Radioactive Cesium Precipitated in Lake Onuma of Mt. Akagi Emitted by the Fukushima Daiichi Nuclear Power Plant Accident

In August and September, 2011, Wakasagi (Hypomesus nipponensis) that was captured in Lake Onuma on the top of Mt. Akagi of Gunma Prefecture contained higher levels (640 Bq kg−1 and 650 Bq kg−1) of radioactive cesium than the temporal regulatory limit for food (500 Bq kg−1). We have investigated the distribution of radioactive cesium (Cs-134 and Cs-137) in deposits of Lake Onuma and Lake Konuma, as well as soils in the inner region of the somma rim of Mt. Akagi to elucidate the contamination mechanisms of the radioactive cesium. Radioactive cesium in the center site of Lake Onuma had remained within the uppermost layer (ca. 5 cm) of the lake deposits. On the other hand, those in the inlet and outlet sites of the lake were found in layers of 15–20 cm depth. This difference might be caused by that in grain sizes (silt) of the lake deposits, i.e., smaller grain sizes in the center site, and/or the disturbance of lake deposits due to the movement of bottom water of these sites of the lake. The main minerals of lake deposits were cristobalite, quarts and plagioclase with large amounts of non-crystalline amorphous silica and aluminosilicates. The non-crystalline materials may be related to the retention of radioactive cesium in the upper layer of the deposits. Most of the soil samples collected from the inner regions of Mt. Akagi had contained radioactive cesium, ranging from 500 Bq kg−1 to 5000 Bq kg−1. These values are at the same level as those of soils in mountainous areas of Gunma Pref. The estimated inventories (December 20, 2011) of radioactive cesium in the inner regions of Mt. Akagi were calculated to be 33 – 104 kBq m−2, which were close to the inventory values (30 – 100 kBq m−2) around Mt. Akagi published by the MEXT Aircraft Monitoring (November 5, 2011).