Officials say two-and-a-half months after learning of its presence, they still have not pinpointed the source of Iodine-131 in Philadelphia's drinking water.

At a public forum Wednesday, officials said weekly tests of the city's drinking water have shown levels of the radioactive material well under the federal maximum of 3 picocuries per liter since April. They stressed the water poses no public health risk, but said they are still working to determine where the contamination is coming from so they can stop it.

Local officials this spring were surprised to hear that federal testing had found levels of the radioactive substance that exceeded the federal limit five times between 2005 and 2010. Since then, the Environmental Protection Agency, Pennsylvania’s Department of Environmental Protection and the Philadelphia Water Department have worked together to test source water and waste-water treatment plants. DEP’s Lisa Daniels said they do not know much more about where the material, which is a carcinogen in high doses, is coming from than they did in April.

"We definitely have more questions than answers at this point,” Daniels said.

DEP found elevated levels of Iodine-131 flowing from waste-water treatment plants in Ambler and Abington in April. Daniels said nuclear power plants and hospitals have been ruled out as sources of the contamination. Waste-water from patients taking iodine as treatment for hyperactive thyroid or thyroid cancer is the likely cause, but that makes it hard to prevent.

“If it is coming from individual folks, what do you do about that?” Daniels asked. “There's not a building or facility that you can point to to say ‘Ok, you guys need to lower you levels.’”

Officials said they will continue to try to determine the source of the iodine, ramping up efforts in the dry summer months when they expect concentrations may grow higher.

The elevated levels of iodine were documented by the federal program RadNet, which for three decades has been used to track environmental releases of radioactivity from nuclear weapons tests and nuclear accidents. The data has been posted online for years, but was never formally given to local water officials because it was not intended to be used for public health purposes. The Environmental Protection Agency said now a policy is in place to share the data with local authorities as soon as it is gathered.

"There was this question of well, why didn't drinking water folks know about it?” said the EPA’s Bill Arguto. “Part of it was that the information wasn't intended for drinking water compliance issues, but I think we've come to realize that the information's important for everyone now."