Comparative study of 137Cs partitioning between solid and liquid phases in Lakes Constance, Lugano and Vorsee

Journal of

Environmental Radioactivity 58 (2002) 1–11

Comparative study of 137Cs partitioning betweensolid and liquid phases in Lakes Constance,

Lugano and Vorsee

A. Konopleva,*, S. Kaminskib, E. Klemtb, I. Konoplevac,R. Millerb, G. Ziboldb

aScientific Production Association ‘‘Typhoon’’, Lenin Av., 82, 249038 Obninsk, Kaluga reg., RussiabFachhochschule Ravensburg-Weingarten, University of Applied Sciences, 88250 Weingarten, Germany

c Institute of Agricultural Radiology and Agroecology, 249020 Obninsk, Kaluga reg., Russia

Received 25 October 2000; received in revised form 5 March 2001; accepted 23 March 2001

Abstract

The methodology for estimating radiocaesium distribution between solid and liquid phasesin lakes is applied for three prealpine lakes: Lake Constance (Germany), Lake Lugano

(Switzerland) and Lake Vorsee (Germany). It is based on use of the exchangeable distributioncoefficient and application of the exchangeable radiocaesium interception potential (RIPex).The methodology was tested against experimental data. Good agreement was found between

estimated and measured 137Cs concentrations in Lake Constance and Lake Lugano, whereasfor Lake Vorsee a discrepancy was found. Bottom sediments in Lake Vorsee are composedmainly of organic material and probably cannot be described in terms of the specific sorptioncharacteristics attributed to illitic clay minerals. r 2001 Elsevier Science Ltd. All rights

reserved.

Keywords: Radiocaesium; Prealpine lakes; Sediments; Distribution coefficient

1. Introduction

The distribution coefficient Kd characterising the partitioning of a radionuclidebetween solid and liquid phases remains a basic parameter in prediction ofradionuclide behaviour in aquatic ecosystems (IAEA, 1994). The value of the total

*Corresponding author. Tel.: +7-08439-71896; fax: +7-08439-44204.

E-mail addresses: [email protected] (A. Konoplev).

0265-931X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 6 5 - 9 3 1 X ( 0 1 ) 0 0 0 7 5 - 3

distribution coefficient K totd , which is the ratio of the total radionuclide concentration

in the solid phase to its concentration in solution, is very sensitive to radionuclidespeciation in the solid phase (Konoplev, Bulgakov, Popov, a Bobovnikova, 1992;Konoplev a Bulgakov, 2000; Wauters et al., 1996). In the immediate term, only theexchangeable portion of a radionuclide contributes to solid–liquid interphaseexchange. Therefore, the notion of an exchangeable distribution coefficient Kex

d wasintroduced as a ratio of the concentration of the radionuclide in exchangeable formin the solid phase to its concentration in solution at equilibrium (Konoplev et al.,1992). The advantage of Kex

d is that its value is governed by ion exchange and can becalculated on the basis of environmental characteristics such as the capacity ofsorption sites and the cation composition of the solution.

It is well proven now that the high retention of radiocaesium in soil and bottomsediments is determined by two different processes: fixation and reversible selectivesorption. Fixation describes the ‘‘permanent’’ (or at least long-term) replacement ofinterlattice K- by Cs-ions. Reversible selective sorption of radiocaesium occurs onfrayed edge sites (FES), located at the edges of micaceous clay particles (Cremers,Elsen, De Preter, a Maes, 1988). The ability of a solid to sorb radiocaesiumselectively is characterised by the capacity of the selective sorption sites (FES) or bythe so-called radiocaesium interception potential (RIP), which is the product of theFES capacity and the selectivity coefficient of radiocaesium in relation to thecorresponding competitive ion (Sweeck, Wauters, Valcke, a Cremers, 1990).Cremers and co-workers (Cremers et al., 1988; Sweeck et al., 1990) developed aspecial method for the quantitative determination of FES capacity ([FES]) and RIP.The method is based on using silver thiourea Ag(TU)+ as a maskingagent for regular exchange sites (RES), which correspond to the planar and easilyexchangeable sites. RES selectively bind this complex. At the same time Ag(TU)+

does not interact with FES because of the molecule’s large size. Thus, the maskingblocks RES and allows the study of caesium sorption–desorption on FES.

This method, however, ignores Cs fixation during the equilibration time (24 h) andthus overestimates the amount of reversibly sorbed caesium. In the case of highlyorganic soils and bottom sediments, collapse of the interlayers of micaceous claydoes not allow the determination of [FES] using this method. To avoid thesedisadvantages, modifications of the procedure were proposed (Konoplev a

Konopleva, 1999), which included

* An additional step of ammonium acetate extraction after the equilibration time.This step avoids the influence of fast fixation on [FES] or RIP determination.Moreover, sorbed caesium in this case is measured directly in the ammoniumacetate extraction and the error of such a measurement is lower than the errorgenerated using Cremers et al. (1988) method, which takes the difference of twomeasurements to determine sorbed caesium.

* The [FES] is calculated using the linearised form of the Langmuir isotherm and itsvalue is given by the intercept on the ordinate of the graph of the inverseconcentration of reversibly sorbed caesium versus inverse equilibrium concentra-tion of caesium in solution. To obtain the FES capacity for highly organic soils

A. Konoplev et al. / J. Environ. Radioactivity 58 (2002) 1–112

and bottom sediments, the range of the isotherm before saturation (or inducedinterlayer collapse) can be used;

* Taking the initial range of the isotherm at low caesium concentrations one candetermine the capacity of high affinity sites [HAS] located between layers ofmicaceous clay particles. HAS have a much higher selectivity for caesium ascompared to average FES and therefore are occupied by caesium in the first stage.HAS represent 1–10% of FES.

The objective of this paper is to test the ability of the proposed modifiedmethodology to predict partitioning of 137Cs between sediments and water in threeprealpine lakes: Lake Constance (Germany), Lake Lugano (Switzerland) and LakeVorsee (Germany). They showed similar deposition levels on the water surfaceranging between 17 and 28 kBq/m2 of 137Cs originating from Chernobyl on1.05.1986; however, these lakes have very different limnological characters. Alocation map of the lakes is presented in Fig. 1.

2. Materials and methods

2.1. Study sites

2.1.1. Lake ConstanceLake Constance is a large and deep prealpine hardwater lake. It represents a

typical example of a large lake ecosystem with a very high self-purification ability(Kaminski, Konoplev, Lindner, a Schroeder, 1998). The main tributaries of thelake are the Alpine Rhine, Bregenzer Ache and Argen. The catchment area of theAlpine Rhine amounts to 6119 km2, which is the largest part of the total catchmentarea. From all the tributaries, about 11 km3 of water flows into the lake annually.The only outflow is the Rhine River. Lake Constance is morphologically subdivided

Fig. 1. Location map of the lakes.

A. Konoplev et al. / J. Environ. Radioactivity 58 (2002) 1–11 3

into two parts: the deeper Upper Lake Constance and the shallow Lower LakeConstance in the west. Illite, kaolinite and chlorite represent the typical clay mineralcomposition of sediments of the lake, amounting to about 45% of dry sediment inUpper Lake Constance (Robbins et al., 1992). The mean calcite content in the sedimentis between 20% and 30% in Upper Lake Constance and more than 40% in LowerLake Constance. The average value of organic matter in the sediments is about 5%.

2.1.2. Lake LuganoLake Lugano is one of the large drinking water reservoirs of southern Switzerland,

situated in the foothills of the southern Alps. Lake Lugano is divided into two partsby a morainic front (about 5 km south of Lugano) on which an artificial dam wasbuilt. The deeper northern basin with a maximum depth of 288m close to the villageGandria has a water residence time of 30 y, whereas the water residence time in therest of the lake is only 2–3 y (Niessen, 1987). The main tributaries of Lake Luganoare the Cassarate, Vedeggio, Magliasina and Cuccio. The outflow leading to LagoMaggiore is the river Tresa, with a mean outflow rate of 24.2m3/s (HydrologischesJahrbuch der Schweiz, 1995), which means that about 0.8 km3 of water flows out ofLake Lugano per year. The mineralogy of the sediments can be characterised asfollows: the dominant mineral components in all basins are quartz and mica (biotiteand muscovite); in the eastern parts of the basins, carbonates (calcite and dolomite)are frequent (Niessen, 1987). About 15–20% of the sediment dry mass is of anorganic nature. The removal of radiocaesium from the water column of LakeLugano was much slower than in Lake Constance. 137Cs residence times, calculatedin 1988 (Santschi, Bollhalder, Zingg, Luck, a Farrenkothen, 1990), were 5 monthsfor Lake Constance, 14 months for the southern basin of Lake Lugano, and 21months for the northern basin.

2.1.3. Lake VorseeVorsee is situated 40 km north of Lake Constance. It is a glacially- formed,

eutrophic lake. Bottom sediments have a very loose consistency down to a depth of7m, with a high water content (still 96% at 60 cm sediment depth); the rest is mainlyorganic matter (from 70% at the sediment surface to 55% at 60 cm sediment depth,percentage of dry sediment). Lake Vorsee shows a slightly higher initialcontamination than Lake Constance, because weather conditions have been verydifferent for the two lakes on 1.05.1986 due to local thunderstorms resulting indifferent amounts of 137Cs washout.

Table 1 summarises the main characteristics of the lakes under study.The comparison of 137Cs behaviour in these three lakes was aimed at verification

of the proposed methodology for the lakes with diverse limnological andhydrochemical characteristics.

2.2. Sampling

Bottom sediment cores from Lake Constance and Lake Lugano were collected in1996 using a gravity sampler (Meischner a Rumohr, 1974) with a 5.8 cm inner

A. Konoplev et al. / J. Environ. Radioactivity 58 (2002) 1–114

diameter. The sediment cores were split longitudinally and then sliced in layers of 1or 2 cm thickness. Sediments from Lake Vorsee were taken in 1996 with a gravitycorer specially constructed for soft sediments, which allowed the taking of porewater and solid material from different sediment depths. After freeze-drying, thetotal activity concentrations of 137Cs and 134Cs in the sediments were measured bygamma-spectrometry using HPGe detectors.

Lake and river waters were collected in 1996 and 1999 using ‘‘Midiya’’, a largevolume water sampler developed in SPA ‘‘Typhoon’’ (Makhonko, 1990) and capableof performing both collection of suspended material and fixation of dissolved 137Cs.Water volumes from 1000 to 3000 l were filtered through sets of ‘‘Petryanov-filters’’(Makhonko, 1990). Dissolved 137Cs was fixed using columns containing ‘‘Fezhelsorbent’’ based on wood cellulose coated with ferric ferrocyanide (Remis, 1996). InOctober 1996, water samples from four different depths were collected in the central(deepest) part of Upper Lake Constance to study the vertical distribution of 137Cs inthe lake water. About 600 l of lake water from the surface layer at the same locationwere tangentially filtered using the ‘‘Millipore system’’.

2.3. Physico-chemical analysis

2.3.1. The capacity of the FES (modified method)Air-dried sediment samples (0.1–1 g) were pre-equilibrated with 30ml of

a 0.015mol/l solution of silver thiourea (AgTU+4 ), shaken overnight, centrifuged

(30min, 5000 rpm) and the supernatants discarded (the same procedure isalso applicable to soil samples). Caesium sorption isotherms were measuredby equilibrating (24 h) sediment samples with AgTU+

4 solutions (0.015mol/l)containing increasing levels of CsNO3 (137Cs labelled) from 0.2 to 1.6mmol/l.Then samples were centrifuged and exchangeable Cs was extracted fromsediment samples with 30ml of 1mol/l ammonium acetate solution. 137Cs wascounted in the extract using an HPGe detector. Results were plotted as the inverseconcentration of exchangeable Cs in the solid phase versus the inverse Csconcentration in the equilibrium solution. According to a linearised form of theLangmuir isotherm, which describes the adsorption on homogeneous sites withsaturation at high concentrations, the capacity of FES was taken as the inverse

Table 1

Characteristics of the lakes under study

Lake Constance Lake Lugano Lake Vorsee

Surface area (km2) 572 48.9 0.09

Mean water depth (m) 85 134 0.6

Maximum water depth (m) 254 288 2.2

Mean residence time (y) 4.1 7 0.24

Drainage basin (km2) 11,487 615 1.27

Initial Chernobyl 137Cs fallout (kBq/m2) 17 24 28

A. Konoplev et al. / J. Environ. Radioactivity 58 (2002) 1–11 5

intercept of this linear dependence:

1

Cs½ �ads¼

1

FES½ �þ

const:

FES½ �1

Cs½ �sol: ð1Þ

2.3.2. Exchangeable radiocaesium interception potential RIPex(K) and RIPex(NH4)0.5 g samples of air-dried sediment were pre-equilibrated with 30ml of 0.015mol/l

Ag(TU)4+ solutions containing different KCl concentrations ranging from 2 to

10meq/l. Phase separation was made by high speed centrifugation (30min,5000 rpm). Liquid phase was then discarded. Sediment samples were thenequilibrated with the same Ag(TU)4

+/KCl mixtures containing labelled 137Cs. After24 h of shaking and centrifugation, exchangeable 137Cs was extracted from thesediment by 30ml of 1mol/l ammonium acetate solution. After phase separation,137Cs was measured in extracts using HPGe detectors. The measured Kexd was thenmultiplied by the appropriate molarity of potassium mk and the product Kexd Csð Þmk

plotted against mk. The exchangeable radiocaesium interception potential, i.e. theplateau value of Kexd Csð Þmk attained at high mk, was then read off from the graph.

The same experimental protocol was followed to obtain RIPex(NH4) except thatNH4Cl was used instead of KCl. The selectivity coefficient KFES

c NH4=K� �

wascalculated as a ratio of RIPex(NH4) and RIPex(K).

2.3.3. Measurement of major ionsMajor cations Ca2+, Mg2+, K+ and Na+ in water and in extracts were measured

by atomic absorption spectroscopy (AAS); ammonium was measured using aspectrophotometric technique.

3. Results and discussions

Table 2 presents the data on the cation compositions of the lake waters under study.As can be seen from Table 2, all lake waters have rather similar ionic

compositions. All three lakes are characterised by relatively high concentrations ofcalcium and relatively low concentrations of potassium. The substantial differencebetween the lakes is the different ammonium concentrations. There are extremely lowconcentrations of ammonium in Upper Lake Constance, measurable levels in LowerLake Constance, medium concentrations in Lake Lugano and high concentrations inVorsee particularly in bottom sediment pore waters (up to 90mg/1). Ammonium isthe strongest natural competitor of radiocaesium for FES. Taking into account therelatively low concentrations of potassium in all the lakes, one can expect thatradiocaesium will have the highest mobility in Vorsee, less mobility in Lake Luganoand Lower Lake Constance and the lowest mobility in Upper Lake Constance.Cation composition, concentrations of 137Cs and its exchangeability in bottomsediments of the lakes under study are presented in Table 3.

As an illustration, isotherms of Cs selective sorption on FES of bottom sedimentsfrom Upper and Lower Lake Constance are presented in Fig. 2.

A. Konoplev et al. / J. Environ. Radioactivity 58 (2002) 1–116

For the lakes under study, the highest capacity of FES was found for bottomsediments from Lake Lugano (7.0meq/kg) and the lowest for bottom sediments fromVorsee (1.1meq/kg), as can be seen in Table 4. It is also interesting to note that thecapacity of FES in Lower Lake Constance is about a factor 2 lower than in UpperLake Constance. We think that the main reason for the difference in FES capacitybetween the Upper and Lower parts of Lake Constance is the different compositionsof their bottom sediments. In particular, bottom sediments from Lower LakeConstance contain much more organic material and less micaceous clay minerals.

Table 4 summarises the data obtained on radiocaesium selective sorptioncharacteristics of bottom sediments. Using the data of Table 4 together with thedata on the cationic compositions of the lake waters (Table 2), it is possible toestimate the radiocaesium distribution coefficient (Kd) and its concentration in waterusing the following equation (Wauters et al., 1996):

Kexd ¼RIPex Kð Þ

mk þ KFESc NH4=K

� �mNH4

; ð2Þ

Table 2

Cation compositions of lake waters under study

Lake [K+]

(mg/l)

[NH+4 ]

(mg/l)

[Ca2+]

(mg/l)

[Mg2+]

(mg/l)

[Na+]

(mg/l)

pH

Upper Lake Constance 1.1–1.3 o0.01 48.7–54.1 6.7–9.7 4.4–4.7 7.5–8.1

Lower Lake Constance 0.8–1.2 0.01–0.17 41.3–53.7 6.6–8.3 4.2–4.5 7.5–8.5

Lake Lugano (Agno basin) o0.4 0.23–0.52 25–32 5–8 8–18 7.7

Vorsee 0.9–2.0 0.04–1.0 53–118 6.9–8.6 3.5–5.0 7.4–9.4

Table 3137Cs concentration, 137Cs exchangeability, cation exchange capacity and exchangeable cation composi-

tion of the studied bottom sediments

137Cs,

(Bq/kg)

aex (%) CEC

(meq/kg)

[Ca]ex(meq/kg)

[Mg]ex(meq/kg)

[K]ex(meq/kg)

[Na]ex(meq/kg)

Upper Lake Constance

Bottom sediments

(upper 4 cm)

460790 2.971.0 240717 222715 1872 0.270.1 0.5070.07

Lake Lugano (Agno basin)

Bottom sediments

(upper 10 cm)

2800 [email protected] 553 6.2 97.5 0.7 0.1

Lake Vorsee (bottom sediments)

Layer 0–10 cm 3110 9 630 540 59 29 2

Layer 10–20 cm 3890 7.7 600 510 61 23 2

Layer 20–30 cm 3210 9.8 600 525 55 14 2

Layer 30–40 cm 860 18.3 710 650 50 6 2

Layer 40–50 cm 650 15.4 640 590 43 5 2

A. Konoplev et al. / J. Environ. Radioactivity 58 (2002) 1–11 7

where KFESc NH4=K

� �is the selectivity coefficient of ammonium sorption on FES in

relation to potassium, mk and mNH4are the molarities of potassium and ammonium

in solution, respectively. The results of these calculations for the lakes under studyare presented in Table 5. To calculate Ktotd , the measured values of the 137Csexchangeability (Table 3) have been used:

Kexd ¼ aexKtotd : ð3Þ

The 137Cs concentration in water was calculated by assuming that its concentrationin surface sediments is the same as in suspended matter. As can be seen from Table 5,the 137Cs concentration in Lower Lake Constance is expected to be 3–10 times higherthan in Upper Lake Constance. There are two main reasons for this difference: (1)the lower ability of bottom sediments in Lower Lake Constance to selectively bindradiocaesium (Table 4); (2) the generation of ammonium in eutrophic Lower LakeConstance during late summer–autumn and radiocaesium remobilisation due tocation exchange with ammonium.

To test the applicability of the method of estimation of distribution coefficients,the 137Cs activity concentrations in water and in suspended matter of both parts of

Fig. 2. Isotherm of Cs selective sorption on FES of bottom sediments from Lower Lake Constance: open

dots, and Upper Lake Constance: filled dots.

Table 4

Characteristics of radiocaesium selective sorption by bottom sediments (upper 10 cm) from lakes under

study

Lake [FES]

(meq/kg)

[HAS]

(meq/kg)

RIPex(K)

(meq/kg)

RIPex(NH4)

(meq/kg)

KFESc

(NH4/K)

Upper Lake Constance 4.1 F 1300 434 3.0

Lower Lake Constance 1.7 F 478 187 2.6

Lake Lugano (Agno basin) 7.0 0.38 224 130 1.7

Vorsee 1.1 0.12 87 60 1.45

A. Konoplev et al. / J. Environ. Radioactivity 58 (2002) 1–118

Lake Constance were measured as well as in Rhine river inflowing to and outflowingfrom Lake Constance using the ‘‘Mydiya’’ sampling system. The Alpine Rhine Riverflows into Upper Lake Constance and the River Rhine flows out from Lower LakeConstance. These results are presented in Table 6.

To assess the heterogeneity of 137Cs activity concentration with depth in LakeConstance, the vertical distribution of 137Cs in lake water was studied (Fig. 3). Thecalculated values agree with the experimental values, which vary depending on thedepth by a factor of 5.

137Cs concentrations measured in the water of Lake Lugano is 1mBq/1 forthe surface layer and 5mBq/1 for the layer close to the bottom (Fig. 3).The agreement between estimated and measured values for Lake Constanceand Lake Lugano is encouraging, taking into account the possible spatialheterogeneity of the characteristics of each lake. The values of the distributioncoefficients estimated as described have been used to model the 137Cs verticaldistribution in bottom sediments of Lake Constance (Konoplev et al., 1996).Good agreement of the calculated results and observations also confirms theapplicability of the method.

At the same time, for Vorsee there is a factor of 2–3 difference betweenthe estimated and measured values of 137Cs concentration. The possible reasonfor this discrepancy could be the fact that the bottom sediments in Vorsee mainlyconsist of organic matter and water. In this case, the proposed approach for Kdprediction, based on selective sorption by illitic clay minerals, is probably onlypartially valid.

Table 6

Results of measurements of 137Cs activity concentration in solution and in suspended matter and measured

in situ values of total distribution coefficients in Lake Constance and inflowing Alpine Rhine and

outflowing Rhine River in 1996

Water object 137Cs in solution

(mBq/1)

137Cs in suspended matter

(Bq/kg)

Ktotd(1/kg)

Alpine Rhine 0.0770.05 1072 1.4� 105

Upper Lake Constance 0.2370.04 120730 5.2� 105

Lower Lake Constance 1.2170.07 56713 4.6� 104

Rhine River 0.2970.06 8007300 2.7� 106

Table 5

Results of estimation of distribution coefficients and concentrations of dissolved 137Cs in waters of the

lakes under study

Lake Kexd (1/kg) Ktotd (1/kg) [137Cs] (mBq/1)

Upper Lake Constance 32,500 106 0.2

Lower Lake Constance 3750–12,000 105@3*105 0.7–2.0

Lake Lugano 5200 157,000 2–7

Lake Vorsee 1150 8100 380

A. Konoplev et al. / J. Environ. Radioactivity 58 (2002) 1–11 9

4. Conclusions

* Estimated and measured values of Ktotd for 137Cs in Upper Lake Constance arevery high (up to 106 1/kg). The main reasons for this are a high content of clayminerals in the sediments of the lake and very low concentrations of competingions (K+, NH4

+). A high concentration of Ca2+ in water is favourable forenhanced fixation of 137Cs by illitic clay minerals.

*137Cs remobilisation is essentially higher in Lower Lake Constance comparedwith Upper Lake Constance because of the lower binding ability of the sedimentsand the existence of a considerable concentration of ammonium during latesummer and autumn.

* Estimated 137Cs activity concentrations in Lake Constance and Lake Lugano arein good agreement with measured data.

* Disagreement by a factor of 2–3 between the measured 137Cs activityconcentrations and estimated values for Lake Vorsee indicates that themethodology fails to predict the radiocaesium distributions in lakes with highlyorganic sediments.

Acknowledgements

This work was partly supported by the Alexander von Humboldt Foundation(Germany), which provided a research fellowship to A. Konoplev, and by the CECProject IC15 CT98-0205 (AQUASCOPE). The authors would like to thank O.Voitsekhovich and V. Kanivets from the Ukrainian Hydrometeorological Institutefor their help in water sampling.

Fig. 3. Depth distribution of the 137Cs activity concentration in Lake Constance and in Lake Lugano.

A. Konoplev et al. / J. Environ. Radioactivity 58 (2002) 1–1110

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