3,000,000 Bq/m2 of cesium from the dam in Fukushima

Following up this article..Fukushima local gov is dumping radioactive mud from decontamination to Abukuma river

Ministry of Education, Culture, Sports, Science and Technology found out high level of cesium is concentrated in the bottom of the lake called Horai dam. This dam is to generate electricity, located in Nihonmatsu (60km North west to Fukushima plants), where is in Abukuma river.
Prof. Onda from Tsukuba university reported the mud sample taken in 20 cm deep underground of the bottom of the lake contained 3,000,000 Bq/m2 of cesium, where other samples taken in 4 other reservoirs around Horai lake showed 200,000~400,000 Bq/m2 of cesium.(July and August in 2011) 3,000,000 Bq/m2 is as contaminated as soil of 20km area.

Prof. Onda explains, cesium doesn’t melt into the water. Water of downstream is not harmful to drink.
However, they also found out that when the soil is highly contaminated, water of the downstream river is also highly contaminated, which proves Prof. Onda is wrong. Cesium was attached to the small swimming sand, which is smaller than 0.1mm diameter.
Ministry of Land, Infrastructure, Transport and Tourism states there is the risk of the contaminated mud to flow into the downstream in flood, but there is no plan of decontamination at the moment.

Other nuclides such as strontium or plutonium were not tested.

Source
放射性物質：ダム底の泥に　雨で川へ流れ濃縮か　福島
阿武隈川中流域にある発電用の「蓬萊（ほうらい）ダム」（福島県二本松市、福島市、貯水量約３８０万立方メートル）の底の泥に大量の放射性セシウムが蓄積していることが１３日、文部科学省の調査で判明した。付近の貯水池と比べ１０倍の濃度だった。東京電力福島第１原発事故で土壌に蓄積したセシウムが、雨で土と一緒に川へ流れ込み、ダムでせき止められて濃縮したとみられる。

同省の委託で調査した恩田裕一筑波大教授（水文学）が同日のシンポジウムで報告した。恩田教授は昨年７～８月、原発の西北西約６０キロにある蓬萊ダムの底の泥を深さ２０センチまで採取、乾燥後のセシウム濃度を周辺の４カ所の貯水池の泥と比べた。その結果、蓬萊ダムの泥は１平方メートル当たり約３０００キロベクレルで、原発から半径２０キロ圏内の警戒区域内の土壌汚染濃度に相当した。他の貯水池の同２００～４００キロベクレルに比べ約１０倍あった。