When the Fukushima Daiichi nuclear plant suffered a catastrophic breakdown in 2011 after being battered by a tsunami, a lot of Americans worried about radiation leaking from the plant and making its way across the Pacific into the waters off the North American coast.

Fukushima radiation did eventually show up off British Columbia the following year, according to a study published in 2014 in Proceedings of the National Academy of Sciences, and it’s been continuing to drift this way ever since. Now, according to new research, an increased number of sites off the U.S. West Coast are showing signs of contamination from the crippled nuke plant.

That sampling includes the highest detected level to date, from waters about 1,600 miles west of San Francisco.

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But before you stop eating fish caught in the Pacific, it’s important to keep things in perspective. Even that sample with relatively high radiation is about 500 times lower than the U.S. government’s safety standard for radioactivity in drinking water.

The research was conducted by Ken Buesseler, a marine radiochemist with the Woods Hole Oceanographic Institution and director of its Center for Marine and Environmental Radioactivity, who plans to present his findings at the American Geophysical Union conference in San Francisco next week.

“These new data are important for two reasons,” Buesseler, one of the first scientists to track Fukushima radiation, said in a press release. “First, despite the fact that the levels of contamination off our shores remain well below government-established safety limits for human health or to marine life, the changing values underscore the need to more closely monitor contamination levels across the Pacific.”

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“Second, these long-lived radioisotopes will serve as markers for years to come for scientists studying ocean currents and mixing in coastal and offshore waters.”

Buesseler, working with Japanese scientists, also continues to monitor the continuing leakage from the Fukushima plant, which includes collecting water samples from as close as a half mile away and sampling marine organisms, sediment and groundwater along the coast. The radiation levels there, while decreased, remain 10 to 100 times higher than those off the U.S. coast.

Tokyo Electric Power Co. is putting in place a frozen soil barrier in an effort to keep groundwater from flowing through the wrecked plant. The company also is building a seawall and drainage system to keep more contaminated water from reaching the ocean, Bloomberg News recently reported. Eventually, the entire facility must be dismantled and hauled away.