A year after Citgo Petroleum agreed to donate Petty's Island to New Jersey as a nature preserve, cleanup is moving ahead, according to the state Department of Environmental Protection.

The 392-acre island, in the Delaware River between Pennsauken and Philadelphia, was used as an oil-processing and storage facility for almost a century. Considerable cancer-causing pollutants have leached into the soil and groundwater, according to an environmental assessment the company submitted last year.

Citgo, the U.S. subsidiary of Venezuela's state-owned oil company, has begun demolishing the tanks and processing facilities that dot the island, but it is still working on how to clean up the soil.

"The majority of the island has some contamination issue," said Larry Hajna, a spokesman for the DEP. "We're just in the beginning stages of mapping out the strategy Citgo will follow."

A Citgo spokesman said the company would not comment for this article or allow access to the privately owned island, whose two-lane access bridge is staffed by a security guard.

Citgo bought the island in 1983 from another oil company to be used as a distribution center for petroleum products brought in by ship. The facility closed in 2003.

New Jersey Sierra Club director Jeff Tittel visited Petty's Island with Citgo officials shortly after the announcement of the handover last year. He said he was satisfied with their cleanup plans.

"They're actually doing more than is legally required," he said. "Our concern has always been that it's cleaned up to a level that's protective - that you actually remove the parts that are dirty, not just doing a 'pave and wave,' as is often the case in New Jersey."

The process is expected to extend at least until 2020, when Citgo is scheduled to officially deed the island to the state. Shipping company Crowley Maritime still operates near the center of the island, moving shipping containers from ships to trucks headed to the mainland.

But across much of Petty's Island, nature has already begun to return. Herons, deer, and foxes roam between the remnants of old oil storage tanks. Even bald eagles show up from time to time.

Duke Riley, a Brooklyn, N.Y., artist with an interest in what he called "urban waterfront," sneaked onto the island last year. On the top surface of a fuel tank, he painted a portrait of an Irish pig farmer who had lived there in the late 1800s and been dubbed "the king of Petty's Island." Riley photographed the giant painting from a helicopter for an exhibit at the Historical Society of Philadelphia.

Over his five or six visits, Riley said, the extent to which the island has returned to its natural state surprised him.

"There's a ton of deer," he said. "The perimeter of the island is heavily littered with flotsam and jetsam, but once you get past that, it's pretty much all pristine, unmolested nature."