One of the biggest and most controversial public works projects in U.S. history - a massive tunnel and reservoir system designed to reduce water pollution and flooding in the Chicago area - was portrayed by the General Accounting Office today as a grandiose and probably unaffordable scheme that might never accomplish its goal.

The GAO, in a 391-page, six-volume study prepared at the request of Sen. Charles H. Percy (R-Ill.), recommended that federal funds for the Chicago Tunnel and Reservoir Plan (TARP) be cut off temporarily as soon as a $1 billion segment of the system now under construction is completed in 1984.

Although the GAO said further study is needed before it would recommend either abandoning or continuing the project, its estimates of the final costs are so astronomical that Percy expressed doubt about whether "the most cost-conscious Congress we've had in decades" would permit construction to continue.

The complete system, including 123 miles of tunnels and reservoirs with a total capacity of 42 billion gallons, would cost $11 billion, or $21,500 for each of the 510,000 households it would serve, the GAO estimated. Seventy-five percent of the costs would be paid by the federal government through Environmental Protection Agency grants.

That estimate includes about $1.5 billion for "associated projects," such as new sewage treatment facilities that will be needed even if the tunnel project is abandoned. But not counting the related projects, Percy pointed out at a press conference here today, the system would cost more than the Trans-Alaska oil pipeline ($9.2 billion), the Washington, D.C., subway ($7.8 billion), or a new Panama Canal ($5.2 billion).

The GAO estimated that the system, if completed, would cost $56 million a year to operate, or about $110 per household in Chicago and the 53 surrounding communities served by the Metropolitan Sanitary district or Greater Chicago. Yet the annual cost of flood damage in the area is $15 million to $20 million a year, about one-third of what it would cost just to operate the system, according to the report.

The report also noted that the system, with its pumps and giant fans to combat odors, would use as much electricity as 27,000 homes consume. The report notes, "A serious question exists concerning the federal funding of an energy-intensive project while at the same time encouraging conservation."

The 54 communities in the sanitary district are among 1,143 U.S. cities and towns that have "combined sewers" - that is, sewers that carry both sewage and storm water. If tunnel and reservoir systems were adopted as the solution to water pollution and flooding nationally, the GAO estimated that the construction bill would be about $600 billion, or nearly half of one year's gross national product.

TARP was conceived by the sanitary district in 1965 and began 10 years later with federal funding. It would collect and store water during storms, reducing river pollution by about 90 percent and flooding by 65 percent.

The improvement would not be enough, however, for the Chicago River system to meet federal goals of making waterways safe for swimming.

"You can boat and fish in the Des Plaines River (part of the Chicago River system) now, but you can't swim in it," Percy told reporters. "We can spend $11 billion on this project, and you'll be able to boat and fish in the Des Plaines River but you still won't be able to swim in it."