This weekend is Harbor Splash. What does the data show about water safety?

This Sunday, Baltimore Mayor Brandon Scott, Maryland Comptroller Brooke Lierman and a group of 150 registrants will do something once considered revolting: jump into the Inner Harbor.

The event, called Harbor Splash, is a milestone for a 14-year campaign that originally aimed to make Baltimore’s waterfront fishable and swimmable by 2020, led by the city’s Waterfront Partnership.

It comes after the Partnership collected data at five sites in the Harbor five days of the week from April to October last year, and found that the bacteria levels were safe for swimming the vast majority of the time — as long as it hadn’t rained over the prior two days, sending pollutants from the land into the water.

Yet, on Tuesday, another local nonprofit Blue Water Baltimore released its annual report card for the health of Baltimore’s waterways, handing the Inner Harbor its latest in a long line of failing “F” grades.

The two data sets may seem contradictory, but that letter grade doesn’t factor in bacteria levels — the most important indicator for swimming safety. Instead, it is focused on other measures of ecosystem health, such as dissolved oxygen, water clarity and nutrient levels.

“The health of the water from an ecosystem perspective is really about the animals that don’t have a choice but to be in the water,” said Eric Schott, an associate research professor at the University of Maryland Center for Environmental Science, who has worked alongside the Waterfront Partnership. “They can actually survive very high fecal coliform levels pretty happily, as long as they have those ecosystem factors working.”

Blue Water does monitor for bacteria levels at each of its 49 monitoring stations, which are tested once per week, no matter the weather. And in 2023, though some of Blue Water’s stations in the region were still struggling with poor bacteria levels, some stations in the Inner Harbor saw their best scores since monitoring began a decade ago.

Notably, the monitoring station in the harbor’s northwest portion, across from Locust Point, was safe for swimming 100% of the times it was sampled last year.

That was “big news,” said Alice Volpitta, Blue Water’s Baltimore Harbor Waterkeeper.

Even the Inner Harbor station that is typically the most heavily polluted, right where the Jones Falls stream meets the Patapsco River, was considered safe for swimming 69% of the time. Other stations were safe between 60% and 80% of the time, irrespective of the weather.

“Obviously that means that’s not good enough and we’re not there yet,” Volpitta said. “We haven’t reached the finish line. But that’s really good news.”

Still, Volpitta said she won’t be taking the plunge this weekend.

Having served as the city’s waterkeeper for years, she’s seen the good, bad and ugly, tracking fish kills, sewage overflows and treatment issues at the city’s two wastewater plants. Bacteria levels are “extremely volatile” from day to day, she said. And even during dry weather, the water’s safety isn’t guaranteed.

Wet weather is well-known to overwhelm Baltimore’s aged network of sewage pipes, causing backups into residents’ homes and overflows into bodies of water. Under a consent decree with the Environmental Protection Agency, the city has shuttered many overflow points and worked to shore up the system, causing the number of overflows to plummet. But the events still occur, and they can occur even during dry weather, due to other blockages or breaks in the pipes.

That’s why it’s important for Marylanders to have the most up-to-date data at their fingertips before swimming in an urban waterway, rather than mere annual averages, Volpitta said.

“This is just part of the nature of being in an urban ecosystem. We do have to recognize the realities of living with this old, leaky infrastructure under our feet,” she said. “It can be unpredictable.”

Since the Waterfront Partnership announced Harbor Splash, it has seen the full spectrum of responses, said Adam Lindquist, the nonprofit’s vice president.

For one thing, with only 150 slots available to the general public, the event sold out in 10 minutes. Now, the waitlist includes about 1,000 people, Lindquist said.

“I have people emailing me every day,” Lindquist said. “They’ve all got their reasons for why we should let them into this event.”

One person said they completed a swim in the Arctic Circle last month, and were desperate to swim in the Inner Harbor event next. Another said they had 50 friends and family members coming to watch them swim — but they failed to get a ticket.

But then there’s the other side of that spectrum: The people who argue there shouldn’t be a swimming event at all — and that those who participate will regret it.

“I don’t know how to engage those directly, because often they’re not based on a reading of what is there,” Schott said. “It’s more an approach of: The harbor has historically been an unsafe place to swim or recreate — but it isn’t anymore.”

Lindquist said he hopes the swimming event shows the level of demand for recreation on Baltimore’s waterfront.

“It’s not a victory lap,” he said. “We’re certainly celebrating the progress that has been made, but we’re also hoping that people will see the potential of an even healthier Baltimore harbor.”

Staffers at the Waterfront Partnership will be sampling the water Sunday at Bond Street Wharf, and if it fails, the event will be postponed, Lindquist said. Similarly, the event will be rescheduled if it rains Saturday. There is a 20% of thunderstorms in Baltimore after 2 p.m., according to the National Weather Service.

On Wednesday, Lindquist himself jumped into the Harbor at Bond Street Wharf to test the waters, and check on the swimming ladders. On a hot day, the water was the perfect escape, he said.

“It was hard to get out,” he joked.