Swimming in the Seine: The Olympics and Water Safety

Will they or won't they? This was the question at the Olympics in Paris, France this past week as swimmers and fans waited to hear whether the famously polluted Seine River would be clean enough to hold races safely. The verdict? A hard... maybe? The Olympics served as a goal post for France, which pledged to clean up the Seine upon being selected to host the Games back in 2015. $1.5 Billion later, the Seine has seen great improvement, but bacterial contamination continues to plague the busy waterway 7 out of 10 days, on average. In today's post, we'll learn how this waterway became so contaminated, and why bacterial contamination can be so difficult to eradicate in urban riverways.

Rivers have long served as a cradle of civilization, from the Tigris and Euphrates to the Nile to the Mississippi, providing a source of continuous fresh water and food. Trade, agriculture, and industry flourished on riverbanks, leading to the greatest cities of antiquity and today. As cities grew and became more densely populated, however, the rivers took on yet another responsibility: A place to dump waste (both human and industrial). When transportation, agriculture, industry and population skyrocketed during the 1800s, once idyllic city rivers became cesspools of filth and disease, no longer safe as a source of fresh water or food. The results can be seen globally: The Sarno in Italy, the Citarum in Indonesia, the Ganges in India, the Passaic in the US, and the Matanza in Argentina show just how widespread this type of pollution has become. In some cases, the pollution is literal trash dumping - plastic, chemicals, refuse, and other solid objects. In other cases, the pollution is more insidious and difficult to eradicate: The dumping of human waste and its accompanying bacterial load.

The Seine followed this same path towards becoming a "dead" river that has been unsafe for swimming, let alone drinking, for over 100 years. As Paris grew, the city had to contend with two major water concerns: Sourcing fresh water and dumping waste. As with many cities across time and geography, the decision was made to source fresh water from upstream, bringing it in from distant regions via aqueducts at first, and later pipes. Waste, including human, agricultural and industrial, would be dumped directly into the Seine to be carried away downstream. As the centuries passed and technologies improved, efforts were made to divert waste into flat lands where it could be converted into fertilizer, or diverted away from the city (and dumped elsewhere), and within the last century, water treatment plants were established to treat the most contaminated water before returning it to the environment. 

What the clean-up efforts have to contend with today is the result of those initial improvements, a combined sewer system, where wastewater and storm water (rain runoff) use the same pipes, with a diversion in place for dumping rainwater into the Seine. However, during times of heavy rain, there is too much water for the pipes, so they back up, releasing not just excess rainwater but also the waste water into the Seine. Look at this illustration from the Environmental Protection Agency, which explains how this happens (POTW means "publicly owned treatment works", that is, water treatment plants.)

So how did Paris finally begin the reversal process of decades of intense pollution? Efforts in the past had failed, notably one started in the 1990s that fizzled out when its dynamic promoter, former President Jacques Chirac, passed away. What spurred this effort was the promise to the Olympics Committee that, should Paris be selected for the 2024 games, the Seine would be cleaned up and swimmable. Plans began in 2015 that focused on the creation of a massive underground rainwater storage tank (a cistern), which would take the pressure off of the city pipes and prevent wastewater from being dumped into the river.

The tank was completed earlier this year - but did it work? Water tests would say yes, for the most part. On average, the contamination levels of the Seine are safe for swimming 7 out of 10 days, with far fewer safe days when there is prolonged heavy rain. This, combined with upgraded sewer systems and an initiative to get upriver residences to divert waste to public systems rather than dumping into the river have made an impact, as do stricter regulation and surveillance of upriver industries, especially agriculture. 

So with all these efforts, especially the limit of human waste dumping, why is bacterial contamination so difficult to return to safe levels? One reason is the formation of biofilms, dense concentrations of bacteria that form a layer on river beds (as well as on hospital surfaces, by the way). These films are extremely difficult to eradicate as they have formed to survive in the niche of a highly polluted, anaerobic river. Which brings us to the second reason: A highly polluted river becomes anaerobic, that is, lacking in the free oxygen required by fish and other marine life. There are bacteria that flourish in this type of environment, and even if they are depleted by cleaning efforts, one good rain can re-introduce the bacteria and levels rise once again. 

This was the case this past week in Paris, when contamination levels were low enough to host a few events in the Seine - only to see them rise again. (Some of those swimmers have since become sick, with one even hospitalized with a bacterial infection.) Hospital rooms face this same dilemma - after terminal cleaning, the levels of bioburden - surface contamination - immediately start to rise again as bacteria, which are always present, colonize surfaces and reproduce. This bioburden quickly leaves the benign zone and enters levels sufficient to lead to disease transmission, putting vulnerable patients at risk. Thankfully, biocidal surfaces (like EOScu) in hospitals can help keep bioburden levels in the benign zone between cleanings - now we just have to figure out how to do the same in our city rivers!