The Fukushima Dai-ichi Nuclear Power Plant experienced a meltdown following the earthquake that hit Japan in March 2011, dumping cesium-134 into the Pacific Ocean and causing spectacular concern about what would happen when the radiation made its way over to North America. That day has finally come, according to scientists at the Woods Hole Oceanographic Institution (WHOI). Traces of the radiation have been detected 160 km (100 miles) off the coast of Northern California, but the levels are far too low to present any immediate health risks.

Ken Buesseler is a marine chemist from WHOI and is heading up the monitoring project. He will be presenting the findings at the annual meeting of the Society of Environmental Toxicology and Chemistry in Vancouver on Thursday. While Buesseler is monitoring the cesium from Fukushima, he also monitors that which has been present in the water for decades.

"Most people don't realize that there was already cesium in Pacific waters prior to Fukushima, but only the cesium-137 isotope,” Buesseler said in a press release. “Cesium-137 undergoes radioactive decay with a 30-year half-life and was introduced to the environment during atmospheric weapons testing in the 1950s and '60s. Along with cesium-137, we detected cesium-134 – which also does not occur naturally in the environment and has a half-life of just two years. Therefore the only source of this cesium-134 in the Pacific today is from Fukushima.”

WHOI states that the Fukushima radiation found near California exists in levels lower than 2 Becquerels per cubic meter. The EPA has deemed that levels as high as 7400 becquerels per cubic meter are perfectly safe for drinking water. Even if someone spent considerable time in the water for a year, the radiation dosage is 1000 times lower compared to getting a regular x-ray at the dentist. These levels are deemed much too low to to cause immediate adverse effects in humans who might swim in the water or marine animals who live in it.

Over time, however, the radiation levels could build up in larger fish who regularly consume smaller fish. Fish caught for human consumption in the Pacific have shown increased levels of radiation following Fukushima’s meltdown, but are still nowhere near the levels that should be cause for concern.

So far, the radiation has not reached any shores in North America, and predicting if and when that could happen isn’t entirely clear. Models suggest that the radiation will turn south along the coast, but there is some ambiguity in those predictions.

"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. Mixing is hindered by coastal currents and near-shore upwelling of colder deep water," Buesseler continued. "We stand to learn more from samples taken this winter when there is generally less upwelling, and exchange between coastal and offshore waters maybe enhanced.”

There are no U.S. federal agencies tasked with monitoring ocean radiation levels, and Buesseler has led crowdfunding efforts and seeks citizen scientist volunteers to take and process samples. Earlier this week, he answered questions on Reddit.