Tracking the spread of that cesium has become the personal crusade of Dr Shinzo Kimura.
He said, "If the radiation is really high, around 2 or 3 micro sieverts, then, that is a hotspot and I start surveying the area to find out if there are any residents."
Dr Kimura made a startling discovery. As he travelled around Fukushima he found hotspots of radiation, areas where readings on his instruments were much higher than expected. By feeding the information from his Geiger counter into a computer, he's been able to build up a map of radiation levels far greater than anything the government has produced so far.
It is time consuming, and costly. The equipment alone costs tens of thousands of dollars. And it is a lonely path he walks. He was forced to resign his job with a government research lab after he was told this research was not considered important. Today he relies on locals to donate what they can so he can carry on his work.
This mountainous area is well outside the government's exclusion zone. Radiation levels are high, about twenty times normal background radiation butwell within government safety limits.
Then he hits a hotspot. The Geiger counter starts to climb, it settles on 6.8 micro siverts per hour. Before the disaster normal background radiation in this area was about 0.03 micro sieverts per hour. This reading is over 200 times that.
Dr Kimura said," There are many similar locations in this region and they measure high radiation levels as well. Like this place, it's generally in areas where water has flowed down from the mountains."
He tells me he has found many areas like this, places that go unnoticed by government monitoring which takes aerial readings and general measurements. But he doesn't want to scare people away.
"Of course it is dangerous. But if people leave this region one by one, their 1000-year-long history, tradition, culture and community will vanish because of the nuclear disaster. It is better if we find a way for them to keep on living here safely so that they can pass on their traditions to the next generation." He said.
In a bid to do that, he's helping local farmers to clean up their land. He explains to pickle manufacturer Chuhei Sakai how to remove the topsoil, taking the cesium with it. But there are complicated equations to be done, not easy science for a farmer to grasp.
And its just one man, and thousands of square kilometers of contamination.