Is it safe to dye the Hillsborough River green? Here’s what we know.

For twelve years, the city of Tampa has soaked hundreds of pounds of yellowish powder and sprayed the resulting electric green liquid into the Hillsborough River, all in the name of St. Patrick’s Day.

The city’s water experts, the county’s environmental regulators and the company that concocts the kelly green mixture all maintain it’s a food-grade dye with no risk to human or environmental health. The river flushes out soon after the event finishes, they say, and it’s such a small concentration that it hardly makes a difference in a waterway that ebbs and flows with the tides.

But anglers who rely on the river for their living say dyeing the river green could be causing ecological harm and it sends the wrong message about how we treat our natural resources. They point to a document from dye-maker Kingscote Chemicals, reviewed by the Tampa Bay Times, that says the dye should be prevented from entering into soil ditches, waterways and groundwater.

“This product is not classified as environmentally hazardous,” the company’s safety data sheet says about its product, called Bright Dyes Yellow Green liquid. “However, this does not exclude the possibility that large or frequent spills can have a harmful effect on the environment.”

That document refers to large amounts of highly concentrated dye, and it’s not uncommon for chemical sheets to have warnings about using high concentrations, according to Chris Pratt, a senior environmental manager at the Environmental Protection Commission of Hillsborough County. The concentration of dye entering the river is less than 1 milligram per liter, Pratt noted, which is considered safe and non-toxic.

High quantities of most chemicals can become dangerous to a waterbody like the Hillsborough River if they’re densely concentrated, environment experts told the Times. They pointed to sugar, the fluoride in our drinking water and sulfuric acid as examples.

“If you had a ton of this stuff fall off the back of a truck, you’d want to clean it up as best as you can,” Pratt said. The same type of dye is used by researchers who want to track the flow of water, including aquifer groundwater. The concentration and the duration of the event are small, he added. “If the city was trying to dye the river green year-round, that would be a concern.”

Still, cleaning up spilled dye at high concentrations comes with its share of safety concerns, according to the document from Ohio-based Kingscote Chemicals. Protective gear should be worn to prevent eye and skin irritation, according to the safety sheet.

Environmental advocates, including area anglers, have raised concerns about the dyeing of the river in the past. In 2016, a citizen complained to the regional office of the U.S. Environmental Protection Agency that the dye was hurting marine life, emails from the city’s water department show. Last year, a petition logged more than 2,000 signatures and was backed by big-name Florida angling organizations such as Captains for Clean Water and Florida Sportsman magazine.

This year, a petition is again circulating that asks the city to stop dyeing the river green.

Boat Captain Dustin Pack started the petition that, as of Tuesday, had more than 6,100 signatures. He said that the dye lingered last year for at least 48 hours in Tampa’s waterways. He and other environmental advocates took photos and video of parts of the Hillsborough River in downtown, along with the Garrison Channel in the Port of Tampa the day after the St. Patrick’s Day event and the water was still bright green. He shared the photos with the Times.

Hundreds of animal species rely on the 60-mile Hillsborough River for shelter and sustenance, from dolphins and manatees to wood storks and snowy egrets, according to the Southwest Florida Water Management District. The Florida Department of Environmental Protection considers the river “impaired,” which means it’s a waterbody in need of restoration.

Before last year’s event, a spokesperson with the Environmental Protection Commission told the Times that the dye dissipates in a few hours.

When asked about the images Pack took of the river two days after the event, which appear to depict a bright green tint from the dye, John Ring, the City of Tampa’s Water Production Manager, said the chemical can remain in the water for several days after it’s applied. It takes time for it to mix with oxygen as it basks in daylight, but it’s not densely concentrated.

“It’s not going to just disappear, but it’s not going to look like it did when we applied it, either,” Ring said.

Crews this year will mix about 250 pounds of the yellowy powder into roughly 300 gallons of water, Ring said. That water will then be spread into the river at about a gallon of mixture per minute. The annual tradition began in 2012 when Bob Buckhorn was mayor and back then the city used 80 pounds of the dye.

“I understand the concerns from fishermen, and how they may see this as a risk. I probably can’t make them comfortable with it, but it is safe,” Ring said. “We wouldn’t be putting this into the environment if we thought this would have a detrimental effect.”

Still, Pack said, the optics of dying the river aren’t good for preserving Florida’s nature as it is.

“Showing kids that we can treat the river however we want, that’s just not a good idea.”