Simulating 131I pathways from Fukushima to Kanto: a case study for March 2011

Abstract:

An estimated 150PBq of ¹³¹I was released from the FDNPP accident, partially deposited into rivers and affected the water treatment plants (WTPs) of Japan. Due to the elevated ¹³¹I concentration in tap water, a restriction on drinking water was placed in 15 of 47 prefectures; of those limited, the densely populated Kanto region was significantly affected during this period. In order to better understand the existence of and to investigate the pathways of ¹³¹I for future risk and water resource management in the Kanto region, the IsoMATTRIP land surface and river model was developed. Half-life considerations of radiotracers were implemented and a river map of the Kanto region was manually created. Few simulation studies on the pathways of ¹³¹I have been conducted due to its short half-life and limited validation data. The development of the IsoMATTRIP model serves as an initial step to address this gap in knowledge.

Preliminary runs on the IsoMATTRIP showed that river discharge has a significant effect on ¹³¹I concentration found in WTPs. River discharge was underestimated (by average of 55 m³/s) while ¹³¹I concentration was overestimated (by 301 Bq/kg). However, the model was able to simulate varying response of ¹³¹I concentration for fallout according to basin size. The discrepancy between observed and simulated river discharge is potentially caused by the model’s usage of natural land parameters to simulate an urban environment. Effective river velocity, input precipitation, and discharge were calibrated to successfully identify optimized settings for the current model setup. The IsoMATTRIP simulated comparable values of ¹³¹I to that from the observed in WTPs of the Kanto region.