A new study has found that discharges from the defunct Fukushima Dai-Ichi nuclear power plants peaked one month after a March 11 earthquake and tsunami brought on the nuclear accident, and continued through at least July.

The study was conducted by Woods Hole Oceanographic Institution chemist Ken Buesseler and two colleagues based in Japan - Michio Aoyama of the Meteorological Research Institute and Masao Fukasawa of the Japan Agency for Marine-Earth Science and Technology.

The levels of radioactivity, while high, are not a direct threat to humans or marine life according to researchers. However, the report, which is published in the current issue of the journal Environmental Science & Technology, notes that the effect of accumulated radionuclides in marine sediments is poorly understood.

The release of radioactivity from Fukushima is the largest accidental release of radiation to the ocean in world history.

According to the report, concentrations of one radioactive isotope at the plant’s discharge points to the ocean peaked at more than 50 million times normal levels. The isotope was cesium-137, which has a 30-year half-life.

Concentrations 18 miles offshore were higher than those measured in the ocean after the Chernobyl accident 25 years ago, but thanks to ocean mixing processes, the levels rapidly diluted off the northwest coast of Japan.

The researchers report that the plants remain a significant source of contamination to the coastal waters.

"There is currently no data that allow us to distinguish between several possible sources of continued releases," said Buesseler. "These most likely include some combination of direct releases from the reactors, or storage tanks or indirect releases from groundwater beneath the reactors or coastal sediments, both of which are likely contaminated from the period of maximum releases."

"We don't know how this might affect benthic marine life, and with a half-life of 30 years, any cesium-137 accumulating in sediments or groundwater could be a concern for decades to come," he added.

Published by Medicaldaily.com