Radiation from the Fukushima nuclear disaster has not yet reached ocean waters along the Pacific coast, but low levels of radioactive cesium from the stricken Japanese power plant could arrive by April, scientists reported Monday.

The report came even as some Internet sites continue claiming that dangerously radioactive ocean water from Fukushima is showing up along California beaches - reports that have been denied by health officials and scientists since they first surfaced more than a month ago.

Ken Buesseler, a chemical oceanographer at the Woods Hole Oceanographic Institution on Cape Cod, Mass., reported that four coastal monitoring sites in California and Washington have detected no traces of radiation from the Fukushima Dai-ichi nuclear plant destruction - “not yet,” he said during a telephone press briefing.

The briefing took place in Honolulu during a meeting of the American Geophysical Union’s Ocean Sciences section. The meeting is being held with scientists from both sides of the Pacific to discuss problems caused anywhere in the Pacific by the offshore earthquake and tsunami that hit Japan on March 16, 2011.

Buesseler said no federal or international agencies are monitoring ocean waters from Fukushima on this side of the Pacific, so he has organized volunteer monitors at 16 sites along the California and Washington coasts and two in Hawaii to collect seawater in 20-liter specialized plastic containers and ship them by UPS to his Woods Hole laboratory.

Two specific radioactive isotopes of the element cesium are formed in nuclear accidents, he explained.

One is cesium-137, whose radioactivity decays very slowly - its half-life is 30 years - while the other is cesium-134, which decays rapidly with a two-year half-life. So while cesium-137 is still detectable in the world’s oceans from old nuclear-weapons tests, any traces of cesium-134 that are detected by monitoring instruments could only have come from the Fukushima nuclear accident, Buesseler said.

According to a widely accepted model of the oceans’ circulation patterns, traces of the plume of radioactive seawater from Fukushima should be detectable along the Pacific coast in April.

“We need to know the real levels of radiation coming at us,” said Bing Dong, a retired accountant and self-described activist at Point Reyes Station who has volunteered to collect ocean samples for Buesseler’s project. “There’s so much disinformation out there, and we really need actual data.”

Roger Gilbert, a physician and radiation oncologist who collected water at Fort Bragg along the Mendocino County coast, said he got involved in Buesseler’s monitoring project because he is concerned “over fear-mongering on the Internet about allegedly high levels of Fukushima radiation in California’s coastal waters.”