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Some considerations concerning 137Cs vertical profile in the Danube Delta: Matita Lake core
ELSEVIER
The Science of the Total
Environment
188 (1996)
9-14
Some considerations concerning 137Csvertical profile in the Danube Delta: Matita Lake core Octavian G. Duliu*a, Lucretia C. Dinescub, Marius C. Dinescub, Radu D. Dorciomanb, N:icolae Gh. Mihailescu”, Ion S. Vanghelie” aUniversity ‘Institute
of Bucharest,
Department
of Atomic
and Nuclear Physics, Ma’gureele. PO Box Mg - II. Bucharest, Romania of Physics and Nuclear Engineering, M&urele, PO Box Mg - 06. RO-76900, Bucharest, CGeological Survey of Romania, I, Caransebes str., RO-78344, Bucharest, Romania
RO-76900, Romania
Received4 December 1995;accepted7 March 1996
Abstract
By using y-ray spectrometryand x-ray diffraction, the vertical profile of 137Cs specificactivity as well as the mineralogicalcompositionof a set of corescollected from the Danube Delta-Mat&a Lake was investigated;the observedprofilesare typical for an undisturbedsite. An overlappingof the 1963nucleartestsand 1986Chemobylderived 137Cs specificactivity peak wasobserved.From the experimentalprofiles,two different valuesfor the mixing rate, D, have beendetermined:D, = 1.81 x lo-* cm2s-’ for the upper IO-cmlayer and Dz = 6.20 x lo-* cm’ s-’ for the next lo-30-cm layer. Thesevaluescorrespolndto different bioturbation regimes.The sedimentationrate hasbeen determinedasR = 0.15 f 0.05g cm-’ year-‘. The total cesiuminventory is 3.596f 0.090kBq m-*, a value approximately equalto the sumof the nuclear testsand Chernobyl fallout. Keywords:
Core; Radiocesiumdiffusion; Vertical profile; Sedimentationrate; Total ‘37Csinventory
1. Introduction
Matita lake, with a surface area of 6.44 km2 is located in a lacustrine depression at approximately equal distances (- 12 km) between the Sul.ina and Chilia branches of the Danube Delta [l]. It is connected to the DunZrea Veche by the Lopatna channel. Another channel, Rosca, connects Matita Lake to Merhei Lake (Fig. 1). Both the Lopatna Corresponding
author.
0048-9697/96/$15.00 PII SOO48-9697(96)05
0
l
1996 Elsevier 130-3
Science
B.V.
and Rosca channels have a very meandered pattern and thus, a considerable fraction of the sediments transported by them does not reach Lake Matita. The hydrological regime of the Danube, which is characterized by spring time floods followed by a long period of regression, causes the water to flow in these channels in both directions, to and from Matita Lake, and thus, the sediments deposited in these channels are continuously washed out. Only the finest fraction of the sediments carried by the Danube reach the lake
All rightsreserved
10
O.G.
Fig. 1. Map
Duliu
of the Danube
et al. /The
Delta
with
Science
of the Total
inset showing
[2]. At the same time, dust, transported by wind is deposited onto the lake. The bottom sediments of Lake Matita consist of lacustrine deposits (O-50 cm; O-3500 years BP), brackish deposits (50-120 cm; 3500-7200 years BP) and continental, lessoide deposits (below 120 cm) [31. The core profile we refer to in this paper represents only the upper part of the lacustrine sediments, which are mainly composed of silts of
Environment
the location
188 (1996)
of the coring
9-14
point
at Matita
Lake.
varied granulation and contain specimens as well as fragments of Dreissena polymorpha Pallas and Vivipams sp. Variations in the level of the Black Sea as well as of the main flow of the Danube [4] have resulted in significant differences in the composition of sediments, including shell debris concentrations. Clay, which is one of the main constituents of these sediments, has a remarkable affinity for binding caesium irreversibly [5] and plays a major
O.G.
Duliu
et al. /The
Science
of the Tota/ Environment
role in the process of retention and redistribution of caesium throughout lake sediments. Therefore, the most suitable method for a better understanding of the transport, sedimentation and circulation of the lake sediments consists of the determination of the total cesium inventory [6] followed by a comparison with the total local cesium fallout. The bottom sediments of almost all the lakes of the Danube Delta are significantly bioturbated as a result of the biological activity of various annelid worms, insect larvae (mainly chironomids) and molluscs who spend their life cycle buried in the mud. Traces of their activity is obvious in cores as well as in samples of superficial sediments [7]. The vertical profile of these sediments consists of an alternation of different thickness layers, mainly composed of silt, sand, shells, conglomerates, etc., which also influence the vertical distribution of radiocesium.
188 (1996)
9-14
11
This paper presents the results obtained by the investigation of cores collected from Matita Lake during the 1994 autumn campaign. 2. Experimental The cores were obtained during the 1994 autumn campaign. To obtain sediments with minimum disturbance caused by winter storms, the coring site was chosen in the center of the lake, at a depth of - 2.5 m below the water level. The position of Matita Lake within the Danube Delta as well as the location of the sampling point are shown in Fig. 1. The cores were collected in 7-cm inner diameter polystyrene tubes using a Kullenberg corer. After collection, the tubes were sealed at both ends and stored in a horizontal position at a temperature of between 10 and 14°C. The cores were cut into 2-cm slices and dried for 2 h at 105°C. Cs - 137 sediment specific activity (Bq/kg)
Core lithoiogic structure
0
50
loo
150
200
0
~~, silt with living pelecypodes
and gastrop0d.s
shells
silt with rare shell fragments silt and very rare shell fragments 1-1 silt and few shell fragments
a
b
Fig. 2. (a) Schematic representation of a vertical section through the investigated core showing its lithologic structure. The variation in shell debris concentration is mainly due to the variation of the level of the Black Sea as well as sediment flow of the Danube over the past 3500 years [4]. (b) Vertical profile of the radiocesium specific activity of the sediments corresponding to the same core. The maximum 13’Cs specific activity corresponds to the 1963 maximum radioactive fallout.
12
O.G.
Duliu
et al. / The Science
of the Total Environment
3. Results and discussion core composition
In Fig. 2a a longitudinal section through the core is illustrated which shows an alternation of at least four different layers containing living molluscs and fragments of shells. The lithologic structure of these layers can be correlated with their mineralogical composition (Table 1). The first level (O-10 cm below the sediments surface) consists of a very tine silt which contains
Table 1 Mineralogical Layer
composition Depth (4
9-14
living bivalves as well as other benthic organisms. With a total mineralization of between 69 and 81%, this level is composed mainly of calcite (65-50%), clay (14-22%) and quartz (7-100/o. The presence of shell determines, in our opinion, the predominance of calcite over other minerals. The mineralogical composition is almost constant along this level. The second layer (lo- 14 cm) also contains many entire shells and shell fragments (Dreissena polymorpha Pallas, Viviparus sP.)* The mineralogical composition of this layer is characterized by the quasi-absence of clay, while the calcite and quartz contents are close to those found in the upper layer. The third level (14-25 cm) is composed mainly of silt with rare fragments of shell. The calcite concentration of this layer monotonously decreases with depth while the quartz content doubles its value, compared with the first and the second layer. The clay concentration reaches that determined in the first layer and the feldspar concentration increases from 2% to 6-8%. The fourth level (25-32 cm) is mainly composed of silt with very rare shell fragments. Its mineralogical structure consist of almost equal
The y spectra of each aliquot was recorded for 40 h using a 80-cm3 HPG detector for the first three slices (upper O-6 cm) to detect the presence of ‘34Cs and for 20 h for the rest of the samples. The specific activity of ‘34Cs (795 keV line) and 13’Cs (661.5 keV line) was determined using AIEA-156 and AIEA-367 standards [8]. The lithologic structure of the cores was carried out by visual inspection while the general mineralogical composition of each slice was determined by x-ray diffraction. The relative concentrations of the following classes of minerals were obtained: clay, calcite, dolomite, feldspars and quartz as shown in Table 1.
3.1. Total mineralogical
I88 (I996)
of the core Feldspar (“4
Calcite W)
Dolomite (W
Clay (“W
Quartz (“4
Total (“4
I
o-2 2-4 4-6 6-8 S-10
50 50 39 47 48
0 0 0 0 0
18 22 18 14 16
10 10 10 10
81 84 69 70 16
II
10-12 12-14
45 44
0 0
0 0
10 10
57 56
III
14-17 17-19 19-21 21-23 23-25
44 36 27 24 26
0 0 3 3 3
18 18 22 20 25
12 15 25 20 21
76 71 82 13 83
IV
25-27 27-30 30-32
21 22 22
2 3 3
31 25 25
29 18 22
91 15 78
O.G.
Duliu el al. /The
Science of the Total Environment
188 (1996)
9-14
13
dating [3]. The fact that the tail of ‘37Cs distribution reaches a depth of 30 cm can be explained by considering an increased spreading of 137C~, mainly due to the biological activity of benthic organisms. We determined the following value for average sedimentation rate: R = 0.15 f 0.05 g cmm2 year-’ 10.00
:
h
1.00
’ 0
s ’
’
z h ’ 10 Depth
’
’
’
8 ’ 20
’
’
1 30
(cm)
Fig. 3. Logarithmic plot of the previous vertical profile illustrating the radiocesium specific activity of the sediments. Two core segments of different mixing rate values can be distinguished. Discontinuity of the specific activity appears at the boundary between two sediment layers of different lithologic compositions.
The presence of living organisms below the sediment surface [7] results in a certain degree of bioturbation. Therefore, the radiocaesium distribution within the sediment will be determined by both diffusion and the biological activity of living organisms. In Fig. 3 a semilogarithmic plot of 137Csspecific activity (A) over sediment depth (x) is represented. The experimental points lie on two straight lines of different slopes; the specific activity variation with depth is exponential, given by the equation: A = &.exp
fractions of clay (25-31%), calcite (21-22%) and quartz (18-29%). This is the last level which still contains a small amount of r3’Cs. 3.2. ‘37Cs specific activity vertical profire
The 13’Cs specific activity profile shows a maximum at 2-4 cm below the sediments upper surface and falls to zero at a depth of -30 cm (Fig. 2b). The 134Csspecific activity was - 1 Bq kg-’ (which was within the detection limit) in the upper 6 cm of sediment and vanished completely in the next layer. Taking into account the experimentally determined 134Cs to 13’Cs ratio, the Chernobyl ‘37Cs represents < 10% of the total ‘37Cs found in the first 6 cm of sediment. Hence, the observed 13’Cs vertical profile represents a superpos#ition of the 1963 atomic tests debris maximum over more recent 1986 Chenobyl fallout. Consequently, the observed 13’Cs maximum can be attributeld to the 1963 atomic tests fallout and is explained by taking into account an extremely reduced sedimentation rate, which is in agreement with radiocarbon
(-k-x) (1)
where k is a numeric coefficient which has different values for each layer. From this profile, two values of the mixing ratio were derived: D1 = 1.81 x lo-* cm2 s-’ for the sediment and 1O-cm layer upper D2 = 6.20 x 10d8 cm’ s-’ for the next lo-30-cm layer. Similar patterns of vertical radiocaesium profiles have been reported before 19,lo]. The values obtained correspond to different bioturbation regimes and characterize the downward penetration of 137Csin the lake sediments. 3.3. Total ‘j7Cs inventory
By adding the total radiocaesium activity over all slices, the total cesium inventory is equal to 3.60 f 0.11 kBq rne2, while the local value of nuclear tests and Chernobyl total fallout as calculated from other data 111,121 is - 3.345 kBq rnm2. The numerical value thus obtained is consistent with our assumption concerning the relative isolation of Matita Lake with respect to the main branches of the Danube.
14
O.G. Duiiu
et al. / The Science
of the Total Environment
4. Conclusion
By the investigation of *37Cs vertical profile in lacustrine sediments the sedimentation rate of recent lake sediments can be determined while the total cesium inventory permits a better understanding of sedimentation dynamics. In the case of Lake Mattita both sedimentation rate and total cesium inventory are in good agreement with the assumption of the relative isolation of this lake with respect to the Danube’s main branches. The penetration of radiocesium in sediment is well correlated with its lithologic structure and is significantly influenced by the activity of benthic organisms. Acknowledgements
The authors thank Prof. D. Barb for useful discussions, Dr. 0. Sima for helpful advice and Dr. 1. Papaianopol for the determination of mollust debris. References [l] [2]
[3]
A.C. Banu and L. Rudescu, Danube Delta, Edit. Stiintifi&, Bucharest, 1965, p. 185 (in Romanian). C. Diaconu, I.D. Nichiforov (Eds), The Danube Emptying Zone. A Hydrological Monograph, Water State Committee, Bucharest, 1963, 78 pp (in Romanian). J.E. Noakes and N. Hertz, University of Georgia radiocarbon dates VII, Radiocarbon, 25 (1983) 919-929.
188 (1996)
9-14
The evolution of the fluvial network of [41 N. Mihailescu, the Danube Delta in Pleistocene and Holocene, Trav. Mus. d’Hist. Nat. ‘Grigore Antipa’, 355-366. W. Penington, R.S. Cambray and E.M. Fisher, Observations on lake sediments using fallout 13?Cs as a tracer, Nature, 242 (1973) 342-346. of 161 D.E. Walling and He. Qingping, Interpretation cesium- 137 profiles in lacustrine and other sediments; the role of cat&intent-derived inputs, Hydrologia, 235/236 (1992) 219-230. S. Radan. L. Artin, V. Iosaf, I. Vanghelie, I71 N. Mihailescu, N. Iosipescu, S. Roman, S.C. Radan and M. Radan, Geographic and Geophysics Study of the Danube Delta (Rosu-Rosculet Sector), Arch. Geological Survey of Romania, 1982, 187 pp (in Romanian). Quality Control Services, IAEA, Vienna, PI Analitical 1994-1995,41 pp. [91 J.-L. Reyes, P. Bonte, S. Schmidt, F. Legeleux and C. Organo, Radiochemical study of the EUMELI sites sediments, Ann. Inst. O&anogr., Paris, 69 (1993) 43-45. rto1 J.-L. Reyes, S. Schmidt, F. Legeleux and Ph. Bonte, Mesure de radioactivite dans l’environement a I’aide de detecteurs ‘germanium puits’ de grand eflicacite et de tres faible bruit de fond, Actes des ‘Joumes de Spectrometrie Gamma et x’, Octobre, 1993. illI D.E. Walling and T.A. Quine, Use of Cesium-137 as a Tracer of Erosion and Sedimentation: Handbook for the Application of Cesium-137 Technique, UK Overseas Development Administration Research Schemes, R 4576, Department of Geography, University of Exeter, 1993, pp. 17-110. m S. Sonoc, I. Osvath and C. Dovlete, Gamma emitter radionuclide concentration in aerosol and deposition sampled during May-June 1986 in Romania, Meteorol. Hydrol. Special Rep., 1989, pp. 13-21.
The Science of the Total
Environment
188 (1996)
9-14
Some considerations concerning 137Csvertical profile in the Danube Delta: Matita Lake core Octavian G. Duliu*a, Lucretia C. Dinescub, Marius C. Dinescub, Radu D. Dorciomanb, N:icolae Gh. Mihailescu”, Ion S. Vanghelie” aUniversity ‘Institute
of Bucharest,
Department
of Atomic
and Nuclear Physics, Ma’gureele. PO Box Mg - II. Bucharest, Romania of Physics and Nuclear Engineering, M&urele, PO Box Mg - 06. RO-76900, Bucharest, CGeological Survey of Romania, I, Caransebes str., RO-78344, Bucharest, Romania
RO-76900, Romania
Received4 December 1995;accepted7 March 1996
Abstract
By using y-ray spectrometryand x-ray diffraction, the vertical profile of 137Cs specificactivity as well as the mineralogicalcompositionof a set of corescollected from the Danube Delta-Mat&a Lake was investigated;the observedprofilesare typical for an undisturbedsite. An overlappingof the 1963nucleartestsand 1986Chemobylderived 137Cs specificactivity peak wasobserved.From the experimentalprofiles,two different valuesfor the mixing rate, D, have beendetermined:D, = 1.81 x lo-* cm2s-’ for the upper IO-cmlayer and Dz = 6.20 x lo-* cm’ s-’ for the next lo-30-cm layer. Thesevaluescorrespolndto different bioturbation regimes.The sedimentationrate hasbeen determinedasR = 0.15 f 0.05g cm-’ year-‘. The total cesiuminventory is 3.596f 0.090kBq m-*, a value approximately equalto the sumof the nuclear testsand Chernobyl fallout. Keywords:
Core; Radiocesiumdiffusion; Vertical profile; Sedimentationrate; Total ‘37Csinventory
1. Introduction
Matita lake, with a surface area of 6.44 km2 is located in a lacustrine depression at approximately equal distances (- 12 km) between the Sul.ina and Chilia branches of the Danube Delta [l]. It is connected to the DunZrea Veche by the Lopatna channel. Another channel, Rosca, connects Matita Lake to Merhei Lake (Fig. 1). Both the Lopatna Corresponding
author.
0048-9697/96/$15.00 PII SOO48-9697(96)05
0
l
1996 Elsevier 130-3
Science
B.V.
and Rosca channels have a very meandered pattern and thus, a considerable fraction of the sediments transported by them does not reach Lake Matita. The hydrological regime of the Danube, which is characterized by spring time floods followed by a long period of regression, causes the water to flow in these channels in both directions, to and from Matita Lake, and thus, the sediments deposited in these channels are continuously washed out. Only the finest fraction of the sediments carried by the Danube reach the lake
All rightsreserved
10
O.G.
Fig. 1. Map
Duliu
of the Danube
et al. /The
Delta
with
Science
of the Total
inset showing
[2]. At the same time, dust, transported by wind is deposited onto the lake. The bottom sediments of Lake Matita consist of lacustrine deposits (O-50 cm; O-3500 years BP), brackish deposits (50-120 cm; 3500-7200 years BP) and continental, lessoide deposits (below 120 cm) [31. The core profile we refer to in this paper represents only the upper part of the lacustrine sediments, which are mainly composed of silts of
Environment
the location
188 (1996)
of the coring
9-14
point
at Matita
Lake.
varied granulation and contain specimens as well as fragments of Dreissena polymorpha Pallas and Vivipams sp. Variations in the level of the Black Sea as well as of the main flow of the Danube [4] have resulted in significant differences in the composition of sediments, including shell debris concentrations. Clay, which is one of the main constituents of these sediments, has a remarkable affinity for binding caesium irreversibly [5] and plays a major
O.G.
Duliu
et al. /The
Science
of the Tota/ Environment
role in the process of retention and redistribution of caesium throughout lake sediments. Therefore, the most suitable method for a better understanding of the transport, sedimentation and circulation of the lake sediments consists of the determination of the total cesium inventory [6] followed by a comparison with the total local cesium fallout. The bottom sediments of almost all the lakes of the Danube Delta are significantly bioturbated as a result of the biological activity of various annelid worms, insect larvae (mainly chironomids) and molluscs who spend their life cycle buried in the mud. Traces of their activity is obvious in cores as well as in samples of superficial sediments [7]. The vertical profile of these sediments consists of an alternation of different thickness layers, mainly composed of silt, sand, shells, conglomerates, etc., which also influence the vertical distribution of radiocesium.
188 (1996)
9-14
11
This paper presents the results obtained by the investigation of cores collected from Matita Lake during the 1994 autumn campaign. 2. Experimental The cores were obtained during the 1994 autumn campaign. To obtain sediments with minimum disturbance caused by winter storms, the coring site was chosen in the center of the lake, at a depth of - 2.5 m below the water level. The position of Matita Lake within the Danube Delta as well as the location of the sampling point are shown in Fig. 1. The cores were collected in 7-cm inner diameter polystyrene tubes using a Kullenberg corer. After collection, the tubes were sealed at both ends and stored in a horizontal position at a temperature of between 10 and 14°C. The cores were cut into 2-cm slices and dried for 2 h at 105°C. Cs - 137 sediment specific activity (Bq/kg)
Core lithoiogic structure
0
50
loo
150
200
0
~~, silt with living pelecypodes
and gastrop0d.s
shells
silt with rare shell fragments silt and very rare shell fragments 1-1 silt and few shell fragments
a
b
Fig. 2. (a) Schematic representation of a vertical section through the investigated core showing its lithologic structure. The variation in shell debris concentration is mainly due to the variation of the level of the Black Sea as well as sediment flow of the Danube over the past 3500 years [4]. (b) Vertical profile of the radiocesium specific activity of the sediments corresponding to the same core. The maximum 13’Cs specific activity corresponds to the 1963 maximum radioactive fallout.
12
O.G.
Duliu
et al. / The Science
of the Total Environment
3. Results and discussion core composition
In Fig. 2a a longitudinal section through the core is illustrated which shows an alternation of at least four different layers containing living molluscs and fragments of shells. The lithologic structure of these layers can be correlated with their mineralogical composition (Table 1). The first level (O-10 cm below the sediments surface) consists of a very tine silt which contains
Table 1 Mineralogical Layer
composition Depth (4
9-14
living bivalves as well as other benthic organisms. With a total mineralization of between 69 and 81%, this level is composed mainly of calcite (65-50%), clay (14-22%) and quartz (7-100/o. The presence of shell determines, in our opinion, the predominance of calcite over other minerals. The mineralogical composition is almost constant along this level. The second layer (lo- 14 cm) also contains many entire shells and shell fragments (Dreissena polymorpha Pallas, Viviparus sP.)* The mineralogical composition of this layer is characterized by the quasi-absence of clay, while the calcite and quartz contents are close to those found in the upper layer. The third level (14-25 cm) is composed mainly of silt with rare fragments of shell. The calcite concentration of this layer monotonously decreases with depth while the quartz content doubles its value, compared with the first and the second layer. The clay concentration reaches that determined in the first layer and the feldspar concentration increases from 2% to 6-8%. The fourth level (25-32 cm) is mainly composed of silt with very rare shell fragments. Its mineralogical structure consist of almost equal
The y spectra of each aliquot was recorded for 40 h using a 80-cm3 HPG detector for the first three slices (upper O-6 cm) to detect the presence of ‘34Cs and for 20 h for the rest of the samples. The specific activity of ‘34Cs (795 keV line) and 13’Cs (661.5 keV line) was determined using AIEA-156 and AIEA-367 standards [8]. The lithologic structure of the cores was carried out by visual inspection while the general mineralogical composition of each slice was determined by x-ray diffraction. The relative concentrations of the following classes of minerals were obtained: clay, calcite, dolomite, feldspars and quartz as shown in Table 1.
3.1. Total mineralogical
I88 (I996)
of the core Feldspar (“4
Calcite W)
Dolomite (W
Clay (“W
Quartz (“4
Total (“4
I
o-2 2-4 4-6 6-8 S-10
50 50 39 47 48
0 0 0 0 0
18 22 18 14 16
10 10 10 10
81 84 69 70 16
II
10-12 12-14
45 44
0 0
0 0
10 10
57 56
III
14-17 17-19 19-21 21-23 23-25
44 36 27 24 26
0 0 3 3 3
18 18 22 20 25
12 15 25 20 21
76 71 82 13 83
IV
25-27 27-30 30-32
21 22 22
2 3 3
31 25 25
29 18 22
91 15 78
O.G.
Duliu el al. /The
Science of the Total Environment
188 (1996)
9-14
13
dating [3]. The fact that the tail of ‘37Cs distribution reaches a depth of 30 cm can be explained by considering an increased spreading of 137C~, mainly due to the biological activity of benthic organisms. We determined the following value for average sedimentation rate: R = 0.15 f 0.05 g cmm2 year-’ 10.00
:
h
1.00
’ 0
s ’
’
z h ’ 10 Depth
’
’
’
8 ’ 20
’
’
1 30
(cm)
Fig. 3. Logarithmic plot of the previous vertical profile illustrating the radiocesium specific activity of the sediments. Two core segments of different mixing rate values can be distinguished. Discontinuity of the specific activity appears at the boundary between two sediment layers of different lithologic compositions.
The presence of living organisms below the sediment surface [7] results in a certain degree of bioturbation. Therefore, the radiocaesium distribution within the sediment will be determined by both diffusion and the biological activity of living organisms. In Fig. 3 a semilogarithmic plot of 137Csspecific activity (A) over sediment depth (x) is represented. The experimental points lie on two straight lines of different slopes; the specific activity variation with depth is exponential, given by the equation: A = &.exp
fractions of clay (25-31%), calcite (21-22%) and quartz (18-29%). This is the last level which still contains a small amount of r3’Cs. 3.2. ‘37Cs specific activity vertical profire
The 13’Cs specific activity profile shows a maximum at 2-4 cm below the sediments upper surface and falls to zero at a depth of -30 cm (Fig. 2b). The 134Csspecific activity was - 1 Bq kg-’ (which was within the detection limit) in the upper 6 cm of sediment and vanished completely in the next layer. Taking into account the experimentally determined 134Cs to 13’Cs ratio, the Chernobyl ‘37Cs represents < 10% of the total ‘37Cs found in the first 6 cm of sediment. Hence, the observed 13’Cs vertical profile represents a superpos#ition of the 1963 atomic tests debris maximum over more recent 1986 Chenobyl fallout. Consequently, the observed 13’Cs maximum can be attributeld to the 1963 atomic tests fallout and is explained by taking into account an extremely reduced sedimentation rate, which is in agreement with radiocarbon
(-k-x) (1)
where k is a numeric coefficient which has different values for each layer. From this profile, two values of the mixing ratio were derived: D1 = 1.81 x lo-* cm2 s-’ for the sediment and 1O-cm layer upper D2 = 6.20 x 10d8 cm’ s-’ for the next lo-30-cm layer. Similar patterns of vertical radiocaesium profiles have been reported before 19,lo]. The values obtained correspond to different bioturbation regimes and characterize the downward penetration of 137Csin the lake sediments. 3.3. Total ‘j7Cs inventory
By adding the total radiocaesium activity over all slices, the total cesium inventory is equal to 3.60 f 0.11 kBq rne2, while the local value of nuclear tests and Chernobyl total fallout as calculated from other data 111,121 is - 3.345 kBq rnm2. The numerical value thus obtained is consistent with our assumption concerning the relative isolation of Matita Lake with respect to the main branches of the Danube.
14
O.G. Duiiu
et al. / The Science
of the Total Environment
4. Conclusion
By the investigation of *37Cs vertical profile in lacustrine sediments the sedimentation rate of recent lake sediments can be determined while the total cesium inventory permits a better understanding of sedimentation dynamics. In the case of Lake Mattita both sedimentation rate and total cesium inventory are in good agreement with the assumption of the relative isolation of this lake with respect to the Danube’s main branches. The penetration of radiocesium in sediment is well correlated with its lithologic structure and is significantly influenced by the activity of benthic organisms. Acknowledgements
The authors thank Prof. D. Barb for useful discussions, Dr. 0. Sima for helpful advice and Dr. 1. Papaianopol for the determination of mollust debris. References [l] [2]
[3]
A.C. Banu and L. Rudescu, Danube Delta, Edit. Stiintifi&, Bucharest, 1965, p. 185 (in Romanian). C. Diaconu, I.D. Nichiforov (Eds), The Danube Emptying Zone. A Hydrological Monograph, Water State Committee, Bucharest, 1963, 78 pp (in Romanian). J.E. Noakes and N. Hertz, University of Georgia radiocarbon dates VII, Radiocarbon, 25 (1983) 919-929.
188 (1996)
9-14
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