Caesium-137, strontium-90 and plutonium-239,240 in Fucus

loviisa

In 1975 – 1976 (before the start-up of the power plant), the activity concentrations of ¹³⁷Cs varied between 14 and 28 Bq kg^-1 d.w. in the Fucus samples collected from sampling site A, and between 16 and 23 Bq kg^-1 d.w. in those collected from sampling site B, both situated in Hästholmsfjärden Bay (Fig. 63). During the next ten years, the concentrations of the weapons-tests-fallout-originated

137Cs declined slowly, and in 1985, the concentrations were 12 – 13 Bq kg^-1 d.w.

in the samples from site A and 8 – 11 Bq kg^-1 d.w. in those from site B. The Chernobyl fallout caused a sudden rise in the concentrations in 1986. At the end of May, the concentration of ¹³⁷Cs was 4 900 Bq kg^-1 d.w. in Fucus from site A, and 2 700 Bq kg^-1 d.w. in that from site B. However, the concentrations soon started to decline at a good rate. Already by the end of August, the concentrations had decreased to a quarter of their top values: to 1 100 Bq kg^-1 d.w. at site A and to 700 at site B. Since then, the concentrations have steadily decreased. In 1990, the concentrations were 160 – 240 Bq kg^-1 d.w. at the site A and 150 – 190 Bq kg^-1 d.w. at the site B. In 2007, the corresponding values were 25 – 38 and 22 – 31 Bq kg^-1 d.w. Based on these results, the effective ecological half-life of ¹³⁷Cs in Fucus was 4.5 ± 0.1 years between 1986 and 2007 at the Häst- holmsfjärden sites.

At the sampling sites situated outside Hästholmsfjärden Bay, the activity concentrations of ¹³⁷Cs were somewhat lower than at the sampling sites A and B situated closest to the power plant. In 1987, the concentrations were 240 – 570 Bq kg^-1 d.w. at sampling site C (Lilla Djupberget), when they were concurrently 400 – 770 Bq kg^-1 d.w. in Hästholmsfjärden Bay. In 1993, when sites D and E (Boistö and Storskarven) were brought into the programme, the concentrations were 67 – 88, 72 – 86 and 46 – 72 Bq kg^-1 d.w. at sites C, D and E, respectively. In 2003, sites D and E were changed for sites F and G (Yttre Täktarn and Hudö), and in 2007 the activity concentrations of ¹³⁷Cs were 19 – 22, 21 – 25 and 23 – 26 Bq kg^-1 d.w. at sampling sites C, F and G, respectively. At site C, the calculated ecological half-life of ¹³⁷Cs in Fucus was 5.5 years between 1986 and 2007, i.e. one year longer than in Hästholmsfjärden (see above).

In 1975 – 1976, the activity concentrations of ⁹⁰Sr varied between 24 and 48 Bq kg^-1 d.w. in Fucus samples taken from the Hästholmsfjärden Bay. Just before the Chernobyl accident, the concentrations had declined to 14 – 18 Bq kg^-1 d.w.

The fallout from the accident raised the values to 83 Bq kg^-1 d.w. at site A, and to 48 Bq kg^-1 d.w. at site B. In 1993, the ⁹⁰Sr concentrations were 14 and 16 Bq kg^-1 d.w.

at sites A and B, and in 2007, 11 and 9 Bq kg^-1 d.w. at sites A (Hästholmsfjärden) and G (Hudöfjärden), respectively. In 1976 – 1982, the activity concentrations

of ^239,240Pu varied between 0.21 and 0.48 Bq kg^-1 d.w. in the samples taken from sampling sites A and B in Hästholmsfjärden Bay. In all samples taken from sites A, E and G in 1993 – 2007, the ^239,240Pu concentrations were 0.027 – 0.18 Bq kg^-1 d.w. On the grounds of these results, the effective ecological half-lives of ⁶⁰Sr and ^239,240Pu in Fucus from site A were 8.4 and 6 years, respectively.

In 2008, the quality assurance of Fucus sampling was tested at sampling site C at Loviisa by taking five parallel samples from the littoral zone of the rocky islet of Lilla Djupberget. The samples were taken at the same time by two independent scuba divers from a depth of 1.7 – 2.3 m. The samples were pre-treated with the same standard method used in our laboratory, and analysed gammaspectrometrically. The activity concentrations of ⁷Be, ⁴⁰K and ¹³⁷Cs (Bq kg^-1 d.w.) and the relative uncertainties of the gamma analysis (1σ) were as follows:

The results show that the uncertainty due to sampling was rather small.

The ratio of the standard error to the mean value, expressed as a percentage, was 2% in the case of ¹³⁷Cs and 2.5% in the case of ⁴⁰K.

olkiluoto

In 1974 – 1976 (before the start-up of the power plant), the activity concentrations of ¹³⁷Cs were 8 Bq kg^-1 d.w. in the Fucus samples collected from sampling site A situated in Iso Kaalonpuhti Bay in front the cooling water outlet of the power plant (Fig. 64). During the next ten years, the concentrations of the weapons-tests-fallout-originated ¹³⁷Cs stayed about at the same level, and in 1985, the concentrations were 7 Bq kg^-1 d.w. in the samples taken from sites A and B (Iso Kaalonpuhti and Kalliopöllä). In 1986, the Chernobyl fallout caused a sudden rise in the concentrations. On the 28^th of April, the concentration of ¹³⁷Cs was still 25 Bq kg^-1 d.w., but on the 21^st of May it was 1 300 Bq kg^-1 d.w. in the Fucus sample collected from site A. However, in the same way as at Loviisa, the concentrations started to decline during the next months at a good rate. Already by the 14^th of August, the concentrations had decreased to 280 and 300 Bq kg^-1 d.w. at sites A and B. After that, the concentrations have steadily decreased.

In 1990, the concentrations were 110 – 140 Bq kg^-1 d.w. at sites A and B, and in

2006, the corresponding values were 31 – 39 and 24 – 37 Bq kg^-1 d.w., respectively.

Based on these results, the effective ecological half-life of ¹³⁷Cs in Fucus was 6.8 ± 0.7 years between 1986 and 2007 at the sites A and B.

At sampling sites C, D and E (Iso Pietari, Iso Siiliö and Vähäkrunni), the activity concentrations of ¹³⁷Cs were somewhat smaller than at the sampling sites A and B situated closest to the power plant. In 1987, the concentrations were 150 – 220 Bq kg^-1 d.w. at sampling site C in Iso Pietari, at the same time as they were 220 – 370 and 230 – 410 at sites A and B in Iso Kaalonpuhti and Kalliopöllä. In 1993, when sites D and E (Iso Siiliö and Vähäkrunni) were brought into the programme, the concentrations there in Fucus were 69 – 81 and 54 – 78 Bq kg^-1 d.w., respectively, while they were concurrently 81 – 130, 70 – 120 and 77 – 86 Bq kg^-1 d.w. at sites A, B and C (Iso Kaalonpuhti, Kalliopöllä and Iso Pietari), respectively. In 2006, the concentrations were 31 – 39, 24 – 37, 26 – 28, 25 – 27 and 27 – 31 Bq kg^-1 d.w. the sites A, B, C, D and E, respectively. At site C, the ecological half-life of ¹³⁷Cs in Fucus was 6.7 years between 1986 and 2007.

In 1974 – 1976, the activity concentrations of ⁹⁰Sr were 15 – 17 Bq kg^-1 d.w.

in Fucus samples taken from Iso Kaalonpuhti Bay. The Chernobyl fallout raised Fig. 85. Small-sized Fucus vesiculosus typical of the Loviisa area.

the values only slightly, so that in 1987 the values were 25 and 24 Bq kg^-1 d.w. at sites A and B. In 1993, the ⁹⁰Sr concentrations were 12 – 14 Bq kg^-1 d.w. at sites A, B and C (Iso Kaalonpuhti, Kalliopöllä and Iso Pietari), while in 2007, they were about 7 Bq kg^-1 d.w. at sites A and C. In 1976 – 1982, the activity concentrations of ^239,240Pu varied between 0.05 and 0.70 Bq kg^-1 d.w. in the Fucus samples taken from sampling sites A and B, while in 1993 – 2007, the concentrations were 0.023 – 0.14 Bq kg^-1 d.w. in all the samples taken from sites A, B and C.