Groton - After 24 years as a federal Superfund site, the Naval Submarine Base is about two years away from completing cleanup of contaminated areas of the 576-acre site and just marked another milestone in the multimillion-dollar effort.

On Monday, the Environmental Protection Agency informed the sub base that it has accepted its report on completion of the remediation and restoration of a 19.6-acre wetland at the north end of the base. The wetland, contaminated with DDT bricks placed there in the 1960s - as well as PCBs, petroleum residues and heavy metals from dredged soils dumped there - is one of the largest of about two dozen areas on the base identified by the EPA for cleanup when it placed the site on the list of the nation's most polluted properties.

"There were very high levels of contamination there," Kymberlee Keckler, remedial project manager for the EPA, said Tuesday.

Tracey McKenzie, installation restoration program manager at the base, said the $1.3 million wetland cleanup entailed excavating and removing more than 3,000 tons of contaminated soil and replacing it with clean soil, then digging out four stream channels, planting 508 trees and shrubs, seeding with wetlands grasses and reducing stands of invasive phragmites with cutting and selective application of pesticides.

"The beauty of what we're seeing here is that we're seeing birds nesting in the areas we've planted and a greater diversity of birds," she said, looking out over the wetland, in a low area between a weapons storage facility and a parking lot. Killdeer, Baltimore oriole, wood ducks, mallards, egrets and herons are using the area, she said, along with turtles, frogs and salamanders.

In its letter Monday, the EPA said while it has accepted the work at the wetland as complete, some areas should have been elevated higher to conform to the original restoration plan. As the base continues to monitor the area, the EPA said, it may require different plantings more suited to the current elevations, or modifications to raise some areas.

This spring, McKenzie said, button bush, Virginia sweetsprire and swamp azalea will be planted to replace some of the shrubs that haven't survived. The new plantings will be better suited to lower elevations and wetter soils, she said.

With the wetland project complete, the cleanup is now focused on completing work on the two largest remaining areas, both in the lower sub base along the Thames River. Soils contaminated with lead and petroleum residues were dug up from a former battery storage area, and a new asphalt cap will be spread over the site this month, McKenzie said. The site - crisscrossed with underground utilities, high-pressure water lines and other equipment - was difficult for work crews to maneuver in because of close proximity to other base facilities and high levels of activity there.

"They actually had to do some of the excavation with shovels," she said.

Work on the second area on the lower base will begin next year, when 30,000 cubic yards of river sediment laced with lead and other heavy metals will be dredged from the areas around the docks. That project is expected to take about eight months.

Overall, the massive remediation, costing roughly $70 million thus far, is progressing well, with about 20 projects completed, including cleanup of three landfills, Keckler said.

After the lower base projects are complete, remaining sites slated for cleanup will include a couple of small areas, one of them a tank farm, she said.

"Our goal is to complete this by the end of 2016," McKenzie said.

j.benson@theday.com

Twitter: @BensonJudy