Swimming has been off-limits in the long-polluted Seine River in Paris for more than a century. So with Olympic swimming events on tap for the river, the city poured in $1.5 billion (1.4 billion euros) to try to clean it up.

With the Paris Games underway, officials are keeping a close eye on water quality. Athletes could feel health effects from swimming in a river with higher-than-accepted levels of E. coli or other bacteria.

Here are key things to know:

What made the Seine so dirty?

Paris, like many old cities around the world, has a combined sewer system, which means that the city’s wastewater and stormwater flow through the same pipes. With heavy or prolonged periods of rain, the pipes can…