The Seine River Was Swimmable … Today

Last night, in Paris, thunderstorms threatened to pummel the Seine. The men’s triathlon had already been postponed after levels of bacteria in the river measured high, and more rain would have made it worse. Downpours can sweep trash and grime into the river and overwhelm treatment plants as well as old combined pipes where stormwater mingles with wastewater; even the massive tanks Paris installed to keep that mess from pouring into the Seine can be bested by pounding rains. And rain did fall in the hours just before the triathletes dove into the river today. It slowed as they prepared to race, plunging goggles into the water before sealing them against their face. But samples taken yesterday from the Seine apparently passed the water-quality tests, though an independent monitoring group classified the health risks as being in a murky “gray zone” based on samples taken around the same time. Paris wanted to prove that its years-long gamble on the river has paid off, and today, the city and the athletes are flush with victory (and hopefully not E. coli).

The problem that the city faces—the very old, very tricky, very ongoing job of wrangling sewage—is not a uniquely Parisian challenge: Systems that dump waste into the river during heavy rain are common in the United States, the United Kingdom, and beyond. But Paris happens to be a place that recently made particularly loud public promises about curbing this type of pollution. The city managed to fulfill those promises, and proved that even a famously dirty river can be made swimmable—just not necessarily on any one particular day.

The Seine met its technical marks, but only by a hair. Under World Triathlon rules, for the athletes to swim, competition waters must fulfill certain laboratory criteria, and also pass a sniff test and a visual once-over. Athletes cannot jump into waters that reek of phenol or are slicked with oil, thick with birds or algal blooms, ribboned with sewage, or studded with garbage. The waters must also meet particular limits for E. coli and enterococcus, a genus of bacteria that proliferates in animals’ intestines and serves as a proxy for disease-causing bacteria that travel in feces. Some samples from the Seine failed to meet these standards—which exceed the limits recommended by the U.S. Environmental Protection Agency—during tests last month after heavy rains, and some samples collected this week revealed E. coli levels that were unacceptably high following a downpour that made for a soggy opening ceremony.

The results from additional, more recent samples aren’t yet publicly available but are reportedly coming soon. Testing takes time. Because bacteria is a living organism, the best way to measure it is to let it grow and replicate, which involves incubating a sample for about 24 hours before analyzing it, Alice Volpitta, the lead water-quality scientist at the advocacy group Blue Water Baltimore, told me. Once that sample is ready, “it’s a look into the past, but it doesn’t tell us what’s happening right now,” she said. “That matters when we’re talking about sewage ecosystems, because things can change at the drop of a hat.”

The river is probably the cleanest it’s been in centuries. Well before swimming in the Seine was prohibited about 100 years ago, the river had already earned a reputation as a fetid open sewer. As Elaine Sciolino wrote in The Seine: The River That Made Paris, in the 18th century, the water was “blood-red from animal carcasses, brown from garbage and excrement, black from tanners’ dyes and rot.” In the 19th century, Baron Haussmann and his brigade of engineers and laborers expanded the sewer network, and once-open trenches were hidden underground. But some trenches still disgorged untreated wastewater into the river. Downstream, where almost 1 million pounds of sewage entered the water each day, the water was reportedly sluggish and black; debris mounded into mud shoals that resisted dredging, as the historian Donald Reid has noted. In the 1870s, the Irish Builder, a trade journal that catered to the engineering-minded, reported that the river was laden with liquids from foundries, rubber makers, and laundries—which caused the river to bubble with 132 tons of soap each year—as well as refuse from the process of making poudrette, a fertilizer fashioned from dried excrement. Well into the 20th century, pollution was blamed for waters gone ominously fallow. Over the past several years, Paris put more than $1.5 billion into several types of improvements, including the tanks meant to catch sewer overflows, to shore up the river before the Games.

Even when they’re not facing the spotlight of a major international spectacle, places with combined sewage and rainwater pipes have been attempting to stave off spills. Some places build retention ponds or tunnels; others rely on sensors to redirect water by opening or closing gates. These tactics can slash the number of spills and the volume of sullied water they introduce, but data are patchy. In the U.S., 700 municipalities still have these combined pipes; as of two decades ago, they were disgorging 850 billion gallons of fouled water each year, according to the EPA. (The Government Accountability Office has pushed the EPA to release updated numbers by next year.)

In the meantime, the pipes are doing what they were designed to do. Overflows function as “emergency escape hatches, basically off-ramps for sewage” when a system sees a big influx of water, Volpitta said. Tamping down on them helps keep rivers cleaner, but it also creates different hazards: When combined sewers can’t spit up water as they previously did (a problem that will only worsen as water levels inch higher in a wetter, flood-prone world), pipes sometimes belch their contents into streets or basements.

One of experts’ main ideas, beyond lining, replacing, or separating pipes, for addressing these issues is maximizing spongy intercepting ponds, rain gardens, reedbeds, and other green infrastructure to hold or slow water and “minimize the shock of rainfall into the system,” says Franco Montalto, a professor of civil, architectural, and environmental engineering at Drexel University, in Philadelphia. Philadelphia incentivizes sponginess by charging for impervious surfaces; bit by bit, Montalto told me, cities could also recontour streets to promote short-term flooding of parks or athletic fields instead of roads that dump into sewers.

Urban rivers are generally healthier now than they were a few decades ago. But at the moment, “there’s no such thing as a risk-free environment in an urban waterway,” Volpitta told me. “We may see lower bacteria levels but are still exposing people to pathogens and toxins.” In New York, for instance, Hudson River Park shares daily measurements of water temperature, cloudiness, and more, and worked with several partners to produce a public-facing calendar that indicates likely risks of bacteria connected to sewer overflows. (The calendar draws on data gathered a few years ago, on rainy spring days.) Most days in the past year have been low risk; about three dozen have been high (the model doesn’t allow for zero). The goal for any river, Volpitta said, is “more swimmable days overall.” Which is, to be fair, what Paris has managed—some days when the river is swimmable.

Claire Robertson, an ecologist and the Oxford Rivers project officer at the England-based environmental charity Thames21, agrees that people belong in rivers. Vienna’s Danube, Munich’s Isar, and many rivers squiggling across the U.K. and France all have a long history of hosting swimmers. Robertson lives on a boat and swims in the Thames several times a week during the warm months—and sometimes during the colder ones too. Swimming in urban rivers is “absolutely a realistic goal,” she told me. Making a river swimmable depends on where you set the threshold and how swimmers think about individual risk tolerance—but planning a dunk for a specific day is tough. Good-enough water quality requires planning, funding, patience, and humility in the face of rain—which is unstoppable and utterly indifferent to medal counts.