Mechanisms of 137 Cs leaching based on long-term observations in forested headwater catchments in Yamakiya, Fukushima Prefecture, Japan.

Dissolved radiocesium (mainly ¹³⁴ Cs and ¹³⁷ Cs) is thought to be leached mainly from suspended sediment in downstream rivers, while organic matter, such as leaf litter, contributes to catchments in forested headwater streams. It is also known that dissolved ¹³⁷ Cs in headwater streams exhibit seasonal variation with water temperature. Some mechanisms have been proposed as the causes: ionic competition of potassium ion (K ⁺ ) and ammonium ion (NH ₄ ⁺ ), leaching associated with the decomposition of organic matter, and thermodynamic adsorption-desorption processes. We investigated the relationship between K ⁺ and the dissolved organic carbon (DOC) concentrations and seasonal changes in dissolved ¹³⁷ Cs concentration using a large number of samples from a headwater's small catchments. We examined temporal trends in ¹³⁷ Cs concentrations in coarse organic matter, suspended sediments, and dissolved forms at four sites (one decontaminated site and three undecontaminated sites) in the Yamakiya area since 2011. The distribution coefficients (Kdcss and Kdorg) of dissolved ¹³⁷ Cs concentrations relative to suspended sediment and coarse organic matter ¹³⁷ Cs concentrations were calculated, and differences in temporal changes due to decontamination were investigated. In addition, we examined the relationship between water temperature and DOC, K ⁺ , NH ₄ ⁺ and ¹³⁷ Cs concentrations in the headwater catchments. The suspended sediment ¹³⁷ Cs concentrations at the decontaminated headwater site (IBO) decreased significantly after decontamination and remained low thereafter. In contrast, dissolved ¹³⁷ Cs concentrations decreased temporarily during the decontamination period, but returned to pre-decontamination levels. Almost no NH ₄ ⁺ has been detected in headwater streams in our catchments. In the SET and ISH watersheds, where the distance from the groundwater spring is short, a correlation was found between DOC concentration and dissolved ¹³⁷ Cs concentration. In contrast, in the IBO watershed, where the distance from the groundwater spring is long, temperature dependence and a good correlation between K ⁺ and ¹³⁷ Cs were observed. Therefore, microbial decomposition of organic matter may have a significant effect on the seasonal variation of dissolved ¹³⁷ Cs in forested headwater streams at short distances from the spring, but the influence of competing ions is expected to increase gradually as the water flows downstream.
Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Yuichi Onda reports financial support and travel were provided by University of Tsukuba. Yuichi Onda reports a relationship with University of Tsukuba that includes: employment.
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