Scientific paper

1. Quantitative assessment of radiocaesium bioavailability in
forest soils
By A. v. Konoplevr'x, R. Avila2, A. A. Bulgakovl, K-J. Johanson3, L V. Konoplevaa and v. E. popovl
1 SPA "Typhoon",249020 Obninsk, Russia
' Swedish Radiation Protection Institute, S-17116 Stockholm, Sweden
3 Swedish University of Agricultural Sciences, 5-75007 Uppsala, Sweden
" Russian Institute of Agricultural Radiology and Agroecolo gy,249020 Obninsk, Russia
(Received August 18, 1999; accepted March 31, 2000)
Radiocaesium / Forest / Soil-plant transfer /
Pqrameterisation / Bioavailabilin factor
Summary. A method for quantitative characterisation of the
radiocaesium availability to plants in forest ecosystems has
been developed. For this purpose an expression has been
proposed to calculate the radiocaesium availabiliry factor
in soils, which is a combination of key soil characteris-
tics: radiocaesium exchangeability, exchangeable calcium in
soil and effective selectivity coefficient. The experimental
dependencies of the radiocaesium soil to plant concen-
ffation factors for fern and bilbeny, on the availability
factor calculated by the above equation were satisfactory
described by linear function. The advantage of this method
to characterise bioavailability of radiocaesium and to esti-
mate site-specific values of concentration factor is that the
necessary soil characteristics may be taken from the ref-
erence literature, evaluated by experts or determined with
a simple experimental procedure. The method can be used
in development of a radioecological Geographic Information
Systems.
Introduction
A key component in dose assessment is the estimation of
the soil-plant transfer of radionuclides. For this purpose the
concentration factor, Cfl is normally used which is the ratio
of radionuclide concentration in plant and its concentration
in the upper soil layer. The values of CF determined ex-
perimentally in different conditions for a particular plant
can differ by hundreds or even thousands of times [1]. Ac-
cordingly, using a plant-average CF leads to a considerable
uncertainty in prediction of bioaccumulation of the radionu-
clide. For this reason, numerous attempts were made to
develop methods for estimating site-specific values of CF
and a lot of efforts went into deriving empirical dependen-
cies of radionuclides accumulation in plant on soil proper-
ties [2]. An essential drawback of purely empirical models
is that they are not all-purpose. In order to be applied for
soils different from those for which they were obtained they
should be justified based on information about mechanisms
Radiochim. Acta 88, 789-792 (2000)
O by Oldenbourg Wissenschaftsverlag, Miinchen
of sorption-desorption of radionuclides in soil and their soil-
plant transfer.
Today it is generally taken that one of the key factors
goveming soil-plant transfer of radionuclides is the part
of the exchangeable form of radionuclides. Although this
hypothesis is rather well-founded, the analysis of experi-
mental data has shown that it is not sufficient [3]. This is
probably explained by the fact that along with the part of
exchangeable form of the radionuclide in soil, the value
of CF is influenced by other factors the most important
of which are the radionuclide and element concentrations
in soil solution [4-6]. For caesium isotopes two hypothe-
ses were proposed accounting for the effect of the com-
position of soil solution on radionuclides transfer to plant.
By the first hypothesis, the radiocaesium concentration in
plant is proporlional to the ratio of its concentration in soil
solution and potassium concentration [7]. The comparison
with experimental data, however, has shown that there is
no meaningful correlation between the concentration factor
and r37Cs7K ratio in soil solution [7]. Another hypothesis,
which seems more sound, is that the concentration factor
is proportional to the part of radionuclide in the root ex-
change complex dependent on the composition of soil so-
lution [4]. Using this hypothesis a method for parameteri-
sation of radiocaesium bioavailability in soils was proposed
in [8]. The factor of radiocaesium bioavailability in soils
was introduced which is calculated by sorption characteris-
tics of soil and the composition of soil solution. The method
validation has demonstrated a good agreement with experi-
mental data which leads us to believe that it can be used
for obtaining fairly accurate soil-specific estimates of CF.
For doing this, however, one should know such soil char-
acteristics as the content of mobile forms of ammonium
cation and exchangeable Radiocaesium Interception Poten-
tial (RIP*). The values of these parameters are known for
a limited number of soils. Methods for their theoretical esti-
mation have not been developed and the methodology used
for experimental determination of RIP" is rather compli-
cated. Therefore, a simpler method is proposed in this paper
for parameterisation of the factor of radiocaesium availabil-
ity to plants using only those soil characteristics which can
either be taken from reference books or estimated usins
known correlation ratios.
+Author for correspondence (E-mail: konoplev@iem.obninsk.ru).

2. 790 A. V. Konoplev, R. Avila, A. A. Bulgakov er a/.
Theoretical approach
According to [8] the radiocaesium soil-plant concentration
factor can be presented as follows:
CF=β xム ,
where B is the parameter dependent on plant characteristics;
A is the availability factor. The availability factor A is taken
to be proportional to the part of radiocaesium in the "root
exchange complex" and is parameteized through the com-
position of the porous solution and the sorption properties of
soils as follows [8]:
o _ a"^SPAR t)
RIP". -/
where cy"* is the part of exchangeable form of 137Cs of
its total content in soil; SPAR is the Sorption Potassium-
Ammonium Ratio (mMos 1-os;'
SPAR= [K]υ 十【が
S(NH4/0[NH4]υ
V[Ca]υ 十[Mg]υ '
whtte[K]切 ,[NH4]の [Ca]り ,[Mg]″ are tllC eqllilibttum
concenttations of cottesponding cations in 、vater phase,
【fSS(NH4/K)is tlle selecttvitt coefncient of potassium ex―
changc for ammonium tt Selectvc Sorpdon Sites(SSS)Of
soll.
The factors goveming paralllcter 』 智e not collsidered
within廿1ls work.Lct us only point out that as a factor it
includes the selecivity coefflcient for tlle exchange of cae―
siunl catons for calcium and magnesiuln cations in the root
exchange coll■ plex of the plant.
A more rcasonおle pttal■ etαis航lon of tlle availability
factor can be del・ived with the equation calculattng radio―
CaeSiLIIIi Concenttation in tlle、 vater phase of thc soil― 、vater
systelll proposed in[9]:
陶 υ=ン : ④
whcrc[137cs]鉱 ,[K]鉱 班c thC COnccnttations of the ex一
changcお le 137cs in Soll(Bq/kg)and potassium(meq/kg),
respecti■ rd】
電
対iS he ettcⅢ e selecttitt cOettcient oftte
potassiunl cation exchange for caesiulll cation in the soll cx―
change complex.
Using(4)we get
A=
αcxPAR
【ダ(Cs/K)[K]鉄 '
where PAR is the Potassium Adsorption Ratio (mMos l 05),
PAR= [K]υ
∼
/[Ca]砂 十 [Mg]砂
`
EqLl航 10n(5)is si血1釘 tO tllat dcl・ ivcd in[8]based On tlle as―
sumption on a dilhcctly propo■ lonal ttlation between RIP鉄
alld[K]cx ThC Only di節研ence is tllat equation(5)incllldes
t13e efFect市 e selecti■rity coettcient.This may a1low a more
accurate estmation of tlle alrallお 1lity ftttor using tlle em‐
piical dependence of【 :T(CS/Ю On s01l prOpel・ tics.A bett釘
accuracy call bc achieved if he vttue of【 芦(CS/Ю iS meas―
ured expe五 rnentそ 述ly.
The value of PAR for forest soils (characterised by low
values of SSS capacities and elevated levels of ammo-
nium) can be expressed through Exchangeable Potassium
Ratio (EPR) using Gapon equation. EPR equals to the ratio
of the exchangeable potassium and the total concentration
of exchangeable calcium and magnesium. From literature
data [10] concerning ion exchange it is known that the selec-
tivity coefficient for K+ in relation to (Ca2+ + Mg'*) is about
10. Thus, PAR can be replaced by EPR in equation (5) as
follows:
A=
10α cx
【ダ(CS/0([Calcx+[Mg]鉱 )
This equation can be further simplified by neglecting the
content of exchangeable Mg in soil. The rational for this
simplification is twofold: 1) the content of exchangeable Mg
in most cases is less than 20Vo of exchangeable Ca; 2) Ca is
generally preferred over Mg on root exchange complex [1 1].
Therefore, finally we get:
(7)
A= (8)
(5)
(6)
WItaterials and lnethods
The applicability of cqutton(8)was teSted witll lllc dtta on
soll― plallt ttansfel・ collected fl・oェ1 0ne fOrcst site of Bり ansk
rcglon(Russia)alld nine forest sites in Uppsala district
(SWeden).Descl■pdon ofthe sites under smdy and solltypes
is presented in Table l. Sal■ ■plcs of solls and plants were
collccted in summers of 1997 and 1998 yetts.Soils of se―
lected sites tte tttributed to podsol typc witll simll釘 tex―
的res.As reference plants we chose fern(Pた ガ冴サ″脅 α?″ サ″―
,れ ″初 と.)and bilbtty(予物CCけれ
'″
初 初ノガサ〃″ざと。).Roots of
these plants occur mostly in tlle organic soll laycr and as
suggcsted by tlle anttysis 9f the vttcal distributon of ra―
dlocacsium,most ofits invcntory is in the samc horizon.The
high variabllity of a3gregated transfer factor, alllong otller
things,ls explalned by direl・ent position of roots、 vith rc―
spect to tlle vttcal dis匂 止bution of thc rattolluclide in soll,
It would,tllerefore,be appropiate to comptte CF(for dリ
weight of plamt and soll)calculated speciflcally for tlle root
zone― C阜 .The S01l charactel・ istcs in tlle equtton account―
ing for tlle a17お labill呼 fractorム wtte also measured for tlle
root zone.For this purpose,、 vater and lヽ /1-almmonium ac―
ctそ近e exttactons wtte perfoコ ■cd.Exchangeable loIIs wcrc
measured in tlle exttacts by Atolllic Absorptton Spectromc―
的′(AAS)and 137cs waS measured in the ammonium cxtract
to gct αcx.Contcnt of organic lnatter in solls、 vas detenllined
as losses on ignition at 450° C.
Results and discussion
To estimate site― speciflc value of effectivc selectivity coe←
flciellt its empiical dependence on organic matter colltellt
(OM,%)Was uSed[12]:
【芦(Cs/K)=13-0.12x OM. (9)

3. Quantitative assessment of radiocaesium bioavailability inforest soils 791
Tablel. Listing of the soils studied, the
symbols used and soil type. Sampling site Symbols Type of forest Soil type Texture
St. Bobovichy
Uppsala 3
Uppsala 11
Uppsala 13
Uppsala 14
Uppsala 21
Uppsala 23
Uppsala 66
Uppsala 71
Uppsala 78
RUSSIA, Bryansk region
StB Pine,birch soddy podsolic
peaty
SWEDEN, Uppsala region
Pine, spruce ferric podsolic
Pine, spruce ferric podsolic
Pine, spmce ferric podsolic
Pine, spruce ferric podsolic
Pine, spruce, birch cambic podsol
Pine, spruce cambic podsol
Up3
Upll
Up13
Up14
Up21
Up23
Up66
Up71
Up78
sand
light loam
sandy loam
sandy loam
sandy loam
sand
flne gravel
sandy loam
sandy loam
sandy loam
Pine ferric podsol
Pine, spruce ferric podsol
Birch humic cambisol
Table2.Soilcharacteristicsandl3TCs soil-plantconcentrationfactorsforrootzoneCF..
Site PHrcr OM,% α.x,% Exchangeable cations. meq/kg
[Ca]鉱 [Mg]ex
C阜 (Bqkg 1/Bq kg 1)
Fcm Bilbery[Klel
St.B
Up3
Upll
Up13
Up14
Up21
Up23
Up66
Up71
Up78
34
3.1
3.5
31
3.0
4.2
4.6
2.9
2.8
2.6
11
70
35
82
26
11
24
767
61
78
18.1
19
27
27
14
2.7
1.4
212
21.2
3,1
17
108
86
204
26
42
228
105
445
51
3
24
11
26
5
7
32
295
115
45
94
5`78
3.31
3.85
0.04
0
6.2
76
06
180
109
1.10
1.41
一
ｍ
２
拓
一
３
６
５
８
４
２
７
御
５ ． ７
︲ ・ ６
This dependence was obtained alsO for podsolic soil types.
Othtt rel釘 allt soll chttactel・ istcs alld values of Ctt for the
plants are prcsented in Table 2.
The derived dcpendencles of radiocaeslum concenttatton
factor on avttlabili呼 勉dOr for tllc two plallts tte adequately
desc五 bcd by a strお ght linc(see Fig.1).Regresslon eqlla―
位ons for plallts under sttdy call be presented as follows:
CFr(fCln)=(0.96± 0.11)× A十 (0.08± 0.59);
C阜 (bllbttry)=(0.17± 0.05)xA十 (0.35± 0.3り ,
In botll cascs deived regressibn eqtlattons tte stattstically
signincant(P<o.01 for fel■ l and P<0.05 for bilbeny).
The good agreementin the tl■ eoredctt and expel■ mental de―
pendencies indicates tllat the propOsed metllod can be uscd
for obtaining sitc― specinc concentration factors of radiocac―
sium in plalats,Pを 配 of tlle pttamettts in cquation(8)for
tlle卸おlabllllj7 factor can be found by expert judgemellt or
measured using a ratller simple procedure(α cx,【:r),While
anotller is impoltallt agrochelllical indicator and for mally
regions call bc taken fl・ om refettnce literam的 ([ca]欲).ThiS
allows using the proposed metllod for mapping tlle radio_
caesium avallabllity to plants. Incorporation of such maps
into Geographic lnformatton Systems(GIS),whiCh are ac―
tively being developed in the wOrld,will lead to a signincallt
reductton in unce■ainty in dose assessment.
The uncertaintt in esdmation of C■ uSing tlle proposcd
method can be signincantly reduced by inttoduction of cOr―
recttons for non― uniformity of the radiocaesium veltical dis―
tnbution and Of exchangeably sorbed and dssolved catlons
in the upper soil layer. In development of practical recom-
mendations for prediction of radionuclides accumulation in
plants the problem of vertical non-unifonnity of the soil and
radionuclides distribution in this soil is reduced to determin-
ation of the layer the characteristics of which can be used for
calculation of CF and A. The vertical distribution of r37cs
in the root zone in all the soils studied was rather uniform
and hence, the value of CF was weakly dependent on which
horizon the radionuclide concentration was determined in.
As demonstrated by the experimental data obtained in
the latest years [13-15] the radiocaesium transfer to plants
is dependent not only on the value of PAR, but also on
the absolute potassium concentration in the soil solution.
The radiocaesium concentration in plants is decreasing in al-
most inverse proportion to the potassium concentration in
14
12
10
、 8
°
6
4
2
0
Fern ./ R'?- 0,9'l2i
●
／
●
Bilberry
R' - 0,61og
A
Fig.l. Dependence of 13?Cs
soil-plant concentration factor for root
zone (CF.) on availability factor A for the sites under study.

4. 792 A. V. Konoplev, R. Avila, A. A.Brigakov et al.
the nutrient solution with the increase of the last to 1 mM,
following which it remains practically unchanged |3,741.
This effect may be rather noticeable in arable soils impov-
erished in potassium, but in forest soils the potassium con-
centration in the soil solution is usually rather high and its
effect on radiocaesium accumulation in plants can be ig-
nored. In all the soils studied, as shown by the analysis of
the water extractions, the potassium concentration in the soil
solution is much higher than 1mM. High nutrient content
of the soil solution of forest soils allows us to neglect both
the effect of the potassium concentration on radiocaesium
accumulation in plants and the difference in the composi-
tion of soil solution in the root layer and in the soil on the
average. Therefore, equation (8) seems sufficient for pre-
diction of the radiocaesium bioavailabilitv at least in forest
soils.
Acknowledgment This work was supported by Swedish Radiation Pro-
tection Institute.
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