The Adriatic Sea-water Radioactivity
Report of the International Atomic Energy Agency (IAEA)
Research Contract No. 302-K4-CRO-8844, B5-CRO-24175
The Adriatic Sea is the northernmost part of the Mediterranean Sea. It is a relatively shallow and partially enclosed sea (by the Apennines, Alps and Dinarides), under a strong impact of the Po river. This is true especially of northern part of the Adriatic Sea where depth does not exceed 80 m. Generally, the Adriatic Sea is characterized by low precipitation, high evaporation, low tidal action, low nutrient content, low suspended load and low biological productivity.
The Department of Radiation Protection of the Institute for Medical Research and Occupational Health has been in charge of a programme of radioactivity measurements of the Adriatic Sea as part of an extended monitoring programme of the Croatian environment since 1963 /1,2,3/. Systematic long-term measurements of 90Sr and 137Cs in surface sea-water, (performed at four locations twice a year on the Croatian coast of the Adriatic Sea) and fallout (performed in the town of Zadar) showed that 90Sr and 137Cs fallout activities highly affect sea-water activity. Relatively long observed residence time for 90Sr in the surface water identifies the Adriatic Sea to be sensitive to potential discharges of radionuclides or various other non-radioactive contaminants. Also, sea- water activity of the North Adriatic region exhibits greater activities compared to those of the south region, the North Adriatic therefore being more endangered by contamination.
The aim of the project was to provide results of long term radioactivity measurements of the Adriatic sea water.
This report gives data 90Sr and 137Cs activity concentrations measured from 1963 to 1995 on four locations on the Croatian part of the Adriatic sea.
90Sr and 137Cs activity concentrations were measured on four locations:
1. Rovinj (45 04' N, 13 37' E),
2. Rijeka (45 19' N, 14 26' E),
3. Split (43 26' N, 16 25' E) and
4. Dubrovnik (42 37' N, 18 07' E).
Sea-water samples were collected usually twice a year (in May and October, if
feasible) 3 km from the shore, at a depth of 0.5 m.
The volume of collected sea-water samples varied from 50 L in 1960s to 150 L
in 1990s per sample.
For the determination of strontium and cesium, were used radiochemical
The radioactivity of 90Sr was determined by beta-counting its decay product, 90Y,
in a low-background anti-coincidence, shielded Geiger-Muller counter.
A gamma-ray spectrometry system based on a Ge(Li) detector (FWHM 1.82 keV
at 1.33 MeV) coupled to a computerized data acquisition system (4096-channel pulse
height analyzer and personal computer) was used to determine radiocesium levels in the
samples from their gamma-ray spectra. The detector is shielded with 10 cm thick lead
lined with 2 mm of cadmium and 2 mm of copper. Samples were measured in cylindrical
plastic containers of appropriate volume that were placed directly on the detector.
Counting time depended on sample activity, but was never less than 60,000 s.
The efficiency calibration was carried out using sources provided by the
International Atomic Energy Agency (IAEA) and World Health Organization (WHO).
On Figures 1-8 (will be posted later) are shown 90Sr and 137Cs activity concentrations, the relative zero on the time scale being 01 January 1963. 90Sr activity concentrations on locations Rovinj, Split and Rijeka were measured from early sixties, while systematic measurements on the Location Rijeka started in 1978. Systematic measurements of 137Cs activity concentrations on all four locations started in 1978.
90Sr activity concentrations were fitted to the exponential curve:
where A(t) is time-dependent activity concentration in the sea water and A(0) is activity for the reference date (01 January 1963).
Observed residence times for overall period, being reciprocal values of constant k are 10.2, 11.6, 9.6 and 10.6 years for Rovinj, Rijeka, Split and Dubrovnik respectively, the average of all locations being 10.5 0.7 years. That values are similar to the results of previous calculations /4/.
The same procedure was used for calculation of observed residence times of 137Cs for the post-Chornobyl period. The values of 1.2, 1.8, 0.8 and 1.1 years were obtained for Rovinj, Rijeka, Split and Dubrovnik, the average being 1.2 0.3 years. Coefficient of correlation, is r>0.95 except for Rijeka, the r value being 0.65, leading to higher standard error. Therefore, the residence time of 1.75 years for the location Rijeka might be over-estimated.
The lowest value for residence time, both for 90Sr and 137Cs, was observed on the location Split.
The absence of significant variations in 90Sr and also in 137Cs residence times (except Rijeka) for different locations implies similarity of oceanographical factors at individual locations concerned.
The Chornobyl nuclear accident did not cause significant increase of 90Sr activity concentrations in the Adriatic sea. Since refractory components of the Chornobyl debris were deposited closer to the accident location than the more volatile constituents, due to the volatile nature of cesium and the refractory nature of strontium the strontium did not reach the Adriatic coast. Also, radioactive air plumes after the Chornobyl accident had rather complicated trajectories over Europe. Consequently, the Chornobyl debris did not travel from the accident site straight to the Adriatic. Therefore, the fallout deposition at different locations varied considerably, depending on meteorological, geographical and various other factors.
The base line level of 137Cs activity concentration in the Adriatic surface water in 1985 has been around 4 Bqm-3, like in the rest of the Mediterranean /5/. The essentially constant levels of 137Cs in the pre-Chornobyl period increased by two orders of magnitude immediately after the Chornobyl accident, but they decreased again to lower levels within a few years, probably either by sedimentation or dispersion by coastal water movements.
Apart from 90Sr and 137Cs, direct gamma spectrometric analyses of the Adriatic sea-water after the Chornobyl accident showed the presence of some radionuclides. However, only a few of these (i.e., cesium isotopes) were responsible for most of the radioactivity while the others (cerium, ruthenium, zirconium, etc.) were only qualitatively determined, since their activity was below the minimum detectable activity. The exception was 131I, whose measured activity concentrations on 10 May 1986 were 320 Bqm-3 in Dubrovnik and 940 Bqm-3 in Rovinj /4/.
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can be accessed here.
Last Update: 14 Oct 1997.