Wildlife was thriving, so six years ago Largo officials turned an old landfill into a nature preserve where people could watch otters, birds, snakes, mama alligators and their hatchlings.

The new Largo Central Park Nature Preserve, they hoped, would provide a forum for environmental education.

Ironically, what city officials are studying out there now involves arsenic in the ground.

The Florida Department of Environmental Protection this month told the city that it might not have done enough to protect the public from exposure to arsenic that ended up in the landfill.

That came as a surprise at City Hall, where officials thought the problem was resolved years ago.

"They've finally got around to saying we're not in compliance and asking us to go back in that area searching and documenting for high levels of arsenic," City Manager Norton "Mac" Craig told city commissioners on Jan. 15. "It's going to be a major project."

About two years before the preserve opened, the U.S. Environmental Protection Agency began studying the landfill site to assess contamination there.

The EPA found elevated levels of numerous chemicals on the site, which was used as a landfill from the early 1960s until 1981. Waste was placed in large unlined areas 10 feet deep, the study said. Some city employees said they buried waste in trenches up to 40 feet deep as well as in a pond and in wetlands.

The question was whether the EPA should make the landfill a Superfund hazardous waste cleanup site.

No, the agency decided. But it forwarded its findings to the Florida Department of Environmental Protection.

In April 2002, a city study found elevated arsenic levels in several locations. Measurements around the new observation tower were particularly high - more than three times Florida's target level for clean-up of arsenic in industrial areas. Largo officials say pressure-treated wood was used to build the tower on site, so sawdust containing arsenic may have contaminated surrounding soil. In 2004, Largo officials sent a letter to the Florida Department of Environmental Protection listing actions the city had taken to protect the public from arsenic. Those steps included:

- Putting up signs warning the public to stay on designated trails.

- Putting dirt caps planted with vegetation over contaminated areas.

- Fencing off land near the observation tower and another area.

When they didn't hear anything back, Largo officials considered the matter closed.

Not so fast, the DEP now says, four years later.

In a Jan. 10 letter to the city, Jill A. Seale, an environmental specialist in the Division of Waste Management, says Largo must further assess arsenic contamination at the preserve. The state has ongoing concerns, she said, including:

- EPA's finding of extensive contamination at the Largo site.

- No one knows the full extent of the contamination, how far out it has spread or how deep it goes.

- The potential for arsenic to leach into waterways.

Largo officials said they hope to set up a meeting with DEP soon.

"The first thing we want to do is understand what DEP is asking of us," City Engineer Leland Dicus said. "It's not clear what the additional concerns are and what actions that they are going to require of us."

The city's primary concern, he said, is making sure there are no adverse effects on the environment or the public.

"I don't know if this is just tying up their paperwork or if there are any real concerns - they've let it sit for four years," he said. "So hopefully, we'll get to the bottom of this soon."

Times staff writer Lorri Helfand contributed to this report.

Fast facts:

Arsenic hazards

Arsenic occurs naturally in the environment, and nearly 90 percent of manmade arsenic is used to make pressure-treated wood, according to the Florida Department of Health. Though other health agencies say any exposure to arsenic increases the risk of cancer in humans, activities that disturb the soil expose the public to much more risk.