Investigations on the Caesium–137 Household of Lake Lugano, Switzerland
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Caesium–137 Household of Lake Lugano INVESTIGATIONS ON THE CAESIUM–137 HOUSEHOLD OF LAKE LUGANO, SWITZERLAND J. DRISSNER, E. KLEMT*), T. KLENK, R. MILLER, G. ZIBOLD FH Ravensburg Weingarten, University of Applied Sciences, Center of Radioecology, P. O. Box 1261, D 88241 Weingarten, Germany M. BURGER, A. JAKOB GR, AC Laboratorium Spiez, Sektion Sicherheitsfragen, Zentrale Analytik und Radiochemie, CH 3700 Spiez, Switzerland *) [email protected] SedimentCaesium–137J. Drissner, E. HouseholdKlemt, TH. of Klenk Lake et Lugano al. samples were taken from different basins of Lake Lugano, and the caesium 137 inventory and vertical distribution was measured. In all samples, a distinct maximum at a depth of 5 to 10 cm can be attributed to the 1986 Chernobyl fallout. Relatively high specific activities of 500 to 1,000 Bq/kg can still be found in the top layer of the sediment. 5 step extraction experiments on sediment samples resulted in percentages of extracted caesium which are a factor of 2 to 8 higher than those of Lake Constance, where caesium is strongly bound to illites. The activity concentration of the water of 3 main tributaries, of the outflow, and of the lake water was in the order of 5 to 10 mBq/l. 1 Introduction Lake Lugano with an area of 48.9 km2 and a mean depth of 134 m is one of the large drinking water reservoirs of southern Switzerland, in the foothills of the southern Alps. The initial fallout of Chernobyl caesium onto the lake was about 22,000 Bq/m2 [1], which is similar to the initial fallout of about 17, 000 Bq/m2 onto Lake Constance, which is located in the prealpine area of southern Germany (north of the Alps). The removal of caesium from the water column was much faster in Lake Constance. In 1988, Santchi calculated 137Cs residence times of 5 months for Lake Constance, 14 months for the southern basin of Lake Lugano, and 21 months for the northern basin [1]. In 1996, we measured a 137Cs activity concentration in the water of Lake Lugano which was at least a factor of 20 higher than the activity concentration in Lake Constance [2]. The mobility of caesium introduced into freshwater lakes strongly depends on their limnological properties, which determine the potential for dissolved caesium to become reversibly bound on organic matter or to become trapped in clay mineral particles. The main purpose of this study is to describe the vertical distribution of 137Cs in the sediments of Lake Lugano, to examine the association of 137Cs to the different geochemical fractions of the sediment, to measure the concentration of competing ions, and to determine the activity concentration in the main tributaries, in the outflow, in the lake water, and in the pore water of the sediment. With this information, it should be possible to explain the persistently elevated level of 137Cs in the water of Lake Lugano as compared to Lake Constance. Czech. J. Phys. 49/S1 (1999) 133 J. Drissner, E. Klemt, TH. Klenk et al. 2 Materials and Methods In the years 1994 to 1997, sediment samples were taken at the positions shown in Fig. 1 using a gravity corer with 1.20 m long PVC tubes, which have an inner diameter of 58 mm. The cores were sectioned into 1.0 cm thick slices, freeze dried, ground, and filled into calibrated beakers for 137Cs and 134Cs analysis by gamma spectrometry on HPGe detectors. Fig. 1. Sediment sampling positions in Lake Lugano Separation of pore water from the top 30 cm of the sediment cores was done using a filter with a pore size of 0.45 µm. Taking into account colloids with diameters of less than 0.45 µm we obtain an upper limit of the pore water activity concentration. 25 l water samples were taken from the tributaries, the outflow, and from the surface of Lake Lugano. Deep water samples close to the sediment of the lake were taken with 5 l Niskin bottles [3]. 10 ml of 0.1 M CsCl were added in order to prevent adsorption of 137Cs to the walls of the sampling vessel. The water samples were filtered through paper filters (pore size 50 µm) or through filters with 0.45 µm pore size. The dissolved 137Cs was collected by coprecipitation on AMP (ammonium molybdophos- phate) at pH = 4. A modified five step selective extraction procedure according to Robbins et al. [4] was used to investigate the geochemical association of caesium in the sediment. In the five steps using the uppermost 10 or 30 cm of a sediment core, (I) the directly exchangeable 137Cs was displaced by ammonium acetate (1 M), (II) carbonates were dissolved by addition of 5 % hydrochloric acid until pH=5 was reached, (III) organic o matter was decomposed by the addition of 30 % H2O2 at 85 C, (IV) oxides and hydroxides of Fe and Mn were extracted by the addition of 1 M NH2OH⋅HCl in 25 % (v/v) acetic acid and (V) amorphous silicates were dissolved in hot (80 oC) 0.1 M NaOH for 40 minutes. With this method clay and other minerals remain in the residue. The concentration of the competing ions was determined photometrically (extinc- + tion of aco colour) in the case of NH4 and standard atomic absorption spectrometry (AAS) in the case of K+. 134 Czech. J. Phys. 49/S1 (1999) Caesium–137 Household of Lake Lugano 3 Results and Discussion Two 137Cs depth profiles of Lake Lugano sediments are shown in Fig. 2. The position LU3 at a water depth of 24 m is close to the mouth of the river Cassarate (see Fig. 1), but not directly in the stream of the river, so that no coarse grained material from the river is deposited there. The well resolved maximum at a depth of 8 to 9 cm can be related to the Chernobyl accident in 1986 which released both 137Cs and 134Cs radio- nuclides. A much broader maximum at a depth of about 30 to 35 cm can be related to the fallout from the nuclear weapons testing with a maximum fallout in 1963 where besides other radionuclides mainly the 137Cs isotope was released. At the position LU3, the inventory of 137Cs from Chernobyl amounts to 72.5 kBq/m2, which is more than a factor of 3 higher than the direct input through the water surface after the accident. The specific 137 Cs activities in the upper sediment layers of about 1 kBq/kg are also very high, about 10 times the values as measured in Lake Constance sediments [5]. Both is an indication for a continuous input of 137Cs-rich fine particles through the Cassarate river. Fig. 2. 137Cs depth distribution of sediments of positions LU3 and SB3 Czech. J. Phys. 49/S1 (1999) 135 J. Drissner, E. Klemt, TH. Klenk et al. The position SB3 at a water depth of 85 m is located in the middle of a basin (see Fig. 1), far away from the main tributaries and from the outflow. The sediment core exhibits two well resolved maxima at depths of 3 to 4 cm and 12 to 14 cm (Information on the sediments of other sampling positions is given in Table 2). They can again be related to the fallout from the Chernobyl accident and to the fallout from nuclear weapons testing. Here, by chance, the Chernobyl 137Cs inventory of 22 kBq/m2 equals the direct input through the water surface after the accident, but the specific 137Cs activities in the upper sediment layers of about 1 kBq/kg have the same high values as in the samples of position LU3. At this position in the middle of the basin, reasons for the high specific activity of the uppermost sediment layer might be direct uptake of 137Cs from contaminated water, or a sedimentation of suspended particulate matter. Again a continuous input of 137Cs-rich fine particles has to be taken into account: The annual mean inflow of water from the tributaries Cassarate, Vedeggio, and Magliasina amounts to only about 34 % of the outflow of the river Tresa into the Lago Maggiore according to Table 1 [6]. This means that an essential amount of water is provided by other mainly small tributaries or surface water flowing down the steep mountains into Lake Lugano, thereby transporting fine grained matter contaminated with 137Cs to the position SB3. In this rough estimate of fluxes, rainfall and evaporation have not been considered yet. Table 1. Cs-137 activity concentration and water fluxes (6) of tributaries and outflow of Lake Lugano Cassarate Vedeggio Magliasina Tresa Activity concentration (mBq/l) 5.2 ± 0.5 3.9 ± 0.4 2.5 ± 0.5 5.3 ± 0.4 Annual mean in/outflow (m3/s) 2.57 3.3 1.19 24.2 The 137Cs activity concentrations of the tributaries Cassarate, Vedeggio, and Ma- gliasina were measured 11 times during the years 1995 to 1997. They are in the order of 5 mBq/l when filtered with a pore size of 0.45 µm, and in the order of 10 mBq/l when filtered with the paper of 50 µm pore size. The activity concentrations given in Table 1 were measured in October 1997 with a 0.45 µm filter. The outflow Tresa also has an activity concentration of about 5 mBq/l as measured in October 1997, thus transporting 11 years after the Chernobyl accident an activity of about 4 GBq/a into the Lago Maggiore. In the surface water of 6 positions of Lake Lugano we measured (see Table 2) again a 137Cs activity concentration in the order of 5 10 mBq/l.