When sewage leaks, Baltimore's waterways are not just fouled by the waste. They also become contaminated with chemical residues of pharmaceuticals, personal care products and even illicit drugs, researchers say.
And such substances can have effects beyond those listed on warning labels.
Traces of amphetamines from prescription drugs and methamphetamine make algae produce less oxygen, speed the growth of midge flies and change the mix of bacteria on stream bottoms, according to scientists who have been gathering data from the Gwynns Falls for nearly two decades.
While drugs have long been detected in urban waterways, the researchers have shown for the first time that the chemicals can cause chain reactions up the food chain long after their effects wear off for users.
"We have not done this kind of research before," said Emma Rosi-Marshall, an aquatic ecologist at the Cary Institute of Ecosystem Studies in New York and co-director of the Baltimore Ecosystem Study. "We didn't know this was the case."
Scientists say that while they do not yet understand how the cocktail of chemicals carried in human waste affects streams in combination, they can infer that the drug residues are having an ecological impact.
"It's not really better or worse; it's just different," Rosi-Marshall said. "We can't make a value judgment on what that means."
Over the past 18 years, a team of researchers has been studying the Gwynns Falls through the Baltimore Ecosystem Study. The National Science Foundation-funded project has produced similar research papers explaining the effects of antibiotics, antibacterial soaps and antihistamines on aquatic ecosystems.
In this case, researchers built eight artificial streams in a lab at the Cary Institute. Half served as control subjects, and the others were exposed to amphetamines at concentrations similar to those found in the Gwynns Falls.
The researchers found a clear difference from the presence of amphetamines, a type of stimulant that includes the attention-deficit and hyperactivity disorder drug Adderall as well as the illegal drug methamphetamine. The results were published this month in the journal Environmental Science & Technology.
In the layer of single-cell algae and bacteria that makes stream beds so slick — known as biofilm — they found the drugs' presence stunted photosynthesis and changed the mix of organisms present. The biofilm is a key food source for insects that themselves sustain fish, spiders, birds and bats.
The amphetamines also affected the insects' growth. Midge larvae feeding on the biofilm developed into flies more quickly, and more of them emerged from the streams that received the drugs than from the streams that did not.
That might seem like an advantage for creatures that feed on flies, but Rosi-Marshall said there is not enough information to know whether that is the case. The flies affected by amphetamines could be smaller, for example.
Researchers cannot say whether the effects they found in the lab are present in the Gwynns Falls or other streams. There are many other stressors, such as nutrients that include nitrogen and salt runoff, that influence the ecosystems, Rosi-Marshall said.
But one water-quality advocate said the study underscores the need to prevent sewage from leaking into waterways. After missing a 2015 deadline to complete sewer system repairs, Baltimore officials and federal regulators entered a new agreement in June that requires $1.2 billion in work over the next 15 years.
"Fixing these sewer leaks is the biggest challenge we have to improving our waterways, making sure they're safe and restoring their ecological health," said David Flores, the Baltimore harbor waterkeeper.