On peculiarities of vertical distribution of 239,240Pu, 238Pu and 137Cs activity concentrations and their ratios in lake sediments and soils

On peculiarities of vertical distribution of 239,240Pu, 238Puand 137Cs activity concentrations and their ratios in lakesediments and soils

B. Luksien _e • E. Maceika • N. Tarasiuk •

E. Koviazina • V. Filistovic • S. Buivydas •

A. Puzas

Received: 28 August 2013 / Published online: 16 February 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract Distributions of 239,240Pu, 238Pu and 137Cs activity

concentrations in the cores of sediments in the shallow lake,

flooded and upland forest soils taken in the vicinity of Vilnius

city were analyzed. The radiochemical, a-spectrometric and

mass spectrometric methods were used for the plutonium

evaluation and c-spectrometry was used for the radiocesium

evaluation. The only peak of enhanced radionuclide activity

concentrations was determined for the lake bottom sediments,

whereas vertical profiles of the radionuclide activity concen-

trations in flooded and upland forest soil cores were distin-

guished by two peaks. The obtained values of the activity

concentration ratio 238Pu/239,240Pu and the isotopic ratio240Pu/239Pu indicated that the global fallout was a source of

plutonium in the investigated environment. Chernobyl-

derived radiocesium was detected solely in the surface layers

(2–11 cm) of the studied sample cores. The contribution of the

Chernobyl deposits amounted to about 2.26, 6.11 and 20.9 %

of the total radiocesium inventory in the bottom sediments, the

upland soil and flooded soil, respectively.

Keywords 238Pu � 239,240Pu � 137Cs � Origin � Lake

sediments � Upland and flooded soils � Vertical profiles

Introduction

Soils and bottom sediments of natural water basins are the

main abiotic components which accumulate the largest

quantities of man-made radionuclides and turn into radio-

nuclide depositories [1]. Actinides have entered the envi-

ronment as a result of nuclear activities, especially due to

atmospheric weapons testing and several accidents in civil

and military nuclear facilities. Actinides, and especially

plutonium isotopes, are obvious long-term radioactive

pollutants due to their high radiotoxicity, long half-lives

and high persistence in the environment. Activity ratios of

plutonium isotopes, such as the alpha-emitters 238Pu

(T� = 87.8 year), 239Pu (T� = 2.4 9 104 year), 240Pu

(T� = 6.6 9 103 year) and the beta-emitter 241Pu

(T� = 14.4 year), are useful tools to identify the origin of

contamination [2, 3].

To assess the long-term environmental impact of radio-

active contaminants on ecosystems, information on source

and radionuclide mobility is of high importance. Each plu-

tonium contamination source shows an individual compo-

sition of plutonium isotopes. Therefore, the 238Pu/239,240Pu,240Pu/239Pu and 241Pu/239Pu activity ratios are widely used to

identify the origin of plutonium contamination. Besides,239,240Pu may be used to identify 137Cs sources according to239,240Pu/137Cs or 137Cs/239,240Pu activity ratios when 134Cs

cannot be detected because of its radioactive disintegration.

Eutrophication makes lakes decline, and they accumu-

late sludge (silt or sapropel), lake shores get covered with

excess vegetation and, consequently, all this causes a

serious ecological problem [4]. These processes are espe-

cially typical in case of small lakes. In Lithuania there is

about 1 billion m3 of pure sapropel which lies mostly in

1–2 m thick layers but in some places layers are up to 5 m,

and there is about 6 billion m3 of sapropel in mixture with

other sediments. About 4.5 billion m3 of sapropel underlie

the peat layer of lacustrine swamps [5].

It is known that sapropel from lakes in Lithuania may be

extensively used in agriculture (as bio-compost, growth

stimulator for vegetation, fertilizer and forage additive), in

industry (for diluents, binders in panel production) and in

medicine (as curative mud, bio-preparations and masks),

B. Luksien _e (&) � E. Maceika � N. Tarasiuk � E. Koviazina �V. Filistovic � S. Buivydas � A. Puzas

State Research Institute Center for Physical Sciences and

Technology, Savanoriu Ave. 231, 02300 Vilnius, Lithuania

e-mail: [email protected]

123

J Radioanal Nucl Chem (2014) 300:277–286

DOI 10.1007/s10967-014-3026-0

but it is indispensable to carry out an extensive analysis of

sapropel before starting to use it [4, 5].

Lake sediments as well as soil cores are an important tool

for reconstructing artificial radionuclide depositional trends in

order to determine changes in local, regional, and global

radionuclide fluxes to the environment. Modeling of the

regional and global effects of radionuclides from these

emission sources and migration properties is based on data

from sediment and soil cores. Furthermore, considering a

probability of sapropel use in agriculture, plutonium and ra-

diocesium investigations in this medium and in soil are of

great relevance from the point of view of radiation protection.

The aim of the present study was to investigate the vertical

profiles of 239,240Pu and 238Pu activity concentrations in lake

sediments as well as in flooded soil and upland forest soil, to

compare distribution of plutonium activity concentration and

inventory in studied profiles with those of 137Cs and, based on

experimental results, to identify the Pu and 137Cs origin in

studied compartments.

Materials and methods

Sampling area

Eutrophic Lake Juodis (54�4604900 N, 25�2602900 E) was

chosen to study the plutonium and radiocesium vertical

distribution in sediment core samples. Lake Juodis (tunnel

Fig. 1 A map of the studied

area with sampling sites:

(a) filled square upland soil

sampling site; (b) filled circle

sediment sampling site; (c) filled

square flooded soil sampling

site

278 J Radioanal Nucl Chem (2014) 300:277–286

123

valley lake, groove type) is located 17 km to the northeast

from Vilnius city (Fig. 1). Its inflows reach their maximum

values of *80 L s-1 in spring, while in summer after rains

they vary in the range of 20–30 L s-1, and during dry periods

they may decrease down to 7–10 L s-1. It has no other

groundwater feeding sources. Banks of the lake are rush-

grown, with large marshy zones. The southern part of the

lake is wider and deeper (up to 3.5 m depth). The northern

one is a shallow bottom terrace (depth *1.0–1.6 m). The

surface area of the lake is equal to 0.1 km2, the drainage

basin -3.5 km2, hydraulic retention time—0.3 year [6].

The typical amount of dry material in the surface layer of

the sapropelic type sediments in Lake Juodis varies in the

range of 23–30 g L-1. The thickness of the sediment layer in

the northern shallow part of the lake was evaluated to be

about 5–7 m. The mineral content of the surface water is

maximum in winter (*174 mg L-1). The organic matter

content of sediments was assessed to be very high (mean

68.8 %). Earlier studies [6] showed that sedimentation in the

lake was mainly affected by the vital cycle of green algae.

Photosynthetic activity of the algae in the northern shallow

part of the lake under permanent aerobic conditions induced

formation of carbonate deposits, which proved to be a barrier

for radiocesium migration. In the deeper southern part of the

lake, radiocesium migration takes place under natural con-

ditions related to the formation of the anaerobic zone in the

near-bottom water in winter.

Sampling and gamma spectrometry

A sediment core was taken on 4 July 2012 in the profundal

southern part of Lake Juodis using an improved version of

the Ekman–Birge type sampler (*40-cm height). It was a

steel tubing with a square cross-section (14 9 14 cm) and

had a manually operated spring bottom shutter. The sam-

pling was carried out with the weight compensation, where

an additional float controlled the depth of the sampler sank

into sediments. Sediment samples without the water layer

above the sediment surface were discarded.

A sediment core was sliced into layers of about

2–2.5 cm thickness. Considering that the sampler was not

waterproof, the slicing was conducted in shallow waters

near the bank using a special spoon to fill the plastic bottles

of standard volume and gradually moving the sampler up to

the bank. Bottles were held for some time for sediments to

settle, and real sediment volumes were determined.

The soil cores on the upland western bank of Lake

Juodis and in the dry but regularly flooded part of an old

channel connecting earlier Lakes Tapeliai and Lydekinis

(Fig. 1) were taken by stabbing a steel tube of 30 cm height

(diameter 12.5 cm) on 25 May 2012.

A sampling site adjacent to Lake Juodis was located in a

dry forestry zone. A sampling site in the old channel was

located about 30 m from the shore of Lake Tapeliai and its

surface was mainly covered with spruce and pine needles.

The channel was blocked about 40 years ago when the road

crossing it was asphalted. Part of the channel adjacent to

Lake Tapeliai soon went dry. In spring and after rains

during warm seasons, this site is flooded but over dry

periods it usually gets more dry and passable.

Sediment and soil samples were air-dried at room tem-

perature. Their weights were determined using VLV-100

scales where samples were held under thermostatic con-

ditions (in the 40–50 �C temperature range) till the con-

stant weight. Results of the organic matter content in soil

and sediment samples in this study were provided using the

loss-on-ignition (700 �C) method.

Sediment and soil samples were analyzed for 137Cs

using a CANBERRA c-spectrometric system with a HPGe

detector (model GC2520, 26.2 % relative efficiency, res-

olution—1.76 keV/1.33 MeV) according to the gamma

line at 661.62 keV of 137mBa (a daughter product of 137Cs).

Measurements were carried out in standard geometry and

calibrated at known efficiencies according to densities of

samples. Measurement errors of radiocesium activity con-

centrations in samples were evaluated by the GENIE

software program. They were less than 5 % (standard

deviation) for active samples and not larger than 15 % for

less active layers of sediment and soil cores. Activity

corrections to the sampling date were not made because

measurements were carried out shortly after the sampling.

Radiochemical method for plutonium analyte

purification and measurement

To determine plutonium analytes the sample destruction is

needed. Organic matter, which interferes with the chemical

separation, was decomposed by heating it at 550 �C

overnight and after that to 700 �C for 2 h. Samples were

spiked with 0.0125 or 0.0250 Bq 242Pu tracer (depending

on the probable plutonium activity concentration in the

sample) as a yield monitor. Acid leaching with aqua regia

and concentrated HNO3 was employed. The leachate was

filtered through a glass-fiber filter and evaporated until wet

residue. The residue was dissolved by adding concentrated

HNO3, then adjusted to the acidity of 8 mol L-1 of HNO3

by adding de-ionized water. The method of chemical sep-

aration was based on two separation column methods. An

initial separation of Pu from uranium/fission products and

other matrix elements was accomplished on a Bio RaD

AG-1X8 anion exchange column followed by the chemical

separation methods described by Talvitie [7] and Holm [8]

J Radioanal Nucl Chem (2014) 300:277–286 279

123

with small modifications. Before loading the sample solu-

tion onto the anion-exchange column, a reduction–oxida-

tion procedure with Na2SO3 and NaNO2 was performed to

convert plutonium into the tetravalent state. Sample solu-

tion was loaded onto the column, which was pre-condi-

tioned with 50 mL of 8 mol L-1 HNO3 with 0.5 g of

NaNO2. This column strongly retained Pu but had affinity

for fission products and some matrix substances. The col-

umn was washed with 80 mL of 8 mol L-1 HNO3 to

remove U, Fe and Pb from the column. Thorium was

removed with 60 mL of 10 mol L-1 HCl. After that, Pu

was eluted with 100 mL of 4 mol L-1 HCl solution with

Na2SO3 (0.6 g), where Pu(IV) was reduced to Pu(III). The

eluted Pu solution was evaporated and processed for further

purification by the extraction chromatography method [9].

Plutonium was purified using TRU and UTEVA resin

columns. The TRU resin showed preferential retention of

tetravalent actinides in a wide range of acidities. The plu-

tonium eluate from the TRU resin was prepared for elec-

trolysis. Electro-deposition was used to obtain thin layer

plutonium sources for alpha spectrometric measurement.

Plutonium was electrodeposited for 1 h under direct

0.6 A cm-2 current onto a one-side polished stainless steel

disc-electrode in sulphate medium.

The plutonium isotopic composition (238Pu and 239,240Pu)

and their amount in the samples were determined by use of

the semiconductor Alpha spectrometer OctetePlus with an

ORTEC large square (450 mm2) Si detectors (BU-020-450-

AS) with the resolution capability of 20 keV. The spectra

were analyzed applying AlphaVision and Maestro programs.

The detection limit for the counting time of 86,400 s was

about 10-3 Bq for plutonium isotopes [10, 11].

After a-spectrometric measurement, electrodeposited

plutonium was re-dissolved from discs with ultra pure

nitric acid to obtain targets suitable for the quantitative

measurements of 239Pu and 240Pu by Inductively Coupled

Plasma Mass Spectrometer (ICP-MS). Noticeable, that

usage of analytical grade HNO3 for this purpose is not

suitable. Pu removal from the discs was controlled by the

measurement of the discs a-spectrometrically.

The mass spectrometric (ICP-MS) analysis was applied to

determine 239Pu and 240Pu at trace or ultra trace levels. The

measurement was performed with a double focusing high

resolution sector field ICP-MS Element 2 (Thermo Fischer

Scientific) with a conical U-Series nebulizer of 1 mL min-1

(Glass Expansion). The conditions of the measurements

were as follows: the forward power—1,100 W; the auxiliary

gas flow rate—0.75 L min-1; the nebulizer gas flow rate—

1.32 L min-1 and the cooling gas flow rate—14.0 L min-1.

The solution uptake rate with a peristaltic pump was

0.70 mL min-1. Prior to the analysis the instrument was

calibrated using a 2 ng g-1 natural uranium solution and was

tuned for the best sensitivity [12, 13].

Results and discussion

Plutonium isotope activity concentrations, their ratios

and inventories

Plutonium isotopes in Lake Juodis sediments

The investigation of anthropogenic plutonium in lake bot-

tom sediments is of great importance since they constitute

the major depositories of this kind of contaminants.

Based on 239,240Pu inventory values in sediment layers

(Table 1) we can observe rather high inventory of pluto-

nium isotopes in each sediment/sapropel layer in the pro-

fundal zone (approximately 3.0 m water depth) of the lake.

Inventory of 239,240Pu of *104.3 Bq m-2 was rather

high compared to the inventory range of 5.14–31.6 Bq m-2

in the sediment cores from four lakes in NW Greenland

(76�330 N, 68�540 W) [14]. Sources of plutonium isotopes

include airborne particulates from nuclear weapon testing

or accidental releases. Artificial radionuclides are deposited

on land or water. Radionuclides deposited on land may also

enter water bodies due to soil erosion and run-off processes

folowed by their accumulation in lake sediments. For

instance, forest canopies are very efficient in the initial

Table 1 Activity concentrations and inventories of 239,240Pu in sed-

iment layers of Lake Juodis in 2012

Thickness of sediment

layer (cm)

Activity

concentration ± r(Bq kg-1)

Inventory

(Bq m-2)

0–1.2 1.54 ± 0.15 0.50

1.2–3.0 1.82 ± 0.16 0.95

3.0–4.8 1.86 ± 0.18 0.94

4.8–6.7 1.70 ± 0.13 0.88

6.7–8.7 3.38 ± 0.28 1.64

8.7–10.5 3.86 ± 0.29 1.79

10.5–12.4 5.12 ± 0.37 2.76

12.4–14.6 7.93 ± 0.65 4.70

14.6–16.5 9.50 ± 0.72 4.90

16.5–18.6 15.70 ± 1.22 9.20

18.6–20.7 16.30 ± 1.23 9.52

20.7–22.3 23.70 ± 1.97 10.41

22.3–23.7 25.00 ± 2.48 14.03

23.7–25.7 22.80 ± 3.04 9.95

25.7–27.8 22.04 ± 1.71 7.19

27.8–29.6 14.07 ± 1.07 12.47

29.6–31.8 9.20 ± 0.73 5.57

31.8–33.8 6.69 ± 0.53 3.58

33.8–35.4 3.86 ± 0.29 1.74

35.4–37.5 1.90 ± 0.16 1.10

37.5–40 0.78 ± 0.08 0.48

Mean: 9.46 ± 0.83 Total: 104.3


123

interception of radioactive materials from the contaminated

air [15]. In general, understanding the fate of plutonium

isotopes released to the aquatic environment through the

global fallout, planned disposal and accidents meets con-

siderable difficulties because this radionuclide can exist in

several oxidation states at the same time. In particular, the

concentration of plutonium is significantly influenced by

anoxicity and redox cycles. The oxygen-depleted and iron-

and manganese-rich bottom waters contain dissolved Pu 5

times more than oxygenated surface waters [16].

Usually, the 238Pu/239,240Pu activity concentration ratio

was used to determine the radionuclide origin in soil. For the

Chernobyl accident, based on UNSCEAR (2000) estima-

tions [17] of releases, the 238Pu/239,240Pu activity concen-

tration ratio corrected to January 2006 was 0.392 ± 0.068.

The isotopic composition of Pu (especially the 240Pu/239Pu

ratio) proved to be useful for identifying radioactive pollu-

tion sources, because the 240Pu/239Pu ratios significantly

vary among different sources. For instance, the isotopic ratio

of 240Pu/239Pu in weapon-grade Pu is typically around 0.05,

while that of global fallout is about 0.18 [18], and the debris

of the Chernobyl accident had a ratio of about 0.4 [19]. The238Pu/239,240Pu activity concentration ratio typical values,

which were found to be 0.026 [20], 0.03 [21], 0.041 [22] and

0.046–0.061 [23] in soils, indicated that in studied areas

plutonium deposition was caused by atmospheric nuclear

tests.

Results of investigations of plutonium distribution in

sediments of various lakes have shown that in most lakes

the plutonium origin according to the mean isotopic240Pu/239Pu ratio value of 0.18 [24, 25] or 238Pu/239,240Pu

activity concentration ratio value of 0.026 [14] was

attributed to the global fallout with the exception of Lake

Chenghai in SW China [26].

Ketterer et al. [27] testing forest soils and peat bogs

determined that the 241Pu/239Pu isotopic ratio was more

sensitive than 240Pu/239Pu isotopic ratio or 238Pu/239?240Pu

activity concentration ratio when detecting small Cher-

nobyl 239?240Pu inputs.

The 238Pu/239,240 Pu activity concentration ratio in the

sediment core from Lake Juodis down to the 40 cm depth

was in a range of 0.012–0.040 with the mean value of 0.024

(Fig. 2). The isotopic ratio 240Pu/239Pu was analyzed in

sixteen layers of Lake Juodis sediments. This ratio varied in a

range of 0.17–0.23. Hence, values of the 238Pu/239,240Pu

activity concentration ratio and the 240Pu/239Pu isotopic ratio

in Lake Juodis sediments indicated the global fallout origin

for plutonium [18].

Figure 2 shows that the peak activity concentrations were

obtained for both 238Pu and 239,240Pu at the *16–25 cm

depth. Top layers of the sediment core down to the 6.7 cm

depth were distinguished by quite uniform (within the error

limits) activity concentrations for 238Pu and 239,240Pu in the

range of 0.061–0.086 Bq kg-1 and 1.54–1.86 Bq kg-1,

respectively. It may be related to the steady low level of

plutonium activity concentrations in lake water over last

years. Possible explanation could be based on the statement

of [28] that sediment mixing, ranging from 1 to 15 cm, is

probably not an experimental artifact but a result of the

burrowing and feeding activities of abundant zoobenthos

populations. Plutonium can also be biogenically solubilized

because of microbial activity. All microbial life is based on

metabolic processes driven by chemical energy extracted

from the oxidation of inorganic or organic compounds [29].

During respiration process, a reducible external electron

acceptor is used (e.g. oxygen, nitrogen, sulphur, iron or

manganese).

As in the case of radiocesium, plutonium isotopes can

participate in the sediment self-cleaning processes related

to the thermodynamic buoyancy mechanism, which was

determined during recent investigations [30] carried out in

Lake Juodis.

Plutonium isotopes in the flooded soil core

The core of flooded soil was rich in organic matter (mean

79 %), which implied a possible large microbial activity in

the sample. The uppermost flooded soil layer contained

leaves, needles and some moss, therefore, its organic

matter was the highest (94.2 %). Inadequate activity con-

centration values of both Pu isotopes in the uppermost

layer of flooded soil (Fig. 3) have conditioned an unex-

pectedly high 238Pu/239,240Pu activity concentration ratio

(*0.14). The value of the ratio should be considered with

caution because of a small amount of surface material and

large measurement error of 238Pu activity. The ratio in all

layers of flooded soil varied from 0.0195 to 0.0427 with the

0.0 0.3 0.6 0.9 8 16 24

36

32

28

24

20

16

12

8

4

0

239,240Pu

238Pu

Sed

imen

t dep

th, c

m

Activity concentration, Bq·kg-1

0.00 0.02 0.04 0.06

0.0 0.3 0.6 0.9 8 16 24 0.00 0.02 0.04 0.06

36

32

28

24

20

16

12

8

4

0

Sed

imen

t dep

th, c

m

238Pu/239,240Pu activity ratio

238Pu/239,240Pu

Fig. 2 Plutonium isotope activity concentrations and activity con-

centration 238Pu/239,240Pu ratios in the sediment vertical profile from

Lake Juodis


123

mean value of 0.031, and with the mean value of 0.036

when the top soil layer ratio (*0.14) was accounted for.

Measurements showed two characteristic peaks in the

vertical profile of Pu activity concentrations in flooded soil

in a forested environment. The mean value of the activity

concentration ratio 238Pu/239,240Pu being 0.031 ± 0.012

(Fig. 3) indicated that plutonium isotopes in the flooded

soil core originated from the global fallout [16]. The iso-

topic 240Pu/239Pu ratio determined in fifteen flooded soil

layers was in a range of 0.19–0.24 and also indicated the

global fallout origin of plutonium.

The obtained 239,240Pu activity concentrations and

inventory values can be seen in Table 2.239,240Pu activity concentration was the lowest

(0.61 Bq kg-1) in the zero layer and it gradually increased

up to 2.11 Bq kg-1 in the 4 cm layer in the flooded soil. At

the depth of 5–8 cm, 239,240Pu activity concentration has

shown peak values of 2.88–3.15 Bq kg-1. The second less

intensive peak of activity concentration values covered the

12–15 cm depth interval. Afterwards, activity concentra-

tion values gradually decreased down to 0.21 Bq kg-1 for239,240Pu in the last sample layer used for the analysis of the

30 cm depth (Fig. 3).

Plutonium isotopes in the upland forest soil core

Data on the obtained 239,240Pu activity concentrations and

inventory values measured in layers of the upland forest

soil core taken in the surroundings of Lake Juodis are

presented in Table 3.

The 239,240Pu activity concentrations and inventory in

the upland forest soil core were the lowest taking into

account sediment and flooded soil cores. The 239,240Pu

activity concentrations in the upland forest soil layers were

mainly in the range of 0.05–0.59 Bq kg-1 and only peak

concentrations were higher, i.e. from *0.89 to

1.45 Bq kg-1 in the first peak and from *0.63 to

0.70 Bq kg-1 in the second one (Table 3; Fig. 4).

The mean 239,240Pu activity concentration was

0.46 ± 0.04 Bq kg-1 and the soil layer inventory values

ranged from 0.04 to 3.94 Bq m-2 (Table 3).

The activity concentration 238Pu/239,240Pu ratio values varied

from 0.011 to 0.046 with the mean value of 0.029 indicating the

origin of plutonium from the weapon testing (Fig. 4).

Comparison of vertical profiles of 239,240Pu and 137Cs

activity concentrations in sediment and soil cores

The vertical profiles of 239,240Pu and 137Cs activity con-

centrations in the sediment core from Lake Juodis, in soil

32

28

24

20

16

12

8

4

0

239,240Pu

238Pu

Soi

l dep

th, c

m

Activity concentration, Bq·kg-10.00 0.05 0.10 0.8 1.6 2.4 3.2 0.00 0.05 0.10 0.15 0.20

0.00 0.05 0.10 0.8 1.6 2.4 3.2 0.00 0.05 0.10 0.15 0.20

32

28

24

20

16

12

8

4

0

Soi

l dep

th, c

m

238Pu/ 239,240Pu activity ratio

238Pu/ 239,240Pu

Fig. 3 Vertical profiles of plutonium isotope activity concentrations

and activity concentration 238Pu/239,240Pu ratios in the flooded soil

core in 2012

Table 2 Activity concentrations and inventory of 239,240Pu in the

flooded soil core in 2012

Soil layer

(cm)

239,240Pu in the flooded soil core

Activity concentration ± r(Bq kg-1)

Inventory

(Bq m-2)

0 0.61 ± 0.10 0.38

0–1 1.56 ± 0.15 1.24

1–2 1.90 ± 0.18 1.42

2–3 1.94 ± 0.17 3.08

3–4 2.11 ± 0.17 4.13

4–5 2.88 ± 0.31 5.61

5–6 3.15 ± 0.34 7.01

6–7 2.64 ± 0.18 5.24

7–8 2.65 ± 0.21 6.02

8–9 2.79 ± 0.23 6.25

9–10 2.21 ± 0.17 4.80

10–11 1.73 ± 0.13 3.86

11–12 2.04 ± 0.16 4.57

12–13 2.28 ± 0.16 5.47

13–14 2.63 ± 0.19 6.86

14–15 1.92 ± 0.14 5.04

15–16 1.85 ± 0.15 4.02

16–17 1.88 ± 0.24 3.99

17–18 1.26 ± 0.15 2.72

18–19 1.27 ± 0.15 2.74

19–20 0.94 ± 0.06 1.52

20–21 1.09 ± 0.07 1.96

21–22 0.73 ± 0.05 1.31

22–23 0.62 ± 0.05 1.07

23–24 0.47 ± 0.04 0.81

24–25 0.31 ± 0.03 0.50

25–26 0.23 ± 0.02 0.38

26–27 0.21 ± 0.02 0.33

Mean:1.64 ± 0.14 Total: 92.33


123

cores taken near the lake and in the old channel are shown

in Figs. 2, 3, 4, 5, 6 and 7. A single distinct peak

(*295 Bq kg-1) at the *23 cm depth with a steep

activity slope down to deeper sediment layers and with a

rather smooth one up to the sediment surface was charac-

teristic of the radiocesium vertical profile in the sediment

core from Lake Juodis (Fig. 5). It was suggested in earlier

studies [6] that the peak was due to global fallout and could

be dated to the notification of the bomb test moratorium in

1963. The main arguments for the absence of consequences

related to the Chernobyl accident were that its impact was

very short-term and dry radioactive deposits on the lake

water surface could be diminished because of the lake

surface self-cleaning mechanism. It was nevertheless true

that passage of the radiocesium peak activity into deeper

layers of the lake sediments was as fast as the rate of

sedimentation, which might be estimated to be

0.00 0.03 0.06 0.4 0.8 1.2 1.6

20

16

12

8

4

00.00 0.03 0.06 0.4 0.8 1.2 1.6

239,240Pu

238Pu

Soi

l dep

th, c

m

Activity concentration, Bq·kg-10.00 0.02 0.04 0.06 0.08 0.10

0.00 0.02 0.04 0.06 0.08 0.10

20

16

12

8

4

0

Soi

l dep

th, c

m

238Pu/239,240Pu activity ratio

238Pu/239,240Pu

Fig. 4 Vertical profiles of plutonium isotope activity concentrations

and activity concentration 238Pu/239,240Pu ratios in a forest soil core of

the upland western bank of Lake Juodis in 2012

0 10 20 100 200 300

40

30

20

10

00 10 20 100 200 300

29±3

239,240Pu

137Cs

Sed

imen

t dep

th, c

m


0 10 20 30 40 50

0 10 20 30 40 50

40

30

20

10

0

Sed

imen

t dep

th, c

m

137Cs/239,240Pu activity ratio

137Cs/239,240Pu

Fig. 5 Vertical profiles of 239,240Pu and 137Cs activity concentrations

in the sediment core taken in Lake Juodis in July, 2012. Vertical lines

indicate 137Cs/239,240Pu ratio for the global fallout (29 ± 3 value,

reference year 2012)

0.0 0.5 1.0 1.5 20 4030

25

20

15

10

5

00.0 0.5 1.0 1.5 20 40

29±3

239,240Pu

137Cs

Soi

l dep

th, c

m


0 10 20 30 40 50

0 10 20 30 40 50

30

25

20

15

10

5

0

Soi

l dep

th, c

m


137Cs/239,240Pu

Fig. 6 Vertical profiles of 239,240Pu and 137Cs activity concentrations

in the forest soil core. Vertical lines indicate 137Cs/239,240Pu activity

concentration ratio for the global fallout (29 ± 3 value, reference

year 2012)

Table 3 Activity concentrations and inventories of 239,240Pu in the

upland forest soil core in 2012

Soil layer

(cm)

239,240Pu in the forest soil core

Activity concentration ± r(Bq kg-1)

Inventory

(Bq m-2)

0–1 0.050 ± 0.017 0.04

1–2 0.447 ± 0.054 0.47

2–3 1.445 ± 0.117 3.94

3–4 1.094 ± 0.085 2.23

4–5 0.894 ± 0.070 1.82

5–6 0.595 ± 0.055 1.21

6–7 0.702 ± 0.072 1.43

7–8 0.657 ± 0.06 1.34

8–9 0.608 ± 0.054 1.24

9–10 0.631 ± 0.065 1.29

10–11 0.475 ± 0.048 0.97

11–12 0.466 ± 0.041 0.95

12–13 0.381 ± 0.034 0.78

13–14 0.312 ± 0.039 0.64

14–15 0.234 ± 0.024 0.48

15–16 0.211 ± 0.022 0.43

16–17 0.23 ± 0.024 0.47

17–18 0.177 ± 0.019 0.36

18–19 0.128 ± 0.016 0.26

19–20 0.113 ± 0.015 0.23

20–21 0.108 ± 0.014 0.22

21–22 0.074 ± 0.011 0.15

Mean: 0.46 ± 0.04 Total: 20.95


123

*4.7 mm year-1. This result was in good agreement with

the values published for the lake in [6].

A common feature of the 239,240Pu and 137Cs vertical

profiles in two soil cores (Figs. 3, 4, 6, 7) was the forma-

tion of two peaks of their activity concentrations. In the

upland soil core taken in the vicinity of Lake Juodis, the

main peak (*56 Bq kg-1) of 137Cs activity concentrations

was observed in the surface layer (at the 2 cm depth) and

the second peak may be recognized by the elevation in the

slope of the radiocesium vertical profile possibly at the

5–8 cm depth. It must be pointed out that the corre-

sponding surface peak of plutonium activity concentration

in its profile was determined in the deeper 3 cm layer

(Figs. 4, 6).

Studying radionuclide vertical profiles in the flooded

soil core taken in the old channel (Figs. 3, 7), a surface

layer consisting of coniferous tree needles was separated

and noted as a ‘‘zero’’ layer. It was specially done to fix a

possible contribution to the radionuclide surface activities

resulted from the Fukushima accident. An evident eleva-

tion of the radiocesium activity concentration in the first

soil layer was measured. Possibly, it may be assigned to the

effect of the Fukushima NPP accident.

The peak radiocesium activity concentration

(*146 Bq kg-1) in the profile was measured at the 5 cm

depth.

The second less active peak (*64 Bq kg-1) was visible

at the 13–14 cm depth, which coincided with the respective

depth of the second less active peak of plutonium (Figs. 3,

7).

Determination of the 137Cs origin

The ratios of plutonium isotope activity concentrations

(238Pu/239,240Pu) in their vertical profiles in sediment and

two soil cores have evidently shown that plutonium

deposits were caused by the global fallout (Figs. 2, 3, 4). It

means that the appearance of radioactive air masses related

to the Chernobyl accident over the studied area was not

followed by new plutonium deposits. That result was very

helpful in identifying the origin of radiocesium activity

peaks in its vertical profiles in sediment and soil cores.

Studying the vertical profiles of the radiocesium to pluto-

nium ratios, data of Bunzl et al. [31] on the plutonium to

radiocesium ratio (*0.018 ± 0.0024) were used as typical

of the global fallout in our latitudes. Owing to the radi-

ocesium decay, the radiocesium to plutonium ratio in 2012

was estimated to be *29 ± 3 and it was taken as a level of

the Chernobyl effect deduction.

The vertical profile of the radiocesium to plutonium

ratio in the sediment core of Lake Juodis showed that

mainly the surface layers of sediments down to the

*10.5 cm depth were affected by the Chernobyl deposits

(Fig. 5). However, that contribution was evidently small

(B2.5 % of the total radiocesium load in the core

(*1,518 Bq m-2; *0.041 Ci km-2).

The vertical profile of radiocesium to plutonium ratios in

the soil core taken near Lake Juodis (Fig. 6) showed the

presence of the Chernobyl input in the soil surface layer

down to the 2 cm depth. That contribution comprised

*84 % of the total radiocesium load of the surface layer

and *6.1 % of the total load in the core (*1,294 Bq m-2;

*0.035 Ci km-2). The inventory value of

*1,294 Bq m-2 was comparable to the total areal activity

density in the forest soils in Bavaria (Germany) varying

between 640 and 61,166 Bq m-2 [32].

Possibly, present data on plutonium and radiocesium

vertical profiles in the soil core also imply that deeper

peaks of radionuclide activities were related to the mobile

fractions of the radionuclide load in the core. The vertical

profile of radiocesium to plutonium ratios in the flooded

soil core taken in the old channel (Fig. 7) showed the

presence of the Chernobyl origin radiocesium in the sur-

face layer down to the 11 cm depth. That contribution

amounted to *21 % of the total radiocesium load in the

core (*3,082 Bq m-2; *0.083 Ci km-2) and *32 % of

the radiocesium amount in the *11 cm surface layer. The

second less active and deeper located peak of radiocesium

activity concentrations (at the 13–14 cm depth) in the soil

profile could also be attributed to the global fallout (the

radiocesium to plutonium ratio was in the range of global

variations). A larger depth of the second peak showed a

larger rate of its deepening as compared with that of upland

soil. Probably, it was related to the radiocesium leaching

processes in the percolating drainage water [33], which was

more abundant in the old channel. It might be proved by

consistency in the distribution of the respective peak in the

vertical profile of plutonium activity concentrations

0 1 2 3 40 80 120 16030

25

20

15

10

5

00 1 2 3 40 80 120 160

29±3

239,240Pu

137CsSoi

l dep

th, c

m


0 20 40 60 80 100

0 20 40 60 80 100

30

25

20

15

10

5

0

Soi

l dep

th, c

m


137Cs/239,240Pu

Fig. 7 The profiles of 239,240Pu and 137Cs activity concentrations in

flooded soil. Vertical lines indicate 137Cs/239,240Pu activity concen-

tration ratio for the global fallout (29 ± 3 value, reference year 2012)


123

(Figs. 3, 7). Equal rates of deepening in soil of different

radionuclides were also established earlier [34]. In the case

of the former soil core taken in the upland site on the

western bank of Lake Juodis, radionuclide leaching pro-

cesses might be only because of precipitation, which

determined lower rates of deepening of smaller radionu-

clide activity peaks. One of the striking features of the

vertical profile of the radiocesium to plutonium ratio in the

flooded soil core from the old channel (Fig. 7) was

appearance of the ratio values exceeding the global fallout

ones (and their variation limits) at the bottom of the core.

It might be suggested that leaching processes in the

percolating water of radiocesium of the Chernobyl origin

were more intensive and induced the formation of a more

mobile radiocesium fraction. It was known [31] that radi-

ocesium was more mobile in deeper soil layers due to an

increase of its exchangeable fractions with depth. Conse-

quently, this needs further studies. A noticeable increase in

the ratio values might also be seen in the respective data in

the forest soil core (Fig. 6). However, plutonium mea-

surements in the core were carried out only down to the

22 cm depth (Figs. 4, 6).

In general, this study of plutonium isotopes and 137Cs

provided an additional information on their vertical dis-

tributions in lake sediments and soil. Hereafter, it would be

very useful to examine radionuclide mobility dependence

on various geophysical and geochemical factors in more

detail.

Conclusions

Results of investigation of plutonium isotopes and 137Cs in

three different environmental systems showed a differing

radionuclide distribution in sampled cores.

Vertical profiles of plutonium activity concentrations in

the studied soil cores were determined to be formed

because of the global fallout and included two peaks. The

most intensive peaks were formed in the soil surface layers

at the 3–5 and *4–10 cm depth in the cores of the upland

and flooded soil samples, respectively. The less active

plutonium peaks were formed owing to the supposed more

mobile radionuclide fraction in deeper layers (at the

7–10 cm and at the 12–17 cm depth in the cores of the

upland and the flooded soil samples, respectively).238Pu/239,240Pu activity concentration and isotopic240Pu/239Pu ratios have shown that the global fallout from

nuclear weapons testing was the main Pu contamination

source in the studied environmental systems.

Vertical profiles of radiocesium activity concentrations

in the studied soil cores were formed as a result of the

global fallout with the additional impact of the Chernobyl

deposits. These profiles also included two peaks. The most

intensive peaks were formed in the soil surface and rep-

resented a sum of Chernobyl and global fallout deposits.

Chernobyl impacts were observed down to the *2 cm

depth and down to the *11 cm depth in the upland and the

flooded soil cores, respectively. The contribution of the

Chernobyl deposits amounted to *20.9 % and *6.11 %

of the total radiocesium inventory in the old channel

(*3,082 Bq m-2) and the upland soil (*1,294 Bq m-2)

cores, respectively. Due to Chernobyl deposits, a common

most intensive radiocesium peak in the vertical profile of

the upland soil core was also shifted to the 2 cm layer

while a component related to global fallout peaks at the

*2.6 cm depth. Less intensive peaks of radiocesium

activity were also caused by the global fallout and were

observed at larger depths (at the 5–8 cm and 13–14 cm

depth in the upland and in the flooded soil cores, respec-

tively). Probably, these results were mainly related to dif-

ferent radionuclide leaching effects in percolating drainage

waters in both cases.

The single peak at the *17–26 cm depth was charac-

teristic of vertical profiles of plutonium and radiocesium

activity concentrations in Lake Juodis sediments. For both

radionuclides, those peaks were induced by the global

fallout. The depths of peaks were dependent on the rate of

sedimentation (*4.7 mm year-1). Effects of the Cher-

nobyl deposits in the radiocesium vertical profile were

observable down to the *10.5 cm depth and the contri-

bution of that component makes up *2.26 % of the total

radiocesium inventory of the sediment core.

Besides, the inventory of studied radionuclides in the

sediment-sapropelic deposits in Lake Juodis was rather

high (104.3 Bq m-2 for 239,240Pu and *1,518 Bq m-2 for137Cs). Therefore, from the radiological point of view, it

would be harmful to use sapropel from the lake for agri-

cultural needs.

Acknowledgments This research was funded by a Grant (No. MIP-

041/2012) from the Research Council of Lithuania. We also wish to

acknowledge the helpful comments and suggestions of the editors and

of two anonymous referees.

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On peculiarities of vertical distribution of 239,240Pu, 238Pu and 137Cs activity concentrations and their ratios in lake sediments and soils

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