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Retention, loss and translocation of radionuclides applied to foliar surfaces of wheat
I::*, is,ml**,mai ¢md I'v*,~e,.,.ged Buta~:l. \ ,,] ;2. X<) I p p ~9] 011. 19q2 h i . l , ' d h, (;l
[1(1!)1{ ~{172 qo S~.,III i ql !1(I ~ 19!i2 I',[~, ,i.,,l~ I'~¢'.- I.Id
R E T E N T I O N , I,OSS A N D T R A N S I , O C A T I O N OF R A D I O N U C I , I D E S APPLIED TO FOLIAR S U R F A C E S 01: W H E A T G. SHAW,* M. J. MINSKI* and J. N. B. BELLt * (',('ntr(' tot :\nalx|ical Rcse,u'ch in the Environment, lmpcrial (:ollcg(. at| Silx~ood Park. Ascot. Berkshire SI,,5 TI'E, I . K . + l ) q ) a r m w m ()fBiulogy, lml)('vial C()llcgc at Sih~()od l'ark. ,\sc~)t, B('rkshirc S1,5 71)Y, ('.K. ', Received I 0 l)~'ccmbcr 19!) 1 : aeeepled i1~ ~,'e'i~ed,/brm ',3 ,]u(r 19921 SHAW (;.. MINSKI M..]. ao.d BLI:I: ,J. N. B. Relelllion, /os~ alld hamloeatioll q/radiotlucli:crc I ) O t tbund to hc [ l - i l l l S ] O C ~ l l c d [ ' r o l l l toliagc to grain. Field loss of radi,mctivity was most strongly corrclatcd with thc mcau dail\ decline it+ crop relative growth rate tbllowil|g c
INTRODUCTION lhRECT c o n t a m i n a t i o n of plant ti:)liar surlaces m a y o c c u r as a result o f a m l o s p h e r i c d e p o s i t i o n of r a d i o n u c l i d e s arising from t h u h c o n d i t i o n s w i t h i n n u c l e a r p o w e r reactors. R a d i o a c t i v i t y ill Ihcsc cases m a y ])e associated with aerosols as a dry deposit or with r a i n c h o p s as a Yvet deposit: it was d w latter w h i c h resulted in the m a j o r p o r t i o n of g r o u n d a n d plant c o n t a m i n a t i o n in Britain and o t h e r l)arts ot'X'Veslern E u r o p e tblloxx ing the (:her3!)[
nob xl a c c i d e n t . ~ W h a t e v e r the p h y s i c o - c h e n l i c a l n a t u r e o t the deposit, o n c e a planl surlhce b e c o m e s c o n t a m i n a t e d with r a d i o a c t i v i t y two p o t e n t i a l p a t h w a y s into the f o o d c h a i n are possible: (i~ c o n s u m p t i o n of c o n t a m i n a t e d forage by r u m i n a n t s , and
392
G. SHAW el al.
nuclide burden with time, whether one considers total contamination burden or the acuvitv per unit of surlhce area or biomass. 'l'he term "tield loss'" has ]tee,~ coined by CHADVv'I(IK a n d CHAMBERt.aIN 5 tO describe this overall timedependent reduclion in contamination, which is thoughl In be due to a variety of biological and cnviromnental processes including plant surthce weathering, "(lilution" by plant growth, and absorption and translocation to organs undex eloped ill the time of contamination. As a result of these processes potential ingestion dose will decrcase x.tith time tbllowing a contamination event at a rale which will be determined I)7 the complex of ti~ctors responsible tor the overall loss of activity. Following .NIII,BOURNand TA','LOR'S'J:~ study wilh ~:~'Srthis rate is usually quantitied tbr the purposes of modelling by assigning it a iielct loss halt:lit;~ of approximately 14 days, a h h o u g h it is clear front the literature that such halt'-liti~ wdues can v a r \ considerably and that they are especially influenced 1)y seasonal t:actors (tbr a review of tield loss half-live values and associated uncertainties see NI1Lt,ER and tlOFFMAN It ). CO[:(;HTRF,Y el a[. 'u have highlighted the diflicuhies i n v o h e d in ol)taining reliable estinlates of loss half-lives ti'om tield measm'ements. In particular these authors stressed the need [br complementary data on plant growth and precipitation after initial interception in order to identit\.' the causes and their quantitative relationships to the observed rates of loss. A turther interl)remlive ditBcuhy, according to (1oughtrey and colleagues, is the evaluation o f " a p p r o p r i a t e models tor telention fron] field data because of the conlbunding etl'ects of fbliar absorption and subsequent translocation, or root uptake from the associated soil pool". A notable study on the translocation of radionuclide translocation from tbliage to developing i)lant organs was that ot A A R K R O G - ~,~.hich demonstrated a strong time-dependency between the activities of several fission and activation products at the time of their application to cereal leaves and their recovery in the grain at harvest. Studies elucidating relationships between radionuclide retention, loss and translocation under field conditions, with respect to detailed phmt growth characteristics, however, have not previously be.en reported. The experiment described in this
paper was carried out d u r i n g the s u m m e r o f 1989
at Imperial College's field station al Silwood Park, m'ar Ascot IU.K.) attempting to link quantitatively these thciors tbr a wheat crop growing under enxironmental conditions representative of those of southern Britain. MATERIALS AND METHODS
:at Experimenlal In order to thllow time courses of radionuclide behaviour in wheat lollowing an external cnnlamination event it was necessary to nse a contamination method which resulted in the heaviest and mosl unilbrm covering of plant surt~aces with radioactivit}; planls were theretbre immersed in solutions containing cocktails of the radionuclides to be studied. In order to t~tcilitate this process spring wheat phmts ( Trilicum aeslivum, cv. Tonic) were grown individuany in polyethylene containers containing a 3:1 peat :sand mixture, amended wilh CaC():~ (sufficient to bring the pH of the compost to neutrality) and R orison's nut rient solution, la \Vhen sealed at each end these containers provided a discrelc rooting environment tbr individual plants which precluded the possibilit} of the compost becoming radioactively contanfinated by washoff fi'om the leaves (this was conlirmed by radioassay of the compost, t?om which roots had been separated, at the end of the experiment) the root uptake pathway of radionuclides was theretbre eltbctively excluded from the experiment with clean water being provided Ihrough an irrigation port. A 25 1 liquid co~ ktail ofS:'Sr, "~tiRu, 117Cs, ~'4Mn and ~'°Co was prepared t~om soluble, carrier-ti'ee chloride sourccs of each radionuclide, obtained ti'om Amersham International, U.K. In the case of the l"~;Ru a small (2/~M) amount of nonradioactive carrier was added to prevent irreversible sorption of I°~;Ru onto the walls of the polyethylene containers used in the experiment. The final cocktail contained individual radionuclides at a specific activity o f 5 0 0 kBq/1. Contamination of plants was carried out at three intervals during the growth cycle of the crop (contamination episode Nos. 1, 2 and 3 at 42, 66 and 93 days after seedling emergence, respectively) and was achieved by inverting and completely immersing individuals in the radioactive solution; immersion
R:kl)I()NU(:LIDE RETENTI()N, I,()SS ANI) 'IRANSIX)CATI()N IN WHEAT was fiir a p p r o x i m a t e l y 30 sec d u r i n g which time the solutions were continuously stirred. After lifting c o n t a m i n a t e d plants [?om the radioactive sohltions t h % were laid on their side to air dry in o r d e r to prevent any radioactive runoff" tiom c o n t a m i n a t i n g the compost in which the plants were growing. I m m e d i a t e l y after the plants were dry a I)at('h of 10 individuals was sainpled to d e t e r m i n e the initial radionuclide b u r d e n associated with the slioot tissues. Plants wen" sutisequently grown in groups of 10 individuals, fully exposed to a m b i e n t e n v i r o n m e n t a l conditions [see Fig. 6 ,:topoi and harvested at the intervals indicated in Fi K. I. T h e overall c r o p p i n g period lasted 120 d a \ s . G r o u p s of 10 plants were sanipled at each harvest wilh the entire shoot system being taken tbllowing excision at the slem base. After d r y i n g at {{()"(: tot 24 hr, plants \~.ere divided into straw, c h a t f a n d grain , the chaff" bein.~ separated [,tom the grain l/v riotation ()n deionized water) which were weighed. Each s a m p h ' was t h e n / r o u n d to horno/eneity in a I)lender, sul)-samples packed into i)olyethylene capsuh:s of C()liSlall[ oeOlTIClry a l l d a s s a v c d for radioa('tivitx t) 3 7-ray spectrometry using a (;el,i detector coupled to a multi-channel anah.ser N u c l e a r D a m hw.). Actual activities (if each radionuclid(' \vere ('alculated t`rom it calibration
3
D
o--o
[]
0
0
0
0
0
"o
o
Q. 0 cO
curve d e t e r m i n e d from a mixed 7-standard {obtained fiom the National Physical L a b o r a /ory, U.K. ) o['identical geometry to the unknown samples. All activities were back-corrected tor radioactive decay (to the nearest dav i to the date of initial a p p l i c a t i o n of the radioactive cocklail to leaf" surfaces. b)
Dala ana!>i.s (i ! l'lanl gro~clh. Both the total biomass of shoot
tissues :sti'ax~, chaff and grain) and the mass o[ grain produced were recorded tor each illclix (dual plant used in the experiment. T o allow a quantitative interpretation o1 the eflT:et of growth ( m the loss of radionuclides t`rom leaves and their subsecluent translocation to grain, logistic curves were titted t(~ lhe d a t a ])v a least squares method. Overall growth curves were obtained [,tom these fits as well as their derivatives which represent ['unctiol> of relative growth rate of total tissue and grain. ,ii~ -l"ic/d /,.~.('. l ) u r i n g a stochastic llil('lcar c(mlanlin,tti(m excnt plant tissm's receive a discrete, initially external, radionuclidc t)urdt'n assuming a relatively slow rate of tool uptake of radioactivity t`reshly deposited to the soil surfacei~. CHAMBERI,AIN ~ modelled lhe time-depelldeni loss of surli/ce radioactivity from plains in such circtunstanccs using all exponential model identical to that describing the radioactive oh'ca\ process itselt~ In the present study d a t a relatin K lo the oxerall loss of r a d i o a c t i x i t ) with tiIrlC [iom c o n t a m i n a t e d tbliage were [it ted by a least s( Iuares incthod to dw [bllowing equation l:, -: t:, o i ( "
[]
E 0
0
0
o
{o
4'0
i
60
8'o
I
loo
I
1"1(;. 1. lmyout of experimental time cours('s employed Io dctcrnlinc loss and translocation of radionuclidcs will1 time tbllo~ing tbliar application t() x~hcat plants at dn('(' stages within th(' c.op's growth cych'. The beginning of the experiment was taken iobc the ti,nc of germination oflhc crop 126,]unc 1989); contamination ('pisodc Nos. I, 2 and 3 (indicatcd I)) the square plol svmholsl wcrc at 42, 66 and 93 days, respectively, t})l]owi,lK this date with subscqucnl salnpling al finlcs indi('atcd 1)v circuh~r plot synibols.
J'~
ili
Her(!, F, is the fractional retention of any radionuclide at any harvest interval ~initial vahlc F, o ) - 1.0 ~, calculated [ioni
12o
Crop age (days)
393
I;, -
specilic a c t i v i l y o[tbliage at time of harvest (*) specific a('tiviiy of tbliage at time ( ) f c o n t a m i n a l i o n
{* back-corrected tor radioactive decay to time of COil(am(nation ; and /q~ is a rate constant of loss of activity t`rom fi)liage. Vahies (d'kl, d e t e r m i n e d t`rom least Stluare
G. SHAW et al.
394
fits to the model were used to calculate biological h a l f lives of radionuclide retention by ff)liage. (iii) Translocation. Translocation of radionuclides fi'om [bliage to grain may only occur once absorption has taken place across the leaf surlhce and the nuclides become available ibr loading into the plant's vascular system ( S H A ~ r el al. '~"' have shown in the case ofradiocaesium that it is the process of absorption and not translocation which is rate limiting in a physiologically unrestricted plant). Assume (a) that the rate of absorption across the leaf surfhce is proportional to the specific activity of the contaminating radinnuclide (i.e. first order kinetics assumed fbr carrier-li'ee radionuclide contaminants, after SHAW and BV.LL~la), (b) that the loss of leaf surthce activity is exponential with respect to time [as in Equation ( 1)] and (c) that further loss of activity (due to weathering and growth dilution) is likely to occur fi'om the grain once the nuclide has t)een transh)cated there, then the fbllowing conceptual model is applicable: k,:, e ~' ~ [[R].,,,,,,]
--+ k,.
(3)
where [R]v<. is the specific activity of a radionuclide (R) in the grain, kli, is a rate constant of foliar absorption [initially kldo)], 2 is a decay constant relating to the loss of tbliar specific activity (this should be the same as k, above) and /i~v is a rate constant of radionuclide loss ti+om grain. The tollowing equation describes the timedependc'ncy of the radinnuclidc specific activity (corrected tbr radioactive decay) within the grain according to the model [1¢1(1) =
k,~,(o ~ (e ; " - e klv /+
/', '/ ~ .
(4)
Grain translocation factors defined as specific activity of grain at time of harvest (*) specific activity of leaf and stem tissue at time of contamination (~) (5) (* 1)ark-corrected tbr radioactive decay to time of contamination ~grain was undeveloped at the time of contamination)
were titted to this model fbllowing the procedure outlined by I,'~rHIGKERand SCHUI,TZ. '24 RESULTS
(a) Planl growlh The growth of wheat under the experimental culture conditions appeared normal with plants completing their litb-cycle within the adopted cropping period of 120 days. The total hiomass of the crop increased throughout the experimental period; instantaneous relative growth rate (RGR, defined as l/l.V'dW/dl, where H7 is biomass; HUN'(1~) declined at each period (Table 1). The growth curves tbr total biomass and grain in Figs 2 and 3(a) show a characteristic sigmoidal shape, with grain production hecoming measurable after 40 days. The RGR fbr total biomass peaked at ~.55 clays [Fig. 4 (top)] whereas that to( grain lagged slightly behind this at ~ 70 days [Fig. 3(a)]. Grain eventually accounted tbr approximately 1/4 of the t()tal biomass at final harvest ( 120 days, see Fig. 2). (b) Reten6on and loss ~radionuclides bL/bliage (i) Field loss half-lives. Following contamination the initial specific activities of each radionuclide associated with wheat tissues were of the order of 75 kBq/kg, with an average coefficient of variation in this value between replicates of3.80i,. All radionuclides applied to wheat tissues decreased with time following application at three stages in the plants' ontogeny. Table 1 shows the results of a series of least squares fits of experimental data to the exponential loss model [Equation (1)1; biological half-lives derived tbr each radionuclide during the experimental period are shown. The associated correlation coefficients are high (miuimum r e value = 0.841, P < 0.001) implying that the data are adequately described I)y this model; the tield loss half-lives given in the tat)le should theretbre give a reliable indication of the rates of loss of activity tollowing each contamination event. Some variation in halt:-li[~ was apparent betwccn radionuclides at each contamination episode, although values were generally similar within data sets derived fi-~r each contamination episode. Each radionuclide (with the exception of ""+Ru, which tbll below detection limits at sam-
R , \ I ) I ( ) N U ( : I J D E R E T t ' N T I ( ) N , l,OSN AND ' I ' R A N S I , ( ) ( : A T I ( ) N
IN WHE,XT
395
Table 1. "'Fidd /,,~.~" ha(/:lives (i, dq>) q/radi,lmclh/e.~ ap/died Io /idiar ~,!/hct'., q/zchcal a,~ a ,~o/uh/c cm'klail al Ihree ~lage,s q/plan! grm~'lh ( I~m tamination cpiso(h'
RCR ,per day,
No. I
0.0t-2
No. 2
0.040
No. 3
0.018
H,u vest
0.009
~"Nr
'""Ru
9.94 0.953) 26.08 ,0.949 19.80 ().991,
16.86 :0.841, NA 13.3}{ ~0.983 ~
~;'('.s 15.{)4 ~0.966a 23.70 , I).':)80~ ll).(i7 ,().995 ~
~i~lll
""Co
15.!)7 (0.942 41.90 0.994 1!t.71 : 1.()()~
15.97 (0.898 35.90 (0.995 } 18.03
{0..qg2
l"igures in brackets represent correlati¢m coefl~ciems i~" values, n = 301 assocmted with h'asl squares lits ¢,t'experimental dam Io a modcl of exponential loss [F,quation :1)]. R(;R ix lfic i n s l a n t a n e o u s relative girox~th rate a! each e(mtalnin;.llion episode determined by fittin~ a logistic curve to tola] 1)iomass data tblh}win.< the procedure o['Hu',:'r, i:! plin£ periods followin£ the second c o n t a m i l m t i o n episode) disphtyed the hmgest half-litb tbllowing the second c o n t a m i n a t i o n episode at {36 days. StMn a n d ""(Io exhibited the longest biological halt:lives of all the nuclides e x a m i n e d , r e a c h i n g a m a x i m u m of 42 a n d 36 days, respectively, tbll o w i n g the second c o n t a m i n a t i o n episode. H o w ever, their field loss half-lives xxere b r o a d l y similar
Total 4.0
3.0 E c~ 2.0 i1)
0
0
40 80 Crop age (days)
120
l:m. 2. (;rowd~ curves ti)r total biomass and grain during the experiment showing typical sigmoidal shape. I,ogistic thnctions were fitted to the data tbllowing a linearization procedure given by HVXT. ':~' \Tertical Bars represent SEM ' n - 10.
to those of the other r a d i o n u c l i d e s tbllowing the first a n d third c o n t a m i n a u o n episodes. iii) Rdalion.ship helweenfield lo.ss and relalh)e growlh tale. F i g u r e 4 (top, indicates that the R(iR of total biomass t h r o u g h o u t the e x p e r i m e n t a l period showed a m a r k e d m e a n decline from the tirst c o n t a m i n a t i o n episode (No. l) until the end of the e x p e r i m e n t a l period. T h e ox erall m e a n daily decline in R(;R over the period tbllowing each c o n t a m i n a t i o n episode up to the end of the experim e n t (i.e. RGR at c o n t a m i n a t i o n m i n u s R G R at harvesl, divided b~ the time i n t e r v a l in da,vs~ is plotted in relation It) the m e a n lield loss half-lilb tot all r a d i o n u c l i d e s over that period in l:ig. !b o t t o m ) . T h e r e was a s t r o n g positive c o r r e l a t i o n b e t w e e n these two properties, suggesting that field loss ['rein the p l a n t s ' tissues is to some extent controlled I)\ v a r i a t i o n in p l a n t RGR d u r i n g the course of the g r o w i n g season. T h e relatively small degree of" v a r i a t i o n in m e a n field loss halfLlif~' in He-. 4 i l)ottom', also suggests that tile r;tte ()floss of'radioactivity tixma the p l a n t tissues in this study is i n d c p e n d e n l o(" the r a d i o n u c l i d e .
c ) 7"ramlocation ~!/ radionudide,, to grain (i) t'allerm ~j lramlocalion ~J individual radio,uclich',~. No m e a s u r a b l e t r a n s l o c a t i o n of ~:'Sr or t°'~Ru fiom tbliage to g r a i n occurred d u r i n g the e xperin~ent. T r a u s l o c a t i o n of 'CCs, -"lMn a n d "°(io tiom [bliage to grain [as m e a s u r e d by the grain translocation thctor, F,,, E q u a t i o n (5)1 was d e t e r m i n e d
396
G. SHAg" el aI.
A
0.05
e"
No.1
No.2
No.3
(a) Grain biomass (W).,.
0.04
1.C
"0
:i
!
Q:
0.6
Q: 0.02
L9
E= °
~.~
0.03
n
0.8
"O
0.4
E
0.01
0.2
0.%
0.0 20
O
7
40
60 80 Crop age (days)
100
4'o
8o Crop age (days)
120
40 (R 2 = 0.991)
31] (b)
•~
~ z
m .£ = 0.8
~ ~
RaR
e-
w t h dilution
-"
20 u_
0.6
1C
0.2
z~
0.0 20
,
i
4.00e-4 5.00e-4 6.00e-4 Mean daily decline in RGR (per day 2)
o.4
E~
No.3
"O
4()
60
80
100
120
Crop age (days)
FIG. 3. (a) Plot of growth data tbr grain. The fitted growth curve (/2 = 0.919) is a logistic function a
H/
1 + be "
(i)
where W = biomass (g dry weight), a, b and c arc constants (1.02, 23813 and 0.1097, respectively) and t is the crop age (days). The plot of relative growth rate (RGR) is the derivate of this function l
dW abce '~ IV" dl -- (1 + be ")~'
(ii)
with constants as above. (b) Grain translocation factors (F,,.) following contamination episode No. 1 plotted in relation to the fitted RGR function in (a). F~,.values are means derived from the data tbr ~:~TCs,~'~Mn and "°Co normalized to the maximum at 66 days. The growth dilution curve is calculated as the reciprocal of the grain growth curve [Equation (i)] and is normalized to the 66 day value.
Fro. 4. (top) Fitted function of relative growth rate tbr total biomass calculated as in Equation (ii). Constants (a = 4.22, b = 8.73, c = 0.042, r 2 = 0.84) were derived ti'om total biomass data shown in Fig. 2. Nos. 1, 2 and 3 indicate contamination episodes 1, 2 and 3 (see Fig. 1). (bottom) Correlation of mean field loss halt:life tbr all radionuclides following each contamination episode with the mean daily decline in RGR over the field loss period. Horizontal and vertical errors bars represent SEM associated with mean estimates of field loss halt: life and daily decline in RGR, respectively.
following the first c o n t a m i n a t i o n e v e n t at 42 days. F , values for 1:~7Cs, 54Mn a n d G°Co r e a c h e d a m a x i m u m (in the o r d e r 137Cs > ~°Co > 54Mn) shortly after a p p l i c a t i o n a n d t h e r e a f t e r d e c r e a s e d with time. F i g u r e 5 shows these d a t a in r e l a t i o n to E q u a t i o n (4); the constants d e r i v e d for the t h e o r e t i c a l r e l a t i o n s h i p are g i v e n in T a b l e 2. It was p r e v i o u s l y stated t h a t if E q u a t i o n (4) was to a p p l y to the e x p e r i m e n t a l d a t a , then the rate c o n s t a n t o f field loss d e r i v e d for E q u a t i o n (1) (kit) should be the s a m e as t h a t d e s c r i b i n g the
RADIONUCLIDE RETENTION, LOSS AND TRANSLOCATION IN WHEAT
397
(ii) Relationship between translocation of radionuclides and grain development. Figure 3Ca) shows a
10 c
101
37Cs
Ft,
o
2'o
t (days)
do
do
logistic growth curve fitted to the data [br grain development over the experiment (r2= 0.919) and includes the derived function tbr instantaneous relative growth rate. Figure 3(b) indicates that the mean normalized F,, values ~)r J~TCs, ~;')Co and 54Mn w e r e closely related to the latter function. The similarity between these two curves suggests that the process of translocation of radionuclides from leaves to grain in wheat is strongly related to the rate ofbiomass increase of the grain.
Fro. 5. Translocatiou time courses tbr HTCs, ~'~Mn and ';'~Co following contamination episode No. 1. The points represent experimental data, each comprising the mean F, value obtained tor bulked samples from 10 plants. The fitted curves are plots of Equation (4) calculated using the constants presented in Table 2.
(d) RaiT~all during the experiment The rainl~all pattern during the course of the experiment was culled from the regular meteorological records at Silwood Park, and this is shown in Fig. 6 (top). The daily average rainfall from 26 J u n e 1989 to 23 October 1989 was 1.68 ram/day. During this period, however, there were three exponential decline in radionuclide input rate to rainlhll events of approximately, 20 m m / d a y or grain in Equation (4) (2). The close similarity greater, and several which were between the between these two derived constants tbr ~:~TCs, mean value and 10 ram/day. Rainfall did not ;4Mn and (s~Co can be seen in Table 2, suggesting appear to be meaningfully correlated with field that Equation (4) is an appropriate model for the loss halflif~" [Fig. 6 (bottom)]. description of the tin'm-dependent translocation of these radionuclides within wheat plants. DISCUSSION Following translocation to grain the rate of decline in concentration of radionuclides within The potential radiological dose to man via the grain tissues tbllowed the reverse pattern to the consumption of contaminated vegetable their m a x i m u m F,, values; thus, 54Mn was lost materials is known to be dependent on a host of most rapidly, with ~°Co being lost at an intert~.ctors which can be broadly categorized under mediate rate and l:~7Cs being most strongly the term "seasonality". c~ One of the major maniretained. This loss appeared to proceed expo- festations of seasonal variation in environmental nentially with time, according to Equation (4). variables such as temperature and irradiance is the cycle of plant growth and senescence which
Table 2. Rate vonstanl,~ derivedfrom least square,~,fits ~!/ consequently aft?ets ingestion dose to man/4 I f predictions ofradionuclide flux within tbodchains erperimental data to Equations (2) and (4) Rate constant (per day) Radionuclide 1~7Cs ~,.iMn '"'Co
k,v
).
kt~
1.62 15.0 8.60
0.044 0.038 0.040
0.046 0.043 0.043
The constants kl~ and ,:. refer to Equation (4) and k, is the rate constant in Equation (2). If Equation (4) is to apply then the constants 2 and k, should be equal.
are to be suceesslul then plant growth itself should be a fS.ctor implicitly taken into account by radiological assessment models. In order to achieve this wc need to understand the nature of the relationships between plant growth rate, the biokinetics ofradionuclides within plant tissues and environmental thctors such as rainfall intensity and duration. The initial capture of radioactive fhllout from the atmosphere will vary seasonally according to the extent of ground cover by vegetation. 2
398
(;. SHAW el al.
301 23/10/89
26/6/89 I E E
20.
¢-
Mean= 1.684 -
~)!,0
200
250 Julian day
300
3so
40
30 ¢u ¢-
"~ 20 "O u_
lO
50
i
100 Cumulative rainfall during period following contamination (ram)
t
150
Flf;. 6. (top) Rainfall during the expcrimental period the datcs delineating the beginning and end of the cropping period are indicatcd. (bottom) Relationship })elwccn mean field loss half-lit~ and the cumulative raintM1 during the field h)ss period.
CHAMBERI~AIN :6: has previously recognized this in a m a t h e m a t i c a l [brmulation of the dependence of the interception fraction on several variables and the idea hits also been i n c o r p o r a t e d it) radiological assessment models such as ECOSYS/l:''17 The m e t h o d o l o g y used in the current study was not designed to investigate this initial capture of
atmospheric deposits; rather the lilt( of deposits alter c a p t u r e by leaf surtaces has been explored as the effects oftactors such its plant growth during this period have 1 l O t previously been a d e q u a t e l y addressed. T h e final radionuclide I)urden of edible tissues igrain) at harvest will
resuh from a balance between loss t~om the system ("fi(:ld loss") and retenti()n tbllowing absorption and translocation, each of which ~,ill I)e aft'coted by such filctors fbllowing an external conl a m i n a t i o n even t. T h e rate oflield loss ofradiouuclides from phmt tissues has bccn shown by (IHAI)V¢I('K ) l i d (.]IIAMBE.RI,AIN'" l() Vill'y seasonally, with observed loss rates ill sumlner exceeding those in ~Vinler. These authors expressed the loss of radioactivily li~om a grass sward on a g r o u n d area basis, \ d 6 c h rules O u t growth dilution as a thctor c o n t r i b u t i n g directly to the observed loss. T h u s it is likely thal some plant g r o w t h - r e l a t e d elt~'ct other than simple dilution was responsible lot (]HAl)WICK ill)(] (IHAMBF,RI,AIN'S observations, a h h o u g h flwy did not speciti(all) allude to this possibility. In tile present study tield loss rates t?om wheat {be live fission and activation products have been shown to t)e a d e q u a t e l y described by a simple exponential t'unction and the absolute halt:-lit~: values ohtained in this study c o m p a r e with those ill several other studies (e.g. SIMMONI)Sez) . As a means of quantit}'ing radionuclide persistence ill vegetation systems, then, the use of a simple biological haltZlilb seems just)liable though it is also clear that this half-lil;~r will vary according to the stage or, rather, lhc tale of growth of the planls' tissues. Plants rarely exhibit linear growth patterns with time and consequently the rate of growth shows a time-det)endem variation. M a n y tune)ions have been invoked to describe these sometimes complex patterns but it is tilt" logistic f u n c t i o n which classically has been used to describe the typical sigmoidal growth curves seen in the present study. T h e derivate of this relati(n> ship provides us with :ill inslantaneous measure of growth ("relative growth rate" or RGR) which can be considered to be potentially the most useful p a r a m e t e r tor incorporation into radioecological models. The fimcti(mal relationship of R(;R with crop age needs to he understood since, because it is complex [Fig. 4 (top) l , all(re is no simple correlation ltetween instantaneous ,~rowth rate at the time of c o n t a m i n a t i o n and the sut)sequent rate o1" loss. However, it strong correlalion does exis! between the overall dail> decline ill R G R lbllowing t o u t ) h a ) n a t i o n and ticld loss haltklifi: [Fig. 4 (hot)ore)l, thus reintorcing the notion of
RAI)I()NU(:IAI)E RETENTION, IX)SS AND TRANSI,O(:ATI()N IN WHEAT a causal relzttionship between plant growth and field loss. M a n y factors have been put tbrward as possihle reasons behind the field loss process, a h h o u g h it seems intuitively likely that rainlhll should phty a m a j o r role. All the raintM1 events recorded d u r i n g the course (~['the current study were titirh unitbrmlv distributed throughout the experimental period [Fig. 6 Itop) ] and it is therefi~re likely dmt their eltix'ts on radionuclidc retention/loss xs'ere e \ e n l y sl)read t h r o u g h o u t Ill(- expcrinaent. T h e r e \x as n() o h \ i o u s correlation hetween the altlOUiH of rainfall in till' period lollowing external cont a m i n a t i o n and tile [ield loss half-lifb d u r i n g this period, hmxevev [Fig. (5 (bottom)]. (Mmparison o [ t h i s resuh \sith that shown in Fig. '1. ,hotlonl) suggests that plant growth itsel['had it much more i m p o r t a n t etti'ct on overall tieht loss than did v a i n l h l l . . \ theory of the ticld loss mechanism which has survived since the mid-1960s is that of epicuticular wax m e d i a t e d loss proposed by ~hI,Bt)URN alld
"I'AYI,(IR. l:;: T h e
loss ot' \x:txv
material [i'~ml phmt leaf surlitces is known to i m r e a s e with the rale of phmt growth and although, as it causal mechanism /)f" field loss of vadionuclidcs, this theors still awaits conclusive I)roofthe dcl)cndency of the biological hal filth of cxlClmll deposits on RGR lends [hrdaer circumstanlial evidence in its thxour. Furtiwrmore, the r c l a t i v c h narrow error limits around the nleatl values in Figs 4 and 6 :l)OtlOn?,l tbr [ield loss half-lives of five r a d i o m . : l i d e s with widely d i s p a r a l c chemistries SllggCsl Ill:it the rate of loss is pr(~bably a function of some t h n d a n w l m d p r o p e r l y ot Ihc h'at" sml'ace. :'~ Ho\vever, wht'lt intcrl)reting the present resulls it llltlSl he ])()rllc in mind that the radionuclide source applied to I)lalll SUI'[ilC{'S\vas initially in a highly soluble tbrm which was suhsequently allowed to dry. This may not he ,in a d e q u a t e model tbr c o n t a m i n a t i o n alier a nu('lcar ac('idenl where hoth soluhh" and insoluble materials m a \ he deposile(t and \vherc thevc m a y not he a period alhtwing fin the d r y i n g of the solul)lc inatcrial. T h e initial interccl)tion of total radioactive deposits from Ihc atmosl)hcve l/y grain, protected within the husk, is of tile order of o n h 0.05 1.9",. '-':-~ Suhsequellt absorption and translocation o1 radionuclidcs deposited on tilt' h'af surf:aces is therct()re (fl i)articular i m p o r t a m c to
399
dose assessment its it m a y result in a significant increase in c o n t a m i n a t i o n of the grain w i t h tnne t b l l o w i n g tile initial d e p o s i t i o n e v e n t , as predicted hy E q u a t i o n +:4', +see also AARKROG 2 ). This model indicates that the input term is proportional in a simph" m a n n e r to the degree o f h ' a f ,~urlace contanfination which, as we have ah'ead} seen, is Stll)lecl to an exponential decline sxilh time; if this model is indeed applicahh" then the m a g n i t u d e of tilt: raw constams fin" Ihc loss of a c t i v i t \ from the external surlhces of the plant field loss~ and the decline in al)sorption rate with time should hc identical. T h e similarity hetwecn these constants in T a h l e 2 gives confidence in dw applicahility of F,quation (4) to tllis situation. Translocation to grain in the cturent experiment was clearly radionuclide specilic, with Sr and Ru exhibiting no measurahh" degree of movement within the tissues analysed. Sr has prev i o u s h heen shown to be r e l a t i v e h immobile in hiological systems-' while COt;(;HTREY and "I'HORNE !' have reviewed d a t a fi'om tlAxDLv,Y and BAB(:t)(:K I~ and ;~ARKROC-I which suggest that, ti)llowing a p p l i c a t i o n as a spray, m"Ru is subject to only lilnited tbliar absorption. T h e mohilitx of the r e m a i n i n g three radionuclides in die current study is p r o h a b l y it reltcctioll of their esscntialhy, in t h e c a s c s o f "(illCo a n d - ' l , \ l n , or the ability to mimic an essential clement in the (asc ,,f" I ~r{:s. i!, Translocati(m of radionuclides wilhin the plants" tissues ill this stu¢h was also clearly dependent ttptm R(;R. XI{?LI,ER and PR6m, t~; have previously shown Ihat translocation of radiouuclides t~ grain will reach it m a x i m u m at "torl~. filling" which can I)e taken to coincide with the period of exponential growth of grain tissues. Indeed, d u r i n g the present study the maximal mcan grain translocation thctor (F,, ; tbr C:lch o{" the radionuclides which exhibited measurahle translocation {~7(Is, '"'Co and ;*Mnj was almost synchronous with the peak in grain R(;R [l:ig. 3,hi], 'Fhe suhsequent decline in /"~, xxith u s e f a n he altril)uted to scveral [itctors, v~;.~. {i, a redu('lion in the int)ut rate of translocate to the o-r,till tissues ,:l/rOl/ortional to till' lield loss of vadioactivitx ['l'Oll/ the external tissues,, ii~ dilution of the activity translocated to the tissue duc to ~rain expansion ,:"tilling", and :iii:, r c n l o \ a l of activity tiom the grain due t . lield
G. SHAW el al.
400
loss. It may be considered that the latter process is almost negligible as the grain is effectively protected tiom the n o r m a l processes of weathering by the husk. Figure 3(hi includes the growth dilution curve tbr the grain (calculated as the reciprocal of the growth curve normalized to the 66 day F,. value) a n d this indicates that the rate ot" loss o[" activity from the grain was less than that expected if lranslocation had ceased and only growth dilution was occurring. This dilution curve does not take into account the rcmoval of activity due to field loss fiom the grain which, even if small, would be expected to result in an even higher rate of loss. It may be concluded that the main source of"loss" of specific activity fiom grain is in thct a general reduction in the rate of translocation of activity, related to a decline in both the input term (i.e. external c o n t a m i n a t i o n ) and growth r a l c . Thc rate COrlstant klg in Equations (3) and (4) is an empirical parameter which includes growth dilution and anv other unspecified "loss'" processes. While the derived values tbr this conslant appear relatively large (Table 2) it may be concluded that this "loss" is mainly apparenl and that Ollce translocated to the grain the major part o[" the lola/activity is conserved within the grain tissue. If" this is the case then these tissues could represent a seasonally variable sink of radionuclides within agricuhural ecosystems.
Ackuowledgement The work described in this paper was carried ()tit as part of a study funded by the Ccntral Electricity Generating Board (U.K.) and is published xxith the permission of Nuclear Electric (U.K.).
REFERENCES 1. :~ARKRO(;A. i 1975) Radionuclidc levels in mature grain related to radiostrontium contcnt and timc ofdircel eontamiimtion. HIIh P/l>. 28, 557 562. 2. AARKROG A. 11983) Translocation of radiomwlides in cereal crops. In Ecological a@eets of radionuclide relea.w, P. ,J. (]OU(ltlTREY, J. N. B. BELL and T. M. ROBERTS, eds. Special puhlicati
aquatic environments and acquisition of data tbr that purpose ("VAMP"). IAEA, Vienna. 4. BOERI(,. (1989) Environmental processes and parameters influencing tile consequences of all accidental release of radioactivity: weather and season. In The iq/tuence ¢?/~ea,~onalconditiom on the radiological con,sequeace,s of a nuclear accident, Proceedings o/'a NEA work.~h@, OECD, Paris, 1989. 5. CHADWICK R. (I. and CHAMBERLAINA. (]. (1970) Ficld loss of radionuclides ti'om grass. :llmos. Envir. 4, 51 56. 6. C~HAMBFRLAIN A. (], (197/)) Interception and retention of radioactive aerosols by vegetation. A!mos. Envir. 4, 57 78. 7. CLARKM.J. and SMITh F. B. (1988) Wet and dry deposition of(:hcrnobyl releases. A?~ture 332, 245 249. 8. (-]OUGHTREYP. ,]., Kmrox J. A. and N{rrCHELL N. G. (1992) Evahlation of tbodchain lransfi:r data fin" use in accident consequence assessment. In Proceedings q/a Seminar on Metho&' and Code.~Jbr Asses,Jng Off-Jle Conrequences oJ" Nuclear Accidenl,~, Athens. CEC (in press). 9. COUGHTRE',"P.]. and THORNEM. C. (1983) Pages 280 320 in Radiouuclide distribution and tramporl in terrestrial ecosystems. Ruthenium. Balkema, Rottcrd am. 10. HANDL~:YR. and BABCOCKK. L. ('1972) Transk)cation of ++'Sr. ~+TCs,and ~'~+Ruin crop plants. Radial. Bol. 12, 113 119. 11. HFwrrr E. ,J. (19(i6) Sand and water culture melhodr in the .~!uqv of plant nutrition, second edition. Commonweahh Agricuhural Bureaux Technical Con> municafion, No. 22. 12. HUNT R. (1983)Planl growth curves: the/hnclional @proach Io planl grmvth anal>is. Edward Arnold, London. 13. Mn,BOURN G. M. and '|'AYLOR R. (1965) The contalnina/ion of grassland with radioactive strontium 1 Initial retention and loss. Radial. Bol. 5, 337 347. 14. ~IILtA.;R (]. ~'V. and HOFFMAN F. O. (1983)An examination of the environmental half-time [br radionuclides deposited t))l vcgetaticm. Illth Pto'r. 45, 731 7H. 15. MOLLm~H. and PR6HL (;. (1988) Caesium traitsport in tbodchains coml)arison of model predictions and observations. In G. DESMV=r, ed. Reliabilil)' o~ radioactive !ran.jer mode[.~. Elsexier Applied Science, l:ondon. 16. Me'LEER H. and PR6HI, G. (1989) The role of seasonal, climatic and meteorological conditions in modit)ing nuclear accident consequences, lu The influeuce o/" wa.~oual conditions on the mdiological
R A I ) I O N U C I A D E R E T f ' N T I O N . LOSS AND T R A N S I ~ O C A T I O N IN W H E A T
17.
Ig.
19.
21).
comequence.~ oJ a nuclear a, cidenl, Proceedit~,qs o/a .VI,LI work.~hop, O E C D . Paris. 1989, PR(iHL (;., I~J(iLI,ER H., JACOB P. and PARETZKE It. (;. (1988i The dynamic radioecoh)gical model ECOSYS a tool tbr tim management of nuclear accidents' consequences, lVlh lnternalional A_'rmpo.~ium ,m radioecolog]': The b@act o/nuclear ori:,in acci&',/~ ot~ /h~ emironment, ('adarachc, 14 19 March l!)8/L Sn:,w G. and BELL.]. N. B. '19891 The kinetics O['C;~ICSiLIIII absorption by roots of\vii|Pdr wheal alld lhc possible conscqucnccs [}31"the derivation ot'soilto-plant transfer thctors tin" radiocacsium. ,]. Era,it. Radioac/. 10, 213 23 I. SUAW (i. and B~,~H,.]. N. B. (1991 (1~nnpclitivc effects of l)Otassiuln and i/llll]lOlli/llll 011 ('a('situla uptake kim'tics in whcat. ,]. Envir. RadioacL 13, 283 296. StI..XW (;., HEWAMANNA R., LIIAA'WItlTE ,1. and BV.LL J. N. B. 11991 Radiocacsium uptake and transh)catioll it] wheat with r(-fiwencc to thc transtk'r t'aclov concept and ion competition effects. ,]. Era,it. Radioa~/. 16, 167 180.
,I01
21. SIMMONDS,l" (198',{) Estinaation of contanfination on plant surthccs due to deposition otradionuclidcs fi'om the atmosphere. In Ecolo2ical aspecl,~ o/ radionuclide re/ea,~e, P.J. (;otrCnTRI,:v,.]. N. B. BELL and T. M. ROBERTS, cds. Special publications series , t the British Ecological Society, No. 3. Bla.:k~cll Scicmific Publications, Oxtord. 22. Vuop, l S. ~19VJ~, Review of evaluations concerning radib',l accidcnt. /tz R~(cttl adz:am'es in react:, accident (oHwquota: a.~,te'~smetl/, ()ECD, NEA CSNI Report N~). 145, l:cbruar\ 19~W,. 23. ~\'ATTt'RSON,J. 1). 1989; Interception and retention o[ aerosols h?, vegctation in relation to their surtScc cuticular characlcristics. Phi) thesis, hnpcrial Colh'gc of Science, Technol~gy and Mcdicin<'. I ;ni\ersity of l:md
[1(1!)1{ ~{172 qo S~.,III i ql !1(I ~ 19!i2 I',[~, ,i.,,l~ I'~¢'.- I.Id
R E T E N T I O N , I,OSS A N D T R A N S I , O C A T I O N OF R A D I O N U C I , I D E S APPLIED TO FOLIAR S U R F A C E S 01: W H E A T G. SHAW,* M. J. MINSKI* and J. N. B. BELLt * (',('ntr(' tot :\nalx|ical Rcse,u'ch in the Environment, lmpcrial (:ollcg(. at| Silx~ood Park. Ascot. Berkshire SI,,5 TI'E, I . K . + l ) q ) a r m w m ()fBiulogy, lml)('vial C()llcgc at Sih~()od l'ark. ,\sc~)t, B('rkshirc S1,5 71)Y, ('.K. ', Received I 0 l)~'ccmbcr 19!) 1 : aeeepled i1~ ~,'e'i~ed,/brm ',3 ,]u(r 19921 SHAW (;.. MINSKI M..]. ao.d BLI:I: ,J. N. B. Relelllion, /os~ alld hamloeatioll q/radiotlucli
INTRODUCTION lhRECT c o n t a m i n a t i o n of plant ti:)liar surlaces m a y o c c u r as a result o f a m l o s p h e r i c d e p o s i t i o n of r a d i o n u c l i d e s arising from t h u h c o n d i t i o n s w i t h i n n u c l e a r p o w e r reactors. R a d i o a c t i v i t y ill Ihcsc cases m a y ])e associated with aerosols as a dry deposit or with r a i n c h o p s as a Yvet deposit: it was d w latter w h i c h resulted in the m a j o r p o r t i o n of g r o u n d a n d plant c o n t a m i n a t i o n in Britain and o t h e r l)arts ot'X'Veslern E u r o p e tblloxx ing the (:her3!)[
nob xl a c c i d e n t . ~ W h a t e v e r the p h y s i c o - c h e n l i c a l n a t u r e o t the deposit, o n c e a planl surlhce b e c o m e s c o n t a m i n a t e d with r a d i o a c t i v i t y two p o t e n t i a l p a t h w a y s into the f o o d c h a i n are possible: (i~ c o n s u m p t i o n of c o n t a m i n a t e d forage by r u m i n a n t s , and
392
G. SHAW el al.
nuclide burden with time, whether one considers total contamination burden or the acuvitv per unit of surlhce area or biomass. 'l'he term "tield loss'" has ]tee,~ coined by CHADVv'I(IK a n d CHAMBERt.aIN 5 tO describe this overall timedependent reduclion in contamination, which is thoughl In be due to a variety of biological and cnviromnental processes including plant surthce weathering, "(lilution" by plant growth, and absorption and translocation to organs undex eloped ill the time of contamination. As a result of these processes potential ingestion dose will decrcase x.tith time tbllowing a contamination event at a rale which will be determined I)7 the complex of ti~ctors responsible tor the overall loss of activity. Following .NIII,BOURNand TA','LOR'S'J:~ study wilh ~:~'Srthis rate is usually quantitied tbr the purposes of modelling by assigning it a iielct loss halt:lit;~ of approximately 14 days, a h h o u g h it is clear front the literature that such halt'-liti~ wdues can v a r \ considerably and that they are especially influenced 1)y seasonal t:actors (tbr a review of tield loss half-live values and associated uncertainties see NI1Lt,ER and tlOFFMAN It ). CO[:(;HTRF,Y el a[. 'u have highlighted the diflicuhies i n v o h e d in ol)taining reliable estinlates of loss half-lives ti'om tield measm'ements. In particular these authors stressed the need [br complementary data on plant growth and precipitation after initial interception in order to identit\.' the causes and their quantitative relationships to the observed rates of loss. A turther interl)remlive ditBcuhy, according to (1oughtrey and colleagues, is the evaluation o f " a p p r o p r i a t e models tor telention fron] field data because of the conlbunding etl'ects of fbliar absorption and subsequent translocation, or root uptake from the associated soil pool". A notable study on the translocation of radionuclide translocation from tbliage to developing i)lant organs was that ot A A R K R O G - ~,~.hich demonstrated a strong time-dependency between the activities of several fission and activation products at the time of their application to cereal leaves and their recovery in the grain at harvest. Studies elucidating relationships between radionuclide retention, loss and translocation under field conditions, with respect to detailed phmt growth characteristics, however, have not previously be.en reported. The experiment described in this
paper was carried out d u r i n g the s u m m e r o f 1989
at Imperial College's field station al Silwood Park, m'ar Ascot IU.K.) attempting to link quantitatively these thciors tbr a wheat crop growing under enxironmental conditions representative of those of southern Britain. MATERIALS AND METHODS
:at Experimenlal In order to thllow time courses of radionuclide behaviour in wheat lollowing an external cnnlamination event it was necessary to nse a contamination method which resulted in the heaviest and mosl unilbrm covering of plant surt~aces with radioactivit}; planls were theretbre immersed in solutions containing cocktails of the radionuclides to be studied. In order to t~tcilitate this process spring wheat phmts ( Trilicum aeslivum, cv. Tonic) were grown individuany in polyethylene containers containing a 3:1 peat :sand mixture, amended wilh CaC():~ (sufficient to bring the pH of the compost to neutrality) and R orison's nut rient solution, la \Vhen sealed at each end these containers provided a discrelc rooting environment tbr individual plants which precluded the possibilit} of the compost becoming radioactively contanfinated by washoff fi'om the leaves (this was conlirmed by radioassay of the compost, t?om which roots had been separated, at the end of the experiment) the root uptake pathway of radionuclides was theretbre eltbctively excluded from the experiment with clean water being provided Ihrough an irrigation port. A 25 1 liquid co~ ktail ofS:'Sr, "~tiRu, 117Cs, ~'4Mn and ~'°Co was prepared t~om soluble, carrier-ti'ee chloride sourccs of each radionuclide, obtained ti'om Amersham International, U.K. In the case of the l"~;Ru a small (2/~M) amount of nonradioactive carrier was added to prevent irreversible sorption of I°~;Ru onto the walls of the polyethylene containers used in the experiment. The final cocktail contained individual radionuclides at a specific activity o f 5 0 0 kBq/1. Contamination of plants was carried out at three intervals during the growth cycle of the crop (contamination episode Nos. 1, 2 and 3 at 42, 66 and 93 days after seedling emergence, respectively) and was achieved by inverting and completely immersing individuals in the radioactive solution; immersion
R:kl)I()NU(:LIDE RETENTI()N, I,()SS ANI) 'IRANSIX)CATI()N IN WHEAT was fiir a p p r o x i m a t e l y 30 sec d u r i n g which time the solutions were continuously stirred. After lifting c o n t a m i n a t e d plants [?om the radioactive sohltions t h % were laid on their side to air dry in o r d e r to prevent any radioactive runoff" tiom c o n t a m i n a t i n g the compost in which the plants were growing. I m m e d i a t e l y after the plants were dry a I)at('h of 10 individuals was sainpled to d e t e r m i n e the initial radionuclide b u r d e n associated with the slioot tissues. Plants wen" sutisequently grown in groups of 10 individuals, fully exposed to a m b i e n t e n v i r o n m e n t a l conditions [see Fig. 6 ,:topoi and harvested at the intervals indicated in Fi K. I. T h e overall c r o p p i n g period lasted 120 d a \ s . G r o u p s of 10 plants were sanipled at each harvest wilh the entire shoot system being taken tbllowing excision at the slem base. After d r y i n g at {{()"(: tot 24 hr, plants \~.ere divided into straw, c h a t f a n d grain , the chaff" bein.~ separated [,tom the grain l/v riotation ()n deionized water) which were weighed. Each s a m p h ' was t h e n / r o u n d to horno/eneity in a I)lender, sul)-samples packed into i)olyethylene capsuh:s of C()liSlall[ oeOlTIClry a l l d a s s a v c d for radioa('tivitx t) 3 7-ray spectrometry using a (;el,i detector coupled to a multi-channel anah.ser N u c l e a r D a m hw.). Actual activities (if each radionuclid(' \vere ('alculated t`rom it calibration
3
D
o--o
[]
0
0
0
0
0
"o
o
Q. 0 cO
curve d e t e r m i n e d from a mixed 7-standard {obtained fiom the National Physical L a b o r a /ory, U.K. ) o['identical geometry to the unknown samples. All activities were back-corrected tor radioactive decay (to the nearest dav i to the date of initial a p p l i c a t i o n of the radioactive cocklail to leaf" surfaces. b)
Dala ana!>i.s (i ! l'lanl gro~clh. Both the total biomass of shoot
tissues :sti'ax~, chaff and grain) and the mass o[ grain produced were recorded tor each illclix (dual plant used in the experiment. T o allow a quantitative interpretation o1 the eflT:et of growth ( m the loss of radionuclides t`rom leaves and their subsecluent translocation to grain, logistic curves were titted t(~ lhe d a t a ])v a least squares method. Overall growth curves were obtained [,tom these fits as well as their derivatives which represent ['unctiol> of relative growth rate of total tissue and grain. ,ii~ -l"ic/d /,.~.('. l ) u r i n g a stochastic llil('lcar c(mlanlin,tti(m excnt plant tissm's receive a discrete, initially external, radionuclidc t)urdt'n assuming a relatively slow rate of tool uptake of radioactivity t`reshly deposited to the soil surfacei~. CHAMBERI,AIN ~ modelled lhe time-depelldeni loss of surli/ce radioactivity from plains in such circtunstanccs using all exponential model identical to that describing the radioactive oh'ca\ process itselt~ In the present study d a t a relatin K lo the oxerall loss of r a d i o a c t i x i t ) with tiIrlC [iom c o n t a m i n a t e d tbliage were [it ted by a least s( Iuares incthod to dw [bllowing equation l:, -: t:, o i ( "
[]
E 0
0
0
o
{o
4'0
i
60
8'o
I
loo
I
1"1(;. 1. lmyout of experimental time cours('s employed Io dctcrnlinc loss and translocation of radionuclidcs will1 time tbllo~ing tbliar application t() x~hcat plants at dn('(' stages within th(' c.op's growth cych'. The beginning of the experiment was taken iobc the ti,nc of germination oflhc crop 126,]unc 1989); contamination ('pisodc Nos. I, 2 and 3 (indicatcd I)) the square plol svmholsl wcrc at 42, 66 and 93 days, respectively, t})l]owi,lK this date with subscqucnl salnpling al finlcs indi('atcd 1)v circuh~r plot synibols.
J'~
ili
Her(!, F, is the fractional retention of any radionuclide at any harvest interval ~initial vahlc F, o ) - 1.0 ~, calculated [ioni
12o
Crop age (days)
393
I;, -
specilic a c t i v i l y o[tbliage at time of harvest (*) specific a('tiviiy of tbliage at time ( ) f c o n t a m i n a l i o n
{* back-corrected tor radioactive decay to time of COil(am(nation ; and /q~ is a rate constant of loss of activity t`rom fi)liage. Vahies (d'kl, d e t e r m i n e d t`rom least Stluare
G. SHAW et al.
394
fits to the model were used to calculate biological h a l f lives of radionuclide retention by ff)liage. (iii) Translocation. Translocation of radionuclides fi'om [bliage to grain may only occur once absorption has taken place across the leaf surlhce and the nuclides become available ibr loading into the plant's vascular system ( S H A ~ r el al. '~"' have shown in the case ofradiocaesium that it is the process of absorption and not translocation which is rate limiting in a physiologically unrestricted plant). Assume (a) that the rate of absorption across the leaf surfhce is proportional to the specific activity of the contaminating radinnuclide (i.e. first order kinetics assumed fbr carrier-li'ee radionuclide contaminants, after SHAW and BV.LL~la), (b) that the loss of leaf surthce activity is exponential with respect to time [as in Equation ( 1)] and (c) that further loss of activity (due to weathering and growth dilution) is likely to occur fi'om the grain once the nuclide has t)een transh)cated there, then the fbllowing conceptual model is applicable: k,:, e ~' ~ [[R].,,,,,,]
--+ k,.
(3)
where [R]v<. is the specific activity of a radionuclide (R) in the grain, kli, is a rate constant of foliar absorption [initially kldo)], 2 is a decay constant relating to the loss of tbliar specific activity (this should be the same as k, above) and /i~v is a rate constant of radionuclide loss ti+om grain. The tollowing equation describes the timedependc'ncy of the radinnuclidc specific activity (corrected tbr radioactive decay) within the grain according to the model [1¢1(1) =
k,~,(o ~ (e ; " - e klv /+
/', '/ ~ .
(4)
Grain translocation factors defined as specific activity of grain at time of harvest (*) specific activity of leaf and stem tissue at time of contamination (~) (5) (* 1)ark-corrected tbr radioactive decay to time of contamination ~grain was undeveloped at the time of contamination)
were titted to this model fbllowing the procedure outlined by I,'~rHIGKERand SCHUI,TZ. '24 RESULTS
(a) Planl growlh The growth of wheat under the experimental culture conditions appeared normal with plants completing their litb-cycle within the adopted cropping period of 120 days. The total hiomass of the crop increased throughout the experimental period; instantaneous relative growth rate (RGR, defined as l/l.V'dW/dl, where H7 is biomass; HUN'(1~) declined at each period (Table 1). The growth curves tbr total biomass and grain in Figs 2 and 3(a) show a characteristic sigmoidal shape, with grain production hecoming measurable after 40 days. The RGR fbr total biomass peaked at ~.55 clays [Fig. 4 (top)] whereas that to( grain lagged slightly behind this at ~ 70 days [Fig. 3(a)]. Grain eventually accounted tbr approximately 1/4 of the t()tal biomass at final harvest ( 120 days, see Fig. 2). (b) Reten6on and loss ~radionuclides bL/bliage (i) Field loss half-lives. Following contamination the initial specific activities of each radionuclide associated with wheat tissues were of the order of 75 kBq/kg, with an average coefficient of variation in this value between replicates of3.80i,. All radionuclides applied to wheat tissues decreased with time following application at three stages in the plants' ontogeny. Table 1 shows the results of a series of least squares fits of experimental data to the exponential loss model [Equation (1)1; biological half-lives derived tbr each radionuclide during the experimental period are shown. The associated correlation coefficients are high (miuimum r e value = 0.841, P < 0.001) implying that the data are adequately described I)y this model; the tield loss half-lives given in the tat)le should theretbre give a reliable indication of the rates of loss of activity tollowing each contamination event. Some variation in halt:-li[~ was apparent betwccn radionuclides at each contamination episode, although values were generally similar within data sets derived fi-~r each contamination episode. Each radionuclide (with the exception of ""+Ru, which tbll below detection limits at sam-
R , \ I ) I ( ) N U ( : I J D E R E T t ' N T I ( ) N , l,OSN AND ' I ' R A N S I , ( ) ( : A T I ( ) N
IN WHE,XT
395
Table 1. "'Fidd /,,~.~" ha(/:lives (i, dq>) q/radi,lmclh/e.~ ap/died Io /idiar ~,!/hct'., q/zchcal a,~ a ,~o/uh/c cm'klail al Ihree ~lage,s q/plan! grm~'lh ( I~m tamination cpiso(h'
RCR ,per day,
No. I
0.0t-2
No. 2
0.040
No. 3
0.018
H,u vest
0.009
~"Nr
'""Ru
9.94 0.953) 26.08 ,0.949 19.80 ().991,
16.86 :0.841, NA 13.3}{ ~0.983 ~
~;'('.s 15.{)4 ~0.966a 23.70 , I).':)80~ ll).(i7 ,().995 ~
~i~lll
""Co
15.!)7 (0.942 41.90 0.994 1!t.71 : 1.()()~
15.97 (0.898 35.90 (0.995 } 18.03
{0..qg2
l"igures in brackets represent correlati¢m coefl~ciems i~" values, n = 301 assocmted with h'asl squares lits ¢,t'experimental dam Io a modcl of exponential loss [F,quation :1)]. R(;R ix lfic i n s l a n t a n e o u s relative girox~th rate a! each e(mtalnin;.llion episode determined by fittin~ a logistic curve to tola] 1)iomass data tblh}win.< the procedure o['Hu',:'r, i:! plin£ periods followin£ the second c o n t a m i l m t i o n episode) disphtyed the hmgest half-litb tbllowing the second c o n t a m i n a t i o n episode at {36 days. StMn a n d ""(Io exhibited the longest biological halt:lives of all the nuclides e x a m i n e d , r e a c h i n g a m a x i m u m of 42 a n d 36 days, respectively, tbll o w i n g the second c o n t a m i n a t i o n episode. H o w ever, their field loss half-lives xxere b r o a d l y similar
Total 4.0
3.0 E c~ 2.0 i1)
0
0
40 80 Crop age (days)
120
l:m. 2. (;rowd~ curves ti)r total biomass and grain during the experiment showing typical sigmoidal shape. I,ogistic thnctions were fitted to the data tbllowing a linearization procedure given by HVXT. ':~' \Tertical Bars represent SEM ' n - 10.
to those of the other r a d i o n u c l i d e s tbllowing the first a n d third c o n t a m i n a u o n episodes. iii) Rdalion.ship helweenfield lo.ss and relalh)e growlh tale. F i g u r e 4 (top, indicates that the R(iR of total biomass t h r o u g h o u t the e x p e r i m e n t a l period showed a m a r k e d m e a n decline from the tirst c o n t a m i n a t i o n episode (No. l) until the end of the e x p e r i m e n t a l period. T h e ox erall m e a n daily decline in R(;R over the period tbllowing each c o n t a m i n a t i o n episode up to the end of the experim e n t (i.e. RGR at c o n t a m i n a t i o n m i n u s R G R at harvesl, divided b~ the time i n t e r v a l in da,vs~ is plotted in relation It) the m e a n lield loss half-lilb tot all r a d i o n u c l i d e s over that period in l:ig. !b o t t o m ) . T h e r e was a s t r o n g positive c o r r e l a t i o n b e t w e e n these two properties, suggesting that field loss ['rein the p l a n t s ' tissues is to some extent controlled I)\ v a r i a t i o n in p l a n t RGR d u r i n g the course of the g r o w i n g season. T h e relatively small degree of" v a r i a t i o n in m e a n field loss halfLlif~' in He-. 4 i l)ottom', also suggests that tile r;tte ()floss of'radioactivity tixma the p l a n t tissues in this study is i n d c p e n d e n l o(" the r a d i o n u c l i d e .
c ) 7"ramlocation ~!/ radionudide,, to grain (i) t'allerm ~j lramlocalion ~J individual radio,uclich',~. No m e a s u r a b l e t r a n s l o c a t i o n of ~:'Sr or t°'~Ru fiom tbliage to g r a i n occurred d u r i n g the e xperin~ent. T r a u s l o c a t i o n of 'CCs, -"lMn a n d "°(io tiom [bliage to grain [as m e a s u r e d by the grain translocation thctor, F,,, E q u a t i o n (5)1 was d e t e r m i n e d
396
G. SHAg" el aI.
A
0.05
e"
No.1
No.2
No.3
(a) Grain biomass (W).,.
0.04
1.C
"0
:i
!
Q:
0.6
Q: 0.02
L9
E= °
~.~
0.03
n
0.8
"O
0.4
E
0.01
0.2
0.%
0.0 20
O
7
40
60 80 Crop age (days)
100
4'o
8o Crop age (days)
120
40 (R 2 = 0.991)
31] (b)
•~
~ z
m .£ = 0.8
~ ~
RaR
e-
w t h dilution
-"
20 u_
0.6
1C
0.2
z~
0.0 20
,
i
4.00e-4 5.00e-4 6.00e-4 Mean daily decline in RGR (per day 2)
o.4
E~
No.3
"O
4()
60
80
100
120
Crop age (days)
FIG. 3. (a) Plot of growth data tbr grain. The fitted growth curve (/2 = 0.919) is a logistic function a
H/
1 + be "
(i)
where W = biomass (g dry weight), a, b and c arc constants (1.02, 23813 and 0.1097, respectively) and t is the crop age (days). The plot of relative growth rate (RGR) is the derivate of this function l
dW abce '~ IV" dl -- (1 + be ")~'
(ii)
with constants as above. (b) Grain translocation factors (F,,.) following contamination episode No. 1 plotted in relation to the fitted RGR function in (a). F~,.values are means derived from the data tbr ~:~TCs,~'~Mn and "°Co normalized to the maximum at 66 days. The growth dilution curve is calculated as the reciprocal of the grain growth curve [Equation (i)] and is normalized to the 66 day value.
Fro. 4. (top) Fitted function of relative growth rate tbr total biomass calculated as in Equation (ii). Constants (a = 4.22, b = 8.73, c = 0.042, r 2 = 0.84) were derived ti'om total biomass data shown in Fig. 2. Nos. 1, 2 and 3 indicate contamination episodes 1, 2 and 3 (see Fig. 1). (bottom) Correlation of mean field loss halt:life tbr all radionuclides following each contamination episode with the mean daily decline in RGR over the field loss period. Horizontal and vertical errors bars represent SEM associated with mean estimates of field loss halt: life and daily decline in RGR, respectively.
following the first c o n t a m i n a t i o n e v e n t at 42 days. F , values for 1:~7Cs, 54Mn a n d G°Co r e a c h e d a m a x i m u m (in the o r d e r 137Cs > ~°Co > 54Mn) shortly after a p p l i c a t i o n a n d t h e r e a f t e r d e c r e a s e d with time. F i g u r e 5 shows these d a t a in r e l a t i o n to E q u a t i o n (4); the constants d e r i v e d for the t h e o r e t i c a l r e l a t i o n s h i p are g i v e n in T a b l e 2. It was p r e v i o u s l y stated t h a t if E q u a t i o n (4) was to a p p l y to the e x p e r i m e n t a l d a t a , then the rate c o n s t a n t o f field loss d e r i v e d for E q u a t i o n (1) (kit) should be the s a m e as t h a t d e s c r i b i n g the
RADIONUCLIDE RETENTION, LOSS AND TRANSLOCATION IN WHEAT
397
(ii) Relationship between translocation of radionuclides and grain development. Figure 3Ca) shows a
10 c
101
37Cs
Ft,
o
2'o
t (days)
do
do
logistic growth curve fitted to the data [br grain development over the experiment (r2= 0.919) and includes the derived function tbr instantaneous relative growth rate. Figure 3(b) indicates that the mean normalized F,, values ~)r J~TCs, ~;')Co and 54Mn w e r e closely related to the latter function. The similarity between these two curves suggests that the process of translocation of radionuclides from leaves to grain in wheat is strongly related to the rate ofbiomass increase of the grain.
Fro. 5. Translocatiou time courses tbr HTCs, ~'~Mn and ';'~Co following contamination episode No. 1. The points represent experimental data, each comprising the mean F, value obtained tor bulked samples from 10 plants. The fitted curves are plots of Equation (4) calculated using the constants presented in Table 2.
(d) RaiT~all during the experiment The rainl~all pattern during the course of the experiment was culled from the regular meteorological records at Silwood Park, and this is shown in Fig. 6 (top). The daily average rainfall from 26 J u n e 1989 to 23 October 1989 was 1.68 ram/day. During this period, however, there were three exponential decline in radionuclide input rate to rainlhll events of approximately, 20 m m / d a y or grain in Equation (4) (2). The close similarity greater, and several which were between the between these two derived constants tbr ~:~TCs, mean value and 10 ram/day. Rainfall did not ;4Mn and (s~Co can be seen in Table 2, suggesting appear to be meaningfully correlated with field that Equation (4) is an appropriate model for the loss halflif~" [Fig. 6 (bottom)]. description of the tin'm-dependent translocation of these radionuclides within wheat plants. DISCUSSION Following translocation to grain the rate of decline in concentration of radionuclides within The potential radiological dose to man via the grain tissues tbllowed the reverse pattern to the consumption of contaminated vegetable their m a x i m u m F,, values; thus, 54Mn was lost materials is known to be dependent on a host of most rapidly, with ~°Co being lost at an intert~.ctors which can be broadly categorized under mediate rate and l:~7Cs being most strongly the term "seasonality". c~ One of the major maniretained. This loss appeared to proceed expo- festations of seasonal variation in environmental nentially with time, according to Equation (4). variables such as temperature and irradiance is the cycle of plant growth and senescence which
Table 2. Rate vonstanl,~ derivedfrom least square,~,fits ~!/ consequently aft?ets ingestion dose to man/4 I f predictions ofradionuclide flux within tbodchains erperimental data to Equations (2) and (4) Rate constant (per day) Radionuclide 1~7Cs ~,.iMn '"'Co
k,v
).
kt~
1.62 15.0 8.60
0.044 0.038 0.040
0.046 0.043 0.043
The constants kl~ and ,:. refer to Equation (4) and k, is the rate constant in Equation (2). If Equation (4) is to apply then the constants 2 and k, should be equal.
are to be suceesslul then plant growth itself should be a fS.ctor implicitly taken into account by radiological assessment models. In order to achieve this wc need to understand the nature of the relationships between plant growth rate, the biokinetics ofradionuclides within plant tissues and environmental thctors such as rainfall intensity and duration. The initial capture of radioactive fhllout from the atmosphere will vary seasonally according to the extent of ground cover by vegetation. 2
398
(;. SHAW el al.
301 23/10/89
26/6/89 I E E
20.
¢-
Mean= 1.684 -
~)!,0
200
250 Julian day
300
3so
40
30 ¢u ¢-
"~ 20 "O u_
lO
50
i
100 Cumulative rainfall during period following contamination (ram)
t
150
Flf;. 6. (top) Rainfall during the expcrimental period the datcs delineating the beginning and end of the cropping period are indicatcd. (bottom) Relationship })elwccn mean field loss half-lit~ and the cumulative raintM1 during the field h)ss period.
CHAMBERI~AIN :6: has previously recognized this in a m a t h e m a t i c a l [brmulation of the dependence of the interception fraction on several variables and the idea hits also been i n c o r p o r a t e d it) radiological assessment models such as ECOSYS/l:''17 The m e t h o d o l o g y used in the current study was not designed to investigate this initial capture of
atmospheric deposits; rather the lilt( of deposits alter c a p t u r e by leaf surtaces has been explored as the effects oftactors such its plant growth during this period have 1 l O t previously been a d e q u a t e l y addressed. T h e final radionuclide I)urden of edible tissues igrain) at harvest will
resuh from a balance between loss t~om the system ("fi(:ld loss") and retenti()n tbllowing absorption and translocation, each of which ~,ill I)e aft'coted by such filctors fbllowing an external conl a m i n a t i o n even t. T h e rate oflield loss ofradiouuclides from phmt tissues has bccn shown by (IHAI)V¢I('K ) l i d (.]IIAMBE.RI,AIN'" l() Vill'y seasonally, with observed loss rates ill sumlner exceeding those in ~Vinler. These authors expressed the loss of radioactivily li~om a grass sward on a g r o u n d area basis, \ d 6 c h rules O u t growth dilution as a thctor c o n t r i b u t i n g directly to the observed loss. T h u s it is likely thal some plant g r o w t h - r e l a t e d elt~'ct other than simple dilution was responsible lot (]HAl)WICK ill)(] (IHAMBF,RI,AIN'S observations, a h h o u g h flwy did not speciti(all) allude to this possibility. In tile present study tield loss rates t?om wheat {be live fission and activation products have been shown to t)e a d e q u a t e l y described by a simple exponential t'unction and the absolute halt:-lit~: values ohtained in this study c o m p a r e with those ill several other studies (e.g. SIMMONI)Sez) . As a means of quantit}'ing radionuclide persistence ill vegetation systems, then, the use of a simple biological haltZlilb seems just)liable though it is also clear that this half-lil;~r will vary according to the stage or, rather, lhc tale of growth of the planls' tissues. Plants rarely exhibit linear growth patterns with time and consequently the rate of growth shows a time-det)endem variation. M a n y tune)ions have been invoked to describe these sometimes complex patterns but it is tilt" logistic f u n c t i o n which classically has been used to describe the typical sigmoidal growth curves seen in the present study. T h e derivate of this relati(n> ship provides us with :ill inslantaneous measure of growth ("relative growth rate" or RGR) which can be considered to be potentially the most useful p a r a m e t e r tor incorporation into radioecological models. The fimcti(mal relationship of R(;R with crop age needs to he understood since, because it is complex [Fig. 4 (top) l , all(re is no simple correlation ltetween instantaneous ,~rowth rate at the time of c o n t a m i n a t i o n and the sut)sequent rate o1" loss. However, it strong correlalion does exis! between the overall dail> decline ill R G R lbllowing t o u t ) h a ) n a t i o n and ticld loss haltklifi: [Fig. 4 (hot)ore)l, thus reintorcing the notion of
RAI)I()NU(:IAI)E RETENTION, IX)SS AND TRANSI,O(:ATI()N IN WHEAT a causal relzttionship between plant growth and field loss. M a n y factors have been put tbrward as possihle reasons behind the field loss process, a h h o u g h it seems intuitively likely that rainlhll should phty a m a j o r role. All the raintM1 events recorded d u r i n g the course (~['the current study were titirh unitbrmlv distributed throughout the experimental period [Fig. 6 Itop) ] and it is therefi~re likely dmt their eltix'ts on radionuclidc retention/loss xs'ere e \ e n l y sl)read t h r o u g h o u t Ill(- expcrinaent. T h e r e \x as n() o h \ i o u s correlation hetween the altlOUiH of rainfall in till' period lollowing external cont a m i n a t i o n and tile [ield loss half-lifb d u r i n g this period, hmxevev [Fig. (5 (bottom)]. (Mmparison o [ t h i s resuh \sith that shown in Fig. '1. ,hotlonl) suggests that plant growth itsel['had it much more i m p o r t a n t etti'ct on overall tieht loss than did v a i n l h l l . . \ theory of the ticld loss mechanism which has survived since the mid-1960s is that of epicuticular wax m e d i a t e d loss proposed by ~hI,Bt)URN alld
"I'AYI,(IR. l:;: T h e
loss ot' \x:txv
material [i'~ml phmt leaf surlitces is known to i m r e a s e with the rale of phmt growth and although, as it causal mechanism /)f" field loss of vadionuclidcs, this theors still awaits conclusive I)roofthe dcl)cndency of the biological hal filth of cxlClmll deposits on RGR lends [hrdaer circumstanlial evidence in its thxour. Furtiwrmore, the r c l a t i v c h narrow error limits around the nleatl values in Figs 4
399
dose assessment its it m a y result in a significant increase in c o n t a m i n a t i o n of the grain w i t h tnne t b l l o w i n g tile initial d e p o s i t i o n e v e n t , as predicted hy E q u a t i o n +:4', +see also AARKROG 2 ). This model indicates that the input term is proportional in a simph" m a n n e r to the degree o f h ' a f ,~urlace contanfination which, as we have ah'ead} seen, is Stll)lecl to an exponential decline sxilh time; if this model is indeed applicahh" then the m a g n i t u d e of tilt: raw constams fin" Ihc loss of a c t i v i t \ from the external surlhces of the plant field loss~ and the decline in al)sorption rate with time should hc identical. T h e similarity hetwecn these constants in T a h l e 2 gives confidence in dw applicahility of F,quation (4) to tllis situation. Translocation to grain in the cturent experiment was clearly radionuclide specilic, with Sr and Ru exhibiting no measurahh" degree of movement within the tissues analysed. Sr has prev i o u s h heen shown to be r e l a t i v e h immobile in hiological systems-' while COt;(;HTREY and "I'HORNE !' have reviewed d a t a fi'om tlAxDLv,Y and BAB(:t)(:K I~ and ;~ARKROC-I which suggest that, ti)llowing a p p l i c a t i o n as a spray, m"Ru is subject to only lilnited tbliar absorption. T h e mohilitx of the r e m a i n i n g three radionuclides in die current study is p r o h a b l y it reltcctioll of their esscntialhy, in t h e c a s c s o f "(illCo a n d - ' l , \ l n , or the ability to mimic an essential clement in the (asc ,,f" I ~r{:s. i!, Translocati(m of radionuclides wilhin the plants" tissues ill this stu¢h was also clearly dependent ttptm R(;R. XI{?LI,ER and PR6m, t~; have previously shown Ihat translocation of radiouuclides t~ grain will reach it m a x i m u m at "torl~. filling" which can I)e taken to coincide with the period of exponential growth of grain tissues. Indeed, d u r i n g the present study the maximal mcan grain translocation thctor (F,, ; tbr C:lch o{" the radionuclides which exhibited measurahle translocation {~7(Is, '"'Co and ;*Mnj was almost synchronous with the peak in grain R(;R [l:ig. 3,hi], 'Fhe suhsequent decline in /"~, xxith u s e f a n he altril)uted to scveral [itctors, v~;.~. {i, a redu('lion in the int)ut rate of translocate to the o-r,till tissues ,:l/rOl/ortional to till' lield loss of vadioactivitx ['l'Oll/ the external tissues,, ii~ dilution of the activity translocated to the tissue duc to ~rain expansion ,:"tilling", and :iii:, r c n l o \ a l of activity tiom the grain due t . lield
G. SHAW el al.
400
loss. It may be considered that the latter process is almost negligible as the grain is effectively protected tiom the n o r m a l processes of weathering by the husk. Figure 3(hi includes the growth dilution curve tbr the grain (calculated as the reciprocal of the growth curve normalized to the 66 day F,. value) a n d this indicates that the rate ot" loss o[" activity from the grain was less than that expected if lranslocation had ceased and only growth dilution was occurring. This dilution curve does not take into account the rcmoval of activity due to field loss fiom the grain which, even if small, would be expected to result in an even higher rate of loss. It may be concluded that the main source of"loss" of specific activity fiom grain is in thct a general reduction in the rate of translocation of activity, related to a decline in both the input term (i.e. external c o n t a m i n a t i o n ) and growth r a l c . Thc rate COrlstant klg in Equations (3) and (4) is an empirical parameter which includes growth dilution and anv other unspecified "loss'" processes. While the derived values tbr this conslant appear relatively large (Table 2) it may be concluded that this "loss" is mainly apparenl and that Ollce translocated to the grain the major part o[" the lola/activity is conserved within the grain tissue. If" this is the case then these tissues could represent a seasonally variable sink of radionuclides within agricuhural ecosystems.
Ackuowledgement The work described in this paper was carried ()tit as part of a study funded by the Ccntral Electricity Generating Board (U.K.) and is published xxith the permission of Nuclear Electric (U.K.).
REFERENCES 1. :~ARKRO(;A. i 1975) Radionuclidc levels in mature grain related to radiostrontium contcnt and timc ofdircel eontamiimtion. HIIh P/l>. 28, 557 562. 2. AARKROG A. 11983) Translocation of radiomwlides in cereal crops. In Ecological a@eets of radionuclide relea.w, P. ,J. (]OU(ltlTREY, J. N. B. BELL and T. M. ROBERTS, eds. Special puhlicati
aquatic environments and acquisition of data tbr that purpose ("VAMP"). IAEA, Vienna. 4. BOERI(,. (1989) Environmental processes and parameters influencing tile consequences of all accidental release of radioactivity: weather and season. In The iq/tuence ¢?/~ea,~onalconditiom on the radiological con,sequeace,s of a nuclear accident, Proceedings o/'a NEA work.~h@, OECD, Paris, 1989. 5. CHADWICK R. (I. and CHAMBERLAINA. (]. (1970) Ficld loss of radionuclides ti'om grass. :llmos. Envir. 4, 51 56. 6. C~HAMBFRLAIN A. (], (197/)) Interception and retention of radioactive aerosols by vegetation. A!mos. Envir. 4, 57 78. 7. CLARKM.J. and SMITh F. B. (1988) Wet and dry deposition of(:hcrnobyl releases. A?~ture 332, 245 249. 8. (-]OUGHTREYP. ,]., Kmrox J. A. and N{rrCHELL N. G. (1992) Evahlation of tbodchain lransfi:r data fin" use in accident consequence assessment. In Proceedings q/a Seminar on Metho&' and Code.~Jbr Asses,Jng Off-Jle Conrequences oJ" Nuclear Accidenl,~, Athens. CEC (in press). 9. COUGHTRE',"P.]. and THORNEM. C. (1983) Pages 280 320 in Radiouuclide distribution and tramporl in terrestrial ecosystems. Ruthenium. Balkema, Rottcrd am. 10. HANDL~:YR. and BABCOCKK. L. ('1972) Transk)cation of ++'Sr. ~+TCs,and ~'~+Ruin crop plants. Radial. Bol. 12, 113 119. 11. HFwrrr E. ,J. (19(i6) Sand and water culture melhodr in the .~!uqv of plant nutrition, second edition. Commonweahh Agricuhural Bureaux Technical Con> municafion, No. 22. 12. HUNT R. (1983)Planl growth curves: the/hnclional @proach Io planl grmvth anal>is. Edward Arnold, London. 13. Mn,BOURN G. M. and '|'AYLOR R. (1965) The contalnina/ion of grassland with radioactive strontium 1 Initial retention and loss. Radial. Bol. 5, 337 347. 14. ~IILtA.;R (]. ~'V. and HOFFMAN F. O. (1983)An examination of the environmental half-time [br radionuclides deposited t))l vcgetaticm. Illth Pto'r. 45, 731 7H. 15. MOLLm~H. and PR6HL (;. (1988) Caesium traitsport in tbodchains coml)arison of model predictions and observations. In G. DESMV=r, ed. Reliabilil)' o~ radioactive !ran.jer mode[.~. Elsexier Applied Science, l:ondon. 16. Me'LEER H. and PR6HI, G. (1989) The role of seasonal, climatic and meteorological conditions in modit)ing nuclear accident consequences, lu The influeuce o/" wa.~oual conditions on the mdiological
R A I ) I O N U C I A D E R E T f ' N T I O N . LOSS AND T R A N S I ~ O C A T I O N IN W H E A T
17.
Ig.
19.
21).
comequence.~ oJ a nuclear a, cidenl, Proceedit~,qs o/a .VI,LI work.~hop, O E C D . Paris. 1989, PR(iHL (;., I~J(iLI,ER H., JACOB P. and PARETZKE It. (;. (1988i The dynamic radioecoh)gical model ECOSYS a tool tbr tim management of nuclear accidents' consequences, lVlh lnternalional A_'rmpo.~ium ,m radioecolog]': The b@act o/nuclear ori:,in acci&',/~ ot~ /h~ emironment, ('adarachc, 14 19 March l!)8/L Sn:,w G. and BELL.]. N. B. '19891 The kinetics O['C;~ICSiLIIII absorption by roots of\vii|Pdr wheal alld lhc possible conscqucnccs [}31"the derivation ot'soilto-plant transfer thctors tin" radiocacsium. ,]. Era,it. Radioac/. 10, 213 23 I. SUAW (i. and B~,~H,.]. N. B. (1991 (1~nnpclitivc effects of l)Otassiuln and i/llll]lOlli/llll 011 ('a('situla uptake kim'tics in whcat. ,]. Envir. RadioacL 13, 283 296. StI..XW (;., HEWAMANNA R., LIIAA'WItlTE ,1. and BV.LL J. N. B. 11991 Radiocacsium uptake and transh)catioll it] wheat with r(-fiwencc to thc transtk'r t'aclov concept and ion competition effects. ,]. Era,it. Radioa~/. 16, 167 180.
,I01
21. SIMMONDS,l" (198',{) Estinaation of contanfination on plant surthccs due to deposition otradionuclidcs fi'om the atmosphere. In Ecolo2ical aspecl,~ o/ radionuclide re/ea,~e, P.J. (;otrCnTRI,:v,.]. N. B. BELL and T. M. ROBERTS, cds. Special publications series , t the British Ecological Society, No. 3. Bla.:k~cll Scicmific Publications, Oxtord. 22. Vuop, l S. ~19VJ~, Review of evaluations concerning radi