GUILDERLAND — By the end of the year, Watervliet Mayor Michael Manning hopes to have started work on doubling the size of his city’s reservoir.

In the heart of Guilderland, the Watervliet Reservoir now takes up about 500 of the 688 acres that the city owns. After the expansion, the surface area is expected to increase by about 5 percent, Manning said.

“We’re not digging it deeper, we’re just letting it go higher,” he said. The reservoir has naturally high walls, he said, so the city plans to raise the damn and collect more water from its 123 square-mile watershed.

Proportionally, the reservoir is small for the size of the watershed, which covers three counties and nine towns, said James Besha, of Albany Engineering Corporation, the firm handling the project.

“The bigger the reservoir, the more stable the water quality,” Besha said.

“The water quality is very unstable. It’s just not a very good water supply,” Thadeus Ausfeld said when the expansion plan was made public in 2005. Ausfeld had run Guilderland’s water-treatment plant until he recently retired.

Water system operators have a hard time making the water drawn out of the reservoir drinkable, he said. He and Charles Reilly, who serve together on the Restoration Advisory Board, which advises the Army Corps of Engineers on the cleanup of the old Army depot, pointed to siltation as one of the main problems with the reservoir. Mud, washed in from the vast watershed, coats the bottom with a thick layer of muck, they said.

The bottom of the reservoir is “like quicksand,” Ausfeld said in 2005. One of the town’s intake pipes is completely clogged, he said. “Swamp water doesn’t make very drinkable water,” Ausfeld said.

“Ted has his own opinions,” Besha responded through the Enterprise when asked about those concerns. “The science is the science,” he said, and his firm has done all the necessary studies, which will be in the Environmental Impact Statement that he expects will be completed and made public by the end of March.

The expansion plan is rooted in a 1964 study of the reservoir’s capacity for growth, Manning said. The study was revisited in 1979 and again in the 1980s, before the city began to seriously undertake the project in 2005, before Manning was in city government.

Watervliet has had the reservoir since 1915, Manning said, and the water filtration system is worn. Part of the city’s master plan for the reservoir is an updated filtration system, he said. Manning estimated the cost of the whole project, which will bring the capacity from 1.5 billion gallons to 3 billion gallons, to be $5 million.

Currently, Guilderland is the only municipal customer buying water from the city — it pays a rate of 40 cents per thousand gallons, Manning said. Watervliet has approached three municipalities about buying water after the size of the reservoir has been increased — Manning declined to name them, and said that he was unsure as to what the city would charge for water. For the first few years, he said, it would be “raw water,” until the new filtration system is built, a project that he would like to undertake with a partner.

Guilderland has agreed to apply for a grant that would cover the cost of a study on the project, Guilderland Supervisor Kenneth Runion said yesterday.

“Our water filtration plant is aging,” he said, and “Watervliet’s is aging as well.” Watervliet is preparing the grant application, which is part of a push from the state for municipalities to consolidate services and equipment, he said.

“There aren’t any concrete plans,” Runion said, but Watervliet and Guilderland are considering their options.