"There's something unusual here and we need to figure out what's going on," said Chris Crockett, the department's acting deputy commissioner of environmental services.
You may ask: Does this have anything to do with the radioactive emissions from the Fukushima Dai-ichi power plant in Japan?
The answer is no.
Although trace amounts of Iodine-131 have blown over to the United States from Japan, Philadelphia has a more serious - and mysterious - problem with an unidentified local source that predates Japan's March nuclear disaster.
U.S. Environmental Protection Agency data show that the iodine in Philly's water has exceeded federal drinking-water limit at least nine times since 2007 at two of the city's three water-treatment plants.
And Philadelphia's water has the highest iodine level among dozens of water systems in the U.S. tested by the EPA since the Japanese disaster.
Water Department officials tell the Daily News that they were not aware of the data until about the time that the Japanese crisis raised concerns about nuclear particles contaminating U.S. air and rainwater, an issue that turned out to be unrelated to the iodine in Philadelphia.
The Water Department is working with state and federal environmental officials to find the local source of Iodine-131, which can cause cancer in high or prolonged doses and is believed to be responsible for thousands of thyroid cancers following nuclear-bomb tests in the Nevada desert in the 1950s and '60s.
"It's not the type of thing you want to hear when you have kids," said Bettina Berg, who lives in the city's Bella Vista section and is worried about the health of her boys, ages 4 and 20 months. "That's insane if it's been at least four years and they haven't done anything about it."
In an attempt to filter out the iodine, the Water Department is using carbon at the Queen Lane Water Treatment Plant, which, along with the Belmont Water Treatment Plant, supplies about 40 percent of the city's drinking water. Both plants use water from the Schuylkill. Lower levels of iodine have been found at Belmont in recent years.
"We want the public to know we have all of our attention focused on this," said department spokeswoman Joanne Dahme.
Berg and other city residents want to know why officials weren't concerned years ago, when the levels of Iodine-131 in the city's drinking water repeatedly exceeded the EPA's "maximum contaminant level" - the highest level of a contaminant allowed under federal regulations.
Crockett said the test results were not shared with the Water Department in 2007. If they were, he said, the department would have begun an investigation to find the local source.
"I personally would have loved to know about it three years ago," Crockett said. "But we only got it now."
Victoria Binetti, associate director of the water-protection division in the EPA's regional office, said the results of those water samples, gathered through the national network RadNet, are not necessarily shared with local water officials.
Binetti acknowledged that Philadelphia's drinking water had exceeded federal limits for Iodine-131, but said those limits are conservative and are based on decades of constant, prolonged consumption.
"It's a level you don't want to exceed, but it's considered safe," she said of the iodine in Philadelphia's drinking water. (See chart for the city's peak Iodine-131 levels).
But why did it take a nuclear incident halfway around the world for officials here to realize that there is a local source of radioactive iodine?
No one seems to have an answer for that.
"If the Japanese disaster had not happened, would we even be reading about this?" asked Berg, a graphic designer whose family now drinks bottled water. "Or would it have continued for another 10 years?"
Most radiation experts say the amount of iodine in Philadelphia's drinking water isn't harmful, especially if the elevated levels are not constant.
"These are very small doses," said Andrew Maidment, chief of the physics section in the Department of Radiology at the University of Pennsylvania.
But Jeff Patterson, past president of Physicians for Social Responsibility, said even low-level radiation should be avoided, if possible, because the exposure is cumulative and the effects are not fully understood.
"There is no 'safe' level" of Iodine-131, said Patterson, whose group opposes nuclear weapons and power.
Maidment and other experts believe that the likely source of Philadelphia's iodine is either a medical facility that is improperly disposing of unused iodine - which is used to treat cancer patients - or the patients themselves when their waste enters the sewer system.
"That pee is radioactive," Maidment said.
Tracking the source could take months. More than 70 wastewater-treatment plants discharge water into the Schuylkill above Philadelphia, Dahme said. The river's watershed encompasses about 2,000 square miles, and all of its tributaries must be considered.
"We're at the very bottom of this watershed," Dahme said. "Our task is huge. It can be like looking for a needle in a haystack."
Or, Crockett added, the iodine could be coming from a "plethora of many little sources," which would further complicate the investigation.
Ambler: The source?
Recent iodine tests performed by the state Department of Environmental Protection produced a hit at the Ambler Borough wastewate-treatment plant. The plant discharges treated water into the Wissahickon Creek, which empties into the Schuylkill.
State officials are awaiting a second round of test results before deciding whether to follow the sewer lines leading to the Ambler plant to find the source, according to DEP spokeswoman Deborah Fries.
"This is an evolving process," Fries said.
Bruce Jones, supervisor of the Ambler plant, said there are no hospitals within the plant's service area, so the source of the iodine is unclear.
"There are some assisted-living homes within the service area. I doubt it's coming from there, but I'm not the expert," Jones said. "If something's found here, I feel an obligation to try to find out where it's coming from."
Iodine-131 has a short half-life, meaning that it decays rapidly. So once the source is eliminated, it should disappear from the city's drinking water.
To Berg, the South Philly mother, that only means that officials could have removed it from the water years ago. Although Water Department officials insist that the city's water is safe to drink, she'd rather not give her children water that's laced with a nuclear byproduct.
"This shouldn't be happening," Berg said. "I can't think of anything more basic than water. If you can't count on your water being safe, what do you do?"