Honolulu city officials are entering the second phase of a cleanup of toxic chemicals at the Waipahu Ash Landfill, which will focus on evaluating the long-term health and environmental risks at the site, the U.S. Environmental Protection Agency said.

The $16 million initial phase of dioxin and heavy metal cleanup included excavation and relocation of ash refuse. It was completed in September 2011.

The agency said the second phase will determine the extent of ash material remaining in the soil and sediment and, if necessary, include installation of additional monitoring wells to sample groundwater.

"The work’s already started," EPA spokesman Dean Higuchi said Thursday.

The cleanup followed a consent order involving several parties, including the city and the EPA.

Higuchi said the duration of work for the second phase might take three years, and the cost is under discussion.

The 54-acre site is on the makai side of Waipahu Depot Road and Kapakahi Stream and near Pouhala Marsh Wildlife Sanctuary.

Residue from the city’s Waipahu incinerator, which burned municipal solid waste, was disposed of at the landfill from 1972 until the incinerator closed in 1991.

The Pouhala marsh serves as a wetland for native wildlife, including the Hawaiian stilt, or aeo.

The marsh once was the site of Hawaiian fishponds. The city has constructed a soccer park east of the landfill site.