"This is just unacceptable that this stuff is showing up," said Chris Crockett, the department's acting deputy commissioner of environmental services.
Perhaps more disturbing: Nobody knows exactly how the Iodine-131 - which can cause thyroid cancer if consumed in large quantities or over a prolonged period of time - is getting into Philly's drinking water.
"At this point, that is not really known," said EPA spokesman David Sternberg. "We're investigating."
Kathryn Higley, a health physicist at Oregon State University, said the most likely source is a nearby or upstream medical facility that treats cancer patients with Iodine-131, which can enter the water supply when patients go to the bathroom.
"That's the big wrinkle. If you saw it last year, it wasn't from Japan," she said.
"It's probably from a hospital."
Higley said the iodine levels found in Philadelphia and other U.S. cities did not pose a health threat.
"The water is safe. We were all drinking it today," said Debra McCarty, the Water Department's deputy commissioner of operations.
But environmental officials at the city, state and federal level are trying to identify the source, and carbon has been added at the Queen Lane plant as a "cautionary measure" to help purify the water.
"We want to have as clear a picture as we can of what the source is," said Katy Gresh, spokeswoman for the state Department of Environmental Protection.
She said that a hospital or other medical facility is "a possibility."
Crockett said the Water Department was informed this month that iodine was found in Philadelphia's water last summer.
"We're not happy about this," Crockett said. "To find that this stuff showed up in the river before [the Fukushima emissions] means that something is coming from somewhere that is not Japan and we need to track that down and stop it."