This city sits next to a sewage plant, but seaweed may be what’s closing its beaches

There’s a sewage treatment plant that pumps treated waste a few miles off the coast from Key Biscayne, but new research hints a rash of beach closures may be tied to a more natural problem: seaweed.

With health warnings about swimming off its beaches on the rise, Key Biscayne commissioned a University of Miami study to diagnose what’s causing the bacteria contamination often linked with unhealthy levels of feces in the water. While preliminary, the November report notes tests for human and animal waste in the water mostly produced unremarkable results but high amounts of sargassum on the beach seemed to be an active source of the same bacteria flagged in the fecal tests.

“The moist sand and decaying seaweed potentially create an environment for the bacteria to persist and grow,” read the report from a research team led by Helena Solo-Gabriele, a UM professor of environmental engineering. The study found sargassum had particularly high bacteria readings over spring and summer when researchers noted a “foul odor” in May and “flies were observed around the seaweed in July.”

The report doesn’t offer any specific conclusions on what’s behind water problems off Key Biscayne beaches, an area that Miami-Dade’s Water and Sewer Department says is a main reason why 2019 was the worst year for swim advisories in nearly two decades.

The nine-month UM study isn’t the final report, and the research is focusing on three potential culprits for bacteria: seaweed, human and animal waste pushed into the sea by tide and storm water runoff, and the Central District Wastewater Treatment Plant, a county facility on Virginia Key.

“We want to get to the bottom of it,” said Mike Davey, Key Biscayne’s newly elected mayor and a resident since 2003. He said the affluent village, located in one of Miami-Dade’s priciest real estate markets, is eager to tackle the water-quality issue if research confirms steps the local government can take. “With beach closings, people have a right to be upset.”

A December analysis by the county’s Water and Sewer Department said about half of Miami-Dade’s swim advisories stem from waters off or near Key Biscayne.

While beach advisories are up, sewage spills are actually down throughout Miami-Dade, according to the county report. In 2018, the county reported 94 sewage “overflows” compared to 258 in 2008. Since 2004, three of the five lowest years in terms of spills occurred between 2015 and 2018.

“Last year was the best year on record for the Water and Sewer Department in the last 15 years for having the least amount of spills,” department director Kevin Lynskey told county commissioners at a Dec. 3 committee hearing. “And it was the worst year in 20 years for beach closures.”

Though preliminary, the UM study found no link between Key Biscayne’s beach problems and a nearby county sewage facility. The plant on Virginia Key, the island between Key Biscayne and the mainland, treats raw sewage and then discharges treated sewage through an underground pipe nearly four miles into the Atlantic Ocean.

Spills and overflows at the plant have caused “no contact” swim advisories in Key Biscayne and beyond, including one in October the county said was tied to a power failure at the facility.

Using reported spill and discharge data, the UM study found “no significant correlations” between recorded spills at the plant and swim advisories for Key Biscayne beaches. Still, that’s only for reported data from the plant itself, and would not take into account malfunctions like the ruptured discharge pipe off Key Biscayne that a diver for the watchdog group Miami Waterkeeper discovered was leaking unnoticed in 2017.

“They’re studying reported data,” Rachel Silverstein, executive director of the group, said of the UM report. “The county doesn’t always know when there’s a leak or a spill.”

Florida’s Health Department issues no-contact advisories for a body of water when bacteria counts associated with feces reach a certain level in water testing. Those advisories typically are referred to as beach “closures” because people are warned that swimming that day can lead to health issues.

The December county report listed 39 beach advisories for Miami-Dade in 2019 due to poor water quality tracked by Florida’s Department of Health. That’s the highest since at least 2001, the earliest year in the county report. The second-highest year was in 2018 with 21 advisories, and 2017 finished fourth with 16.

The spike in beach closures overlaps with an inundation of sargassum on Southeast Florida beaches over the last several years. The spike in seaweed arrivals was tied to an explosion of sargassum growth in the Atlantic Ocean, a trend some scientists link to increased nutrient use along the Amazon River.

Key Biscayne, which maintains its own beaches, buries most of its sargassum under the sand in an effort to keep the areas more pristine. With sargassum linked to bacteria production, the village plans to end that practice. “If that’s the issue, we’re going to take it off the beach,” Davey said. “We’re already talking to the city of Miami about composting” the seaweed.

At a Nov. 5 forum in the Village Council chambers to address beach advisories and potential causes, Solo-Gabriele said Key Biscayne generally shouldn’t be alarmed at a natural flow of sargassum onto the sands. But the recent inundation requires some action.

“Seaweed is a natural thing. It’s part of the ecosystem. You want it there,” she said. “But when you have seaweed levels that are highly excessive .... something else has to be done to minimize the impact.”

Along with removing sargassum, the village of about 13,000 people also hopes 2020 will close out another potential culprit in water-quality issues, as the remaining 200 households still using septic tanks on the island face a July 1 deadline to hook up to the municipal sewer system.

For now, village leaders aren’t ready to express any confidence that they know seaweed, septic tanks or the sewage plant is the cause of Key Biscayne’s beach woes.

“One thing is clear to me: We need more data,” said village manager Andrea Agha. “Right now the village has at least half a dozen working theories.”

This story was originally published January 2, 2020 at 6:00 AM.