Radioisotope concentrations (Cs-137 and I-131) in sewage sludge, obtained on a daily basis in the Fukushima city wastewater treatment plant from end of April 2011 until end of December 2013, have been analysed for their evolution with time, and the time series have been compared to similar data from central Europe after the Chernobyl accident. Additionally, daily rainfall data have been considered for Fukushima. The long-Term trends for Cs-137 are very similar between Japan and Europe, indicating a decrease of concentration with an initial half-life of about 1 year with a tendency towards a slower decrease at later times. Short-Term Cs-137 data indicate a dominating influence of rainfall - each significant rainfall event leads to a sharp increase of sludge Cs-137 concentration. Absolute values (maximum: ca. 6000 Bq kg-1) are relatively low compared to Europe after 1986 - this is attributed to the different types of sewer systems: separated in Fukushima, collecting only small amounts of rainfall with the wastewater, mixed in many cities in Europe, collecting rainwater and wastewater in the same sewer. I-131 was detectable during short intervals and at relatively constant maximum concentration (ca. 1000 Bq kg-1) during the whole observation period. The origin is suspected to be routine medical application of the isotope, which has been shown to contribute significantly to sewage sludge concentration wherever it is in use.
Fischer, H. W., & Yokoo, Y. (2014). Preliminary comparison of radioisotope concentration in sewage sludge after the Fukushima and Chernobyl accidents. In Energy Procedia (Vol. 59, pp. 256–262). Elsevier Ltd. https://doi.org/10.1016/j.egypro.2014.10.375