Aiming to boost its supply of parkland and stave off construction of dozens of town homes, Prince William supervisors this week approved the purchase of 42 acres in Triangle for a park.

The property, sloping and wooded, is owned by the D.C.-based developer The Donohoe Cos. It sits at 18551 Old Triangle Rd. and is bordered by Fuller Heights Road and the Quantico Marine Corps Base at its southern end.

The Prince William County Park Authority negotiated a price of $800,000 for the land, considerably less than the property's assessed value of $1.4 million.

But it could be several years before ballfields or playgrounds are built there. The Park Authority has no money in its budget to develop them, and the only likely way to find it would be to ask voters to approve it in a bond referendum, a county official said.

Prince William's exploding growth has generated a huge demand for parks that the county is only starting to meet, local officials said. A study several years ago by the Park Authority, which acquires and develops parkland, calculated the deficit at 1,000 acres, 83 of them in the Dumfries magisterial district, which is home to the Triangle property.

County officials said Donohoe announced preliminary plans to build town houses on the property several years ago, but was met with community opposition. The developer recently approached the county for a sale, said Patrick Mulhern, the Park Authority's director of corporate services.

About half the purchase money was approved by voters in a bond referendum in 1998 when it was set aside for ballfields at Locust Shade Park. That park, however, is under consideration as the site of a museum planned by neighboring Quantico Marine Corps Base, Mulhern said. The Park Authority then decided to shift the money to the purchase of the Triangle property.

Mulhern estimated the cost of developing the Triangle property at $2 million. "In providing for park services, you often purchase property in hopes of acquiring additional dollars to develop it," he said.

Even with funding to develop the site uncertain, the purchase marks the county's growing effort to preserve open space, which is dwindling quickly in Prince William's eastern end.

"We eliminated homes that would have placed a lot of demands on our overburdened county," said Supervisor Maureen S. Caddigan (R-Dumfries), whose district includes the new property.

"Instead of cookie-cutter homes, we're preserving the land," Caddigan said. "It's going to save us money in the long run."

Donohoe executive Gerard Goeke did not return phone calls seeking comment.

The deal is the county's third purchase of a parkland site in recent years for use as ballfields, playgrounds or other recreational areas. In 1998, the Park Authority bought property off Minnieville Road in Dale City for about $500,000 and on it built several soccer fields now known as Howison Homestead Park.

The Park Authority paid $670,000 last summer for a 101-acre tract in Gainesville, in the county's western end, for eventual conversion to ballfields. Citizen activists fought the purchase, saying the site may not give the county its money's worth and disputing how much of it was usable for ballfields. The purchase also stirred controversy after the district's supervisor, Edgar S. Wilbourn III (R-Gainesville), who received campaign contributions from the site's owner, helped choose the list of six finalists for the park site.