They said levels of iodine-131 have remained well below federal drinking-water limits.
The substance has a half-life of eight days, which means that every eight days, the radioactivity is reduced by half. It would be considered gone after 80 days.
Iodine-131's presence, they said, is likely a new reality of modern medicine.
"The goal is zero. We can't get to that," said David Allard, director of the DEP's Bureau of Radiation Protection.
According to the Water Department, one person receiving iodine-131 treatment would excrete enough of the substance to be measurable in the watershed.
"This is an issue that is not going to go away any time soon," said Caroline Johnson, a physician with the Philadelphia Department of Health. "These are very effective treatments."
Officials here discovered the problem when they increased sampling after Japan's Fukushima nuclear power plant, crippled in the post-quake tsunami, released radioactivity last March.
In a state sampling, the Wissahickon stood out with detectable levels of iodine-131.
At about the same time, officials realized that a little-known national sampling program of the EPA had shown iodine-131 in Philadelphia's drinking water. Among the roughly 50 sites sampled, levels here were the highest.
They were below drinking-water limits but still worrisome, partly because its source was unknown.
Iodine-131 is used to diagnose and treat thyroid disease. It's extremely effective because the thyroid's natural function is to concentrate iodine, and when the iodine is radioactive, it kills cancer cells. However, long-term exposure to high levels can also cause thyroid cancer.
Iodine-131 also is a byproduct of nuclear power plants. But officials have ruled out the Limerick nuclear power plant, located on the Schuylkill south of Pottstown, and any of the region's medical, research, or pharmaceutical firms as the source of the iodine-131.
By excluding everything else, they settled on the patients themselves as the source.
Another factor pointing to patients is that sewage sludge being taken landfills often sets off radiation monitors at entrances, and iodine-131 is regularly the culprit.
Now that officials are looking more closely, iodine-131 has been detected in the Schuylkill, downstream of Reading, Norristown and Pottstown, said Julia Rockwell, project engineer with the water department's source-water protection program.
It has spiked occasionally in the Wissahickon Creek, specifically below the five sewage-treatment plants draining into it. In dry weather, the treated effluent can make up 90 percent of the creek's flow.
"We believe we have confirmed the wastewater-plant effluent is a pathway for Iodine-131," Rockwell said.
A University of Delaware researcher has discovered iodine-131 in river sediment from the Lehigh River to south of Wilmington.
One mystery officials have pondered is why iodine-131 isn't showing up in many other places, or is found at lower levels than they see here.
One reason: most other cities aren't looking for iodine-131.
Plus, the Philadelphia region is a medical center, and a lot of sewage-treatment plants discharge into waterways that then flow past Philadelphia and into its drinking-water intakes.
Cities that use groundwater for drinking or have large reservoir systems that hold the water for long periods would allow the short-lived iodine-131 to decay, keeping it from reaching detectable levels.
Many people travel here for medical care at one of the region's 50-plus hospitals. Among residents of southeastern Pennsylvania alone, the health department's Johnson said, 400 to 500 cases of thyroid cancer are diagnosed every year.
About half of them get iodine-131 treatments.
"The medical community here is very advanced," she said. "If this is a state of the art treatment, we are going to be using it here."
Many patients with Graves disease, an autoimmune disorder that leads to thyroid overactivity, also get treatments.
Joanne Dahme, a spokeswoman for the water department, said the next step is to start talking to the medical community about possible ways to keep iodine out of the region's water.
One thing officials have ruled out from the beginning is asking patients to collect their urine and store it until it can be treated or until the radioactivity has declined.
"No way would patients be able to comply with that," said Kurt Bodison, assistant radiation safety officer with Temple University Hospital.
In addition, it would be unsanitary, there would be a risk of spill, and it might pose a danger to others in the household.
Researchers have said that bringing urine back to a hospital for treatment is not good, either, because the risks are greater when the radioactive material is concentrated rather than dispersed.
Contact Sandy Bauers at 215-854-5147, firstname.lastname@example.org, or follow on Twitter @sbauers. Read her blog, GreenSpace, at www.philly.com/greenspace.
More information can be found at www.phillywatersheds.org/iodine131