Long-term migration of iodine in sedimentary rocks based on iodine speciation and 129I/127I ratio
Abstract
[Introduction] 129I is one of the available indexes of long-term migration of groundwater solutes, because of its long half-life (15.7 million years) and low sorption characteristics. The Horonobe underground research center (Japan Atomic Energy Agency), at which are conducted research and development of fundamental techniques on geological disposal of high-level radioactive waste, is an appropriate site for natural analogue studies, because iodine concentration in groundwater is high in this area. To predict iodine behavior in natural systems, speciation of iodine is essential because of different mobility among each species. In this study, we determined iodine speciation and129I/127I isotope ratios of rock and groundwater samples to investigate long term migration of iodine. [Methods] All rock and groundwater samples were collected at Horonobe underground research center. The region is underlain mainly by Neogene to Quaternary marine sedimentary rocks, the Wakkanai Formation (Wk Fm, siliceous mudstones), and the overlying Koetoi Formation (Kt Fm, diatomaceous mudstones). Iodine species in rock samples were determined by iodine K-edge X-ray absorption near edge structure (SPring-8 BL01B1). Thin sections of rock samples were prepared, and iodine mapping were obtained by micro-XRF analysis (SPring-8 BL37XU). Iodine species (IO3-, I-, and organic I) in groundwater were separately detected by high performance liquid chromatography connected to ICP-MS. The 129I/127I ratios in groundwater and rock samples were measured by accelerator mass spectrometry (MALT, Univ. of Tokyo). Iodine in rock samples were separated by pyrohydrolysis and water extraction. [Results and discussion] Concentration of iodine in groundwater varied widely and was much higher than that of seawater showing a high correlation with that of chlorine (R2 = 0.90). Species of iodine in groundwater was mainly I-. Iodine in rock samples decreased near the boundary between Wk and Kt Fms. Iodine K-edge XANES showed that iodine in rock was a mixture of organic and inorganic iodine. According to iodine and carbon mapping in micrometer scale, iodine was accumulated locally and correlated with carbon, suggesting that iodine existed as organic iodine. The 129I/127I isotope ratios in groundwater were lower than those in rocks and almost constant at various depths, demonstrating that iodine in groundwater was released from layers deeper than co-existing rocks. According to these results, migration of iodine in this area can be expected as follows. (i) During sedimentation of Wk and Kt Fms, iodine was accumulated as organic iodine in siliceous sediments. (ii) Iodine was released as I- from the layers deeper than Wk Fm during diagenetic processes. Subsequently, iodine rich groundwater was distributed to Wk and Kt Fms due to the compaction of the layers. (iii) During uplift and denudation processes, both iodine and chlorine were diluted by meteoric water from the surface. Iodine distribution coefficient (Kd = [I concentration in rock]/[I concentration in groundwater]) of Kt Fm is higher than that in Wk Fm. Diatomaceous mudstones might be more effective than siliceous mudstones as natural barrier for 129I released from deep underground radioactive waste repository. This suggestion should be reinforced by laboratory experiments in future studies.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.H43E1392T
- Keywords:
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- 1051 GEOCHEMISTRY / Sedimentary geochemistry;
- 1832 HYDROLOGY / Groundwater transport