A research team led by the University of Tokyo has found more than 30 concentrations of radioactive cesium in the first full-fledged study of the isotope’s accumulation on the seabed near the crippled Fukushima No. 1 nuclear plant, scientists said Wednesday.

The research, spearheaded by the university’s Institute of Industrial Science, found that cesium-137, which has a half-life of 30 years, tends to get absorbed by clayish soil and concentrate in seabed depressions.

For example, a hot spot 70 meters wide was discovered 32 meters below the ocean surface 5.9 km from the plant.

The team said it found soil there containing radioactive cesium concentrations of 651 becquerels per kilogram.

The research, conducted from last August to July, covered the ocean within 20 km of the nuclear station. In the past scientists had only conducted sporadic samplings for cesium near the plant.

The team found relatively high levels of cesium-137 near the mouth of the Abukuma River in Miyagi Prefecture, 70 km north of the plant.

For example, 1.6 km east of the Abukuma River estuary, the research team found a hot spot with average concentrations of 1,029 becquerels per kilogram of mud. The river runs through both Fukushima and Miyagi prefectures, and the research team believes radioactive cesium was carried by the river to the hot spots in the sea.

It has also been reported that contractors doing decomtamination work have dumped their debris in rivers.

Blair Thornton, a special associate professor at the university, told reporters that the findings will be shared with fishery researchers, who hope it will shed light on the possible effect of radioactive contamination on marine life.

Experts also hope the findings will lead to better contamination mechanisms and the ability to predict the movement of cesium-137 near coastal areas.

University of Tokyo professor Toyoji Kaneko, an expert on maritime creatures, said he doesn’t believe all fish caught at or near the seafloor hot spots is dangerous to eat.

Fish swimming near the sea bottom may eat sand worms that have taken in mud contaminated with radioactive cesium, but fish eventually excrete most of the cesium and little would penetrate from the bowels into the muscle of the fish, Kaneko said.

“Although I can’t guarantee 100 percent safety, it’s not the like (meat from) fish will get immediately contaminated” with radioactive cesium, he said.

Thornton also said most of the cesium now detected in fish off Fukushima is believed to have come from tainted seawater near the plant, not from seabed mud.

National Maritime Research Institute of Tokyo and Kyushu Institute of Technology also participated in the survey.