Why Can You Swim in the Seine but not the Potomac?

"The Cleanup Plan: In 1990, Paris's mayor set a goal to open the Seine to swimmers. Efforts picked up in 2015, when the city announced that the river would be swimmable for the 2024 Olympics. Pipes and wastewater-treatment plants were updated, and an underground storage basin was built to help prevent overflows. The city incentivized houses and houseboats to connect to the city's sewage plant instead of dumping into the water."

"Why It's Polluted: DC's stormwater system washes water runoff (along with trash and chemicals that get caught in it) into tributaries leading into the Potomac. Most of the city's river pollution comes from these small waterways, though heavy rain also sometimes causes overflow in sewer systems that combine wastewater and stormwater. The Cleanup Plan: DC's efforts have been more piecemeal."

Paris's Seine accumulated pollution from human and animal waste, boats, and debris, compounded by a combined sewer system that sent about 60 percent of the city's sewage into the river by the late 1960s. Swimming was banned in 1923. Starting in 1990 and accelerated in 2015, the city upgraded pipes and treatment plants, built an underground storage basin, and incentivized connections to sewage plants, spending an estimated $1.5 billion; the Seine was used in the 2024 Olympics and public swim areas opened despite some elevated bacteria. Washington, DC's pollution primarily stems from stormwater runoff into Potomac tributaries and occasional combined-sewer overflows. Swimming was banned in 1971. Cleanup has been community-driven and municipal, and the 2011 Clean Rivers Project began building large overflow tunnels, with one tunnel completed and ongoing work to reduce overflows.

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