NEWARK — The cost to remediate contaminated groundwater at a former chemical facility on Frelinghuysen Avenue in Newark is expected to reach at least $25 million, according to the U.S. Environmental Protection Agency.

Because of complex rock formations underneath the property, it is not possible to pump out contaminated groundwater at the site, where White Chemical Corp. manufactured acid chlorides and fire retardants from 1983 to 1990.

Cleanup will instead be done by bioremediation, a process that involves injection of chemicals into the ground water to breakdown pollutants.

“The depth, nature and variety of the rock formations would present extreme technical challenges,” the EPA said in a statement.

The agency has already spent about $20 million on cleanup at the 4.4-acre property, which was put on the federal National Priorities List, better known as the Superfund site list in 1991.

A 1999 consent decree between the EPA and the former site owner, AZS Corp., together with four corporate affiliates called for the agency to recover incurred and future cleanup costs.

During its initial site investigation more than 20 years ago, the agency catalogued more than 10,000 55-gallon drums and other containers, most of which held hazardous substances. The majority were precariously stacked or improperly stored and in “various stages of deterioration, fuming or leaking their contents onto the soil,” according to the EPA.

All of the containers were removed 20 years ago, the EPA said. More than 23,000 cubic yards of contaminated soil has since been removed.

The agency then installed 28 monitoring wells on- and off-site.

About 12,000 people live and work within a one-quarter mile radius of the property.

The EPA includes more than 220 New Jersey properties on its cleanup list, about half of which are Superfund sites, the most in the nation.

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