Sediments as sinks and sources of marine radionuclides: Implications for their use as ocean tracers

A Lagrangian transport model for the North Atlantic has been applied to simulate the historical releases of 137Cs, 129I and 236U from the European nuclear fuel reprocessing plants. Advection by currents, mixing and decay are included, as radionuclide interactions between water, sediments and suspended matter. The model was validated comparing predictions with measured radionuclide concentrations in water and sediments in several areas. 129I and 236U signals entering the Arctic Ocean have been compared with the input terms: the 236U signal is distorted, but the 129I signal preserves its shape. In the first moments after the releases, the sediments act as sinks for 236U, but not significantly for 129I and ultimately they become sources of 236U to the open sea. This results in a weaker correlation between input and output signals for 236U than for 129I. The same effects as for 236U have been found for 137Cs signal into the Arctic.

Tracing the upwelling process in the northern Benguela upwelling system (nBUS) by 129I

New data on the presence of ¹²⁹I in seawater in the Southern Hemisphere measured by Accelerator Mass Spectrometry (AMS) is presented. The samples were collected in 2014 along the Namibian coast during a cruise organised by the National Marine Information and Research Centre (NatMIRC), the national laboratories of the Ministry of Fisheries and Marine Resources (MFMR) in Namibia, and the IAEA Environment Laboratories (IAEA NAEL) in Monaco. The Benguela upwelling system is known as one of the most important marine upwelling regions in the world. Strong winds induce an offshore transport of surface seawater which is substituted by cool subsurface water inshore. As this water is nutrient-rich, which leads to high primary productivity, the Benguela upwelling system has a very important role as a fishing production area. The ¹²⁹I concentrations in samples were between (0.66 ± 0.14) × 10⁷ and (1.45 ± 0.30) × 10⁷ atoms/kg. The highest ¹²⁹I concentrations were found in the offshore surface samples. Deep-sea and inshore samples contained lower ¹²⁹I concentrations, possibly as an effect of the upwelling process. A comparison with previously published studies suggests that the presence of ¹²⁹I in the northern Benguela upwelling system (nBUS), is mainly due to the impact of nuclear weapons global fallout, without any evident impact of nuclear fuel reprocessing.

Meteoric 10Be in aerosol filters in the city of Seville

Cosmogenic radionuclides in the one-million-year half-life range, like ¹⁰Be, find application fields in several Sciences. They are powerful tools in Geology and Geochronology, as they are very important tracers on the Earth, being utilized as chronometer. Meteoric ¹⁰Be (T_1/2 = 1.39 × 10⁶ y) associated to aerosols can be used as a tracer of atmospheric processes and specifically as indicators of the cosmogenic interactions in lower Stratosphere, upper Troposphere, the air exchange between both and deposition processes on the Earth surface. The applications of ¹⁰Be are even more relevant when combined with other radionuclides such as ²⁶Al. In order to provide new data about concentration ¹⁰Be in this type of samples, the first atmospheric air filters in Spain have been analysed. Values around 10⁴ at/m³ (atoms per cubic meter of air) for ¹⁰Be have been obtained. Due to the location and the features of the sampling site (urban area, at sea level and mid latitude), a new radiochemical procedure was designed and developed in our laboratory for the Accelerator Mass Spectrometry (AMS) measurement of ¹⁰Be in this kind of samples. The samples were measured in SARA, the 1 MV AMS system at Centro Nacional de Aceleradores (CNA).

Determination of denudation rates by the measurement of meteoric 10Be in Guadiana river sediment samples (Spain) by low-energy AMS

The concentration of meteoric ¹⁰Be in estuarine sediment samples has been measured by Spanish Accelerator for Radionuclides Analysis (SARA) at CNA and subsequently used to assess the denudation rate in Guadiana river basin together with the sediment budget method, on both sides of the frontier between Spain and Portugal. The two methods yielded coincident results. The estimation by the ¹⁰Be method gave the denudation rate of (0.76 ± 0.10) × 10^-2 cm/y. After correcting for an approximate 80% attenuation of the sediment discharge into the ocean, caused by the river dams, the sediment budget method yielded the rate of (0.77 ± 0.17) × 10^-2 cm/y.

Recent evolution of 129I levels in the Nordic Seas and the North Atlantic Ocean

Most of the anthropogenic radionuclide ¹²⁹I released to the marine environment from the nuclear fuel reprocessing plants (NFRP) at Sellafield (England) and La Hague (France) is transported to the Arctic Ocean via the North Atlantic Current and the Norwegian Coastal Current. ¹²⁹I concentrations in seawater provides a powerful and well-established radiotracer technique to provide information about the mechanisms which govern water mass transport in the Nordic Seas and the Arctic Ocean and is gaining importance when coupled with other tracers (e.g. CFC, ²³⁶U). In this work, ¹²⁹I concentrations in surface and depth profiles from the Nordic Seas and the North Atlantic (NA) Ocean collected from four different cruises between 2011 and 2012 are presented. This work allowed us to i) update information on ¹²⁹I concentrations in these areas, required for the accurate use of ¹²⁹I as a tracer of water masses; and ii) investigate the formation of deep water currents in the eastern part of the Nordic Seas, by the analysis of ¹²⁹I concentrations and temperature-salinity (T-S) diagrams from locations within the Greenland Sea Gyre. In the Nordic Seas, ¹²⁹I concentrations in seawater are of the order of 10⁹ at·kg^-1, one or two orders of magnitude higher than those measured at the NA Ocean, not so importantly affected by the releases from the NFRP. ¹²⁹I concentrations of the order of 10⁸atoms·kg^-1 at the Ellet Line and the PAP suggest a direct contribution from the NFRP in the NA Ocean. An increase in the concentrations in the Nordic Seas between 2002 and 2012 has been detected, which agrees with the temporal evolution of the ¹²⁹I liquid discharges from the NFRPs in years prior to this. Finally, ¹²⁹I profile concentrations, ¹²⁹I inventories and T-S diagrams suggest that deep water formation occurred in the easternmost area of the Nordic Seas during 2012.

Estimating the impact from Fukushima in Southern Spain by 131I and Accelerator Mass Spectrometry detection of 129I

After the Fukushima accident, large amounts of radionuclides were discharged to the atmosphere. Some of them travelled long distances and were detected in places as far from Japan as Spain a few days after the accident. One of these radionuclides was ¹³¹I. Its isotope ¹²⁹I (T1/2 = 15.7 × 106 years) was also expected to follow the same pathway. In this work, we present the results for the ¹²⁹I concentration in the same atmospheric samples from Seville (Spain) where ¹³¹I activity was measured in 2011 by Baeza et al. (2012). ¹²⁹I concentrations in aerosol and gaseous samples showed concentrations in the order of 104 and 105 atoms/m³, typically higher in the gaseous form with respect to the aerosol form. Also ¹²⁹I in rainwater was measured, showing concentrations in the order of 10⁸ atoms/L. The results show a very good agreement with the ¹³¹I profile, showing that, if background from other sources is not relevant, it is possible to estimate the impact of similar events years after them thanks to the sensitivity of techniques like Accelerator Mass Spectrometry.

The behaviour of ¹²⁹I released from nuclear fuel reprocessing factories in the North Atlantic Ocean and transport to the Arctic assessed from numerical modelling

A quantitative evaluation of the fate of (129)I, released from the European reprocessing plants of Sellafield (UK) and La Hague (France), has been made by means of a Lagrangian dispersion model. Transport of radionuclides to the Arctic Ocean has been determined. Thus, 5.1 and 16.6 TBq of (129)I have been introduced in the Arctic from Sellafield and La Hague respectively from 1966 to 2012. These figures represent, respectively, 48% and 55% of the cumulative discharge to that time. Inventories in the North Atlantic, including shelf seas, are 4.4 and 13.8 TBq coming from Sellafield and La Hague respectively. These figures are significantly different from previous estimations based on field data. The distribution of these inventories among several shelf seas and regions has been evaluated as well. Mean ages of tracers have been finally obtained, making use of the age-averaging hypothesis. It has been found that mean ages for Sellafield releases are about 3.5 year larger than for La Hague releases.

New insights on the role of sea ice in intercepting atmospheric pollutants using (129)I

Measurements of (129)I carried out on sea ice samples collected in the central Arctic Ocean in 2007 revealed relatively high levels in the range of 100-1400×10(7) at L(-1) that are comparable to levels measured in the surface mixed layer of the ocean at the same time. The (129)I/(127)I ratio in sea ice is much greater than that in the underlying water, indicating that the (129)I inventory in sea ice cannot be supported by direct uptake from seawater or by iodine volatilization from proximal (nearby) oceanic regimes. Instead, it is proposed that most of the (129)I inventory in the sea ice is derived from direct atmospheric transport from European nuclear fuel reprocessing plants at Sellafield and Cap La Hague. This hypothesis is supported by back trajectory simulations indicating that volume elements of air originating in the Sellafield/La Hague regions would have been present at arctic sampling stations coincident with sampling collection.

Influence of releases of (129)I and (137)Cs from European reprocessing facilities in Fucus vesiculosus and seawater from the Kattegat and Skagerrak areas

(129)I is a very long-lived radionuclide (T1/2=15.7×10(6) years) that is present in the environment because of natural and anthropogenic sources. Compared to the pre-nuclear era, large amounts of (129)I have been released to the marine environment, especially as liquid and gaseous discharges from two European reprocessing facilities located at Sellafield (England) and La Hague (France). The marine environment, i.e., the oceans, is the major source of iodine. Brown seaweed accumulates iodine at high levels up to 1.0% of dry weigh, and therefore they are ideal bioindicators for studying levels of (129)I. In this work, (129)I concentrations have been determined in seaweed Fucus vesiculosus and seawater collected in the Kattegat and Skagerrak areas in July 2007. The resulting data were evaluated in terms of (129)I concentrations and (129)I/(137)Cs ratios. (129)I concentrations were found to be in the order of (44-575)×10(9) atoms g(-1) in seaweed and (5.4-51)×10(9) atoms g(-1) in seawater, with an enhancement in the Skagerrak area in comparison to the Kattegat area. Iodine-129 concentrations in both seaweed and seawater were used to determine the concentration factor of iodine in brown seaweed F. vesiculosus. The high levels of (129)I and (129)I/(137)Cs ratios in the Skagerrak area and their gradually decreasing trend to the Kattegat indicates that the most important contribution to the (129)I inventory in those areas comes from Sellafield and La Hague reprocessing plants.

Pre- and post-Chernobyl accident levels of 129I and 137Cs in the Southern Baltic Sea by brown seaweed Fucus vesiculosus

(129)I is a very long-lived radionuclide (T(1/2) = 15.7 × 10(6) years) that is present in the environment both because of natural and anthropogenic sources. In this work (129)I concentration and (129)I/(127)I ratio have been determined in seaweed Fucus vesiculosus collected in the Southern Baltic Sea during 1982 and 1986 (post-Chernobyl accident). The resulting data were evaluated in terms of (129)I concentrations, (129)I/(127)I and (129)I/(137)Cs ratios. (129)I concentrations were found to be in the order of (0.82-5.89) × 10(9) atoms g(-1) in 1982 and (1.33-38.83) × 10(9) atoms g(-1) in 1986. The (129)I/(127)I ratios ranged from (22.7-87.8) × 10(-10) for seaweed collected in 1982 and from (26.1-305.5) × 10(-10) for seaweed collected in 1986. Also a linear relationship was established for (127)I concentrations in seawater and salinity in this area, enabling the estimation of concentration factors for (127)I in F. vesiculosus. The high levels of (129)I and (129)I/(127)I in the Kattegat and their gradually decreasing trend to the Baltic Sea indicates that the most important contribution to the (129)I inventory in the Baltic Sea area comes from Sellafield and La Hague reprocessing plants. With respect to Chernobyl accident, (129)I concentrations in samples collected in 1986 were not much higher than those expected in less contaminated samples from 1982. This supports the view that the contribution of the Chernobyl accident to (129)I in the Baltic region was not significant.

Level and origin of 129I and 137Cs in lichen samples (Cladonia alpestris) in central Sweden

Lichen is a symbiosis between algae and fungi. They have for decades been used as bioindicators for atmospheric deposition of heavy metals, organic compounds and radioactive elements. Especially the species Cladonia alpestris and Cladonia rangiferina are important for the food chain lichen-reindeer-man. The concentration of (129)I was determined in lichen samples (Cladonia alpestris) contaminated by fallout from atmospheric nuclear tests explosions and the Chernobyl accident. The samples were collected at Lake Rogen District (62.3°N, 12.4°E) in central Sweden in the periods 1961-1975 and 1987-1998, and analysed with accelerator mass spectrometry (AMS) at CNA (Seville) to study its distribution in different layers. Data on the (137)Cs activity measured previously were also included in this study. The (129)I concentration ranged from (0.95 ± 0.13) × 10(8) at g(-1) in 1961 in the uppermost layer to (14.2 ± 0.5) × 10(8) at g(-1) in 1987 in deepest layer. The (129)I/(137)Cs atom ratio ranged between 0.12 and 0.27 for lichen samples collected in the period 1961-1975, indicating weapons tests fallout. For lichen samples collected between 1987 and 1998 the behaviour of (137)Cs concentrations reflected Chernobyl fallout. The concentrations of the two radionuclides followed each other quite well in the profile, reflecting the same origin for both. From the point of view of the spatial distribution in the lichen, it appears that (129)I was predominantly accumulated in the lowest layer, the opposite to (137)Cs for which the highest amounts were detected systematically in the topmost layer of lichen. This vertical distribution is important for radioecology because lichen is the initial link in the food chain lichen-reindeer-man, and reindeer only graze the upper parts of lichen carpets.

Iodine-129 in soils from Northern Ukraine and the retrospective dosimetry of the iodine-131 exposure after the Chernobyl accident

Forty-eight soil profiles down to a depth of 40 cm were taken in Russia and Ukraine in 1995 and 1997, respectively, in order to investigate the feasibility of retrospective dosimetry of the 131I exposure after the Chernobyl accident via the long-lived 129I. The sampling sites covered areas almost not affected by fallout from the Chernobyl accident such as Moscow/Russia and the Zhitomir district in Ukraine as well as the highly contaminated Korosten and Narodici districts in Ukraine. 129I was analyzed by radiochemical neutron activation analysis (RNAA) and accelerator mass spectrometry (AMS). 127I was measured for some profiles by RNAA or ion chromatography (IC). The results for 127I demonstrated large differences in the capabilities of the soils to store iodine over long time spans. The depth profiles of 129I and of 137Cs showed large differences in the migration behavior between the two nuclides but also for each nuclide among the different sampling sites. Though it cannot be quantified how much 129I and 137Cs was lost out of the soil columns into deeper depths, the inventories in the columns were taken as proxies for the total inventories. For 129I, these inventories were at least three orders of magnitude higher than a pre-nuclear value of 0.084+/-0.017 mBq m(-2) derived from a soil profile taken in 1939 in Lutovinovo/Russia. From the samples from Moscow and Zhitomir, a pre-Chernobyl 129I inventory of (44+/-24) mBq m(-2) was determined, limiting the feasibility of 129I retrospective dosimetry to areas where the 129I inventories exceed 100 mBq m(-2). Higher average 129I inventories in the Korosten and Narodici districts of 130 and 848 mBq m(-2), respectively, allowed determination of the 129I fallout due to the Chernobyl accident. Based on the total 129I inventories and on literature data for the atomic ratio of 129I/131I=13.6+/-2.8 for the Chernobyl emissions and on aggregated dose coefficients for 131I, the thyroid exposure due to 131I after the Chernobyl accident was estimated for the inhabitants of four villages in the Korosten and of three villages in the Narodici districts. The limitations and uncertainties of the 129I retrospective dosimetry are discussed.

Determination of 129I/127I in aerosol samples in Seville (Spain)

In this work we present results of the (129)I/(127)I ratio in aerosols of Seville, Southwest of Spain (37.4 degrees N,6 degrees W). A radiochemical method is applied to extract the iodine from the aerosols and prepare samples to be measured by accelerator mass spectrometry (AMS) at the ETH facility in Zürich. We have found the possibility of monitoring the (129)I/(127)I isotopic ratio on a two-days basis with sensitivities in the order of 10(4)-10(5) atoms (129)I/m(3), and values of 10(-8)-10(-9) for the isotopic ratio.

Relative influence of 129I sources in a sediment core from the Kattegat area

The depth profiles of the (129)I concentration and the (129)I/(127)I ratio in a surface sediment core from the Kattegat area have been analyzed in order to obtain information about the different sources of (129)I in that core. Therefore, a mathematical model that relates the measured values to the available emission data from the nuclear fuel reprocessing plants and nuclear weapons tests has been applied. Results show that the reprocessing plants at La Hague and Sellafield are the main sources of (129)I in the sediment. Results about the transfer from the release points at the reprocessing plants to the sampling zone agree with other literature data. The model calculates quite fast the sedimentation of (129)I in the sampling place, probably attached to organic matter. Finally, an estimation of approximately 89 kg of (129)I released by Sellafield between 1952 and 1968 has been obtained from the model.

Wet and dry deposition of 129I in Seville (Spain) measured by accelerator mass spectrometry

Iodine-129 (T1/2 = 1.57 x 10(7) yr) concentrations have been determined by accelerator mass spectrometry in rainwater samples taken at Seville (southwestern Spain) in 1996 and 1997. This technique allows a reduction in the detection limits for this radionuclide in comparison to radiometric counting and other mass spectrometric methods such as ICP-MS. Typical 129I concentrations range from 4.7 x 10(7) 129I atoms/l (19.2%) to 4.97 x 10(9) 129I atoms/l (5.9%), while 129I depositions are normally in the order of 10(8)-10(10) atoms/m2d. These values agree well with other results obtained for recent rainwater samples collected in Europe. Apart from these, the relationship between 129I deposition and some atmospheric factors has been analyzed, showing the importance of the precipitation rate and the concentration of suspended matter in it.

Determination of 129I in atmospheric samples by accelerator mass spectrometry

A method for the radiochemical extraction of 129I from atmospheric charcoal filters and its measurement by accelerator mass spectrometry is presented. Either the 129I concentration or the 129I/127I atom ratio can be determined in the sample. With this method, air filters from Seville, in the Southwest of Spain (37.4 degrees N, 6 degrees W) have been analyzed. Sensitivities in the order of 10(4) atoms/m3 for 129I concentrations and 10(-10) for 129I/127I atom ratios are obtained. AMS measurements are performed with the 6 MV tandem accelerator at the ETH-Hönggerberg in Zurich.