Iodine isotopes (¹²⁹I and ¹²⁷I) in the hydrosphere of Qinghai-Tibet region and South China Sea

Highlights

• Variable ¹²⁹I concentrations (1–51) × 10⁶ atoms/L occur in the hydrosphere of China.

• The range of ¹²⁹I/¹²⁷I is (0.03–21) × 10⁻¹⁰ in the hydrosphere of China.

• Depth profiles of ¹²⁹I in the South China Sea water reveal effects of different ocean currents.

• Emissions from the Nuclear fuel reprocessing facilities is the major source of ¹²⁹I in China.

Abstract

The radioactive isotope ¹²⁹I, with a half-life of 1.57 × 10⁷ years, is widely used as a tracer to assess nuclear safety, to track environmental and geological events and to figure out the details of the stable iodine geochemical cycle. This work investigated the ¹²⁹I and ¹²⁷I distribution in water samples collected from the terrestrial (rivers, lakes and springs) and marine water systems (estuary and sea) in China. The measured ¹²⁹I concentrations of (1–51) × 10⁶ atoms/L and ¹²⁹I/¹²⁷I ratios of (0.03–21) × 10⁻¹⁰ shows the variability of ¹²⁹I level in the water systems. The local permafrost and seasonal frozen environment play a key role in groundwater recharge in the Qinghai-Tibet region, which is reflected in the ¹²⁹I distribution in surface water. The depth distribution of ¹²⁹I in the water column of the South China Sea reflects the effluence of different currents. The results also indicate that the hydrosphere of China contains one to three orders of magnitude less ¹²⁹I compared to those reported in Europe. Despite the large distance, the European nuclear fuel reprocessing facilities represent the major source of ¹²⁹I in the hydrosphere of China through atmospheric transport. The contribution of the Fukushima nuclear accident to ¹²⁹I levels in the hydrosphere of China was negligible.

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.