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The Invisible Dangers Lurking in Our Open-Water Swims

From the Tokyo Olympics to local triathlon races, race directors are fighting a constant battle against sewage overflows, algae blooms, and pathogens. What can the average triathlete do?


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In the lead up to the  2016 Rio Olympics, water quality became a major issue for rowing, sailing, swimming, and triathlon events held in open water. A 16-month-long water quality analysis commissioned by the Associated Press, released just a few days before the 2016 opening ceremonies, found that the water was highly polluted and threatened the health of competitors.

The analysis “revealed dangerously high levels of viruses and bacteria from human sewage in Olympic and Paralympic venues—results that alarmed international experts and dismayed competitors training in Rio, some of whom have already fallen ill with fevers, vomiting, and diarrhea,” the AP reported. One expert noted that the data collected by the AP indicated that “international athletes at all water venues would have a 99% chance of infection if they ingested just three teaspoons of water.”

Thankfully, reports of athletes falling ill after contact with the water weren’t as widespread as estimates suggested they could be, but the same cycle of concern about the water quality cropped up again in the lead up to the Tokyo Games. Tokyo Bay reportedly smelled “like a toilet,” and a Bloomberg report noted that in 2019, E. coli—that’s the fecal-based bacteria that can cause severe gastrointestinal illness—was detected in the water at more than twice the limit set by World Triathlon.

Massive efforts were undertake to clean the water, requiring the efforts of some of the biggest names in water quality research. An older city like Tokyo uses a combined sewage system that processes stormwater runoff in the same pipes as sewage. In times of heavy rainfall, the system can becomes overwhelmed, dumping untreated sewage into local waterways, like the Tokyo Bay where Olympic triathletes were set to compete. As the world learned in horrified coverage of the problem, the “toilet smell” descriptor was spot-on.

An elaborate three-layer barrier system was developed to block bacteria from entering the swim area during the Olympics. But the system was a Olympic-size one-off created out of necessity and global pressure—not something most race organizers could create or install. What happens then at smaller races, which struggle to keep competitors safe in open-water swim conditions? Trying to stay ahead of urban runoff, sewage systems, and toxic bacteria in the water is an ongoing struggle—and, in some cases, a losing proposition.

RELATED: How Tokyo Fixed Its Olympic Open Water Problem (Hopefully)

Photo: Tim Radville

Swimming in sewage

Boston’s Charles River was once was so polluted it inspired the Standell’s song “Dirty Water.” The song became a Red Sox anthem and perverse point of pride for generations of Bostonians. Naturally, to see hundreds of wetsuit-clad swimmers dive in every June for the Charles River 1-Mile swim is a bit jarring.

Frans Lawaetz, co-founder of the Charles River Swimming Club (which sponsors the race), knows this. Though the water in the Charles is significantly cleaner than it once was, it still rides a very fine line between safe and not-safe.

“There are established water quality guidelines that the state has dictated for recreational water contact,” Lawaetz said, noting the most concerning pathogen for the Charles River race is the presence of E. coli. As with Tokyo and locations with combined sewer overflows, the Charles River is sensitive to rainfall; heavy rain in the days leading up to the event can be catastrophic. “It’s really very clear. When you look at the Charles River after any kind of heavy rain, the bacteria counts go way up.”

Kate Radville, race director of the Charles River swim, said that rain has forced the postponement or cancelation of the event in the past. “The weather forecast is our annual source of torture,” she said. Though she knows better, Radville starts obsessing about heavy rainfall some 10 days out, and as the race inches closer, she becomes even more preoccupied with local weather reports.

This race, like many others, has a protocol in place. “We basically have a rainfall allowance in the time between when the water sample was taken and the time of the race,” Lawaetz said. “If that allowance gets exceeded, then of course the race is canceled.” This call is made usually 18 to 24 hours before the swim.

While water quality tests are great for telling organizers when there’s danger in the water, they provide another hurdle to planning a race. “Most of these tests, you can’t get instantaneous results. Some of them take 24 hours to run,” Lawaetz said. Because events are planned for a particular date—permits are secured, calendars are locked in—an event months and even years in the making can be thwarted at the last minute by a single water quality test.

“Because the tests aren’t instantaneous, it’s a little bit of a nail-biter waiting for those results to come back,” Radville said.

Lawaetz and Radville hope that one day the Charles River might open up to swimming on a more regular basis. “I personally would feel completely safe swimming in the Charles, because it is completely safe,” Radville said, citing recent findings from the state agency that oversees water quality monitoring, which say the Charles River is swimmable almost every day of the year. “I think it does sound alarming that we’re monitoring for all these things if you don’t have context, but in actuality, it’s very clean. I tell people when they ask that we are very confident that it’s clear. We’re not stretching things or under-evaluating the cleanliness. We’re pretty darn certain that you’ll be fine if you go into the water.”

Photo: Getty Images

The algae equation

Rainfall-induced sewer overflow is a major concern for urban events, but another ever-growing hazard is cyanobacteria. In recent years, blooms of the aquatic microorganism in the Charles also triggered postponement of the swim. “People think of it a blue-green algae, but it’s really a bacterium,” Lawaetz said. “The risk there is that there’s many different types of cyanobacteria, and within a single species, you can have some that produce toxins, but not others. So it’s really hard to have a clear picture as to what’s going on.”

Cyanobacteria and other certain types of algae, which can be lumped together under the header phytoplankton, can release toxins that can be harmful to health when ingested. Local circumstances can cause these organisms to grow wildly out of control and create dangerous swimming conditions. To measure the presence of cyanobacteria, the state environmental agency runs routine testing of the water, which the race organizers rely on to determine whether it’s safe. (In 2021, the World Health Organization released a second edition of its cyanobacteria guidelines for freshwater.)

Some modern farming practices have exacerbated this problem. Roughly half of the land area in the United States, or some 1.2 billion acres, is farmland. To keep that land productive year after year, farmers use a range of products and chemicals to support robust growth of food crops. The Environmental Protection Agency reports that upwards of a half-million tons of pesticides, 12 million tons of nitrogen, and 4 million tons of phosphorus fertilizer are applied to crops annually in the continental U.S. But these products don’t always stay where they’re applied, and when they enter the local watershed, they can encourage the growth of organic material that can lead to the overgrowth of phytoplankton and potentially toxic algal blooms.

Anna Michalak, director of the department of global ecology at the Carnegie Institution for Science located on the campus of Stanford University, studies this process. She said, “if we end up with more nutrients in the water and the water itself is becoming warmer, that can lead to excessive growth of phytoplankton,” which can lead to a potentially harmful algal bloom.

Photo: Getty Images

How these harmful blooms occur

There are three ways in which algal blooms can become harmful. The first is uncontrolled growth of phytoplankton, which can replicate rapidly under the right conditions (as discussed above). In other cases, the problem is related to the specific species. “There’s certain types of populations of phytoplankton that take over in the bloom, and especially for recreational waters, we tend to be concerned with cyanobacteria,” Michalak said.

The third category of potential harm, which is related to the second category, are the toxins produced by the algae. “If the concentrations of these toxins get to be high enough, then you have potential health impacts,” Michalak said. These impacts can lead to mass die-off of fish and other detrimental environmental effects. Eutrophication, the scientific term for excess nutrients getting into bodies of water that encourage overgrowth of algae and other plants, can also cause hypoxic dead zones. These occur when oxygen levels in the water drop below a certain threshold that’s harmful to the ecosystem. This can become hugely problematic for aquatic species that rely on adequate levels of oxygen in the water. It can also be deadly for pets and cause illness in humans who come into contact with the water. When these conditions occur, closure of recreational water areas and canceling swim events is not an overreaction; it’s a necessity.

Photo: Getty Images

Climate change is making it worse

Though many are quick to blame the agricultural industry for exacerbating the problem, Michalak says that’s an oversimplification of a much bigger problem.

“Often this is framed as a sort of adversarial relationship between the environment and the agricultural community,” Michalak said. “But I think it’s important to realize that farmers are no more interested in their fertilizer ending up in the local lake than swimmers are. Instead of pitting farmers against other users of the local environment, I think it needs to be viewed as more of a collaborative process where doing things in a way that fosters conservation is actually helpful for both the people using the water and the people using the land that’s surrounding a particular river or lake.”

That means working together to fight a much bigger enemy: Climate change.

In North America, rainfall patterns are changing. Places that are wet are becoming more wet, and places that are dry are becoming more dry. We’re also seeing changes in the seasonality of rainfall; stronger rainfall patterns in the springtime, which also tends to be when farmers apply fertilizer to their fields. When these rains arrive, they also tend to be heavier or more extreme events.

“All of these things are contributing to more nutrients ending up in the water,” Michalak said. “It’s essentially feeding the system that makes it possible for phytoplankton to grow more and grow more quickly.” This skewing of the weather patterns can also favor the overgrowth of certain kinds of phytoplankton, like cyanobacteria, that may be more harmful than other types. “Some types of phytoplankton outcompete other types of phytoplankton because they’re able to grow more quickly.”

Rising temperatures also play a role, she said. “When the lake water itself gets warmer, certain things change as well. For example, warmer waters might disproportionately encourage the growth of certain species that are harmful that prefer warmer temperatures.”

When the water warms, stratification also increases. This is the phenomenon where the top few inches of the water are significantly warmer than the rest of the water deeper down.

“That layering of the water, with the warmer water being near the top and the colder water being at the bottom, tends to get stronger as the lake gets warmer, because you have a larger contrast between the temperature of the top and the temperature at the bottom,” Michalak said, explaining that the water mixes less because the warmer water is less dense than the colder water. “That tends to favor certain types of cyanobacteria that can float on the top to get better access to the sunlight, and therefore grow faster relative to other types of phytoplankton.”

Over time, these effects snowball. “We’re using land in much more intensive ways than we used to, then we’re having rainfall patterns that are much more intense as well. When you put those together, we’re having more nutrients end up in the water. And on top of that, as the water is getting warmer within the lakes and the rivers themselves, you’re getting more conditions that are conducive to those nutrients being used to support the growth of these harmful algal blooms.”

RELATED: Why Triathletes Should Care About Climate Change

Photo: Getty Images

What else is in the water?

While Michalak and her team are focused primarily on freshwater bodies, similar patterns of eutrophication have also been observed in coastal and marine waterways, leading to dangerous overgrowth of red tide, another type of phytoplankton that can be harmful to people and other animals. And though harmful algal blooms suck up most of the oxygen in the room (pun intended), there are a wide variety of other problematic pathogens that can wreck a day at the beach, including:

  • Bacteria. Escherichia coli, a common food-borne bacteria better known as E. coli, is one of the most common pathogens in the water system. While many forms of E. coli bacteria are actually helpful to the human digestive system, there are some strains that can cause severe gastrointestinal illness. Legionella is another bacteria that can thrive in water. It causes Legionnaire’s disease, a serious form of pneumonia that results in death about 10% of the time. It can also cause Pontiac Fever, a milder form of Legionnaires’ disease with symptoms similar to influenza.
  • Viruses. While bacteria are usually the first pathogens that come to mind when talking about water quality, viruses can be water-borne too. Common culprits include rotavirus, adenovirus, Norwalk virus, and hepatitis A, all of which can cause a variety of unpleasant symptoms. Norovirus, another culprit behind gastrointestinal illness, is also quite common.
  • Protozoa. Though not all single-celled microscopic animals cause harm, the much-feared “brain eating amoeba” Naegleria fowleri is the quintessential bad guy when it comes to protozoa. These tiny creatures love to lurk in the silty bottoms of freshwater lakes, rivers, and springs in warm regions of the world. They thrive in water that ranges from 80 to 115 degrees F and can be kicked up by shuffling feet along the bottom. Once the amoeba is in the water column, it can be transferred to the nasal canal in a droplet of water. There, the pathogen travels to the brain and begins destroying brain cells. Death nearly always occurs and transpires within a few days. The good news is that while N. fowleri is terrifying and increasing water temperatures have brought its range northward in recent years, infections are still very rare; between 1962 and 2021, only 154 known cases of the amoeba have been recorded in the U.S. (Only four people have survived known infections.)
  • Parasites. Among the many microscopic parasites that live in watery environments, some, such as schistosomiasis, a type of parasitic flatworm, can cause skin symptoms in swimmers. Similarly, duck itch, a super common complication of freshwater swimming, is caused by a parasite. Cryptosporidium, which causes acute gastrointestinal illness, is another parasite found commonly in water.
  • MicroplasticsMicroplastics, or tiny bits of plastic debris less than 5 millimeters in length (about the size of a sesame seed), have become a major issue in both drinking and recreational waters in recent years. Once in the marine environment, microplastics enter the food chain as fish and other animals mistakenly eat them. Exactly how these microplastics impact the environment and human health isn’t fully understood yet, but the National Oceanic and Atmospheric Administration has established the NOAA Marine Debris Project to research the problem. In 2017, triathlete and German chemistry professor Andreas Fath of Furtwangen University, swam the 652-mile length of the Tennessee River as part of Tenneswim, a clean water project to bring attention to a crisis of microplastics in the water. During the swim, Fath and his crew took water samples that identified 600 different chemicals and microplastics in the water. Fath also swam the length of the Rhine River in 2014 and the Danube River in 2022 in similar projects.

Where to swim: What to look for in clean water

While there are plenty of potential hazards in the water, there are still loads of places that are safe to swim. Generally speaking, stagnant water should be a no-go; rather than dipping in a small, still pond, it’s best to head for a larger body of water where the water moves and can be exchanged on a regular basis with fresh water.

Bodies of water that are adjacent to farmland or urban roadways may also contain higher levels of pesticides and other agricultural runoff or oil and other pollutants from road surfaces.

Algal blooms sometimes look like foam, scum, or mats on the surface of the water. They can even look like swirls of paint in the water. But Michalak said visual clues don’t tell the full story. “The harm that can come from harmful algal blooms has to do not just with the amount of phytoplankton, but whether the type of phytoplankton that’s growing and whether that type happens to be producing toxins at that time.” Therefore, “if it were me, if it’s my kids, I would not rely on the visual appearance of the water.”

Sure, a still, greenish body of water may look less inviting than clear running water, but even that clear water might have phytoplankton that’s producing toxins. Therefore, “I encourage people as much as possible to rely on actual monitoring,” she said.

That means instead of relying on a visual assessment, it’s best to check with your local environmental group or water testing authority to see if recent tests have indicated there’s anything worrisome in the water. Many of these agencies diligently check the water on a weekly or more frequent basis throughout the swimming season and post results to their website and social media channels. Get familiar with these local authorities and follow their advice; if a test has come back positive for cyanobacteria or another concerning pathogen, skip your swim or head to the pool instead.

It’s true that pools are almost always very safe in terms of water quality. The presence of sanitizing agents like chlorine or bromine kills most any bacteria and inactivates most viruses that may be introduced to the water. And there’s little chance of a harmful local population of algae being present. As long as the facility is being properly maintained and the water quality is checked regularly, enjoy your swim.

If after any swim, whether in the pool or open water, you develop certain symptoms, it’s still best to check in with your doctor. Symptoms that you shouldn’t ignore include:

  • Diarrhea
  • Skin rashes
  • Ear pain
  • Eye pain and redness that doesn’t clear up with over-the-counter eye drops
  • Cough or congestion

Let your health care provider know where you’ve been swimming and for how long, as that may help provide insights to potential exposures that could be causing your symptoms.