Role of natural organic matter on iodine and 239,240Pu distribution and mobility in environmental samples from the northwestern Fukushima Prefecture, Japan

Abstract

In order to assess how environmental factors are affecting the distribution and migration of radioiodine and plutonium that were emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, we quantified iodine and (239,240)Pu 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. Sampling locations were 53-63 km northwest of the FDNPP within a 75-km radius, in close proximity of each other. A ranking of the land uses by their surface soil (<4 cm) stable (127)I concentrations was coniferous forest > deciduous forest > urban > paddy, and (239,240)Pu concentrations ranked as deciduous forest > coniferous forest > paddy ≥ urban. Both were quite distinct from that of (134)Cs and (137)Cs: urban > coniferous forest > deciduous forest > paddy, indicating differences in their sources, deposition phases, and biogeochemical behavior in these soil systems. Although stable (127)I might not have fully equilibrated with Fukushima-derived (129)I, it likely still works as a proxy for the long-term fate of (129)I. Surficial soil (127)I content was well correlated to soil organic matter (SOM) content, regardless of land use type, suggesting that SOM might be an important factor affecting iodine biogeochemistry. Other soil chemical properties, such as Eh and pH, had strong correlations to soil (127)I content, but only within a given land use (e.g., within urban soils). Organic carbon (OC) concentrations and Eh were positively, and pH was negatively correlated to (127)I concentrations in surface water and rain samples. It is also noticeable that (127)I in the wet deposition was concentrated in both the deciduous and coniferous forest throughfall and stemfall water, respectively, comparing to the bulk rainwater. Further, both forest throughfall and stemflow water consisted exclusively of organo-iodine, suggesting all inorganic iodine in the original bulk deposition (∼ 28.6% of total iodine) have been completely converted to organo-iodine. Fukushima-derived (239,240)Pu 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. Plutonium-239,240 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. Together, these finding suggest that natural organic matter (NOM) plays a key role in affecting 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.

Iodine budget in forest soils: Influence of environmental conditions and soil physicochemical properties

“…For iodine, accumulation rates were always positive. Iodine enrichment in humus, compared to litterfall, is supposed to be linked to a process of adsorption as well as the possible iodination of organic matter, since soluble organic iodine is already the dominant fraction in throughfall (Gilfedder et al, 2008;Xu et al, 2016). Furthermore, humus I concentrations were positively linked to humus mass (Table 1; p < 0 .001).…”

Iodine soil dynamics and methods of measurement: a review

“…An important factor that influences iodine biogeochemical dynamics and cycling within the natural environment is pH and Eh (redox potential). Within the literature an inverse relationship between soil-iodine and pH is often found due to lower pH levels increasing the anion exchange capacity of many pH-dependent charge soils, 31 and a desorption of soil iodine into solution when soil Eh falls below 150mV. 32 Under environmental conditions, the speciation of inorganic iodine is generally controlled by pH and Eh.…”

“…33 Shetaya, et al 34 reported that soil pH acts mainly on the initial adsorption of iodate in soils while soil organic matter influences time-dependent sorption. Xu, et al 31 showed that Eh was positively correlated to total iodine concentrations in seven urban surface soils collected from Fukushima Prefecture, Japan. However, this trend was not found in the soil depth profiles, suggesting more complex interactions exist and that multiple soil properties influence iodine geodynacmis.…”

“…34 Soil humus and Fe/Al/Mn hydrous oxides are known to influence iodine speciation and fixation into unavailable forms in soils. [31][32][33] In most soils the majority of iodine is bound to organic matter and soils with high organic matter contents often have higher iodine concentrations. [35][36][37] A common theme reported within the scientific literature was that organic matter is naturally degraded by biotic and abiotic processes in soil leading to the creation of humic substances (HS), which are composed of the operational fractions: humic acids (HA), fulvic acids (FA) and humin.…”

Reconstruction of uranium and plutonium isotopic signatures in sediment accumulated in the Mano Dam reservoir, Japan, before and after the Fukushima nuclear accident

“…Although radiocesium sources have been extensively researched, less information is available in the literature regarding the fate of other radionuclides such as U or Pu. Non-volatile Pu isotopes including 239Pu (Ti/2=24,110 y, alpha-decay), 240Pu (Ti/2=6,563 y, alpha-decay), 241Pu (Ti/2=14.35 y, beta-decay) and 242Pu (Ti/2=376,000 y, alpha-decay) (Evrard et al, 2014;Xu et al, 2016;Zheng et al, 2012b) and U isotopes including 238U (Ti/2=4.469x109 y, alpha-decay), 235U…”