The Isle of Wight Board of Supervisors has put off a decision on a proposed water purchase agreement with Norfolk and Suffolk until later this month.

As county officials elaborated on details of the agreement to a crowded meeting room on Thursday, it became clear that the deal was largely being driven by a desire to meet the anticipated water demands of the intermodal park near Windsor.

“If we shortchange the water, we’ll miss the intermodal park,” Newport District Supervisor Stan Clark said.

If approved by the county and the Suffolk City Council, the 40-year agreement would allow the Western Tidewater Water Authority — which serves both communities — to purchase up to 15 million gallons of water per day from Norfolk.

Eddie Wrightson, the county’s director of general services, told supervisors that there were several merits to the proposal.

“It will secure a water source for the county for the next 20 to 40 years,” Wrightson said. “But most importantly, it reduces Isle of Wight’s future reliance on the existing real estate tax base.”

According to Wrightson’s projections, water demand is expected to rise in tandem with the anticipated development of the Shirley T. Holland Intermodal Park, located on Route 460.

Wrightson said that by 2029, the county could need 5.3 million gallons of water per day, most of which — 3.2 million gallons — may be needed by the Windsor Development Service District, which comprises the intermodal park.

By contrast, the county currently uses half a million gallons of water per day.

Andy Hecker of the engineering firm Moffit & Nichol said the intermodal park eventually could require two to five million gallons of water per day, depending on what types of industries build there, and when.

“It’s difficult to predict when these businesses will come on line,” Hecker said.

Under the terms of the deal, the WTWA eventually would take water from Norfolk’s Western Branch Reservoir, located in Suffolk.

Albert Moor II, Suffolk’s director of public utilities, has said that a new pump station and three miles of underground pipeline would need to be constructed to take water from the reservoir. A water treatment facility also would need to be expanded. Moor estimated the projects would cost $100 million and could be completed by 2014.

Wrightson said Friday that WTWA currently gets its water from several sources, including surface water in Suffolk, groundwater permits and a Suffolk contract with the City of Portsmouth.

The proposed agreement — reached after secret negotiations among the three municipalities — contains a ramped-up “take schedule,” which stipulates that WTWA would start purchasing water from Norfolk at a rate of three million gallons per day.

In 2016, the authority would begin purchasing another one million gallons per day every two years, reaching the limit of 15 million gallons per day by 2038. The agreement continues through 2048, with 75 percent of the water going to Suffolk and the remaining 25 percent being sent to Isle of Wight.

Assistant County Attorney Mark Popovich said the ramped take schedule, which would coincide with anticipated growth in the county, made the agreement particularly attractive to Isle of Wight.

“Part of the negotiation process with Norfolk was to make this a one-of-a-kind contract in Hampton Roads,” Popovich said.

The Norfolk City Council approved the deal on July 21. The Suffolk City Council has yet to vote on it, but may decide to this month.

Burton said Norfolk officials insisted that negotiations between the authority and the city be conducted in private, and that details over the contract be released to the public only after an agreement had been reached.

“That was a preceding condition from the City of Norfolk,” Burton said. “The Virginia Freedom of Information Act recognizes the sensitivity of negotiating contracts. But once you make the deal, everything is public. The agreement is not a private document.”

He added, “the public can judge if the agreement is good, bad or indifferent.”