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

Abstract

Dissolved radiocesium (mainly ¹³⁴Cs and ¹³⁷Cs) 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 ¹³⁷Cs 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 (NH₄⁺), 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 ¹³⁷Cs concentration using a large number of samples from a headwater's small catchments. We examined temporal trends in ¹³⁷Cs 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 ¹³⁷Cs concentrations relative to suspended sediment and coarse organic matter ¹³⁷Cs 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⁺, NH₄⁺ and ¹³⁷Cs concentrations in the headwater catchments. The suspended sediment ¹³⁷Cs concentrations at the decontaminated headwater site (IBO) decreased significantly after decontamination and remained low thereafter. In contrast, dissolved ¹³⁷Cs concentrations decreased temporarily during the decontamination period, but returned to pre-decontamination levels. Almost no NH₄⁺ 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 ¹³⁷Cs 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 ¹³⁷Cs were observed. Therefore, microbial decomposition of organic matter may have a significant effect on the seasonal variation of dissolved ¹³⁷Cs 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.

Keywords: (137)Cs; DOC; Decontamination; Fukushima Daiichi nuclear power plant accident; Kd; Seasonal variation; Stream water.