Harvard’s plans to sell a 62.5-acre parcel of land to the Town of Weston, Mass. have been temporarily derailed after engineers discovered high levels of lead and arsenic in the soil, and the sale—nearly a month overdue—has been put on hold until the University’s cleanup of the land.

Weston, an affluent town of 12,000 located 10 miles west of Cambridge, had already secured financing for the $22.5 million land purchase, said Town Manager Donna S. VanderClock. In 2006, Weston had agreed to purchase the property from Harvard, which had acquired the open plot of land in Weston—called the Case Estates—as a gift in 1942.

But the recent discovery of potentially hazardous levels of arsenic in the soil—likely the remains of lead-based pesticides once used in orchards on the site—has brought the transaction to a momentary stop, according to the town’s Web site.

“We [were] ready with the money. It’s in the bank,” VanderClock said. “Then we discovered this problem with the land.”

The contamination currently poses no health risks since the arsenic remains underground, but Harvard must now “remediate” the soil—to remove or contain the affected soil—to facilitate the completion of the sale, according to James Gray, associate vice president of Harvard Real Estate Services.

While arsenic contamination is typically eliminated by removing affected soil, Harvard is considering “encapsulating” contaminated areas, essentially burying affected soil, Gray said. This option not only costs less but also preserves the site’s aesthetic appeal, as the removal of soil would lead to the uprooting of vegetation. Gray added that the “encapsulating” method is environmentally safe, as long as the land is held in perpetuity as green space.

But Weston’s governing Board of Selectmen—the executive branch of the town’s government—will not agree to purchase any land in which contaminants remain, according to VanderClock.

“We’re ready to buy the land when it is clean, but we will not purchase contaminated land and take on that liability,” she said.

At the Jan. 28 Weston town meeting, Harvard representatives plan to present an alternate plan in which the University would retain ownership of approximately 15 to 17 acres of contaminated land—parcels with permanent restrictions on their usage that would likely preclude any future development or agriculture on the sites. The remaining 45 to 47 acres of uncontaminated land would be sold to the town at a reduced price.

“We think there will be very little cost and very little liability to the University to holding onto the contaminated and encapsulated land, or we wouldn’t be considering it,” Gray said, adding that in the future, Harvard could sell the land to another non-profit group.

A Moody’s Investors Service report issued Thursday stated that Weston intended to sell $32 million in bonds, of which $19.5 million in bonds would finance the purchase of the land plot, according to a Dec. 17 presentation prepared by the town. Other revenue from the bond sales will go towards public work projects, the report stated.

The Case Estates are considered the most desirable open space in Weston, according to VanderClock. The “pastoral” site—which contains both woodlands and meadows—was formerly used as an auxiliary facility for Harvard’s Arnold Arboretum, according to former arboretum director Robert E. Cook ’68.

For years, the premises housed nurseries and experimental growing facilities, but the arboretum has since consolidated those functions into its main, 265-acre reservation in Boston, Cook told The Crimson in 2006.

Harvard began discussing selling the property about four years ago, according to Gray.

“The University has held property for a long time, but has never been able to make effective use of marrying it into the academic mission,” Gray said. “We decided it was in the interest of the University to liquidate the asset.”

—Staff writer William N. White can be reached at wwhite@fas.harvard.edu.