Iodine isotopes (129I and 127I) in the hydrosphere of Qinghai-Tibet region and South China Sea
Introduction
Iodine-129, is a radioisotope of iodine with a half-life of 1.57 × 107 years, that is naturally produced by the spontaneous fission of uranium in the Earth's crust and cosmic-rays induced spallation of xenon in the atmosphere (Aldahan et al., 2007a; He et al., 2013). These processes resulted in a ratio of 129I/127I in the marine reservoir that was between 2 × 10−12 and 6 × 10−13 (Fabryka-Martin et al., 1985; Kilius et al., 1992). The natural inventory of 129I was estimated to be approximately 230 kg (Rao and Fehn, 1999), however, this amount is very small compared to recent anthropogenic releases of 129I. Major sources of anthropogenic 129I include releases from: 1) atmospheric nuclear weapon tests, 2) nuclear accidents, 3) nuclear power plants operation and 4) nuclear fuel reprocessing facilities. Atmospheric nuclear weapons tests in 1945–1980 released about 43–150 kg of 129I (Carter and Moghissi, 1977; Eisenbud and Gesell, 1997a). Nuclear accidents at Chernobyl in 1986 released 6.0 kg of 129I (Aldahan et al., 2007a) and at Fukushima in 2011 about 1.2 kg of 129I (Hou et al., 2013). Emission of 129I from the routine operation of nuclear power plants is suggested to be insignificant (Jin et al., 2009; Zhang et al., 2011). Most of anthropogenic 129I in the environment has been released from nuclear fuel reprocessing facilities through atmospheric and marine discharges. Besides atmospheric releases (about 400 kg by 2007), the Sellafield (UK) and La Hague (France) nuclear facilities have discharged about 6500 kg 129I to the seas (up to 2007)), with an annual discharge of 129I still remaining at a very high level of about 250 kg/y (Liu et al., 2016a). This 129I has contaminated large areas via transport by ocean currents. Furthermore, the re-emission of the reprocessing derived 129I from the contaminated seawater (about 3% of the marine discharges) to the atmosphere has become an additional source of 129I to the atmosphere in recent years (Zhang et al., 2016).
Despite the large amount of 129I in the environment, direct health hazards are minimal owing to its low specific radioactivity (long half-life) (Li et al., 2005). The criteria of the long half-life have, however, strengthened the use of 129I as environmental tracer of both environmental processes and anthropogenic activities.
There has been many published data on the 129I distribution in different Earth reservoirs (atmosphere, hydrosphere, biosphere and soil and sediments) portraying a general picture of 129I contamination and sources in Europe and USA, but such research is meager in China. Some investigations have been performed in selected areas (Hou et al., 2000; Li et al., 2005; Zhou et al., 2010; Zhang et al., 2011, 2014; Ma et al., 2013) such as the analysis of 129I in seaweed from the south coast region of China and human thyroid samples (Hou et al., 2000), as well as grass, seaweed, seawater and pine needles (Li et al., 2005) and local vegetation, soil and precipitation (Zhang et al., 2011). Although these investigations provided valuable data on 129I concentrations, the regional distribution pattern of 129I in China is far from complete. Consequently, further research concerning 129I spatial patterns in China is needed to provide a base line for environmental analysis and prediction. In the investigation presented here, we focus on the distribution of 129I and 127I in some parts of the hydrosphere of China including water samples collected from rivers and lakes in the Qinghai-Tibet region, Yangtze Estuary and South China Sea (SCS). In addition, we explore the sources of 129I in these regions, assess environmental hazards and establish the possibility of using 129I as a chronological indicator. The choice of these water systems is based on the fact that none of them has been analyzed for 129I and that: 1) The Qinghai-Tibet region is the birthplace of the Yellow River, the second-longest river in Asia and the sixth-longest river system in the world, including Gyaring and Ngöring (Sisters) Lakes (Jin et al., 2009). 2) The Yangtze River is the longest river in Asia and the third-longest in the world, and its river basin is home to one-third of the population of China. 3) The South China Sea is a marginal sea that is part of the Pacific Ocean, encompassing an area from the Karimata and Malacca Straits to the Strait of Taiwan of around 3,500,000 square kilometers.
Section snippets
Sampling and sites
Sixteen freshwater samples from the inland water system of the Qinghai-Tibet region, and 2 samples from the Yangtze Estuary were collected. A seawater section in the South China Sea (SCS) with 10 samples covering the depth interval from 0 m to 3800 m (Fig. 1 and Table 1) was also analyzed in this work. The inland water samples were collected during 15th - 19th April and 21st - 25th July in 2014 from Qinghai-Tibet region (Fig. 1), the source area of the Yellow River (SAYR), in four watersheds,
Spatial distribution of iodine isotopes
The 127I and 129I concentrations and 129I/127I atomic ratios in water samples collected from the Qinghai-Tibet region, Yangtze Estuary and SCS are presented in Table 1. The 129I/127I ratios in the inland water of the Qinghai-Tibet region are in (0.18–21.34) × 10−10 with a mean ratio of 8.04 × 10−10 (n = 16), in which a range of (0.80–19.8) × 10−10 in the river water, (0.23–21.3) × 10−10 in the lake water, and (0.18–2.40) × 10−10 in the spring water. As for 129I concentrations, they span in
Conclusion
Based on the results and discussion above, it can be concluded that:
- 1.
The 129I concentrations in the hydrosphere of China reflect anthropogenic input with varying degrees being more on land hydrological system (rivers and lakes) than in the sea.
- 2.
The 129I depth profiles in the SCS reflect distribution pattern largely controlled by monsoon drift in the surface and cyclonic currents at depth.
- 3.
Major source of 129I in China is attributed to atmospheric transport from the NFRPs emissions (direct
Acknowledgements
This research was funded in part by the Special Fund of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering [Grant No. 20165042512 and No. 20155045612], the Fundamental Research Funds for the Central Universities [Grant No. 2017B10314 and No. 2016B04214], Postgraduate Research & Practice Innovation Program of Jiangsu Province [Grant No. KYZZ16_0277]. The first two authors contributed equally to this paper and should be considered co-first authors.
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The first two authors contributed equally to this paper and should be considered co-first authors.