R. Bojanowski, Z. Radecki, S. Uscinowicz and D. Knapinska-Skiba
Abstract
Large areas of the Southern Baltic floor are covered by sand and gravel. It is a common belief that such materials do not hold appreciable amounts of pollutants because their sorption capacity is low. Our measurements have shown, however, that the activities of caesium and Pu radionuclides locked in these loose sedimentary structures are not negligible.
Actually, half of the total Cs-137 inventory was found to be associated with hard bottom, which occupies 64 per cent of the Polish Economic Zone.
Although activity concentrations of Cs-137 and Pu-239+240 are an order of magnitude lower in sands, compared to muddy sediments, their cumulative deposition, expressed in total amounts per unit area, differs only by a factor of two.
A relatively large accumulation of these radionuclides in coarse-grained deposits is made possible by incorporation of radioactive particles within the bottom material and the particles’ downward displacement by mechanical forces. The penetration depth depends on hydrodynamical conditions prevailing at the sampling sites, and can vary from several centimetres to a few meters. The knowledge of this parameter is prerequisite for reliable assessment of radionuclide inventories in marine sediments.
Introduction
Monitoring of hazardous substances in the marine environment requires sampling of various materials for analytical purposes. Bottom sediments represent an important part of the marine realm because it is there where many pollutants accumulate. By studying vertical distribution of substances in sediment cores it is possible to reconstruct pollution history of aquatic basins, trace sources of pollutants and estimate their inventories.
Polluting substances combine frequently with fine-grained particles which carry them away to areas where they can settle. Sandy deposits, which normally occur in areas of high hydrodynamic activity, are considered free of these contam-inants and are given little attention. However, our studies of the behaviour of Cs-137 in the Baltic Sea have shown, that the amounts of this radionuclide in sandy sediments are not negligible (Bojanowski et al., 1995a).
Figure 1 Sampling stations of the bottom sediments in the Southern Baltic 56 o N
Penetration of Caesium-137 into sandy sediments of the Baltic Sea 86
Sandy bottom is difficult to sample. The reaching depth of popular devices such as Van Veen and Pettersen grab samplers rarely exceeds 20cm and the retrieved material does not preserve undisturbed structure. Box cores are more suitable, but are difficult to operate due to their weight. As a result, the depth distribution of pollutants in such sediments is poorly known. This is a serious drawback for inventory studies, which must rely on correct assessment of all input/output components.
Part of our laboratory’s activity is devoted to studying the behaviour of radioactive elements in the Baltic Sea. Most of our work is confined to the Polish Exclusive Zone (Figure 1). It is a shallow water body which occupies an area of about 30.5 thousand square kilometres. Two thirds of the bottom area is covered by sand and gravel (Table 1).
Table 1 Area of soft and hard sediments within the Polish Economic Zone.
Our earlier studies showed that Cs-137 is present in the southern Baltic in appreciable amounts and that its occurrence is not limited to soft sediments only. In 1986, before the Chernobyl accident, typical levels of Cs-137 were 0.2 and 0.5 kBqm–2 for sandy and muddy bottoms, respectively, but during 1991–93 they increased respectively to 1 and 2 kBqm–2, on average. The total inventory of this radionuclide was divided between hard and soft type bottoms in the proportion 3:2.
Our inventory estimations were based on the belief, that the sampling depth of our device (which was then about 15±5cm on average) was sufficient for this purpose. At that time we had no technical means to prove it. It is only recently that we have gained access to longer, undisturbed cores, which allowed us to test that assumption.
Methods
Cores of sandy deposits were collected at the stations shown in Figure 2 with a vibration corer 10cm in diameter. It collected relatively undisturbed cores up to 3m long. The material was cut into segments and kept in plastic bags until analysis.
Figure 2 Location of sampling stations
Each segment was homogenised and transferred to counting vessels of the Marinelli type. The measurements were made by a gamma spectrometer equipped with a HPGe detector (20% rel. eff. and 1.8keVFWHM resolution for a 1.33MeV 60Co line) and the standard S100 Canberra electronics. The counting efficiency was determined using a standardised mixture of radionuclides provided by the PTB Braunschweig (Germany). Analytical quality control was assured by using the reference materials IAEA-300 (Baltic sediment) and IAEA-375 (soil). Stability of counting efficiency and
Gdansk Bay Southern Baltic Total Area of mixed bottom [km2] - 2 147 (8%) 2 147 (7%) Area of soft bottom [km2] 1 733 (58%) 7 080 (26%) 8 813 (29%) Area of hard bottom [km2] 1 266 (42%) 18 307 (66%) 19 573 (64%)
Total area [km2] 2 999 27 534 30 533
18o 19o 20o E
54o 55o N
8 2
10 21 23
1611
R. Bojanowski, Z. Radecki, S. Uscinowicz and D. Knapinska-Skiba 87
background variation was monitored throughout the whole period of measurements. During the applied conditions (about 1 kg sample and 80.000 s. counting time) the minimum detectable activity was close to 0.2Bqkg–1 d.w.
Results and Discussion
In this paper we present results for the first seven cores which were collected in the shallow parts, of the Gulf of Gdansk.
This region is known for the relatively high Cs-137 levels in sediments, so the chance of observing any activity gradients in the sandy bottom seemed promising.
Table 2 Vertical distribution of Cs-137 and Cs-134 in the analysed cores at the stations 2, 10 and 23.
Table 2, Figure 3 and Figure 4 shows typical vertical distribution of this radionuclide in the selected analysed cores. With the exception of core 2 (Figure 5) all other cores display the same distribution pattern, i.e. the highest activity is in the uppermost segment and then steadily decreases. The shape of the distribution curves differs from one core to another but the differences are small, considering the variable hydrodynamic conditions in this area and different depths. Some 80 to 90 percent of the total inventory resides in the layer that is 15–20cm thick, but here the activity gradients are the steepest.
It seems that the action of waves and currents affect the surficial sandy sediment structure not much deeper than that in the study area. 10 18o50.29’ 54o22.64’ 13 0.00–0.05 1332+25 14+5
0.05–0.19 1150+23 18+3
0.19–0.24 55+6 <7
0.24–0.36 42+6 <8
0.36–0.45 <12 <8 23 19o36.31’ 54o27.27’ 16 0.00–0.10 2460+45 36+6
0.10–0.20 980+25 15+3
Penetration of Caesium-137 into sandy sediments of the Baltic Sea 88
Figure 3 Vertical deposition of Cs-137 at station 10 (54o22.64'; 18o50.29')
Figure 4 Vertical deposition of Cs-137 at station 23 (54o27.27'; 19o36.31')
Figure 5 Vertical deposition of Cs-137 at station 2 (54o23.61'; 18o56.77')
In Table 3 the characteristic features of the Cs-137 distribution in the examined cores are summarised. We have defined the penetration depth of Cs-137 as the depth in the sea floor, below which less than 5 percent of the integral total inventory remains. In most cases this depth does not exceed 30cm and it does not appear to be related to the water depth nor to the magnitude of total deposition. The latter varies by an order of magnitude and is much higher on average than in other parts of the Southern Baltic (Bojanowski et al., 1995a and 1995b). That attests to the Vistula river as a major source of Cs-137 in the Gdansk Bay.
R. Bojanowski, Z. Radecki, S. Uscinowicz and D. Knapinska-Skiba 89
Table 3 Summary of Cs-137 deposition characteristics in sandy deposits of the Baltic Sea.
Presenting Cs-137 activities in activity concentrations units (Bqkg–1 of sediment) has little meaning because such values depend, to a large extent, on how the sample was taken. It is however, interesting to note, that in the uppermost layer the concentrations varied a lot with ranges of 6 to 101 Bqkg–1 d.w., whereas below the penetration depths they in some cases approached detection limits.
The observed features are not claimed to be applicable to sandy sediments as a whole. The distribution at station 2 is at present difficult to explain but it demonstrates that such cases do exist. Some records also have evidence of homoge-neous distribution of Cs-137 down to 15cm sampling limit in Pomeranian Bay sediments (Bojanowski et al., 1995b).
Clearly, more information is needed on this subject, and measurements on cores from other regions are underway.
Conclusions
The most important conclusion resulting from our measurements is that the Cs-137 occurs down to about 30cm in some Baltic near-shore sandy sediments. This observation can be extended to include other pollutants that are bound to partic-ulate matter and are of recent origin, such as the Chernobyl fallout. Sampling to that depth is thus a prerequisite for quantitative inventory assessment of such substances.
References
R. Bojanowski, D. Knapinska-Skiba, Z. Radecki, J. Tomczak, T. Szczepanska, 1995a: Accumulation of radioactive Caesium (137Cs) in Southern Baltic Sediments. Prace Panstwowego Instytutu Geologicznego CXLJX, 145–150 R. Bojanowski, Z. Radecki, D. Knapinska-Skiba. 1995b: Distribution of 137Cs, 239+240Pu and 210Po in the Pomeranian
Bight ecosystem. Bulletin of the Sea Fisheries Institute, 3 (136)
Station