Suffolk and Isle of Wight County plan to spend millions purchasing water from Norfolk over the next 40 years, the three localities announced Friday.

“This agreement will allow the city of Suffolk and Isle of Wight County to continue to grow and prosper,” said Norfolk Mayor Paul Fraim at a press conference in Norfolk.

The agreement, which takes effect when official papers are signed in a few weeks, will provide for the Western Tidewater Water Authority, which comprises Isle of Wight and Suffolk, to purchase up to 15 million gallons a day from Norfolk. The water will come from Norfolk’s Western Branch reservoir, which is on Route 10 near Reid’s Ferry in Suffolk.

According to Albert Moor II, Suffolk’s director of public utilities, the water will be pumped from the Western Branch reservoir and piped underground to Suffolk’s G. Robert House water treatment facility in the Chuckatuck area, near Lone Star Lakes. There, it will be treated and pumped out to customers.

The water doesn’t start flowing until 2014, he added. About $100 million worth of capital investment, including a pump station, three miles of underground pipes and treatment plant expansion, will have to be made in the interim.

Suffolk currently gets water from groundwater sources, Lone Star Lakes, Crumps Mill Pond and a contract with Portsmouth. This agreement supplements those water sources for the anticipated growth over the next 40 years.

Suffolk and Isle of Wight County will be splitting all costs related to the agreement, with Suffolk paying 75 percent of costs and Isle of Wight County the remaining 25 percent. The water also will be split by the same percentages.

Authorities were unclear on Friday exactly how much the agreement would cost taxpayers. Payments will start as soon as the papers are signed, and the costs are increased incrementally as water purchase increases.

The water authority will start out purchasing only three million gallons per day, and the purchase will increase by one million gallons per day every other year, leveling out at 15 million gallons per day and continuing through the 40-year life of the agreement. Suffolk currently uses around 7.5 million gallons per day, Moor said.

The agreement should not affect Norfolk’s ability to provide water to its customers and other localities with which it has agreements, Fraim said.

Isle of Wight County Board of Supervisors Chairman James Brown Jr. called the agreement a “monumental step forward” for the area.

“We will all share clean water,” he said.

Suffolk officials were equally pleased.

“Suffolk is growing rapidly,” Mayor Linda Johnson said at Friday’s press conference. “Growth at this rate requires careful thinking.

City Manager Selena Cuffee-Glenn was excited about the agreement.

“This is a great day for us,” she said. “We’re ecstatic about today.”

Johnson shared her enthusiasm.

“We’re going to have good water to drink for a long time,” she said.