Concentrations of 137Cs in surface and deep water along the Algerian coast

Despite the availability of extensive information about the levels of artificial radioactive contamination of the Mediterranean Sea, the problem of assessment of this radioactive contamination to the south part of the Mediterranean Sea is of current interest. The main sources of contamination are fallout from nuclear tests and Chernobyl accident in 1986. 137Cs which is an anthropogenic radionuclide, conservative in the open sea, remains the potential contributor of contamination in the Mediterranean Sea, although used as radioactive tracer. A total number of 25, between surface and deep seawater samples (40-70 L), were collected in different water masses during Oct.-Nov. 1999, along the Algerian coast, from Ghazaouet (N35 deg. 10'90, W02 deg. 05'000) to Skikda (N36 deg. 56'107, E06 deg. 55'857), covering a distance of around 1000 km, by the 'Laboratoire d'Etudes d'Impact Radiologique' (LEIR/CRNA), on board the M.S. Benyahia, Research Vessel of (ISMAL), to determine 137Cs concentration. Sampling of surface seawater was performed using a pump system, however, for water column, a 50 L Niskin PVC water - bottle sampler was used. Multiparameter probe was also used to determine vertical profiles of temperature, salinity and pH in water column samples at different stations. 137Cs was pre-concentrated on board for some samples and in the laboratory for others, by co-precipitation on ammonium molybdo-phosphate (AMP) technique at pH (1-2). The chemical yield was determined using the injected 134Cs isotope tracer activity. After pretreated and dried the AMP precipitates, they were analysed by direct counting gamma spectrometry, using a high purity germanium detector of relative efficiency of 20.6% and resolution (FWHM) of 1.8 keV at 1332 keV gamma-energy of 60Co. The accuracy of the results is checked by participation to the IAEA-385 intercomparison exercise. Detection efficiency is determined by preparation of two standard samples of same geometries, in which 13 Bq activity of caesium isotope tracers was injected. Concentration of surface seawater ranges from 1.69 to 2.26 Bq/m3 with a mean value of 1.87 Bq/m3 and from 1.72 to 3.31 Bq/m3 for water column samples with an average value of 1.78, 1.93, 2.28 and 2.5 Bq/m3 for Ghazaouet, Mostaganem, Alger and Annaba stations, respectively. After comparison with the literature, we found that our values are close to the mean value of the Mediterranean Sea in coastal regions. According to the results of concentration, and although they were obtained by using simple means of sampling, we observe an increase of activity from western to eastern coast of Algeria, and an activity peak at a depth of about 100 m. Obtained profiles are shown and seem to be similar to those already obtained in the region