HOPEWELL, N.J. Rain fell Friday on south-central New Jersey, and a part of it flowed into Baldwin's Creek.

Some flowed, too, into Duck Pond Run, and Six Mile Run, and Cranbury Brook, Devil's Brook, and Peace Brook, and into dozens more creeks and streams bearing names like Little Bear, Duck Pond, and Ten Mile, that flowed into the Stony Brook, that streamed past a white tent in Hopewell Township, Mercer County.

There, 100 people and 10 waiting shovels were gathered at the Stony Brook-Millstone Watershed Association.

"We think of this as good weather here in the watershed," Jim Waltman, the association's executive director, joked to the crowd.

"It reminds us why we're here today: to protect our water supplies."

They had gathered on this gray day for a ceremonial groundbreaking for the association's ultra-green $7.5 million nature center.

Founded in 1949 and situated on a 930-acre preserve of woods, fields, streams, and hiking trails, the association spent five years raising money for the 13,000-square-foot center.

It will serve, the association predicts, as a regional model of energy conservation, and will demonstrate eco-friendly ways to handle wastewater and excess groundwater.

"The irony," Waltman told the gathering, is that even without cataclysmic events like Hurricane Sandy, New Jersey increasingly finds itself with "too much water, too little, or it's dirty."

The new building, which will include a 10,000-square-foot addition to the existing Buttinger Nature Center, expects to generate more than 85 percent of its energy through the use of on-site renewable resources.

Those resources will include geothermal heating and cooling systems drawing energy from 400-foot-deep wells and powered by solar panels.

Photovoltaic solar panels will generate most of the building's electrical needs and much of its daytime lighting will come from skylights and windows. Hot water will be produced by solar tubes.

The new center will also feature a green roof that will reduce storm water runoff by absorbing and storing rainwater, and rain gardens will use water-loving plants and soil amendments to reduce and purify storm water runoff.

Waltman said he expected the building, designed by architect Michael Farewell, to receive platinum certification from the U.S. Green Building Council's Leadership in Energy and Environmental Design program, its highest rating.

"There are very few buildings in the state rated platinum," he said.

The center will also house classrooms, exhibit halls, a laboratory, a computer learning center, conference rooms, a gift shop, kitchen, and staff offices.

But with 10,000 schoolchildren already visiting each year, there is the messy business of toilets to deal with. For that, eco-friendly wastewater treatment is the second centerpiece of the project.

When toilets start flushing there in the fall - the scheduled opening - their waste and the gray water from showers and sinks won't gurgle into septic systems or sewer lines.

Instead, it will flow first into holding tanks, where solids will be separated, and will then be pumped into a wetlands where specially selected plants with root systems rich in microbes will break down bacteria.

This water will continue to flow laterally into adjacent wetlands, where naturally occurring materials will further filter and clean the water before it descends into the aquifer.

The scope and ambition of the project set up several speakers at Friday's ceremony for some puns. It was, they said, a "groundbreaking event" and a "watershed moment."

The association takes its name from the Stony Brook, which flows past the hilly preserve and into the Millstone River, which drains into the Raritan River. Several of the five counties touched by the 230-square-mile watershed also drain into the Delaware River, which provides drinking water to Philadelphia and other southern counties.

Friday's rain paused long enough to allow Farewell, Holt, and Waltman, along with Michele Siekerka, deputy commissioner of the state Department of Environmental Protection, and others to don hard hats and grip shovels for the ceremonial groundbreaking.

Among them was Pam Mount, a Lawrence Township councilwoman and president of the environmental group Sustainable New Jersey.

"People may say this is a luxury," she said of the new center's ambitions. "But the work needs to be done."