Rain, runoff and other threats can carry pathogens into your local rivers and lakes.
A sewer pipe collapse in Maryland earlier this year dumped more than 360 Olympic-sized swimming pools of sewage into the Potomac River, just upstream from Washington, DC. The incident is perhaps the largest sewage spill in U.S. history, and it is a prime example of fecal pollution, the most common source of illness from natural waterways. Every time we swim in a lake, river or ocean we risk encounter waterborne pathogens.
Most commonly, these microbes infect the digestive tract, causing symptoms such as diarrhea and nausea, but they can also affect the eyes, ears, skin, and more. With summer swimming season here in the United States, public water quality reports can help you assess your risk of getting sick.
In the United States, scientists typically collect water samples, culture them in the laboratory, and then count the number of certain types of bacteria that grow. They focus on one or two types of bacteria associated with fecal contamination, called indicator pathogens, because it would be too expensive to test for all harmful microbes directly. The most common indicator pathogens are Escherichia coli for fresh water and Enterococcus for salt water.
Unfortunately, “this indicator system…is associated with a whole host of problems,” says environmental microbiologist Kelly Reynolds of the University of Arizona in Tucson. E.coli can die in the water before other harmful microbes from the same source, Reynolds says, thus finding low levels of E.coli does not necessarily mean the water is safe. E.coli and enterococci also appear in the feces of many warm-blooded animals, but conventional culture tests cannot determine whether their source is human waste, which carries a greater number of diseases that can harm us.
These limitations were highlighted in a 2024 study, which demonstrated an alternative testing method which 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 collected at 18 ports around the world, detecting fecal pollution in 46 percent of the samples. Meanwhile, only 18 percent exceeded indicator pathogen standards on conventional tests.
But that doesn’t mean cultural testing isn’t useful. Although they may miss short-term periods of contamination, especially if samples are not collected several times per week, these tests can help identify when there is persistent pollution.
Water quality checks and advisories are usually available on government websites. States issue advisories when the number of viable indicator pathogen cells counted in culture tests exceeds their standard. Where McLellan lives in Wisconsin, for example, the state issues range advisories when testing counts E.coli exceed 235 colony forming units per 100 milliliters of water. At this concentration, the EPA estimates that 36 out of every 1,000 swimmers in a given area will get sick. The Potomac River crested at a E.coli concentration almost 12,000 times the safe post-spill recreational standard, according to independent testing by the Potomac Riverkeeper Network. The EPA announced on May 6 that the river recovery goals had been encountered.
Wherever you dive, there are guidelines you can follow to minimize the risk of infection. McLellan recommends avoiding entering water for at least 24 hours after light rain and 48 hours after a downpour of more than three centimeters. Runoff can flow through leaking pipes or faulty sanitary pipes and carry infectious microbes into public waterways.
Cloudy water and algae can also indicate potential pollution, says McLellan. And if you’re unsure of the water quality, avoid submerging your head. A common way to contract waterborne illnesses is to swallow water, and that’s hard to avoid if your face is sagging, Reynolds says. “I’m always afraid of discouraging people from enjoying the water,” McLellan says, but you can cover your bases. “I think that eliminates 95 percent of the concerns.”