But when she looked at the sample from the Wissahickon Creek near Green Lane, just upstream from a city drinking water intake, she froze.
None of the other river samples in the batch showed iodine-131. But this one did.
By 6 p.m. that day, that drinking water intake would be getting extra treatment, and officials would be embarking on a detective mission that has generated interest nationwide.
Since then, officials have found more iodine-131 in the Wissahickon, and at several sewage treatment plants along the creek.
They've also realized that worrisome levels of iodine-131 had been detected long before the Fukushima accident in several Philadelphia drinking water samples taken as part of an obscure monitoring program run by the U.S. Environmental Protection Agency.
Within that limited data set of 59 locations across the country, Philadelphia's levels were the highest in the previous decade, the Water Department discovered.
So Fukushima couldn't be the cause after all.
The source they now suspect was a surprise. Iodine-131 is used to treat thyroid cancer, and they suspect it's coming from patients excreting excess radioactivity in their urine, which then winds up in rivers, and ultimately in Philadelphia's drinking water intakes.
Iodine-131 is not good for you.
When radioactive iodine gets into the body, it concentrates in the thyroid gland. Low doses can impair the gland's activity, according to the EPA. Long-term exposure to high amounts can cause cancer.
Officials from the Water Department, the EPA, and the DEP emphasize that the levels detected are tiny and don't constitute a public health threat. Philadelphia's drinking water meets standards for radioactivity and remains safe, they say.
