ANTHROPOGENIC RADIONUCLIDES IN SURFACE SEDIMENTS OF THE TOULON-BAY AREA
(FRENCH MEDITERRANEAN COAST).
Mireille Arnaud
1
*, Sabine Charmasson, Hervé Thebault, Frédérique Eyrolle, Evelyne Barker
1
Institut de Radioprotection et de Sûreté Nucléaire, Base Ifremer, BP 330, 83 507 La Seyne sur mer, France - * mireille.arnaud@ifremer.fr
Abstract
In the frame of an integrated coastal area and river basin approach, an environmental assessment was carried out in the Toulon Bay area
as defined by the Regional Water Master Plan. Part of this assessment concerns anthropogenic radionuclides within the sediment
compartment. In surface sediments,
137
Cs is the only artificial radionuclide which was regularly detected by gamma spectrometry.
137
Cs
and silts fraction (4-63 µm) are closely correlated, revealing that
137
Cs arise mainly from a diffusive source (i.e. atmospheric inputs).
Keywords: Surface sediment, radionuclides, Mediterranean sea, Toulon Bay contract
Rapp. Comm. int. Mer Médit., 37,2004
168
Introduction
In the Toulon area, a Regional Research Contract (Toulon Bay
Contract) concerning Toulon harbor and its catchment basin was initia-
ted in 1998 [1]. The main objective was to assess a balanced water
resource management and to propose an integrated coastal management.
Since 1984, the site harbor has been sheltering six French Navy
nuclear-powered submarines. The arrival of the Charles de Gaulle,a
new nuclear-powered aircraft carrier, led us to carry out a study of
anthropogenic radionuclide contents in surface sediments of this area.
Materials and methods
The study area
The area includes the harbor and the surrounding coastal area
between the longitudes 5°52.586 E and 6°01.282E, as defined in
the Water Master Plan instituted by the French Water Act (1992). It
represents a coastline of about 70 km for a marine water surface area
of about 60 km
2
and a catchment area of 340 km
2
.
Sampling and analytical methods
51 stations regularly distributed on the study area were sampled in
April 2000 (Fig. 1). Within this area 50 sediment samplings were
realized in September 1997 in the Eastern part of the Bay which is
affected by the releases of the Toulon sewage outfall (Fig. 2). Thus
this last area represents a zoom within the area studied in 2000.
Samples were collected with grab corers (Shipeck and Orange Peel)
and Eckman box corer depending on station depths. They were dried
(80 °C) and measured by gamma spectrometry (N type hyper-pure Ge
detector). Particle-size was measured using a Coulter LS 230 laser
diffraction granulometer.
Fig. 1. Distribution of
137
Cs and silt fraction in surface sediments of the
Toulon Bay obtained by numerical interpolation (Spline method,
Arcview®).
Results and conclusions
137
Cs was the only artificial radionuclide systematically detected in
the sediment samples. Indeed, due to is relatively long half life (30
years), it is the only radionuclide arising from atmospheric deposits
(bomb fallout and Chernobyl) still detected by gamma spectrometry.
In 2000,
137
Cs levels in Toulon area atmosphere do not exceed
1µBq.m
-3
.
137
Cs levels in sediment range between 0.28 (
±
0.09) and
12.20 (
±
0.90) Bq.kg
-1
dry weight. In the literature it is generally
reported that
137
Cs demonstrate high affinity for clay minerals [2],
[3]. Indeed, spatial distributions obtained by numerical interpolation
methods (Figs. 1 and 2) highlight the positive correlation between
137
Cs contents and silt fractions (4-63 µm) and the negative
correlation to raw sands (200-2000µm). The harbor area which is
characterized by fine-grained sediments due to the low dynamic in the
current conditions has the highest
137
Cs contents in relation to the
high specific surface area of the smallest particles.
The scale of observation is of peculiar interest. As a matter of fact,
in 2000 stations surrounding Toulon-East sewage outfall, exhibited
relatively low
137
Cs concentrations (1,2 to 2,8 Bq.kg
-1
dry weight)
and silts fractions (20,1 to 32,7%). In 1997 a higher range of variation
in both
137
Cs levels and silt contents were found in an area included
between these stations (respectively 0,68 to 7,7 Bq.kg
1
dry weight
and 9,7 to 62,6%). If repeated this kind of study, should allow to
resolve the fractal dimension of the distribution of these parameters in
this area.
137
Cs levels found in the study area at both periods (1997 and 2000)
are within the range reported for Mediterranean Sea coastal sediment
only affected by atmospheric fallout i.e. 2 to 20 Bq.kg
-1
dry weight
[4], [5]. Radionuclides such as cobalt, silver, manganese were not
detected demonstrating the lack of contamination by nuclear releases
in that area
Acknowledgement. This work was supported by the SIAT
Contract, and IRSN Rademer programme. Sampling in 1997 was
carried out by IFREMER. Special thanks to L. Saey, for his helpful
technical assistance during fieldwork.
References
1-Henocque Y., 1999. Urban communities and environmental
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Workshop on Coastal Cities. China, Handgzhou, 27-30 septembre 1999,
10 p.
2-Ancellin J., Guéguéniat P. et Germain P., 1979. Radioécologie marine.
Etude du devenir des radionucléides rejetés en milieu marin et applications
à la radioprotection. Ed. Eyrolles, Paris, 256 p.
3-IAEA, 1985. Sediment Kds and concentration factors for radionuclides
in the marine environment. Tech. Rep. Series n°247, Aiea, Vienne, 73 p.
4-Delfanti R., Frignani M., Langone L., Papucci C. et Ravaioli M., 1997.
Accumulation of radiocaesium in fine recent sediments of the northern
Adriatic Sea. Radioprotection, vol. 32, C2: 251-256.
5-Albrecht A. et Beer J., 1997. Assessment of radionuclide distribution in
continental shelf sediments (Gulf of Taranto, Mediterranean Sea).
Radioprotection, vol. 32, C2: 277-285.
Fig. 2. Distribution of
137
Cs and silt fraction in surface sediments around
the Toulon-East sewage outfall (Ñ—-),obtained by numerical interpo-
lation (Spline method, Arcview ®)
