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Rachel Feltman: One week from today the 2024 Olympics in Paris will begin with a parade—not in a stadium but on a river. Thousands of athletes from more than 200 territories will float on boats down the Seine. City officials and event organizers have placed a big bet on this beloved river: that the infamously polluted waters will be safe for Olympic swimmers to compete in.But their efforts have been met with—well, we’ll say skepticism, to say the least. Back in June, when the mayor of Paris, Anne Hidalgo, was set to swim in the Seine to show her confidence in the cleanup efforts, a trending hashtag encouraged folks to poop in the river in—protest? Unclear. Hidalgo did successfully take a dip this past Wednesday and gave the experience rave reviews.[CLIP: Cheering and clapping]On supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.Feltman: But that doesn’t mean the Olympic events will go quite as swimmingly. You know what they say about stepping into the same river twice: those things are always changing and always flowing. And the Seine’s bacterial levels are still fluctuating from day to day.For Scientific American’s Science Quickly, I’m Rachel Feltman. Today I’m joined by associate news editor Allison Parshall, who investigated this high-profile cleanup attempt for us.So, Allison tell me: Are Olympians going to swim in the river or not?Allison Parshall: I would love to be able to tell you—I would peer into my crystal ball—but I think until there are bodies in the water, I’m not going to be able to say one way or the other [laughs]. And that’s mostly because they’ve basically done all that they can at this point from the perspective of, you know, cleaning up the river. The main problem right now is bacteria, and some of the things that could cause bacteria levels to be higher are kind of just at the whims of the weather: It’s if it’s too rainy, bacteria counts can be too high. If it’s not sunny enough, because sun can kill the bacteria—the bacteria counts can be too high.So throughout June, basically, the bacteria counts were much higher than, I think, anyone expected or wanted. And that’s because the—Western Europe, in general, had an unseasonably wet summer, at least in the beginning.So basically, after a very wet June, the organizers, who had been very proudly saying, “There is no backup plan. We’re all in on this end. There is no backup plan. There’s no plan B,” announced a backup plan.So the backup plan for the marathon swimming events, at least, which is one of the ones that would be in the Seine, is this nautical stadium outside of the city. It’s this very fancy facility inaugurated in 2019. It’s already hosting the Olympic and Paralympic canoe, kayak and rowing events, so there’s that.Feltman: Yeah, well, it’s good that they have that backup.From above, this facility definitely looks like that kind of freaky ocean arena from The Hunger Games, but ...Parshall: Hate it.Feltman: [Laughs] I’m, I’m sure it’s lovely, though. I’m sure it’s a lovely place to be and definitely better than a river full of poop, in any case.Parshall: It’s probably hard not to be better than a river full of poop.Feltman: [Laughs]Parshall: But basically this nautical stadium, it’s already hosting those boating events, but the triathlon wouldn’t be able to be relocated there—so that’s the other event that would be swimming in the Seine. So that they would just have to postpone that and hope the bacteria levels go down. Or if they don’t, it could just get downgraded from a triathlon to a duathlon, which I feel like is a different sport.Feltman: Yeah, I think if I had trained for years to specifically be in the triathlon in the Olympics, and swimming got cut, and that was, like, my main strength, I’d be pretty ticked off. It makes you wonder why Paris, like, took such a chance on the Seine in the first place.Parshall: This river is such an important part of their city’s history and culture, and they’ve been trying to clean it for a long time, and so it might be one of the only cities right now where we’re seeing them place such a big bet in the international spotlight on being able to clean this up, especially when it’s kind of at the whims of the weather.But they’re definitely not the only city facing this problem with its urban waterways. Industrialized cities across the world are reaching this kind of new phase of their river cleanup, at least for these rivers that were once so polluted by industry. And it’s possible that after, you know, decades, centuries of being very unsightly waste dumps, we might get to swim in a lot of urban waterways again.So I’ve got kind of, like, a personal touchstone with this. I grew up in Ohio. My Seine, as I like to say, was the Cuyahoga River ...Feltman: Oh, wow.Parshall: Have you heard of the Cuyahoga River?Feltman: I have in the context of it being, like, a river so gross that it inspired us to create the Environmental Protection Agency [laughs], which is ...Parshall: [Laughs] Yeah, when I was a kid ...Feltman: Such a legacy.Parshall: When I was a kid it was just, like, the place that we would go as a family on the weekends. We would walk and bike the towpath, and then there was this farmers’ market where we would get ice cream and corn on the cob; it was very Ohio. But I didn’t realize until I grew up that most of the people like you that knew of the Cuyahoga knew of it because they’d seen pictures of it on fire—like, the surface of the river burning, or at least ...Feltman: Yeah, yeah.Parshall: The industrial waste ...Feltman: It’s striking [laughs].Parshall: Yeah, yeah—that picture in particular. There’s this one particular photo, and it shows these firefighters spewing water onto the surface of the river to, you know, try to put out the fire, and it looks so preposterous because a river’s not supposed to be on fire.So when I picture these urban waterways that have just been so polluted but have since been relatively cleaned up, I picture this infamous image of the Cuyahoga on fire and then what I know it as today, which is kind of a muddy, lazy river but definitely not on fire.And I actually talked to a hydrologist about this—her name’s Anne Jefferson. She researches urban waterways at the University of Vermont, but she spent 10 years of her career at Kent State University, studying the nearby Cuyahoga.Anne Jefferson: The Cuyahoga River didn’t just catch fire once; it caught fire [a] dozen-plus times. It was oil. It was paint byproducts. It was all sorts of industrial byproducts. It—also sewage—the sewage is not the part that’s gonna catch fire, but it’s, you know, if you fell into the Cuyahoga, or if you fell into the Thames in London, the advice was that you take yourself to the hospital immediately.Parshall: I can’t say that I really want to swim in the Cuyahoga River, even these days—like, it generally looks pretty muddy—but it’s no longer a flaming health hazard, so there’s that. And its misfortunes really helped galvanize support for new regulation: that’s the Clean Water Act of ’72.Paris’s river may not have caught fire, but it kind of has a similar story, as do many other urban rivers. After the industrial revolution they just become this dumping ground that carries all of our waste, both of our bodies and of our factories, out and away from cities. And in Paris, this killed what was a really important part of the city’s culture at the time, which is bathing in the Seine.Feltman: That’s so wild. Like, I, I can know intellectually that before cities were super polluted, their rivers were nice places to be, but I still have trouble picturing people, like, you know, bathing in the Seine.Parshall: Yeah, I don’t know that this was all—the case with every industrialized city, but it was definitely the case with Paris. I mean, a lot of cities, you know, they kind of grew up around the industrial revolution. But with Paris there are several very famous paintings by Monet, Renoir, Seurat that depict these riverside scenes, and there’s these famous floating bathhouses that were filled with untreated water from the city—like, basically barges.And swimming in the river was largely banned in 1867. But that was just in the city, and then in the suburbs it was banned in 1923, but some people kept swimming in it. Like, Paris did hold the 1900 Olympics swimming events in the Seine. So this would be—if they do swim, it’ll be upholding this 124-year-old tradition. But by the 1960s the river was just well and truly disgusting, and it had been declared biologically dead.Feltman: I mean, first of all, continuing to swim in it—extremely French. Second of all, what does it, what does it actually mean for a river to be biologically dead?Parshall: Yeah, I asked Anne Jefferson that question because I also had never found a definition. She has never found a definition, so it might be kind of, like, an advocacy phrase.Feltman: A vibe.Parshall: People say it a lot—a vibe. It—basically it means, roughly, there’s no fish, or there’s no “desirable species,” quote, unquote ...Feltman: Fair enough.Parshall: But the bacteria, as undesirable as they may be—or some of them, at least—those were thriving, definitely, in the 1960s. And later—in ’85, I think, was the low point—it was measured—the Seine was measured to have 500,000 colony-forming units of E. coli per 100 milliliters of water. That’s, like, 500 times the current European standard for bathing.Yeah, we’re—I mean, I’m picturing sludge. I imagine it would not be sludge—like, it would still be water consistency—but I’m just picturing a lot of bacteria. But I have to say, the Cuyahoga, during a dry summer around the same time, I think in, like, ’82, the E. coli counts ranged up to 2 million. So not that it’s a competition, but I think we won—or lost.And I mean, it was only a few years later—so in 1988—that the mayor of Paris at the time, Jacques Chirac, he promised to swim in the Seine within three years’ time. Would you like to guess if he kept the promise?Feltman: [Laughs] I’m gonna guess he did not do that [laughs].Parshall: That is correct. He did not do that.Feltman: That just makes me think of the Mary-Kate and Ashley movie Passport to Paris. Do you remember the scene where they’re visiting their, I think, grandfather is, like, the French ambassador or something, and he’s trying to get the French to accept this, like, clean water proposal, and they’re like, “No! We don’t need your stinky, American, clean water.”Parshall: “No!”Feltman: Yeah. And then they surprise the, I guess, president or prime minister at a dinner party with a glass of tap water that’s untreated, and it’s like—it looks like chocolate milk. It’s, like, so disgusting. And I’m sure they took a lot of liberties with crafting the, the untreated Parisian water. But it was a real—I think I saw a TikTok recently that was like, “This full-on Erin Brockovich moment from Mary-Kate and Ashley.” Very formative for me [laughs].Parshall: I somehow missed this movie, but I think I absolutely would have loved it. And, like, I guess to be clear, the French government is not saying that the river needs to be clean enough to drink. That would be a whole other thing entirely.But the, the river is definitely in a better shape now than it was when, you know, Jacques Chirac promised to swim in it. Last summer, actually, the part where the Olympic races are supposed to start from, of the Seine, it was swimmable seven days out of 10, on average, so it’s not that bad. Like, like, people are making it sound like it’s literally, like, a flaming—you know, like the Cuyahoga or something. But in reality it, it’s more variable than that. And the fact that it’s possible at all for any, you know, somewhat safe swimming in rivers like the Seine right now is because of those regulations like the one from Mary-Kate and Ashley movie Passport to Paris or whatever. It’s because of those regulations that targeted the obvious and easy places where waste was being dumped into our water—so like the pipes just dumping industrial waste straight into the water. That’s what Jefferson called the “low-hanging fruit.”Jefferson: So once you’ve taken care of, like, the paint and the oil and stuff going into the river from the factories, what you’re left with is this harder problem that we call nonpoint source pollution. It’s the pollution that’s coming from a million different little places, right?In the air it’s the stuff coming out of the tailpipes of our cars. For water it’s stormwater runoff: it’s all the water coming off the rooftops and pavements, being carried by thousands of pipes, coming into every small stream, every river, you know, from every neighborhood.Parshall: So that stormwater that’s coming from all those pipes, it’s a problem because it’s carrying things like fertilizer, pesticides, bacteria—basically all sorts of stuff that you just don’t want in the water. And even worse, in many cities like Paris—also kind of, like, 60 percent of New York—when it enters stormwater drains, it gets funneled into the same pipes that carry the raw sewage to our wastewater plants, and when it rains too much, you get a bit of a backup.Jefferson: In order to keep this sewage-stormwater mix from backing up into people’s houses, you have what are called combined sewer overflows, so sort of like the safety pressure release valves on the system where now water is being diverted out of the sewage network and directly into streams, rivers and lakes. And this was one of those “it seemed like a good idea at the time” legacies that constrain what we do now.Parshall: And the reason it’s so constraining is because it’s so expensive to fix. Basically what you’d have to do is dig up all of those combined pipes and replace them with two sets: one for sewage, one for stormwater. And some cities like Minneapolis have tried to do that, but probably the more common option is to just find somewhere to store all of that mixed sewer-stormwater stuff until the treatment plant is ready to take care of it, so I reached out to Bruno Pigott, the acting assistant administrator for water at the EPA, and he mentioned some ways that cities are going about doing this.Bruno Pigott: In Indianapolis, for example, they put in a 28-mile tunnel [system] underground that captures all this combined sewage before it gets to a water body, stores it and then sends it to a wastewater treatment plant for cleanup.Parshall: So he actually told me a bit about this time that he got to visit the site of that project. He was working for the state of Indiana at the time, in, like, the 2010s. And this project is still under construction—and it cost $2 billion.Feltman: Wow.Pigott: I went down as they were building the tunnel and went into it—so the sewage that eventually will be in that tunnel was not, luckily, there when I was in it. But it looks very much like a tunnel that you would see in a subway. I mean, it’s that big. It’s tremendously large. You could drive a truck down this tunnel. It is, it’s so deep that it stores millions of gallons of sewage so that the treatment plants can actually treat it and send it out in a clean form back to the river.Parshall: So that’s not too different from Paris’ main solution ahead of the Olympic Games. They also just built this very big basin to hold all of that raw sewage-stormwater mix.Feltman: Oh, yeah ...Parshall: Yeah.Feltman: “The shove it all under the bed” method—tried and true [laughs].Parshall: [Laughs] Well, it’s more like “shove it under the bed, and then take a little bit out of it every day for the next few months until you can finally take care of it all” ...Feltman: Oh, great ...Parshall: So, yeah, yeah ...Feltman: That’s actually really nice.Parshall: So it’s not like—it’s not gonna sit in there forever. It’s actually kind of the logical solution because the whole problem in the first place is just the system does not have enough capacity to deal with all of this water. Their backup plan is dump it in rivers, which is not all that logical—it’s just, like Anne Jefferson said, seemed like a good idea at the time.This is probably just as logical of a solution, besides, you know, replacing all the pipes or scaling up the wastewater treatment’s capacity. But basically this basin, even though it sounds like it would be maybe the cheaper option, is still pretty expensive: they spent €90 million on it; that’s about $97 million. And that reservoir holds 50,000 cubic meters of liquid, so that is 20 Olympic-size swimming pools’ worth of raw sewage mélange that might otherwise get dumped into the river for the actual Olympic swimmers.Feltman: Incredible.Parshall: So that basin was completed in May, and bacteria levels in the water, which is the main concern for the swimming events, were still measuring too high throughout June, and that’s partially just because it’s been so unseasonably rainy, causing a lot more of those combined sewer overflows.For what it’s worth, the Olympics president said that he remains confident that the weather will be fine and the river will be fine come the end of July, so, you know, there’s that. But I did get to speak with someone who actually had the opportunity to swim in the Seine. Her name is Sibylle van der Walt. She’s the president of a clean water advocacy organization based in France.Sibylle van der Walt: It does mean a little bit of courage to, to, to swim in the Seine, firstly because the water is dark. It’s, it’s not transparent, and that is not a good sign.Feltman: How murky are we talking about here?Parshall: She told me that she couldn’t see much further down than about a foot [roughly 0.3 meters], so most of her body was totally obscured by the water.Feltman: Ugh, that’s—yeah, ew [laughs].Van der Walt: But it didn’t smell bad, that I can say, and I didn’t have a problem afterwards, so I was perhaps just lucky, and first you think, “Oh, what is—what am I doing here?” and so, but then you get used to it, and then it was actually very pleasant. It’s, it’s fun.Parshall: So Van der Walt got involved in the movement for swimmable urban rivers when she moved to France after living in Germany and Switzerland, and those are two countries where swimming in rivers is far more common.Van der Walt: I used to work at the University of Bern, where you have a river called Aare, and there even the president of the parliament goes for a swim during the lunch break, and it’s really, like, everyone walks around in a swimming costume and walks up the river, jumps in and comes back.Parshall: Apparently some people in Bern actually use the river to commute one way to work during the summers ...Feltman: What?Parshall: Like, they, like, pack up their belongings in floating, waterproof bags and just go for a dip, and I guess just coast home.Feltman: Oh, my gosh, I—so I assumed you meant, like, boating, but, no, we’re talking about people swimming ...Parshall: No, yeah—in the water.Feltman: Briefcase bobbing along behind them—incredible [laughs].Parshall: [Laughs] I literally did not believe this. I thought that it was one of those Internet stories, made up, and it doesn’t seem like this is something that people do really often, but I did email the Swiss Lifesaving Society—it’s a lifeguard association—to confirm because it sounded so far-fetched, and they warned me that the current in the river that goes through Bern is no joke, and they actually don’t recommend swimming in it right now, but some people appear to actually do this. And this kind of river-swimming culture seems like a total dream to me, that’s something totally unattainable in Paris and definitely in New York City, where we both live, and ...Feltman: Yeah.Parshall: Like, sure, these rivers are no longer full of stinky industrial waste, and cities are turning back toward them by building waterfront parks and business districts, but swimming in them? It feels harder ...Feltman: Yeah, yeah, a river doesn’t have to be on fire for me to not wanna swim in it. There’s, there’s a spectrum, really, from “on fire” to, to “swimmable,” and I don’t really feel like the Hudson is there yet.Parshall: Yeah, apparently they do the triathlon in—the New York City Triathlon—the swimming part is in the Hudson, so, you know, I’m not tempted to do that.Feltman: [Laughs] Yeah, I think I briefly, in, like, a moment of complete delusion a few years ago, I was like, “Maybe I’m gonna try to get into triathlons.” And I honestly don’t remember if the part about the triathlon being in the Hudson River was, like, a selling point to me, or if that was the moment when I was like, “Wait, I’ve suddenly remembered I absolutely don’t want to do a triathlon under any circumstances.”Parshall: I mean, I did go to the beach for the first time since moving to New York last weekend, and I was shocked at how pleasant it was. I think I forgot how much it’s enjoyable ...Feltman: Oh, yeah.Parshall: To swim in, like, natural waters.I mean, they’re installing a pool in the East River now in New York. They call it a, quote, “giant strainer” dropped into the river. So relatively soon you should be able to do that.Feltman: Yeah, I remember when the renderings for that first went around, and I, like, I would totally do it, but I kind of feel like tourists and, like, wannabe influencers are gonna make it terrible even if there isn’t tons of pathogenic bacteria in it. So, yeah, fingers crossed, but I, I have to say, I’m not optimistic.So, yeah, I’m getting the impression that, like, unless a city makes a big effort to make swimming safe, and that that’s been confirmed by outside testing, like, you probably don’t want to go do laps in an urban river. Would you say that’s true?Parshall: I don’t know if it totally needs a big effort in every city—some of them are just not as bad. Like, Sibylle Van der Walt told me about a couple cities in France where the quality is kind of fine already.But basically, if you’re looking at swimming in pretty much any urban waterway, you’re gonna wanna take precautions. And that includes checking the recent bacterial accounts for the water, if that information is available—a lot of cities do make it available. And just, when in doubt, do not swim after heavy rains, especially if the city has combined sewer and stormwater systems.Feltman: Sure, makes sense.Parshall: Yeah, also—whether or not the Olympians are actually able to swim in the Seine, at this point, you know, it’s a little bit of an act of God, but Paris’ mayor, Anne Hidalgo, and other Siene cleanup proponents, they really emphasize that this project will benefit Parisians beyond the Olympics, regardless of whether or not the Olympians are able to swim.So the government of Paris has said that there will be three public swimming sites that will be open for the summer of 2025. That’s next summer.Feltman: Yeah, no, that’s a really good point. And, you know, so much Olympics infrastructure, like, ends up not being useful to cities, you know, kind of infamously, after the fact, but it’s true that a clean, swimmable river is something that is gonna be really impactful.Parshall: Yeah, there’s a lot of kind of schadenfreude surrounding this whole situation, I think, just because the Olympics can be so fraught: There’s so much money involved. There’s livelihoods involved. There’s—France is in the midst of a difficult political time. It makes sense that people are so—have so—focused in on this as kind of a, a symbol of the success of the Olympics.But putting all of that aside, it, it seems like an uncomplicated good to me to be able to say, “Hey, this river is swimmable now,” or at least it’s a lot closer to swimmable, or even if it isn’t swimmable, there is, you know, 50 Olympic swimming pool worth of raw sewage that is, in the worst-case scenario …Feltman: No longer in the river.Parshall: Not being dumped into the river.I think, you know, if, if the Olympics has to be the excuse for them to spend €90 million on it—which, again, like, in comparison, Indianapolis spent $2 billion—so it, it can be, it can be a really fraught topic, but at the same time, hey, clean water—seems like a good thing.Feltman: [Laughs] It’s true. We all love our water to not be full of poop.So, listeners, what’s your take: Would you go swimming in the Seine? Let us know at ScienceQuickly@sciam.com. While you’re there, feel free to share any feedback you have for us or any suggestions you have for topics we should cover. And if you have a second, it would also be great if you could give us a quick rating and review wherever you are listening to this podcast right now.Science Quickly is produced by me, Rachel Feltman, along with Fonda Mwangi, Kelso Harper, Madison Goldberg and Jeff DelViscio. This episode was reported and co-hosted by Allison Parshall. Madison Goldberg and Anaissa Ruiz Tejada edit our show, with fact-checking from Shayna Posses and Aaron Shattuck. Our theme music was composed by Dominic Smith. Subscribe to Scientific American for more up-to-date and in-depth science news.For Science Quickly, I’m Rachel Feltman. Have a great weekend!

The Seine will be the stage for the Paris 2024 Olympics’ Opening Ceremony—and for its marathon swimming events. But this urban waterway is challenging to keep clean.