Radioactive fallout from Japan's ruined nuclear power plants has reached drinking water in Denver and other U.S. cities in trace amounts that authorities Tuesday deemed harmless.

Environmental Protection Agency officials have sampled drinking water as part of their regular monitoring of spreading radioactive particles and said similar elevated radiation is expected in the coming weeks.

The radioactive isotope iodine-131 was detected at 0.17 picocuries per liter in Denver on March 30. EPA officials posted the results late Friday.

The EPA has not set a specific drinking water contaminant standard for iodine-131. However, iodine-131 is a beta emitter, and the EPA has set a limit for total beta emitters equivalent to 3 picocuries per liter of iodine-131.

See the Radiation Network map of volunteer radiation readings from across the U.S.

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