indicator bacteria – A Maryland sewer collapse dumped wastewater into the Potomac River just upstream of Washington, D.C., underscoring how common “indicator” bacteria tests can miss human fecal pollution. New DNA-based methods and better on-the-ground rules—like waiting after rai
The Potomac River didn’t just turn into a headline—it became a warning you can feel on a hot day. Earlier this year. a sewer line collapse in Maryland spilled more than 360 Olympic-sized swimming pools’ worth of wastewater into the Potomac just upstream of Washington. D.C. It was, the incident may be the largest sewage spill in U.S. history, and it’s a quintessential example of fecal pollution, the most common source of sickness from natural waterways.
When summer swimming season hits across the United States, this is the uncomfortable reality: anytime we swim in a lake, river or ocean, we risk encountering waterborne pathogens.
Most often these bugs infect the digestive tract, causing symptoms like diarrhea and nausea. But they can also affect the eyes, ears, skin and more. That’s why public water quality reports matter—and why, increasingly, scientists are questioning how well the tools behind those reports work.
In the U.S. scientists typically collect water samples. culture them in the lab and then count how many of certain types of bacteria grow. The approach is built around indicator pathogens, because it would be too costly to test directly for all harmful microbes. The most common indicator pathogens are Escherichia coli for freshwater and Enterococcus for saltwater.
On paper, it’s a sensible shortcut. In practice, the limits can be brutal.
Environmental microbiologist Kelly Reynolds. of the University of Arizona in Tucson. points to a key blind spot: “this indicator system … has a whole host of problems associated with it.” Reynolds says E. coli can die off in the water before other harmful microbes from the same source. so finding low levels of E. coli doesn’t necessarily mean water is safe.
There’s also the question of what the bacteria are actually coming from. E. coli and enterococci can show up in the feces of many warm-blooded animals, but conventional culture tests can’t determine whether their source is human waste, which carries more diseases that can harm us.
Those limitations didn’t stay theoretical. A 2024 study highlighted them by testing a different kind of method—one that identifies fecal matter using DNA markers unique to human gut microbes. Sandra McLellan. an environmental health researcher at the University of Wisconsin–Milwaukee. and colleagues filtered bacterial DNA from hundreds of samples gathered at 18 harbors worldwide. They detected fecal pollution in 46 percent of the samples.
On conventional tests, meanwhile, only 18 percent exceeded indicator pathogen standards.
None of this means culture tests are useless. McLellan’s work also fits with a more complicated truth about risk: culture tests can miss short-lived periods of contamination, especially if samples aren’t taken multiple times a week. But they can still help identify when there’s persistent pollution.
For swimmers trying to make decisions, the practical question becomes: what do advisories mean, and what do you do with the gaps?
Water quality checks and advisories are typically findable on government websites. States issue advisories when numbers of viable indicator pathogen cells counted in culture tests exceed their standard. Where McLellan lives in Wisconsin, for example, the state issues beach advisories when test counts of E. coli exceed 235 colony forming units per 100 milliliters of water. At that concentration, the EPA estimates that 36 out of 1,000 swimmers in an area will get sick.
The Potomac River provides a stark case study. After the spill, independent testing by the Potomac Riverkeeper Network found the river peaked at an E. coli concentration nearly 12,000 times the safe recreational standard. The EPA announced on May 6 that the river’s recovery goals had been met.
That’s the kind of timeline people want to trust—but science still warns that the lab counts aren’t the whole story, and swimmers can’t afford to treat them like perfect answers.
Wherever you dip, there are some guidelines you can follow to minimize the risk of infection.
McLellan recommends avoiding entering the water for at least 24 hours after light rainfall, and 48 hours after a downpour of more than three centimeters. The reason is runoff: it can flow past leaking pipes or faulty sanitary system plumbing and carry infectious microbes into public waterways.
Cloudy water and algae can also indicate potential pollution, McLellan says.
And there’s one habit that gets less attention than it should. If you’re unsure about the water quality, avoid sub-merging your head. One common way people pick up waterborne diseases is by swallowing water, and that’s hard to prevent if your face goes under, Reynolds says.
“I always worry that I’m discouraging people from enjoying the water,” McLellan says. Then she adds the kind of reassurance that lands differently coming from someone who studies risk up close: “I think that takes away 95 percent of the concern.”
sewage spill Potomac Riverkeeper Network E. coli Enterococcus fecal pollution indicator pathogens DNA markers Escherichia coli waterborne pathogens beach advisories EPA recovery goals