Traces of radioactivity believed to have come from the 2011 Fukushima Dai-ichi Nuclear Power Plant accident were detected 100 miles off the coast of California, the Woods Hole Oceanographic Institution (WHOI) said Monday.

The group found traces of cesium-134, a radioactive element released by the power plant, 100 miles off the coast of Eureka, California.

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The amount of radioactive chemicals in the water is still below levels that are harmful to humans and is 1,000 times below limits set by the Environmental Protection Agency for drinking water. WHOI predicts contaminant levels could increase over the next two to three years.

“We don’t know exactly when the Fukushima isotopes will be detectable closer to shore because the mixing of offshore surface waters and coastal waters is hard to predict,” said Ken Buesseler, the leader of WHOI’s monitoring team, in a statement. “We stand to learn more from samples taken this winter when there is generally less upwelling.”

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Buesseler noted careful and consistent monitoring of radioactive elements along the Pacific coastline is needed. Scientists tested water samples taken from Alaska to California.

The findings confirm data earlier this year showing cesium-134 traveled across the Pacific to the coast of Canada.

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The Fukushima nuclear power plant, operated by Tokyo Electric Power, was damaged during the 2011 earthquake in Japan. Workers were unable to cool the reactors which eventually exploded, leaking radioactive chemicals into the water. The plant has since been decommissioned.