Plutonium Isotopes in a Water Column and Sediment Core along the Algerian Coast

Concentrations of natural (40K, 210Pb, uranium, and thorium series) and artificial radioelements (137Cs, 90Sr, 239+240Pu) were determined in seawater and sediment samples collected from stations along the Algerian coast. Seawater was collected from the surface to a maximum depth of 2000 m; the sediment cores were sampled from a depth of around 1500 m. This work was carried out in August 2001, in the framework of the Regional African project /7/004 (RAF), by the accomplishment of an oceanographic campaign organized by the International Atomic Energy Agency (IAEA) in collaboration with Commissariat à l’Energie Atomique (COMENA) and L’Institut des Sciences de la Mer et de L’Amenagement du Littoral (ISMAL), on board of the research vessel of M.S. Benyahia of ISMAL. In addition to the record of the conductivity (mS) and temperature (◦C) data at each station, seawater samples were treated and preconcentrated on board, those of sediment cores were divided into different layers in order to undergo analyses in the laboratory of radiological impact studies of Algiers. Concentration results were obtained for 137Cs and 90Sr in mBq/L and 239+240Pu in μBq/L in seawater, and also for natural and artificial radionuclides in Bq/kg dry weight in the layers of the sediment cores. The different profiles of 137Cs, 239+240Pu, 90Sr, and 210Pb concentrations against depth were presented to show artificial radioactivity distribution in the water column and sediment core. Concentrations of 137Cs in the sediment core were also used to identify the 137Cs peak in the area of interest.


INTRODUCTION
Natural radioactivity is present since the creation of earth, that is, billions of years, and is introduced into the marine environment through terrestrial and atmospheric routes. The most important radioelements are 40 K and uranium and thorium series known to have a very long half-life and to be abundant in the environment. In addition to that, and since the introduction of the nuclear technology, several artificial radioactive sources were added to the marine radioactive inventory. These sources result from nuclear weapons tests, authorized releases from nuclear facilities, and also from nuclear accidents, particularly that of Chernobyl occurred in April 1986. According to the physicochemical properties of the receiving medium and those of the radioelements of interest, radioactivity can be introduced through several routes into the marine environment. Radioactivity can be both directly introduced into the marine environment by wet or dry depositions or from the terrestrial medium, and dispersed by different processes depending on physicochemical properties of the medium and the radioelements of in-terest. Radioactivity can be diluted, transported, fixed, distributed, and deposited on the marine bottom, and much information can be deduced to determine the behavior and advance of radioelements in the marine environment. Therefore, it is necessary to measure the concentration of some radioelements of interest, namely, 137 Cs, Pu isotopes, 90 Sr, and 210 Pb, in some representative samples such as seawater and sediment. In this work, and in the framework of the radiological surveillance along the Algerian coast and the study of marine process, several oceanographic campaigns were organized. Concentrations of 137 Cs reported in 1997 and 1999 along the Algerian coast show concentrations ranging from 2.05 ± 0.09 to 3.15 ± 0.34 mBq/L with an average value of 2.59±0.26 mBq/L in seawater, and from 0.1±0.04 to 6.29±0.3 Bq/kg in sediment for 239+240 Pu; the concentrations range from 0.16 ± 0.02 to 1.02 ± 0.09 Bq/kg [1]. Concerning natural radioactivity, concentrations of 40 K range from 312 ± 14.6 to 613 ± 25 Bq/kg. Regarding the results of the sampling campaign of August 2001 of the present work, 137 Cs, 90 Sr, Pu isotopes, and 210 Pb concentrations in Bq/kg were determined in several sediment cores and seawater columns,

Sampling
In the framework of the radiological surveillance program of the marine environment along the Algerian coast and the IAEA regional project RAF/7/004, an oceanographic campaign was organized by the IAEA in collaboration with COMENA and ISMAL, in August 2001, on board of the research vessel M.S. Benyahia of ISMAL, in order to collect sediment cores and seawater columns at different stations at the center, the east, and the west ( Figure 1) and also to record some marine data such as temperature ( • C) and conductivity (mS). The purpose of this campaign is to measure 137 Cs, 90 Sr, and Pu isotopes concentrations in a seawater column and that of 137 Cs and 210 Pb in the different layers of the sediment core. On board of M.S. Benyahia research vessel, seawater samples were collected at different depths using a stainless steel bottle of a volume of 250 L, where an amount of 150 L was filtered on a 0.45 μm membrane filter and transferred to a tank of polyethylene, in order to precon- centrate the radioelements of interest, namely, 137 Cs, 90 Sr, and Pu isotopes, for a radiochemical analysis in the laboratory.
Concerning sediment samples, an IAEA box corer (see Figure 2(a)) was used to collect sediment cores at depths reaching a maximum of 1500 m. Sediment cores were carefully sectioned into slices on board with an appropriate device to cut into layers of 0.5, 1, and 2 cm of thickness; see Figure 2(b). Sediment samples were stored in a freezer to determine porosity and to count them by gamma spectrometry.  Porosity is defined as the part of the water content in the whole sediment of analysis [6].

Radioactivity measurement
Seawater samples under ammonium molybdophosphate precipitate (AMP) were put in plastic beakers of 100 cc volume to be analyzed by direct counting by gamma spectrometry using a high-purity germanium detector of 23% relative efficiency and 1.8 keV at 1332 keV energy peak of 60 Co. Samples were put in contact with the detector and counted for 24 hours to 72 hours in order to reach reliable statistic counting. Detection efficiency was determined by preparing a standard sample in the same density and geometry conditions by introducing a liquid source of 134 Cs. The main contribution to the efficiency uncertainty is the standard solution error added to the statistic error.
With regard to sediment samples, they were dried at 80 • C in the oven, crushed into fine powder, homogenized, and transferred to cylindrical plastic beakers of 100 cc volume. They were counted and analyzed with the same HP germanium detector, using a Genie-2000 and an interwinner program to process energy spectra. Detection efficiency was also determined using the same geometry conditions by contamination of the standard using a radioactive liquid source of 152 Eu to cover the range of energy of interest. The main contribution to the concentration uncertainty is the efficiency error plus the statistic error.
Natural and artificial gamma emitting radioelements ( 210 Pb, 137 Cs) were conditioned and evaluated by gamma spectrometry.
However, for 90 Sr and 239+240 Pu, samples undergone radiochemical separations, 90 Sr was extracted using selective crown ether resin, and concentration was determined by liquid scintillation counting through a source prepared by gravimetric precipitation using a stable carrier of 90 Sr as strontium nitrate. For 239+240 Pu, the extraction was carried out by anion exchange, and the source was prepared by coprecipitation with neodymium fluoride, using a radioactive tracer ( 242 Pu), and evaluated by alpha spectrometry.

RESULTS AND DISCUSSION
Sampling locations of seawater column and sediment cores are presented in Figure 1. Records data of temperature ( • C) and conductivity (mS) are given in Table 1 and their profiles versus depth are shown in Figures 3(a) and 3(b). Concentration levels of 137 Cs in mBq/L in the water column of Algiers and Ténes stations are indicated in Table 2, where uncertainty used is 1 σ. 137 Cs concentration in mBq/L at the surface water and at different depths of Ténes station ranges between 1.31 ± 0.08 and 2.20 ± 0.13 mBq/L, with a peak at 1000m, and that of Algiers station ranges from 2.21 ± 0.14 to 2.3 ± 0.15 mBq/L. Regarding 90 Sr and 239+240 Pu in the station of Ténes, the average concentration in seawater is 4.79 ± 0.5 mBq/L and 18.01 ± 2.58 μBq/L, respectively. The obtained results of 90 Sr and 239+240 Pu in the station of Ténes are given in Tables 3 and 4. 137 Cs profiles versus depth in both stations are plotted and shown in Figures 4(a) and 4(b). The average values of 137 Cs concentration in Algiers and Ténes stations are 2.28 ± 0.15 mBq/L and 1.89 ± 0.11 mBq/L, respectively. These values were compared to other works (values ranging from 2.04 mBq/L to 4.4 mBq/L) in the same area, and found to be in the same range [2][3][4][5]7].
Also 90 Sr and 239+240 Pu profiles were plotted against depth for Ténes station, and shown in Figures 5(a) and 5(b). 239+240 Pu concentrations are in the range of those given by different authors; however, those of 90 Sr seem to be overestimated. This could be explained by some systematic errors that might have been undertaken during the radiochemical analysis and not due to a contamination.
Concerning sediment cores, concentrations of 137 Cs vary from 1.26 Bq/kg dry weight to 11.69 Bq/kg dry weight in Jijel station. Concentrations in Bq/kg versus depth of the different sediment core layers gave 137 Cs profiles in Algiers and Jijel

CONCLUSIONS
This study enabled us to determine natural and artificial radioelements concentration. 137 Cs, 90 Sr, and 239+240 Pu distribution is observed in a water column showing subsurface peaks at different layers (250 m and 1000 m depth). These peaks might belong to 1963 nuclear tests and Chernobyl accident. These results confirm the behavior of 137 Cs as conservative and Pu isotopes adsorbed by suspended particles. Concerning 137 Cs and 210 Pb concentrations in the sediment core, they could be used to estimate the radioactive inventory and the sedimentation rate in the area of study.