Radionuclides in algae from Swedish coastal waters for over half a century

Abstract

Samples of Fucus serratus and Fucus vesiculosus have been regularly collected at Särdal (56.76 N, 12.63E) on the Swedish west coast since 1967, for most of the time, every two months. In 2020, sampling of Fucus spp. was extended to 44 other locations along the Swedish west, south, and east coast for comparison with Särdal data. At seven of these locations, water samples and extra samples of Fucus spp. were also taken for 3H analysis. Measurements have been performed by gamma spectrometry, or by radiochemical separation followed by low background beta measurements, alpha spectrometry or accelerator mass spectrometry. Time variations of the measured concentrations of various radionuclides (3H, 14C, 60Co, 99Tc, 129I, 131I, 134Cs, 137Cs, 236U, 238U, 239Pu, and 240Pu) are reported. The concentrations have been correlated with emissions from the nuclear fuel reprocessing facilities at Sellafield (UK) and La Hague (France). Contributions from the nuclear power plants in Ringhals and Barsebäck have also been identified, as has fallout from Chernobyl still stored mainly in the sediments and water of the Baltic Sea. In recent decades, studies have shown increasing levels of 129I and 236U and decreases in 137Cs, 99Tc, and 239 + 240Pu concentrations over time. The 14C analyses show an impact of anthropogenic 14C from activities other than atmospheric nuclear weapons tests in the 1950s and 1960s. The considerable variation in the concentration of different radionuclides over time and along the coasts warrants further studies to determine the possible origin of these radionuclides and to map background data in the event of future releases.

Figure 1

Map including the sampling place at Särdal (56.76 N, 12.63E) on the Swedish west-coast regularly used since 1967. Major ocean currents (adapted from Qiao et al. [27] and Qiao et al. [28]) are indicated (surface water movement in thin water flow arrows and bottom water movement in thick water flow arrows). The Swedish nuclear power plants (R: Ringhals, B: Barsebäck, O: Oskarshamn and F: Forsmark), and the two nuclear facilities at Sellafield (UK) and La Hague (France) are also marked. The site Winfrith (W) in UK, previously hosting research reactors and performing reactor development, is under decommissioning. Several other sites for potential releases of radioactive discharges to water exist in the area [21, 26]. In addition to those inputs, the Baltic Sea is exposed to other potential local and/or regional sources, as e.g. the controlled or accidental discharges from nuclear facilities (e.g. historical releases from the Swedish nuclear facility at Studsvik (58.77 N, 17.39E) have been recently studied [29]). There is also a number of radioactive dumping sites reported all around the Baltic Sea. Most of them not well-documented. Sk., Skagerrak; K., Kattegat; D., Danish straits. Barsebäck NPP is located at the Öresund Strait (Öresund Strait is not marked in the figure). Map: Schlitzer, Reiner, ocean data view, https://odv.awi.de, 2021 [30].

Figure 2

¹³⁷Cs-activity concentration (Bq/kg d.w.) in Fucus serratus and F. versiculosus from Särdal (56.76 N, 12.63E) on the Swedish west-coast during the period 1967–2021. The uncertainty (1 SD) in the individual measurements is typically ±0.3 Bq/kg d.w.

Figure 3

The annual releases of ¹³⁴Cs and ¹³⁷Cs from the Sellafield reprocessing plant located in the Irish Sea and from the Cap de La Hague reprocessing plant in the English Channel [21, 42–47, 50, 51].

Figure 4

¹³⁷Cs in Fucus spp. at the various sites along the Swedish coast in spring 2020. Samples collected within 10 km of the Swedish NPPs are highlighted in the grey areas. Uncertainty bars correspond to ±1 SD. Open circles: MDA. FS, Fucus serratus; FV, Fucus vesiculosus. Site numbers are the same as in [19].

Figure 5

¹³⁴Cs-activity concentration (Bq/kg d.w.) in Fucus serratus and F. vesiculosus from Särdal (56.76 N, 12.63E) on the Swedish west coast during the period 1967–2020. For long periods, the activity concentration was below the detection limit. The uncertainty (1 SD) in the individual measurements is typically ±0.4 Bq/kg d.w.

Figure 6

⁶⁰Co-activity concentration in Fucus serratus and F. vesiculosus from Särdal (56.76 N, 12.63E), 1967–2000. First detected on august 21, 1976. Last time for detectable amount December 29, 2005. Barsebäck NPP was in operation from July 1, 1975 to May 30 2005. Minimum detectable activity concentration 0.3 Bq/kg d.w. The uncertainty (1 SD) in the individual measurements is typically ±0.4 Bq/kg d.w.

Figure 7

Liquid discharges of ⁶⁰Co from Sellafield, Winfrith, La Hague and Barsebäck 1973–2022 [21, 26, 55–58].

Figure 8

¹⁴C in Fucus spp. from Särdal in the period 1967–2023. Additional data from three other places (Glommen (56.93 N, 12.35E), Träslövsläge (57.06 N, 12.27E), and Kärradal (57.19 N, 12.21E), on the west coast are also included. The atmospheric data are representative of CO₂ of the northern hemisphere [59–63]. Prior to the atmospheric nuclear testing F¹⁴C in atmospheric CO₂ was ~1 due to naturally produced ¹⁴C. The modelled marine bomb pulse represents the average global oceanic water down to a depth of 75 m (data up to 1996 are from Reimer et al [63]; values from 1996 to 2020 are based on linear regression of the data from 1987 to 1996).

Figure 9

a. Annual liquid ¹⁴C water-borne discharges from La Hague and Sellafield [21, 51, 64]. b. Annual liquid ¹⁴C water-borne discharges from La Hague and Sellafield [21, 51, 64]^, with dilution factors and delay times according to Eriksson Stenström and Mattsson [19].

Figure 10

F¹⁴C values in samples of Fucus spp. collected along the Swedish coast in spring 2020 (11 March to 13 May) and autumn 2020 (24 September to 15 October). Site 1 (Strömstad) is the northernmost site at the Swedish west coast, located close to the border of Norway, while Site 17 (Mölle) is located in southernmost Kattegat. Site 30 (Skillinge) is a terrestrial and marine reference site located in the south east of Sweden, at the Baltic Sea [65]. At Site 11 (Bua, at beacon) Fucus vesiculosus as well as F. Serratus were sampled in autumn 2020.

Figure 11

Reported releases of ⁹⁹Tc from the reprocessing facilities at Sellafield [42, 55] and La Hague [51, 66].

Figure 12

The ⁹⁹Tc activity concentration in Fucus serratus and F. vesiculosus from Särdal during the period 2001–2024.

Figure 13

¹²⁹I concentration (atoms/kg d.w.) in Fucus serratus from Särdal (1 atom = 1.40·10⁻¹⁵ Bq).

Figure 14

Reported releases of ¹²⁹I from the reprocessing facilities at Sellafield and La Hague [21, 22, 42, 46, 47].

Figure 15

The activity concentration (mBq/kg d.w.) of ^239 + 240Pu in Fucus spp. collected at Särdal in the period 1967 to 2021.

Figure 16

Estimated yearly ²³⁹Pu releases (TBq) [23] from Sellafield and measured ²³⁹Pu activity concentration in Fucus serratus from Särdal. To compare the time course of releases and activity concentration in the algae, the transport time from Sellafield to Särdal must be taken into account, which for ¹³⁷Cs and ⁹⁹Tc was previously determined to be 4 years [2, 12, 14, 15].

Figure 17

²³⁶U concentration (at/kg) in Fucus spp. from Särdal and the estimated releases of ²³⁶U from Sellafield nuclear reprocessing plant according to shell data by Castrillejo et al. [64]. Note the logarithmic scale on the y-axis. During the whole time period releases from Sellafield seems to dominate over the releases from La Hague [22]. To compare the time course of releases and activity concentration in the algae, the transport time from Sellafield to Särdal must be taken into account, which for ¹³⁷Cs and ⁹⁹Tc was previously determined to be 4 years [2, 12, 14, 15].

Figure 18

The ²³⁶U/²³⁸U atomic ratio in Fucus serratus from Särdal at various times between 1976 and 2021 and the same ratio in surface water from the Danish straits in 2013 and 2014 [84].

Figure 19

¹²⁹I/²³⁶U atomic ratio in Fucus serratus from Särdal at various times between 1976 and 2021.