Last week, we looked at evidence gathered in Seattle of tiny amounts of radioactive material that had made its way across the Pacific from the stricken nuclear reactors in Fukushima, Japan.

So it’s no surprise that similar evidence is turning up further down the coast in San Francisco.

Between 16 and 26 March, Eric Norman and pals at the department of nuclear engineering at UC Berkeley placed buckets at various locations in the Bay Area to gather rain water (the earthquake that triggered the accident occurred on 11 March). They then analysed the water they collected, looking for the tell tale gamma rays from radioactive stuff.

As expected, they found it. “Gamma ray spectra measured from these samples show clear evidence of fission products - iodine-131 and132, tellurium-132 and cesium-134 and 137,” say Norman and co.

That’s very similar to the spectrum of stuff seen in Seattle.

Just how much material is in the rainwater is interesting. Norman and co say the US Environmental Protection Agency places a limit for the amount of iodine-131 allowed in drinking water of 4 becquerels per litre, which equates to 4 decays per litre per second.

By comparison, San Francisco’s rainwater contains 16 Bq/litre.

This number needs to be placed in context, however. Norman and co say: ” If a person were to drink a typical amount of water per day containing the EPA limit of I-131, then in one year he or she would receive a whole body dose of < 0.04 mSv (4 mrem). This dose should be compared to the US average annual radiation dose of 6.2 mSv (620 mrem).”

And since Iodine-131 has a half life of 8 days, and so decays very quickly, Norman and co say that it is extremely unlikely that the public will be exposed to anywhere near these levels in drinking water.

They end their paper with this conclusion: “Thus the levels of fallout we have observed in San Francisco Bay area rain water pose no health risk to the public.”

Worth keeping an eye on, though.

Ref: arxiv.org/abs/1103.5954: Observations of Fallout from the Fukushima Reactor Accident in San Francisco Bay Area Rainwater

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