Swimming in Urban Waters: Advice from Baltimore to Paris
Over 150 people jumped in to the harbor for this year’s Harbor Splash event. Photo: Waterfront Partnership
Swimming in urban waterways is certainly having a moment. The Paris Olympics featured swimming in the Seine and here in Baltimore Waterfront Partnership hosted a swim in the Inner Harbor this past June. It makes sense. In many post-industrial cities, former working waterfronts have been transformed into condos, offices, and parks. Waterfront residents and tourists now look to the water as a potential amenity – one that can provide exercise, adventure, and relaxation – without leaving downtown. As climate change continues to warm our cities, residents are also increasingly looking for ways to beat the heat, and a jump in a local waterway has the potential to provide just that.
Waterfront Partnership has long admired Paris’ ambitious plan to make the Seine swimmable in time for the Olympics and we were disappointed to read reports of swimmers getting sick. By now we have all heard the stories. At least eight Olympic athletes have reported falling ill after competing in the Seine. Sweden’s Victor Johansson, Belgium’s Claire Michel, Ireland’s Daniel Wiffen, two swimmers from Portugal, and three from Germany. A disheartening result after the city spent $1.5 billion on sewer system repairs and improvements.
The Olympic Triathlon was first cancelled after failing to meet water quality standards.
Photo: Valentine Chapuis / Getty Images
Over the last decade, Baltimore has spent a comparable amount ($1.6 billion) on sewer system upgrades, but with a significantly different outcome. This past June, ultra-marathon swimmer Katie Pumphrey completed a 24-mile swim from the Bay Bridge to the Inner Harbor and 150 people participated in a public plunge from a floating dock in Fells Point. Both swim events went off without a hitch and, most importantly, nobody got sick. So, how did Baltimore do what Paris seemingly could not?
Swimmers enjoying the water at Harbor Splash. Photo: Mollye Miller / Waterfront Partnership
There are likely as many differences between swimming in the Seine and swimming in the Baltimore Harbor as there are similarities. When deciding to host Inner Harbor swim events, however, there were a handful of boxes that Baltimore needed to check. So, we’ve made this handy checklist as a way of sharing what we have learned with Paris or any city that aspires to make its urban waterways swimmable.
1. Use appropriate water monitoring standards
Waterfront Partnership collects water samples 5 days a week and tests them against the EPA’s recreational water quality criteria. Photo: Waterfront Partnership
In both Baltimore and Paris water samples were taken daily at multiple locations and analyzed by professional laboratories; however, the results were interpreted quite differently. World Aquatics, the governing body for Olympic watersports, sets its own water quality standard for swimming (1,000 cfu/100 mL for E. coli), which is more than four times higher than the standard set by the state of Maryland for swimming beaches (235 cfu/100 mL for E. coli). At no time during the Olympics did water monitoring in the Seine meet the more conservative Maryland swimming standard. In other words, if Baltimore Harbor had gotten the Seine’s water quality results, we would not have gone for a swim.
The inconvenient truth about swimming in any open water is that it is a science of risk. Organisms that can make you sick are always in our waterways, especially in urban, highly developed areas. Because Olympic athletes are by definition in great shape, the assumption is that they will be fine swimming in water that is more polluted than we would recommend for the public. Unfortunately, while being young and fit may mean an illness from the Seine won’t be fatal, it doesn’t mean you won’t get sick.
2. Don’t swim within 48 hours of rainfall
If there is one universal rule that can be applied to all open water swimming it is this – don’t swim if it has rained within the past 48 hours. This is especially true in cities where rain may cause sewers to overflow and stormwater runoff carries pollution from our streets and alleys into our waterways. Thankfully, the organisms that can make us sick don't live for very long in open water. They settle into sediment, are killed by the sun, or are eaten by other organisms, typically within two days' time.
Of course, you can’t reschedule the Olympics and unfortunately Paris has been experiencing an unusually rainy year. Of the five swim events in the Seine, four took place within 48 hours of a rain event. In Baltimore, we were lucky to have an extremely dry June with less than half an inch of rain in the weeks leading up to our swim events and no rain within 48 hours prior.
Photo: Mollye Miller/ Waterfront Partnership
3. Get predictive about water quality
One of the challenges with monitoring for bacteria is that samples require 24 hours to incubate before results can be read. This presents a real challenge because swimmers want to know what the water quality is at the time of swimming, not 24 hours later. That’s where predictability comes into play. Swimmers need to know when the risk of becoming sick from swimming is low and when it is high. If you can’t reliably predict when a waterway will meet water quality standards, it may not be ready for swimming.
In Baltimore, Waterfront Partnership had been testing the Harbor weekly (May-September) since 2020, but it wasn’t until switching to daily testing in 2023 that we realized how predictable water contact advisories had become. At the location for Harbor Splash, 96% of water contact advisories were predictably tied to rain within the prior 48 hours. Throughout Baltimore Harbor, 93% of water contact advisories were associated with rainfall. These findings gave us the confidence to host swim events, even though we would have to wait 24 hours for sampling results.
In Paris, two major factors have made it very difficult to predict changes in water quality. First, as previously stated, Paris is experiencing one of its rainiest years on record. Second, new infrastructure meant to help clean up the Seine only came online in May, meaning there was little time to understand how it would impact water quality before the opening of the Olympic games. Together, these two things made it nearly impossible to use water quality data from previous years to predict water quality during the Olympics.
4. Build giant storage tanks to contain sewage overflows
One thing that both Paris and Baltimore got right was the construction of giant storage tanks to capture sewage mixed with rainwater and prevent it from overflowing into public waterways. In Paris, officials opened the Austerlitz water tank, a 13.2-million-gallon cistern, less than three months before the opening of the Olympic games. Officials left themselves far too little time to test and confirm that the system was working as designed. Given the age and complexity of the Paris sewer system, it is likely additional large infrastructure fixes are needed.
Compounding the issue in Paris, the city’s sanitary sewer is combined with its stormwater system. That means that every storm drain in Paris sends rainwater into the sanitary sewer pipes. When it rains, this combined system is designed to overflow into local waterways. While many US cities also have combined systems, Baltimore isn’t one of them. And, because the Baltimore system isn’t designed to overflow into local waterways it is, at least in theory, easier to fix when it does.
Large sewer overflows in Baltimore are mainly caused by rainwater infiltrating into the sewer system through leaks, cracks, and other flaws in the 100-year old system. To reduce these overflows, Baltimore installed two 18-million-gallon storage tanks and a new 800-million-gallon-a-day pumping station at the City’s Back River Wastewater Treatment Plant in 2021. Both city government and NGOs spent the next three years assessing the effectiveness of these upgrades and sewer overflows have decreased by 76% since 2019.
Photo: Mollye Miller/ Waterfront Partnership
Conclusions
The public’s desire to use urban waterways for recreation is still a relatively new trend. As recently as 25 years ago nobody was talking about swimming in Baltimore Harbor. Today, residents expect to be able to use urban waterways to connect with the natural environment or cool down on hot summer days. While a truly swimmable harbor is a goal worth pursuing, it will take time and money for our aging infrastructure to fully meet these new expectations. In the meantime, cities like Baltimore and Paris exist in a gray zone where swimming is possible, but only if appropriate precautions are taken.
Unfortunately, for Paris, the $1.5 billion effort to make the Seine swimmable appears to have been undersized given the scope of the problem and the age of the city’s infrastructure. While rain certainly played a critical role in impairing water quality throughout the Olympics, the use of a comparatively high threshold for E. coli combined with an unpredictable river resulted in at least eight swimmers getting sick.
Without the pressures of the Olympic games, Baltimore was able to take a far more cautious approach to planning a swim in the Harbor. We were able to monitor both water quality and weather conditions and then select swim dates with the greatest chance of success. While we all had hoped for a better outcome at the Paris Olympics, we are confident that the Seine is moving in the right direction and look forward to one day visiting Paris to jump in ourselves.
To learn more about how we monitor water quality in the harbor, visit our website here.
