Levels of radioactive iodine-131 in a city drinking-water intake rose to their highest level yet earlier this year.

However, city and state officials noted the spike, measured Oct. 17 at the Belmont water plant, is a one-time event. It's not a public health concern, they said, and the water remains safe to drink.

"The biggest message is that it's not a health issue," said David Allard, director of the state Department of Environmental Protection's Bureau of Radiation Protection.

Still, they continue to watch as they have since the issue surfaced in 2011 with more monitoring after an earthquake and tsunami severely damaged Japan's Fukushima nuclear power plant.

Officials then found that a little-known sampling program of the U.S. Environmental Protection Agency had been detecting spikes of iodine-131 in Philadelphia and other places that get drinking water from rivers and streams.

Iodine-131 is used in thyroid treatments.

After months of detective work, the agencies traced the scattered spikes to excretions by thyroid patients. Once they flush toilets, the radioactive material travels through sewage plants and into rivers and streams, where it has a half-life of eight days. So it is detectable but disappears quickly from the environment.

On Oct. 17, the latest date for which readings are available, the level of one sample at the Belmont water plant, on the Schuylkill, reached a record 5.46 picocuries per liter.

The same day, a sample roughly across the river at Queen Lane reached 3.28. The previous high there was 4.42 in February 2005.

A national drinking-water standard sets the permissible limit at 3 picocuries per liter, but that figure is for a yearly average, not a single sample.

From April 2011 to April 2012, the city Water Department tested more than 150 samples from area rivers and streams and only three came in above 3 picocuries per liter, said department spokeswoman Joanne Dahme.

Philadelphia is unusual in that its two water intakes on the Schuylkill are fed by streams whose flow consists of a large portion of treated effluent from sewage plants. Many other towns pump groundwater, which would not have the same issue.

Given that dry conditions led to low river flows in October, Dahme wasn't surprised at the spike.

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