Scientists investigating the Fukushima Daiichi nuclear power plant in Japan have found an unexpected source of radioactive material at the site.

They found that sands and brackish groundwater up to 97 kilometers (60 miles) away had retained some of the radioactive cesium from the disaster in 2011, and this has been released into the ocean.

The findings, led by Woods Hole Oceanographic Institution (WHOI), were published in the Proceedings of the National Academy of Sciences. Using tubes 1 to 2 meters long (3 to 7 feet), they found that cesium levels were 10 times higher in the surrounding sand on eight beaches and groundwater than they were in the seawater in the harbor of the plant itself.

Two isotopes of cesium were found. The first, cesium-137, may have come from the plant or it could have come from nuclear weapons tests in the 1950s and 1960s. They also found cesium-134, however, which must have come from the plant.

Fortunately, these waters are not used for drinking and no one is exposed to them, so the authors said “public health is not of primary concern” in their paper. How, it’s still an alarming example of how radiation from the reactor has spread.

In the days and weeks after the accident, it’s estimated that cesium was transported along the coast and became stuck to sand grains. When it came into contact with saltwater later from the ocean, the cesium no longer stuck to the sand, and was carried back into the ocean.

"It is as if the sands acted as a 'sponge' that was contaminated in 2011 and is only slowly being depleted," said study co-author Ken Buesseler from WHOI in a statement.

The numbers are still low. The team estimate that this radiation, coupled with that from ongoing releases and runoff from the plant, is thousands of times smaller than the release in the days after the disaster.

But the authors note that, with about 200 nuclear reactors in the world situated on a coastline, this sort of data is vital in working out how plants can contaminate waters.

As for Fukushima, it’s a long and slow process to clean up the 2011 meltdown. Scientists have yet to find all the melted fuel from the reactor, and even once that’s done, the plant is not expected to be decommissioned until the 2050s.