OBTmakes up amuchhigher part of the total tritiumactivity in grains (upto 90%), which have a high organic content.

OBT is formed during the day in the green parts of plants and istranslocated to stems, roots, tubers and fruits. OBT can also beproduced in the dark by non-photosynthetic assimilation, althoughat rates three to ten times lower than those resulting from photo-synthesis (Moses and Calvin, 1959; Thompson and Nelson, 1971;Atarashi et al., 1998; Strack et al., 2005). These processes involve themetabolic turnover and synthesis of organic compounds such asproteins, oils, alkaloids, and vitamins using the energy of respira-tion. Later in the growing season, usually after anthesis (flowering),most of the assimilates are used in the development of fruits andtubers (Indeka, 1981). Diabate and Strack (1997) observed that themajor factor influencing the translocation of OBT is the growth stageof the plant, while light conditions do not play a significant role.

For routine airborne releases of HTO, OBT contributes 11%e50%of the total tritium dose received by members of the public(Table 1). In the case of cereals, OBT makes up a significant fraction(85%) of the total tritium dose. And yet, the need for the routinemeasurement of OBT concentrations in environmental samplesremains a continued topic of debate due to the effort and special-ized equipment needed to carry out the measurements and therelatively high uncertainty in the results.

Measuring OBT is expensive, and as of today the data collectedworldwide is still scarce and not representative of thewide range ofenvironmental conditions encountered in the vicinity of nuclearfacilities. Therefore, regulatory algorithms rely on the routinelymonitored air tritium concentration found in the atmosphericwater vapour (air HTO). In particular, the Specific Activity (SA)method uses annual average air HTO concentrations to estimatelong-term averages of HTO and OBT concentrations in plants. With

Table 1Contribution of OBT to the total tritium dose to members of the public for chronicatmospheric releases.

Site OBT dose contribution (%) Reference

Deep River 50 Kotzer and Trivedi (2001)Ottawa 26 Kotzer and Trivedi (2001)Wolsong (Korea) 25 Kim and Han (1999)Germany 20 Gulden and Raskob (1992)Japan 11e18 Hisamatsu et al. (1989)Generic 29e33 Evans (1969)

the SA approach, plant HTO concentrations are assumed to be equalto the concentration of HTO in air moisture (Catm, Bq L�1), witha reduction factor RFp to account for the fact that plants draw someof their tritium from soil water, which typically has a lower HTOconcentration than air moisture does due to the depleted tritiumconcentration in precipitation. HTO concentrations in the aqueouspart of the plant (CHTO, Bq L�1) are given by:

CHTO ¼ RFpCatm (1)

The key problemwith this approach is the empirical estimationof RFp, as available data shows considerable scatter. The mainreason for this scatter is due to the assumption of steady-stateconditions, which is too stringent.

The OBT concentration in the water collected after combustionof the dried plant sample (combustionwater of plants, COBT, Bq L�1)is assumed to equal the plant’s water concentration multiplied bya discrimination factor of IDp that accounts for isotopic effects indry matter production. Studies carried out in controlled conditions(McFarlane,1976; Garland and Ameen,1979; Kim and Baumgärtner,1994) indicate that IDp has a value of 0.54 for maize, barley, andalfalfa. In the absence of other information, the CSA (2008) suggestsan IDp value of 0.8 for all plants, partially due to the inclusion of theexchangeable OBT in plants in dose assessment. Since COBT onlydiffers from CHTO by the discrimination factor of IDp, it dependsupon the same value of RFp:

COBT ¼ RFpIDpCatm (2)

In this currently used model all variables (specific activity oftritium in atmospheric water Catm, tissue free plant water CHTO andcombustible water COBT) are annual averages. In addition, no sepa-rate algorithmpresently defines theOBT in fruits, tubers, and grains.This means that the HTO and OBT of fruits, tubers, and grains areassumed to be equal to levels in the other parts of the plant (e.g., theleaves). The RFp values are subsequently calculated on the basis ofthe plant’s HTO measurements, which vary dynamically as dis-cussed above. HTO-caused dynamical scatter in RFp appears to be anadditional cause of uncertainty in terms of the OBT levels in plants.

Published observations for the value of RFp vary, from 0.1 to 1.1(USNRC, 1977; Murphy, 1984; Dunstall et al., 1985; Hamby andBauer, 1994; Peterson and Davis, 2002). Paunescu et al. (1999)measured an RFp of 0.38, 0.47, and 0.66 in fruits, vegetables (rootand other), and cereals (grain) respectively. Peterson and Davis(2000) presented measurements in support of RFp ¼ 0.9 in leafyvegetables and pasture vegetation, and RFp¼ 0.8 in fruit, vegetables(root and other), and grains.

In complete equilibrium, the theoretical value of RFp ¼ 1.1 couldbe introduced to express the buildup of tritium in the plant due todifferences in water vapour pressure within the leaf and in theatmosphere and between the vapour pressure of water and of HTO.An RFp that is close to unity implies a dynamical quasi-equilibriumbetween the plant’s HTO and the HTO of the air and soil. Rodgerset al. (1996), however, reported that under chronic releases, fullequilibrium between air HTO and soil water HTO is never attained.Apparently, during episodes of plume departure, soil water HTOexceeds air HTO and controls the plant HTO via the transpirationstream. Plant tissue is also dynamic, breaking down and rebuildingcontinuously. Tritium atoms from HTO can be incorporated intonon-exchangeable sites during the OBT rebuilding phase.

Essentially dynamical changes in the atmospheric HTO and inplant HTO are clearly connected, and SA paradigm proposes, thatthese processes can be approximated by a quasi-equilibriumexchange. However, this strong assertion requires knowledge ofa timeline of plant HTO transfer into fruits and tubers. And thistimeline as of yet is not known. Also the observations on this matter

V.Y. Korolevych, S.B. Kim / Journal of Environmental Radioactivity 118 (2013) 113e120 115

are scarce in the literature, whichmakes it difficult to theorize uponthe affiliation between the fruits and tubers HTO and the HTO in theatmosphere. An important question therefore arises: Is a directaffiliation between the HTO in fruits and tubers with the air HTOpossible, as all SA-based regulatory models suggest? And, if so,what modifications to the SA approach are required for quantifi-cation of the HTO in fruits and tubers?

The focusof our study, however, isOBT, andourprimaryobjectiveis to quantify the OBT in harvested fruits and tubers based onavailable measurements of air HTO. We build our study ona hypothesis that stems from the suggestion of Diabate and Strack(1997), who remark that the average air HTO during the pod-filling period (the fastest period of growth for fruits and tubers)provides an appropriate basis for estimates of OBT levels withinfruits and tubers.

The study begins with a description of methods used, includingexplanations of the modified SA formulation and the concept ofa pre-harvest time window for air HTO averaging, which is centralto our study. Next, results pertaining to the right choice of theposition of time window and the window size are organized on thebasis of persistency of the relationship between the OBT and HTO ofthe fruits and tubers and the air HTO of the different analysedgrowing seasons. The study concludes with a discussion of differentrelationships corresponding to air HTO established within alter-native pre-harvest windows.

2. Materials and methods

This study focuses on the HTO and OBT levels within fruits andtuber driven by a fine-scale (continuous) reconstructed atmo-spheric HTO. This allows us to take into account the field-scaleeffects of tritium transfer in the soilevegetationeatmospheresystem. It also allows for the testing of different averaging strate-gies pertaining to driving atmospheric HTO concentrations.

2.1. Concept of window

Modelling the tritium transfer from atmosphere to plants usingaveraging of subject variables not over a whole year, but overa growing season (e.g., two months before harvest, one monthbefore harvest, etc.) was first proposed by Raskob (1993). Diabateand Strack (1997) confirmed the clear affiliation of tritium trans-fer processes to the growth stage of the plant. However, definingthe beginning and end dates of a plant’s growth stage (the window)for measurement purposes has not been made clear in the litera-ture. These values are essential, for large-scale atmosphericprocesses in mid-latitudes are subject to significant changes, whichare reflected in the changing persistency of winds and changes inwind roses, among other things. The sensitivity of plant HTO andOBT concentrations to these changes is presently unknown.

It should be noted that weight-averaging of air HTO (i.e. inte-gral) over the whole growing season corresponds to the exactsolution of the simple dynamical equation of CHTO driven by theatmospheric HTO. The weighting provides discrimination of pastatmospheric concentrations in favour of more recent concentra-tions. This discriminating weighting function could, however, besubstituted for a step function that is a constant within the narrowpre-harvest window and zero elsewhere. This approach allows forthe simple averaging of Catm(t) during a short pre-harvest periodand results in finding the effective reduction factor RFp. Theresulting RFp appears to be similar to that in the current procedure,except for the different air sampling performed during the week-long (or shorter) window preceding harvest:

CHTOðthÞ ¼ RFHTOCatm (3)

The same approach could be used for the procedure to measureOBT levels. In the case of OBT, however, the weighting function isdifferent, and the window is defined over a different interval thatcorresponds to the peak of the biomass growth function. Averagingwithin this window also results in another effective reductionfactor RFOBT defined as the ratio of concentration of OBT (COBT) tothat of the averaged air HTO (Catm). It should be emphasized thatthis way RFOBT is affiliated not with the value of RFHTO, but withsome other, different value RF*HTO which might have been definedvia traditional formulation RFOBT¼ RF*HTO IDp. Nevertheless, RF*HTOwas notmeasured in this study and for this reasonwe refer to RFOBTdirectly, which is different from the formulation (2):

COBT ¼ RFOBTCatm (4)

The proposed approach to SA modification essentially relies onthe validity of assumptions of a stationarity underlying formulae(3) and (4).

2.2. Approach

The primary objective of this study is to quantify the OBT inharvested fruits and tubers based on the available measurements ofair HTO. This study subsequently aims at validating themodified SAmodel recommended by the CSA (2008) by suggesting a directcorrespondence between air HTO and the levels of HTO and OBTfound in agricultural produce at harvest.

The second task is to verify whether the HTO in fruits and tuberscould be related to air HTO, as all regulatory models suggest, and toconfirm whether the SA approach, which underlies all regulatorymodels, could be modified to include the fruit and tuber HTO.

The number of alternative averaging periods of atmosphericHTO was tested in order to verify the hypothesis that the directcorrespondence with air HTO holds for both HTO and OBT in fruitsand tubers. For OBT, a sampling window based on growth rate(particularly the period of pod-filling peak activity) is used. It isimportant to note that the pre-harvest window’s width was notknown a priori. For this reason, pre-harvest windows of differentdurations are used in search of the relationship between air HTOand the HTO in fruits and tubers. The data collected during threedifferent growing seasons (2008, 2010, and 2011) provides a suffi-cient variety of conditions necessary to establish consistency in therelationship between the HTO and OBT of fruits and tubers and theair HTO.

2.3. Experimental site and sampling method

AECL’s Chalk River Laboratories (CRL) site is located in Ontario,Canada, on the south shore of the Ottawa River, about 200 kmnorthwest of Ottawa. Low amounts of radioactive airborne andliquid releases occur from CRL during normal operations, with HTObeing the most abundant form of tritium.

Samples of fruits (tomatoes and red and green peppers) andtubers (potatoes) were collected from an experimental garden plotat the former Acid Rain Site located 2.5 km NW from the reactorstack (Fig. 1). This direction coincides with the prevailing winds atCRL, and therefore the sampling of atmospheric HTO by trackingthe arrival and departure of plumes was relatively easy.

In early spring of each of the three experimental seasons, onepart of the experimental garden plot was stripped of its organic andtopsoil layers and covered by an imported tritium-free mixture oforganic soil and sand to maintain the consistency of the soil at thecontrol part of the garden plot. This “clean” soil was covered witha tarp, and plant seedlings (tomatoes, red peppers, green peppers,and potatoes) were planted in small holes punctured in the tarp at

Fig. 1. Map of the position of the experimental garden plot at CRL’s Acid Rain Site.

V.Y. Korolevych, S.B. Kim / Journal of Environmental Radioactivity 118 (2013) 113e120116

appropriate intervals. The neighbouring “control” part of thegarden was left exposed to the atmosphere (Fig. 2) in order toevaluate the role of soil HTO in the formation of OBT in plants (andparticularly fruits and tubers).

Plant samples from the “clean” tarp-covered garden plot werecollected separately from those grown in the control plot in anambient setting. Each sample (at each particular moment) wasa composite of aminimumoffive fruits, or tubers of the same speciescollected randomly fromeither the “clean”or controlledgardenplot.Experiments included prolonged overnight period, lasted severalhours and samples were collected at sub-hourly intervals. In addi-tion to this, one morning sample and one midday sample werecollected at intervals of several days some 8e12 days before harvest,when tomatoes and pepper were fully grown but not ripe, andduring the typical harvest period, when fruits and tubers were ripe.

2.4. Analytical method

2.4.1. Measurement of atmospheric tritiumHTO in the air moisture was sampled on a weekly basis using

both active and passive samplers to cross-check each measurementfor quality control purposes. The measurements obtained from anactive sampler were used to determine the activity concentration ofHTO in the atmosphere at the garden plot. The active sampler usedwas an AECL-built bubbling sampler that measures low levels oftritium activity in the environment. At the experimental gardenplot, the active sampler recorded fine-scale measurements of air

Fig. 2. Experimental garden plot: (a) the “clean” soil covered with a grey tar

HTO that consistently and significantly exceeded the minimumdetectable activity levels (maximum MDA w15%).

2.4.2. Measurements of gamma radiation and correlation with HTOin the air moisture

One of CRL’s gamma radiation monitors (A-117) is located 150 mfrom the experimental garden plot. As the gamma radiationmonitors present on the CRL property are primarily for emergencyresponse purposes, they are designed to monitor high accidentallevels. Their lower threshold reading is set at about 17 mR/h. For thisreason, all readings from the A-117 monitor below 20 mR/h werediscarded and substituted for the background radiation levels in thevicinity of the experimental garden plot. The background gammaradiation measurements were retrieved by the EXPLORANIUM unit(mobile survey meter), and found to be in the range of0.68 � 0.08 mR/h. Subsequent correlation of ambient gamma radi-ation with the HTO in the air moisture was based on the synchro-nization of half-hourly measurements of atmospheric HTO duringthe diurnal experiment of July 30eJuly 31, 2008.

Averaging of the gamma radiationmonitor readings (initially setat w10 s intervals) over correspondingly positioned half-hourlyintervals resulted in the following empirical relationship:

Catm ¼ dR; (5)

where gamma radiation (R) is measured in mR/h, and d is theempirical conversion factor calculated to be 121.7 Bq h (mR)�1 L�1

p at the ground; (b) the control site (natural setting with exposed soil).

V.Y. Korolevych, S.B. Kim / Journal of Environmental Radioactivity 118 (2013) 113e120 117

before 2009 (i.e., in 2008) and 38.5 Bq h (mR)�1 L�1 after 2009 (i.e.,in 2010 and 2011). The change in the value of d after 2009 is due tothe repair of the NRU reactor vessel and an exchange of the NRUheavy-water moderator in 2010. The difference in d was calculatedusing the quarterly data from CRL’s environmental monitoringreports, as well as the weekly data from gamma radiation monitorA-117 which records the long-term tritium in the air in the vicinityof the experimental garden plot. Empirical formula (5) is valid ataverage summer-time atmospheric humidity of 11 g m�3, which isthe average summer-time value.

The wind speed was factored in formula (5) using the half-timeof Ar-41 decay, which is 1.82 h. For example, strong winds (>5 m/s)would bring the atmospheric Ar-41 plume from the stack, 2.5 kmaway, to the experimental garden plot in less than 8 min with mostof the source activity preserved. Conversely, under calm conditions(with winds <0.5 m/s), approximately 80 min would be requiredfor the plume to arrive at the experimental garden plot, by whichpoint almost half of the Ar-41 activity would have been gone. Thiswas taken into account in the reconstruction of the continuous airHTO at the experimental garden plot (Fig. 3). However, uncer-tainties do arise from wind meandering and intermittent turbu-lence, which cause significant variability in the magnitude of Ar-41activity measured by the A-117 gamma radiation monitor.

2.4.3. The measurement of tissue-free water tritiumMeasurements of tissue-free water tritium (TFWT, or HTO) and

organically bound tritium (OBT) in environmental samples arecarried out routinely at CRL for environmental monitoring, doseassessments, and general research and development programs.Tritium levels in environmental samples from the vicinity of thenuclear facilities are usually less than few hundred Bq L�1.

Samples were collected and stored in a freezer at �20 �C. Forthis study, TFWT was extracted by a freeze-drying method usinga specific laboratory designed apparatus and drying of samples wascompleted in an oven. The tissue-free water was extracted usinga dry ice trap under vacuum pressure for at least 15 h. The collectedwater was measured by liquid scintillation. Following the freeze-drying a drying oven at 55 �C was employed for 24 h to completethe drying process. The total removal of the TFWT was confirmed

Fig. 3. Sample comparison of the continuous air HTO (reconstructed using the formula(5) and the record of Ar-41 activity) with measurements taken from the activeatmospheric HTO sampler. Local time (LT) is indicated on the abscissa.

by weighing the residual samples. Special attention was given toensuring that no contamination occurred from the ambient air.

2.4.4. Measurement of exchangeable and non-exchangeable OBTOnce the tissue-free water was removed completely, approxi-

mately 20 g of the dried sample was chopped and homogenized.The exchangeable OBT was removed by mixing the dehydratedsamples with 30e50 ml of tritium-free water to facilitate theisotopic exchange. These rinsed samples were refrozen, and thewater was removed using the same freeze-drying processdescribed above in Section 2.4.3 for fresh samples. The activityconcentrations of the exchangeable OBT were obtained throughcomparison with the non-rinsed dried samples.

Over 10 g of the rinsed and oven-dried sample was combustedusing a Parr bomb system (Parr Instrument Company) with pres-surized oxygen at 300 psi. Generally, less than 5 ml of combustionwater was collected by a pasture pipette directly from the bomband purified by azeotropic distillation before being counted byliquid scintillation.

2.4.5. Liquid scintillation countingTritium activity concentrations in the water samples extracted

from the plants were determined by mixing 8 ml of the extractedwater with 10 ml of Ultima Gold XR scintillation cocktail (Perki-nElmer) and placing the solution in a 20 ml polyethylene scintil-lation vial (PackardTM). For the OBT measurements, the purifiedcombustion water sample was placed in a 20 ml liquid scintillationcounter vial (Beckman 6500 LSC) with 8 ml of tritium-free watermixed with 10 ml Ultima Gold XR. HTO and OBT activity concen-trations were counted by the Beckman 6500 LSC for 100 min. Theminimumdetectable activity (MDA) was found to be approximately10 Bq/L.

3. Results

By the time of anthesis, the level of HTO in the soil moisture ofthe “clean” plot (under the tarp) was approaching that of thecontrol (ambient uncovered) plot, with a difference of only 30%.Later on, towards harvest, the difference between the “clean” plotand the control plot was most evidently seen following precipita-tion, when there was up to 40% more intense oscillation of HTOconcentrations in control fruits and tubers as compared to “clean”fruits and tubers. This difference was indeed exacerbated by thevery small size of the sample available for direct comparison at eachparticular moment. For example, on September 1, 2011, two daysafter a tritium-free rainstorm, the HTO activity concentration infruits and tubers appeared to be 127 � 6 Bq L�1 at the “clean” plot,versus 78� 19 Bq L�1 at the control plot (with a very small availablesample size of n¼ 5). OBT levels in corresponding plots were foundto be 88 � 22 Bq L�1 versus 73 � 12 Bq L�1 (with a similarly smallsample size of n ¼ 3).

Since the differences were found to be much smaller betweenprecipitation events, in the analysis that follows fruits and tuberscollected from the “clean” plot are not distinguished from thosecollected from the control plot. Similarly, the ripened and un-ripened samples were not analysed separately because the differ-ences in the activity concentrations of HTO and OBT between the“clean” samples and the control samples appear to be within themeasured range of spatio-temporal variability.

3.1. Estimates of RFHTO

Comparisons averaging the reconstructed air HTO over pre-harvest sampling windows of different durations are presentedin Table 2. It could be noted that for all window intervals (except

Table 2Mean, SD and range in RFHTO determined on the basis of the air HTO averagedwithin different pre-harvest windows 1 h,1 day, 3 days, and 9 days wide in 2008, 2010, and 2011;the number of samples each year is n.

Interval 2008 (n ¼ 13) 2010 (n ¼ 14) 2011 (n ¼ 23)

Mean � SD Range Mean � SD Range Mean � SD Range

1 h 5.50 � 4.06 0.27e13.34 1.24 � 0.25 0.98e1.68 0.54 � 0.80 0.07e3.061 day 1.28 � 1.47 0.30e60.91 1.04 � 0.5 0.42e1.68 0.64 � 0.32 0.22e1.363 days 1.26 � 0.55 0.36e2.49 1.14 � 0.37 0.69e1.68 1.19 � 0.75 0.16e2.269 days 1.68 � 0.79 0.37e2.93 0.75 � 0.35 0.40e1.31 1.02 � 0.63 0.15e1.92

V.Y. Korolevych, S.B. Kim / Journal of Environmental Radioactivity 118 (2013) 113e120118

one) e 1-h, 1-day and 9-day e the constant of proportionalityappears inconsistently different in different years. In strikingcontrast the 3-days long pre-harvest window is the only intervalthat provides consistent values throughout the 3 different growingseasons (that of 2008, 2010 and 2011). Since the probability of thishappening by chance is small, the 3-day pre-harvest window issuggested to be the most adequate window to use with formula(3). The fruit/tuber HTO data and affiliated air HTO from the 3-daypre-harvest windows of all three years was subsequently pooled,and the effective reduction factor RFHTO was calculated:

RFHTO ¼ 1:20� 0:63 (6)

It should be noted that the variability of this result reflects thefact, that samples were harvested many times (3 in 2010 and 2011,and 7 in 2008), with sampling separated by a period ranging from 1day to two weeks.

3.2. Estimates of RFOBT

Tritium measurements in both the rinsed and non-rinsed driedsamples were compared in 7 instances (14% of cases) to assess thedifference in tritium activity concentration due to exchangeableOBT. No differences in OBT activity concentration in excess of 20%were noted and exchangeable OBT was not considered in thisstudy.

For OBT, a stronger relationship was observed within windowscentred at the peak of pod-filling activity, as compared to testwindows shifted 15 days before or 15 days after this same period.The pod-filling peak was found to start 22 days after anthesis. Sincethe associated pod-filling window size was not known a priori, 20-day, 15-day, and 10-day wide windows, centred on the samecalendar date, were tested in each case The window start and enddate is marked by the corresponding day-of-the-year (DOY) in

Table 3Mean, SD and range of RFOBT determined on the basis of the air HTO averagedwithin differshifted 15 days after the peak pod-filling (denoted “þ15”), or 15 days before the peak pocorresponding start and end dates counted from the beginning of the year (day-of-the-yeais n.

2008 (n ¼ 13) 2010 (n ¼ 7)

Period/DOY Mean � SD Range DOY Mean �Pod-fill:208e228 0.96 � 0.34 0.46e1.7 228e248 1.12 �210e225 0.93 � 0.32 0.43e1.56 230e245 0.95 �213e223 1.32 � 0.45 0.61e2.22 233e243 0.72 �þ15d:223e243 1.18 � 0.4 0.54e1.98 243e263 0.7 �225e240 1.12 � 0.38 0.52e1.89 245e260 0.65 �228e238 0.98 � 0.33 0.45e1.64 248e258 0.54 ��15d:193e213 0.88 � 0.3 0.41e1.49 213e233 4.61 �195e210 1.59 � 0.54 0.73e2.67 215e230 5.00 �192e208 1.69 � 0.58 0.78e2.85 218e228 5.01 �

Table 3. It is important to note that the standard deviations weremarkedly smaller during pod-filling periods.

Table 3 does not provide any evidence for rejecting thehypothesis that the SA model in modified form based on equation(4) holds, i.e. that the fruit/tuber OBTcould be related to the air HTOconcentrations that prevailed during the formation of the fruits/tubers. In particular, RFOBT values defined within the set ofwindows advanced 15 days before the pod-filling peak (the groupof windows denoted “�15” in Table 3) were different in differentyears. Inter-annual variability appears even more pronouncedwithin the set of windows delayed 15 days past the pod-filling peak(denoted “þ15” in Table 3). In contrast, the pod-filling periodrepresents the only averaging interval where the effective RFOBTvalues were consistent throughout the 3 different growing seasons.Since the probability of this consistency occurring by chance issmall, it is suggested that the pod-filling period is the mostadequate window for formula (4), and that the data collectedduring this period for all three years could be pooled to refine theeffective reduction factor RFOBT:

RFOBT ¼ 0:93� 0:21 (7)

It should be noted that the minimum duration of the pod-fillingwindow should be limited to 15 days, since according to ourcalculations (omitted here for the sake of brevity) a shorter windowin all cases appears to be heavily subjected to the influence ofdynamical variability of air HTO.

It should also be noted that the variability in RFOBT is three timessmaller than that of RFHTO.

4. Discussion

The data presented in Table 2 provides support for amodified SAapproach that could relate fruit/tuber HTO directly to air HTOthrough a constant proportionality factor. The variability of the

ent windows either centred at the peak of the pod-filling period (denoted “pod-fill”),d-filling (denoted “�15”); 20-day, 15-day, and 10-day windows and denoted by ther, or DOY) in 2008, 2010, and 2011, correspondingly; the number of samples each year

2011 (n ¼ 23)

SD Range DOY Mean � SD Range

0.18 0.9e1.38 223e243 0.91 � 0.14 0.70e1.190.15 0.77e1.17 225e240 0.88 � 0.15 0.67e1.150.12 0.58e0.89 228e238 1.15 � 0.18 0.88e1.50

0.05 0.59e0.74 238e258 0.86 � 0.13 0.59e1.010.05 0.55e0.69 240e255 0.75 � 0.12 0.55e0.940.04 0.45e0.57 243e253 0.62 � 0.10 0.46e0.78

0.76 3.71e5.67 208e228 0.41 � 0.07 0.34e0.580.83 4.02e6.15 210e225 0.41 � 0.06 0.31e0.540.83 4.02e6.15 213e223 0.38 � 0.06 0.29e0.53

V.Y. Korolevych, S.B. Kim / Journal of Environmental Radioactivity 118 (2013) 113e120 119

effective reduction factor RFHTO, both in terms of standard deviationand range of measurements, appears to be too high when a relationis sought using the air HTO averaged during one day or overa shorter period prior to harvest. However, this variability appearsto be reasonably limited when the averaging is performed overperiods of 3 pre-harvest days and during 9 days. Further compar-ison of different analysed years reveals that the RFHTO values fromthe 3-day pre-harvest window reoccurs consistently and underdifferent weather conditions, in contrast with values correspondingto 9-day window. It should be noted that 3 days is the minimumperiod for plume reoccurrence e and thus the minimal length ofa single uptake-depuration episode recorded in summer at CRL,where our assumptions of quasi-equilibrium transfer could beemployed. This is most likely the reason why no other shorterwindow (i.e., 1 day, 1 h) worked in our attempts to define theeffective RFHTO. That the longer period (i.e. 9 days) did not workeither was not particularly unexpected, because of the fast HTOexchange between the vegetation and atmosphere.

The value of RFHTO ¼ 1.20� 0.63 measured in this study appearsto be slightly higher than that established in Peterson and Davis(2000), which can be attributed to uncertainty in air HTO andespecially to field scale of our experiment, where the processes ofHTO transport in soil now play important role.

Relationship between fruit/tuber OBT and air HTO fromwindows shifted either 15 days before the pod-filling window or 15days after it appear to be markedly different in different years andthus inconsistent, leaving no grounds for hypothesis rejection. Theratio of fruit/tuber OBT to atmospheric HTO averaged within therecommended pod-filling window is 0.93 � 0.21. The variability inthe ratio of fruit/tuber HTO to air HTO appears three times largerthan that for the OBT of fruits and tubers.

This study demonstrates that the concepts of the SpecificActivity (SA) model are applicable to assessments of the levels oftritium found in fruits and tubers, notwithstanding the fact that thepresence of tritium is driven by highly dynamical planteatmosphere exchange processes. The modified SA model substi-tutes the complicated dynamical tritium exchange with quasi-stationary fluxes (from the atmosphere directly to the fruit andtuber), which are quantified by the effective reduction coefficientsRFHTO and RFOBT, which correspond to HTO transfer and OBTformation.

However, the use of the SA model, even in its modified form ofeqs. (3) and (4), results in larger variability associated with thedirect correlation between the HTO of fruits and tubers with airHTO, as compared to the variability pertaining to OBT. Still, theconsistency of RFOBT values over several years of observations, inconjunction with the smallest standard deviation associated withthe pod-filling window, provides clear support for the use of themodified SA model. This study’s use of essentially different timewindows for OBT and HTO in fruits tubers shows RFOBT’s non-relation to RFHTO, which contrasts with the presently prevailingform of the SA model.

It should be noted, that climatic conditions over 3 years ofexperiment were significantly different and this lends certainsupport to suggestion, that obtained results are not site-specific.This suggestion however merits investigation.

5. Conclusion

Results of this study imply that the present regulatory SpecificActivity model should be modified on the assumption that theaverage air HTO from the pod-filling period is related to the OBTfound in fruits and tubers. It was found that the relation betweenfruit/tuber OBT and the average air HTO for 20-day, 15-day, and 10-day wide windows centred at the peak of the pod-filling period is

consistent throughout three analysed years. Relations betweenfruit OBT and air HTO from pod-filling windows either shifted ordelayed 15 days with respect to the pod-filling period were used forhypothesis rejection. Relationships based on these shiftedwindows, however, appear markedly different in different years(and thus inconsistent), leaving no grounds for rejection of thehypothesis presented in Section 2.1.

The ratio of fruit/tuber OBT to air HTO averaged within therecommended pod-filling window is established to be 0.93 � 0.21.When the air HTO is used for fruit/tuber HTO assessment and airHTO is averaged within a recommended 3-day pre-harvestwindow, the ratio of fruit/tuber HTO to so averaged air HTO isestablished to be 1.20 � 0.63, where the variability appears to bethree times larger than that for fruit/tuber OBT.

It is concluded that, in relative contrast to plant HTO, the level offruit/tuber OBT adequately follows an empirical relationship basedon the average air HTO from the pod-filling window, and thereforeis in clear agreement with the modified SA approach proposedhere.

Acknowledgements

The funding for this study was provided through COG WP-30472. The authors would like to acknowledge the help of BudO’Donnell with tritium monitoring data, Bruce Reavie with mete-orological and gamma radiation measurements, and Mike Bredlaw,Oscar Petersons, Mark Ranjram, Evan McNamara and WayneWorkman with field sampling and measurements of tritium andcritical comments of Marilyne Stuart. This study has benefitedgreatly from numerous discussions with Phil Davis.

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