The urban contamination after the Chernobyl accident studied in sewage water and sludge from Lund and Gothenburg

The concentration of various radionuclides have been measured regularly in samples of outgoing water and sludge from the sewage treatment plant serving the town of Lund in southern Sweden. These measurements started already in 1983 and have gone on since then, so in this investigation we have obtained data not only after but also before the Chernobyl accident. Similar measurements were also started at Gothenburg in western Sweden at the end of April 1986. The total deposition of the various radionuclides was almost the same both at Lund and Gothenburg although the time distribution was different. The activity concentrations measured in the sludge reached a peak value of 87.1 kBq/kg (d.w.) for 103Ru and 3.8 kBq/kg for 137Cs at Gothenburg and of 15.4 and 12.1 kBq/kg respectively at Lund. These maxima were measured about 2-3 days after the onset of the precipitation at Gothenburg and about 14 days after at Lund. The difference in the time delay is due to differences in the construction of the plants. At Lund the total outgoing activity of 137Cs has fallen exponentially from 900 MBq in 1986 to 100 MBq in 1989, and the decrease seems to follow an exponential line. If this time course is extrapolated one would be back to pre-Chernobyl values in 1992/93. But the figures for 1990 are definitely above those predicted by the extrapolation. It will therefore take longer before the activity concentration in the sludge will be back to pre-Chernobyl levels. Of the fallout from Chernobyl, only a minor part is available for run off and of the original deposition of 12400 MBq about 1/3 will have left the town and 2/3 will still be left there