Jeremy Siegel: You’re listening to GBH’s Morning Edition. Just ahead of the Olympic opening ceremonies, the mayor of Paris, surrounded by applause and a field of journalists, took a plunge into the River Seine to celebrate cleanup efforts and prove that the water quality in the river was safe for Olympic events.

Paris Mayor Anne Hidalgo: It’s a very lucky and happy day. It’s also for the planet, you know, and for the river and for the ocean. We did it. We did it.

Siegel: But on Tuesday, officials postponed the triathlon because it failed water quality tests. At the last minute, athletes ended up swimming yesterday, but many complained of nasty water and bad smells. One threw up several times. For more on what’s behind dirty water, we have GBH meteorologist Dave Epstein here with his weekly science deep dive. Good morning Dave.

Dave Epstein: Good morning. No pun intended with the deep dive.

Siegel: I didn’t even think about that. Accidental pun. So why is the river so dirty in the first place, Dave?

Epstein: Yeah, I mean, the Seine River, you know, in some ways similar to Charles, has been, you know, a repository for sewage for many years. So that river has not been swimmable for 100 years. So for a century. Paris, France invested roughly something like $1.5 billion to clean it up. And it’s to, you know, get all those connected sewer systems that are kind of just haphazardly connected and dumping a lot of that water into the river. And, you know, just like they started doing here back in the mid '90s, the EPA, the state government, local governments, a lot of NGOs all got together to clean up the Charles. And we certainly have a much cleaner river. Some folks might even remember pictures of [Gov.] Bill Weld jumping in there back in the mid '90s. But what happens is, even with the clean up, when you get these heavy rain events — and Paris did have some heavier rain, I just looked at the Euro model. It’s fun to use the Euro model for actually parts of Europe. But you know, it’s looking at the Euro model. They’re not going to have much rain today, but when they get the heavy rain, it overwhelms even a new system. And so the system can only be built for so much. And when it gets overwhelmed, you start putting in bacteria. And now the river is not as clean as it was. And so it gets put on hold, as we saw happened yesterday.

Siegel: I want to go back to the comparison you made with the Charles here in Boston. How effective have those cleanup efforts been that started back in 1995?

Epstein: Yeah. It’s tremendous. I mean, you know, the the song "Dirty Water," of course, from from way back and, it’s no longer really that dirty of a river. The Charles River gets routinely graded, I think, in the B range. And we have many more days where it’s fishable and swimmable now. But that said, even the Charles can still get overwhelmed when we get these heavy precipitation events. And of course, we’ve seen this, you know, this year. Look at Vermont just a couple of days ago, another one of these crazy heavy precipitation events. And you can build a system to handle these sort of outlier events, but then it becomes so cost prohibitive and I guess, you know, and Karen, our producer, she and I were talking this morning before the segment: What is enough? And that’s a societal question, because there’s a cost to managing a system for what’s going to be routine high precipitation events. But, you know, what’s acceptable loss, what’s acceptable for the river to be shut down for, you know, is it three, four, or five days? You’d argue, well, if it’s going to be shut down for the entire summer, we need to build a better system. But if it’s only going to be shut down for a week, maybe the system’s good enough.

Siegel: Before I let you go, Dave, quickly, I wanted to ask: What are the problems if nobody is swimming in the Charles? Like if we didn’t have Olympic events going on in the Seine, is there a problem in the first place with there being dirty water?

Epstein: Well, I mean, it’s acceptable, right? The water was dirty for decades and decades and society functioned, right? Society went, oh, Boston was fine, right? People moved around. But it’s like, what type of society do we want to live in? Do you want to live in a great resource like that that’s not usable? So that upper watershed being graded, you know, B-plus, in some cases, you know, even as high as an A seems to me, and this is an opinion, it’s a much better place to live in a city like Boston, where you have this amazingly beautiful resource that you can actually use instead of something that you look at and go, it’s too dirty to go in there. Oh, it’s pretty to look at the water going by, but don’t get too close. So I think that it just improves the lives of everybody that can participate in a place like that. That’s why it’s important to clean it up.

Siegel: GBH Meteorologist Dave Epstein, thanks for diving into this with me.

Epstein: You’re welcome.

Siegel: This is GBH news.

Meteorologist Dave Epstein is our go-to person for pressing weather questions on everything from winter blizzards to summer droughts. He’s also a horticulturist, meaning he’s an expert in anything that grows leaves and flowers. GBH’s Morning Edition asked our audience for weather and gardening questions, and Epstein graciously answered them on the air.

Have a gardening or weather question for meteorologist Dave Epstein? Email us at    thewakeup@wgbh.org or text 617-300-2008.

Just ahead of the Olympic opening ceremonies, Anne Hidalgo, the mayor of Paris, took a plunge into the River Seine to celebrate cleanup efforts and prove that the water quality in the river was safe for Olympic events. She was surrounded by applause and a field of journalists.

But on Tuesday, officials postponed the triathlon because it failed water quality tests. At the last minute, athletes did swim yesterday, but many complained of nasty water and bad smells.

So what is going on with river water quality?

“The Seine River, in some ways similar to Charles, has been a repository for sewage for many years,” GBH meteorologist Dave Epstein said. “That river has not been swimmable for 100 years.”

To prepare for the Olympics, France spent 10 years and €1.6 billion to revamp the local sewage systems, building a large basin and connecting disparate pipes.

“Just like they started doing here back in the mid '90s,” Epstein said. “The EPA, the state government, local governments, a lot of NGOs all got together to clean up the Charles. And we certainly have a much cleaner river.”

But even when sewage systems can handle all the flow coming from homes and businesses, heavy rain can overwhelm them.

“The system can only be built for so much,” Epstein said. “And when it gets overwhelmed, you start putting in bacteria. And now the river is not as clean as it was. And so it [Olympic events] gets put on hold, as we saw happened yesterday.”

Since 1995, cities and towns around the Charles River have also changed their sewage systems to minimize the amount of septic flow into the river. It’s made the river much cleaner — safe to fish in on many days, and even usually safe to swim in. Still, there are no designated swimming areas with lifeguards along its banks, except for one day a year.

Like in Paris, heavy rain can overwhelm sewage systems and make them overflow into the Charles, bringing unwanted bacteria and pollution.

“You can build a system to handle these sort of outlier events, but then it becomes so cost prohibitive,” Epstein said. “What is enough? And that’s a societal question, because there’s a cost to managing a system for what’s going to be routine high precipitation events.”

Society has to decide: Do we want to invest in a system that keeps the Charles swimmable more often?

“The water was dirty for decades and decades and society functioned, right?” Epstein said. “People moved around. But it’s like, what type of society do we want to live in? Do you want to live in a great resource like that that’s not usable?”

Epstein, for one, knows where he stands.

“It seems to me, and this is an opinion, it’s a much better place to live in a city like Boston where you have this amazingly beautiful resource that you can actually use instead of something that you look at and go, it’s too dirty to go in there,” he said.