It will have taken at least $110 million since 2002 to clean up the Standard Chlorine superfund site left behind by the Metachem Products LLC which declared bankruptcy.
The latest phase will be run $17.4 million to install a thick cap and toxic vapor collection system.
It will cover the 23 acres that is the site's main production area.
Ninety percent of the cleanup costs are being funded by the federal Environmental Protection Agency.
The state of Delaware is picking up the other 10 percent.
The project managers will be holding a public meeting at the Delaware City fire hall on April 23 to take questions from residents.
