The solution was to inject 38,000 tons of portland cement into hundreds of cells in the lagoon, an operation that took a year and a half and has now created a stable site where Bass Pro Shops can build. Remediation on other parts of the site is continuing, and some environmental permits are still pending, Mr. O’Neill Jr. said.

Before O’Neill Properties purchased the site, it was the subject of a number of lawsuits between NL Industries, Sayreville’s economic development agency and the state Department of Environmental Protection, all of which were resolved by the purchase, Mr. O’Neill Jr. said. NL Industries did not respond to requests for comment.

The company says that the costly and time-consuming process of reclaiming the site from its toxic history is justified by the unique opportunity to own such a large parcel of developable land in the midst of one of the country’s biggest markets.

“The real estate fundamentals of this site are unparalleled,” Mr. O’Neill Jr. said, noting that the waterfront property promised to be an additional attraction for future residents.

Matt S. Krauser, senior managing director in northern New Jersey for Integra Realty Resources, a national valuation company, agreed that the availability and development of a big parcel in a prime location were rare and promising.

“To be able to acquire 400 acres right off the parkway with ease of accessibility both north and south, it’s really an unusual project in an unusual location,” he said. “It’s more or less a once-in-a-lifetime opportunity to be able to construct something like this.”

He said that the project was a departure from the town center type of development that had dominated in recent years. With its office and residential components, the Sayreville project could be seen as a hybrid, he said.