High levels of radioactive iodine-131 (with a half-life of about 8 days), cesium-137 (with a half-life of about 30 years), and cesium-134 (with a half-life of about 2 years) were measured in seawater adjacent to the Fukushima Dai-ichi site after the March 2011 events. EPA rainfall monitors in California, Idaho, and Minnesota detected trace amounts of radioactive iodine, cesium, and tellurium consistent with the Japanese nuclear incident, at concentrations below any level of concern. It is uncertain how precipitation of radioactive elements from the atmosphere may have affected radiation levels in the marine environment.
Scientists have stated that radiation in the ocean very quickly becomes diluted and would not be a problem beyond the coast of Japan. The same is true of radiation carried by winds. Barring another unanticipated release, radioactive contaminants from Fukushima Dai-ichi should be sufficiently dispersed over time that they will not prove to be a serious health threat elsewhere, unless they bioaccumulate in migratory fish or find their way directly to another part of the world through food or other commercial products. Radioactive contamination of seafood from the nuclear disaster in Japan has not emerged as a food safety problem for consumers in the United States. According to the U.S. Food and Drug Administration (FDA), the damage to infrastructure in Japan limited food production and associated exports from areas near the Fukushima nuclear facility. FDA and Customs and Border Protection continue to screen imported foods from Japan, including seafood, before they can enter the U.S. food supply.
Based on computer modeling of ocean currents, marine debris from the tsunami produced by the Tohoku earthquake was projected to spread eastward from Japan in the North Pacific Subtropical Gyre. Approximately two to three years after the event, the debris plume was projected to reach the U.S. West Coast, dumping debris on California beaches and the beaches of British Columbia, Alaska, and Baja California. However, initial debris is already arriving. Although most of the radioactive release from Fukushima Dai-ichi is believed to have occurred after the tsunami, there is a slight possibility that some of the tsunami debris might also be contaminated with radiation. A related concern is the transport of nonnative, and potentially invasive, species from Japan to U.S. shores on marine debris. Legislation (H.R. 1171, H.R. 6251, and S. 1119) has been introduced in the 112th Congress to address marine debris concerns.
Source: Effects of Tohoku Tsunami and Fukushima Radiation on the U.S. Marine Environment