Organic matter controls on iodine and plutonium in atmospheric depositions, streams, and soils in the Fukushima Prefecture

Abstract:

In order to assess how environmental factors are controlling the distribution and migration of radioiodine and plutonium that were emitted from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, we quantified iodine and ^239,240Pu concentration changes in soil samples with different land uses (urban, paddy, deciduous forest and coniferous forest), as well as iodine speciation in surface water and rainwater. A ranking of the land uses by their stable ¹²⁷I and by their ^239,240Pu concentrations were quite distinct from that of ^134,137Cs, indicating ¹³⁷Cs might not be a good geochemical proxy for radioactive ¹²⁹I or Pu in the long-term, post-FDNPP accident. Being a proxy for the long-term fate of ¹²⁹I, soil ¹²⁷I content was well correlated to soil organic matter (SOM) content, regardless of land use type, suggesting that SOM might be an important factor controlling iodine biogeochemistry. Other soil chemical properties, such as Eh and pH, had strong correlations to soil ¹²⁷I content, but only within a given land use (e.g., within urban soils). Organic carbon (OC) and Eh were positively, and pH was negatively correlated to ¹²⁷I concentrations in surface water and rain samples. It is also noticeable that ¹²⁷I in the bulk deposition was concentrated along the rainwater passage likely due to plant evapotranspiration activity, with all inorganic iodine being completely converted to organo-iodine by plant organic matter. ^239,240Pu activities of all soil samples were well within the global fallout range, but the Fukushima-derived ^239,240Pu was detectable at a distance ~61 km away, NW of FDNPP. However, it is confined to the litter layer, even three years after the FDNPP accident-derived emissions. ^239,240Pu activities were significantly correlated with soil OC and nitrogen contents, indicating Pu may be associated with nitrogen-containing SOM, similar to what has been observed at other locations in the United States, e.g., Savannah River Site (SRS) and Rocky Flats Environmental Technology Site (RFETS). Together, these finding suggest that NOM plays an key role in controlling the fate and transport of I and Pu and may warrant greater consideration for predicting long-term stewardship of contaminated areas and evaluating various remediation options in Japan.