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.
Even if it was getting into streams above Philadelphia, iodine-131 has such a short half-life - half the radioactivity is gone after eight days - that amounts would be much reduced by the time they were swept downstream.
But, said Chris Crockett, the Water Department's deputy commissioner of environmental services, "we don't want any iodine-131 in our water. I don't want it there for me or my kids and my family. And I don't want it for our neighbors and citizens."
Water experts nationwide say iodine-131 is another unwelcome pollutant - along with pharmaceuticals and personal care products - that swirls down the nation's drains, into waterways and, in minute amounts, into many water supplies.
But iodine-131's radioactivity makes it stand out.
Philadelphia's water department, like other cities' utilities, is required to test for radioactivity. But the test measures total radiation, which could come from any of about 160 radionuclides. The city had never found a problem.
But unbeknownst to city water officials, federal radiation officials had detected spikes of iodine-131 in the water here since 2002, through a program called RadNet.
Created during the Eisenhower administration, RadNet started out monitoring radiation from nuclear weapons testing. Now, the purpose is to sample air, milk, rain, and drinking water to make a baseline so that in the event of a nuclear attack or accident, officials can compare levels.
Radioactivity in water is measured in picocuries per liter. Many RadNet cities have detectable amounts, but mostly they are less than one picocurie per liter. Some Philadelphia samples spiked to three and four picocuries per liter.
The EPA's drinking water standard is three picocuries per liter - but only over a long-term average. A single sample that was higher would not constitute an excess. Still, it would be a red flag.
Since RadNet is a tracking program, not a compliance program, the results had never been widely shared. It was only concern about Fukushima that prompted the EPA to send out its latest data.
Now, partly because of the Philadelphia case, the EPA has a new policy that requires more communication.
Tracking the source of water pollution on a river system with many tributaries can be tricky.
But it is one of the Water Department's specialties.
In the 1990s, the department traced a cucumber taste in the water to algae building up in the winter under the ice in New York reservoirs that feed the Delaware River.
But the work requires a lot of testing. And at first, the department couldn't find anyone to do it. The EPA and DEP labs, as well as many private labs, were swamped with test samples related to Fukushima.
When they found a lab, the spring rainy season had begun. The rivers were swollen.
"We're looking for something that's just at our threshold for being able to see," said Crockett. "Now, with the dilution, we can't detect it."
They started ruling out sources. Iodine-131 isn't natural like the radon that seeps into basements in this region.
It is man-made - a by-product of fission occurring in nuclear power plants.
So the Limerick plant on the Schuylkill upstream of Philadelphia was an obvious place to look.
Such plants are required to test regularly for evidence of iodine-131 emissions. It was due to Limerick that the city had the capability to deal with iodine-131 in the first place. When the plant went online, the city was required to have a carbon filtration system at the ready in case of an accidental release.
But Limerick's reports showed nothing unusual.
Officials also checked out a laundry facility in Royersford that washes uniforms worn at Limerick. Again, nothing.
Iodine-131 also has medical uses. It can help diagnose and treat thyroid problems.
Both healthy thyroid cells and malignant ones absorb iodine, and when it's the radioactive kind, they die. The healthy tissue outside the thyroid remains unharmed.
"It's sort of a magic bullet" for thyroid cancer, said John Keklak, director of radiation safety at Thomas Jefferson University Hospital. "They'd love to do this with other kinds of cancer. It takes advantage of natural functions of thyroid cells."
Southeastern Pennsylvania has about two dozen pharmaceutical and medical facilities licensed to handle iodine-131, and each is required to track even the slightest amounts.
DEP staffers began pulling out inspections reports. Within weeks, all those facilities had been cleared, said David Allard, a DEP radiation protection chief.
Any time substances wind up in water, sewage treatment plants are an obvious suspect. So the DEP began checking them. This is when the picture began to get even more complicated.
The Wissahickon, where the initial tests prompted by Fukushima had found the iodine-131, has five sewage plants discharging into it.
In a first round of samples, officials found it in the Ambler plant. Ten days later, the amount at Ambler had dropped somewhat, but the Abington plant suddenly had a hit. North Wales, Upper Gwynedd, and Upper Dublin were all negative.
No licensed iodine-131 facilities discharge to any of those plants.
Stream samples - nearly 150 so far - have remained variable as well.
It reminded Crockett of a Christmas tree with blinking lights. "One week, a light comes on in one part of the stream, the next week somewhere else."
A new possibility emerged.
Patients take iodine-131 in a capsule or a liquid. What isn't absorbed by the thyroid is eliminated, mostly in urine.
Until the mid-1990s, patients stayed at the hospital for several days, so presumably excess iodine-131 would have shown up in the hospital's effluent, if anyone had thought to look for it.
These days, patients are allowed to go home.
They would excrete a lot of iodine-131 at first, but then it would lessen quickly. Plus, iodine-131's short half-life would explain why it would also subside quickly below detectable levels in a stream.
There was another factor implicating human waste. In this region, sludge from sewage plants is often sent to landfills, and when it's fresh, it frequently sets off radiation detectors.
Still, why were radiation levels so high in some streams here and not others?
The nature of the Wissahickon presents an answer. The highly urban stream has very little natural flow. Treated wastewater constitutes upward of 95 percent of the flow during a dry stretch of summer.
This has the effect of maximizing iodine-131 levels.
At the very least, Philadelphia's iodine-131 problem has initiated national discussion.
"People are watching," said Alan Roberson, director of federal relations for the American Water Works Association, an industry group. "They're waiting to see if Philadelphia can figure out what's happening."
Maybe it's in other cities' water and no one has found it because they haven't looked hard enough. "If anything, this has given us reason to look at other locations," said Marcos Aquino, regional radiation protection manager for the EPA.
Other officials wonder if the U.S. drinking water standard for iodine-131, at three picocuries per liter, is too strict. The World Health Organization suggests a limit of 270. Canada's limit is 160.
Cancer experts said that while the theory is plausible, they're skeptical.
Nationwide, the number of thyroid cancer cases is rising, partly due to better detection methods. But because most are early stage cancers, "the tendency is to treat less with radioactive iodine," said James. A. Fagin, chief of endocrinology at Memorial Sloan-Kettering in New York, and president-elect of the American Thyroid Association.
A colleague, Sloan-Kettering medical physicist Pat Zanzonico, considers the levels so minuscule that they provide reassurance rather than concern. It shows "that outpatient treatment of thyroid cancer patients has the expected minimal [effect] on environmental levels of radioactivity, as we would have expected."
Some have suggested requiring outpatients to hold their waste and return it to the hospital for disposal.
"I'm sure it was well-intentioned," said Zanzonico. "But it's the exact thing you don't want to do - hold and concentrate whatever little radioactive waste is created. The best thing is to disperse it."
With the spring rains, the threat was considered gone and on May 5, the department turned off its carbon filtration system.
Since then, weekly tests of intake water - more than 50 samples so far - show that levels remain low.
The Water Department is developing a plan for intensive testing this summer, when stream flows are typically low and concentrations of iodine-131 might be high enough to detect.
They'll be extending their view up the Schuylkill to Conshohocken, Norristown, and Reading.
Like other officials, the department's Kelly Anderson calls this a long-term process. In all likelihood, she said, "we will be working on the issue all this year, and probably for many years to come."
Contact staff writer Sandy Bauers at firstname.lastname@example.org, 215-854-5147, or @sbauers on twitter. Visit her blog at philly.com/greenspace.