That's raising concerns among some Americans including the residents of the San Francisco Bay Area city of Fairfax, California, which passed a resolution on December 6 calling for more testing of coastal seafood.
At the same time, oceanographers and radiological scientists say such concerns are unwarranted given existing levels of radiation in the ocean.
The runoff from the Japanese plant will mingle with radiation released by other atomic stations, such as Diablo Canyon in California. Under normal operations, Diablo Canyon discharges more radiation into the sea, albeit of a less dangerous isotope, than the Fukushima station, which suffered the worst nuclear accident since Chernobyl.
"There's a point to be made that we live in a radioactive world and the ocean just has radioactive isotopes in it," said Ken Buesseler, senior scientist at the Woods Hole Oceanographic Institution in Massachusetts, who forecasts the Fukushima plume will arrive in the US early this year. "People have a limited knowledge of radioactivity."
At Tokyo Electric Power's Fukushima Dai-Ichi station, where three reactors melted down after the March 2011 earthquake and tsunami, about 300 metric tons of contaminated groundwater seep into the ocean each day, according to Japan's government.
Between May 2011 and August 2013, as many as 20 trillion becquerels of cesium-137, 10 trillion becquerels of strontium-90 and 40 trillion becquerels of tritium entered the ocean via groundwater, according to Tokyo Electric.
Cesium isotopes, which emit flesh-penetrating gamma rays, are among the most dangerous radionuclides emitted by the plant, said Colin Hill, an associate professor of radiation oncology at the University of Southern California's Keck School of Medicine.
Strontium-90, which mimics calcium, increases the exposure risk for humans by remaining in the bones of fish for extended periods. While tritium is less radiologically intense than cesium and passes through fish faster than strontium, it can also contaminate sea creatures that encounter the isotope in high levels, Hill said.
Water exposed to radiation from the Fukushima plant would reach the US at levels at least 100 times lower than the US's drinking water threshold, Nuclear Regulatory Commission chairman Allison Macfarlane said at a December 6 briefing in Tokyo.
The assurances haven't eased concerns for some. "I'm terrified," Doreen Jean Dempski, a children's book author, said by phone from her home more than 5000 miles (8046 km) across the Pacific from Fukushima in Carpinteria, California. "My boyfriend is a surfer and he spends hours a day in the water."
Sharing Ms Dempski's worries are the Fairfax city council, which passed the coastal testing resolution, and more than 127,000 signatories to an online petition calling for a United Nations' takeover of part of the Fukushima cleanup. South Korea has already banned imports of fish from Japan's northern Pacific coast.
Part of the issue is general concern about radiation, and the startling amounts that are released into the environment by the 435 nuclear power plants operating worldwide as of January 3. Measurements that puzzle the public - becquerels, rems, curies and sieverts - don't aid transparency. And, worse, scientists disagree on the health risks from low-dose radiation exposure.
A report on the Fukushima disaster by the World Health Organisation in February last year estimated increased cancer risk for those in the most contaminated areas around the plant, but not elsewhere in Japan. However, the report also notes that better understanding of the effects of low-dose radiation may alter risk expectations from the Fukushima accident.
