Uranium and plutonium isotopes and their environmental implications in surface sediments from the Yangtze River catchment and estuary

Highlights

• U and Pu isotopes were identified in the Yangtze River catchment and estuary.

• ²³⁶U concentrations were measured as (3.05–21.1) × 10⁷ atoms/g.

• ²³⁶U/²³⁸U atom ratios were measured as (1.43–6.85) × 10⁻⁸.

• ²⁴⁰Pu/²³⁹Pu atom ratios were 0.168–0.201 in the Yangtze River catchment.

• ²⁴⁰Pu/²³⁹Pu atom ratios were 0.216–0.264 in the Yangtze River estuary.

Abstract

The U-series and Pu isotope signatures in the environment have emerged as important tools for promoting geochemical studies. To enhance the understanding of U and Pu sources and behaviours in river systems, this study examined ²³⁴U, ²³⁵U, ²³⁶U, ²³⁸U and ²³⁹Pu, ²⁴⁰Pu isotopes in surface sediments collected from the Yangtze River catchment and estuary in China, for the first time. The ²³⁴U, ²³⁵U, ²³⁸U and ²³⁶U concentrations were measured in the range of 8.29–20.6 Bq/kg, 0.352–0.789 Bq/kg, 7.39–16.4 Bq/kg and (3.05–21.1) × 10⁷ atoms/g. Additionally, the ²³⁶U/²³⁸U, ²³⁴U/²³⁸U and ²³⁵U/²³⁸U atom ratios were in the range of (1.43–6.85) × 10⁻⁸, (6.08–7.01) × 10⁻⁵ and (7.49–8.08) × 10⁻³ respectively. For all samples, the ^239+240Pu activity concentrations ranged from 0.01 to 3.40 Bq/kg. In the Yangtze River catchment, ²⁴⁰Pu/²³⁹Pu atom ratios presented typical global fallout values (0.168–0.201); while in the Yangtze River estuary, higher values of 0.216–0.264 located between global fallout and PPG (Pacific Proving Grounds) close-in fallout values. From west to east, the ²³⁴U, ²³⁵U, ²³⁶U and ²³⁸U activities generally exhibited decreasing trend from the upstream parts of the river to the estuary. The values of ²³⁶U concentrations and ²³⁶U/²³⁸U, ²⁴⁰Pu/²³⁹Pu, ²³⁶U/²³⁹Pu atom ratios located in the range of global fallout level in environmental samples from other regions, therefore, it may be concluded that ²³⁶U in surface sediments of the Yangtze River catchment and estuary are mainly derived from global fallout.

Introduction

There are three major kinds of uranium (U) isotopes in nature: ²³⁸U (t_1/2 = 4.47 × 10⁹ y), ²³⁵U (t_1/2 = 7.04 × 10⁸ y), and ²³⁴U (t_1/2 = 2.46 × 10⁵ y). Additionally, ²³⁶U with a half-life of 2.34 × 10⁷ y is of growing interest with applications in environmental and geochemical studies (Bu et al., 2017). In general, the naturally occurring sources of ²³⁶U in the surface environment was trace, as the product of natural nuclear reactions of ²³⁵U(n,γ) (σ = 86.7b) and ²³⁸U(n,3n) (σ = 0.4b). Therefore, naturally ²³⁶U/²³⁸U is demonstrated to be ultra-low. In fact, in the background Earth’s crust, unaffected by nuclear fallout or cosmic radiation, the ²³⁶U/²³⁸U atom ratios are in the order of 10⁻¹⁴ (Steier et al., 2008). In the earth surface layer, the ²³⁶U/²³⁸U atom ratio, reaches up to 7 × 10⁻¹³ for typical rocks affected from the neutron flux of cosmic radiation (Sakaguchi et al., 2009). In addition, the relatively high ²³⁶U/²³⁸U atom ratios could be found in the order of 10⁻¹⁰ in U ores (Richter et al., 1999, Zhao et al., 1994).

Artificial ²³⁶U was released due to the human nuclear activities (discharges or releases from nuclear weapon tests, artificial reactor accidents, and nuclear reprocessing plants). It was estimated that ~900 kg of artificial ²³⁶U was released to the environment, while ~30 kg of natural ²³⁶U as reported in the Earth's crust (Sakaguchi et al., 2009, Shao et al., 2019). Therefore, these ultra-low natural ²³⁶U/²³⁸U atom ratios in the surface environment mentioned above are overwhelmed due to the man-made neutron fluxes. It should be noted that the ²³⁶U/²³⁸U atom ratio varies significantly based on the source and production process (Shao et al., 2019). For example, in terms of the source of global fallout, Sakaguchi et al. (2009) reported ²³⁶U/²³⁸U atom ratios in Japanese soil samples ranging from 10⁻⁸ to 10⁻⁷. Similar results were also reported in soil samples from La Palma, Spain (Srncik et al., 2011), soil samples from Hunan Province, China (Shao et al., 2019), soil and river sediment samples from Fukushima Prefecture, Japan (Yang et al., 2017a, Yang et al., 2019). In terms of the source of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, Shinonaga et al. (2014) reported a sharp increase of ²³⁶U/²³⁸U atom ratios up to ~10⁻⁶ in air particles after the hydrogen explosions. In terms of the source of the Chernobyl accident, ²³⁶U/²³⁸U atom ratios of ~10⁻³ was found in hot particles (Boulyga and Becker, 2001). However, as a new tracer for geochemical studies, ²³⁶U has major drawbacks due to worldwide contamination by global fallout-derived uranium and dilution effect from the natural uranium. Therefore, most studies have failed to reveal the release of trace ²³⁶U related to the FDNPP accident using environmental samples, such as seawater, vegetation, litter, soil, sediment samples (Jaegler et al., 2019, Sakaguchi et al., 2012, Schneider et al., 2017). In soil and “black substances” samples with obvious Pu contamination due to the FDNPP accident (Sakaguchi et al., 2014), Yang et al. (2017a) indicated a trace release of ²³⁶U with Pu, because they observed high linear correlations between ²³⁶U and ^239+240Pu activities, and between ²³⁶U and ²³⁸Pu activities. Furthermore, it should be noted that the chemical behaviors of U and Pu differ markedly, considerably more than differences between Np and Pu, therefore, the atom ratios of ²³⁶U/²³⁸U or ²³⁶U/^239+240Pu may be treated as a new and powerful fingerprint for radioactive source identification, along with other tracers, such as ²⁴⁰Pu/²³⁹Pu, ¹³⁴Cs/¹³⁷Cs (Bu et al., 2017). However, unlike ²⁴⁰Pu/²³⁹Pu and ¹³⁴Cs/¹³⁷Cs, it is desirable to obtain more ²³⁶U data since it is limited in the environment.

The anthropogenic isotopes of plutonium (Pu) are present in the environment as a result of the above-ground testing of nuclear weapons, various accidental releases and discharge events from reprocessing plants (UNSCEAR, 2000, Zhuang et al., 2019). The atom ratio of ²⁴⁰Pu/²³⁹Pu has been successfully used as tracer for source identification and Pu transport behavior after release for a long time in different environment, such as sediments from the coastal areas, lakes and soils (Guan et al., 2018, Wang et al., 2017a, Xing et al., 2018, Xu et al., 2018). Recently, ²³⁵U, ²³⁶U, and Pu isotopes have been used as indicators of long-term environmental change, anthropogenic nuclear activities, and natural processes (Hirose et al., 2017, Sakaguchi et al., 2014, Schneider et al., 2017, Shinonaga et al., 2014, Yang et al., 2017b). However, the distribution of anthropogenic U and Pu isotopes in surface river sediments from the Yangtze River catchment is very scarce. The challenge is mainly due to the analysis of ²³⁶U, since Pu isotopes in environment already can be measured routinely by advance mass spectrometric techniques, such as inductively coupled plasma-mass spectrometry (ICP-MS), thermal ionization mass spectrometry (TIMS) and accelerator mass spectrometry (AMS) (Bu et al., 2018).

In such a case to measure ²³⁶U routinely as Pu isotopes, it is required to overcome the great challenge of high cost and limited facilities for ²³⁶U analysis by AMS. Up to date, the data of ²³⁶U in China were only available in two papers (Shao et al., 2019, Wang et al., 2017b). The building of NPPs is expanding in Asian area, especially in China, to solve the challenge due to the soaring energy requirement and the severe air pollution. Therefore, apart from the potential use a geochemical tracer, it is also vital to obtain more data of ²³⁶U in vast region of China to have a preliminary understanding of this artificial radionuclide with more feasible methods. Yang et al. (2016) have suppressed interferences and tailing effects arising from ²³⁵U⁺ and ²³⁸U⁺ effectively during ICP-MS analysis, by proposing a new method with one DGA resin column purification coupled to triple-quadrupole ICP-MS (ICP-MS/MS) analysis. It is feasible to perform routine monitoring of environmental ²³⁶U originating from global fallout and nuclear accident fallout, due to the low-cost method with low method detection limit (3.50 × 10⁻⁶ Bq kg⁻¹ for ²³⁶U) (Yang et al., 2016). This method made it is possible to obtain more data of ²³⁶U/²³⁸U in the whole Yangtze River catchment and estuary economically in short time, which covers a catchment area of 1.8 million km², accounting for one fifth of the China land.

To the best of our knowledge, investigations on anthropogenic U isotopes in sediment samples from China have been limited. The present work is part of a large effort to explore the signature of U in undisturbed regions of China, well-removed from any nuclear weapon testing sites and nuclear reactors. This paper reports first results on ²³⁶U levels in sediments collected from the Yangtze River catchment and estuary to yield atom ratios among ²³⁶U, other U and Pu isotopes. To summarize, the goals of this paper are (1) to determine the ^239+240Pu activity concentrations and ²⁴⁰Pu/²³⁹Pu atom ratios, the ²³⁴U, ²³⁵U, ²³⁶U, ²³⁸U activity concentrations and ²³⁶U/²³⁸U, ²³⁴U/²³⁸U, ²³⁵U/²³⁸U atom ratios in the sediments of our selected areas by using ICP-MS and ICP-MS/MS analyses; (2) to investigate the source term of ²³⁶U and Pu isotopes in the studied areas; (3) to establish a baseline of activity concentrations and atom ratios for U and Pu isotopes to perform future environmental risk assessment in case of potential nuclear accidents since more NPPs are being built along the Yangtze River in the future.