A huge, new crosstown water main, first proposed by city planners more than 30 years ago, may soon move off the drawing boards and into the ground if the City Council approves the project, estimated to cost $33 million to $40 million.

The water main, which would be approximately five miles long and six to nine feet in diameter, would carry water from the Dalecarlia Reservoir into the city in the event that two, century-old tunnels now in use should collapse.

D.C. Department of Environmental Services (DES) officials said they do not know the condition of the two tunnels and there is no way to check their condition.

Furthermore, they said, there is no adequate back-up system that could supply major portions of the city - particularly Anacostia and most of the downtown area - with water in the event of a tunnel collapse.

The two tunnels are the 90-year-old Lydecker Tunnel (also known as the Washington City Tunnel), which feeds the McMillan Reservoir with water from the Georgetown Reservoir, and the 110-year-old Georgetown Conduit, which supplies the Georgetown Reservoir with water from the Dalecarlia Reservoir.

If either of these tunnels were to collapse, more than half a million persons living and working in the District would suffer from severe water shortages for an indeterminate period of time, according to DES officials.

DES officials said the project is not merely an expensive insurance policy, but is essential in order to meet increasing water demands and to ensure that the District will have an adequate water system through the year 2000.

Preliminary studies for the crosstown main are almost complete and will be presented to mayor and City Council within the next two months, according to William Garlow, chief of the hydraulic control branch of the DES Bureau of Design and Engineering.

Once the plans are received, the council will have to make three decisions: whether to go ahead with the project; how to fund it, and which of the five routes that have been studied should be chosen.

If the project is given the go-ahead, a year will be spent working out the final design. construction will take an additional two to three years, depending on which route is chosen, according to Garlow.

Based on the estimated construction and operating costs and the benefits and problems inherent in the construction of each of the five routes, consultant engineers have narrowed the choice to two routes.

Within the next month, the DES will choose one of those routes as its recommendation to the council, Garlow said.

Each of the five routes studied would connect the Dalecarlia Reservoir to the Southeast Transmission Main, a major water line which begins at 15th and N streets NW and extends to the Anacostia Pumping Station at 18th and R streets SE. the Anacostia Pumping Station supplies water to the area east of the Anacostia River.

Of the two routes under active consideration (alignments B and D), Alignment B is the most direct, the most expensive (estimated cost of $37.9 million) and would take the longest to build (approximately three years).

Alignment B, which DES refers to as the "as-the-crow-flies" route, would be a direct tunnel from the reservoir to the Southeast Transmission Main.

Because it would take a direct route, most of the tunnel, which would be about 150 feet below ground, would travel under private residences, and the city would have to purchase underground easements from the property owners. Because its construction method would be tunnelling, above-ground disruptions would be at a minimum, officials said.

If Alignment B is chosen, the only above-ground disturbances would be at the route's five construction shaft sites, according to officials.

The shaft sites would be open construction sites for the duration of the project, Garlow said. In order to ensure a minimum amount of disruption, the DES has chosen for shaft sites areas which are presently undeveloped or are occupied by parking lots or grassland.

The shaft sites along Alignment B would be at 15th and N streets NW; the intersection of Massachusetts Avenue and 22nd and Q streets NW; a vacant grassland spot adjacent to Whitehaven Parkway, off Wisconsin Avenue NW; 44th Street between Edmunds and Garfield streets NW, and the Dalecarlia Reservoir.

Alignment D would be cheaper to construct (estimated cost of $32.8 million) than Alignment B, primarily because more than half of the route would be constructed by the open cut (trench digging) method.

Open cut construction, in addition to being cheaper, is quicker than tunnelling. The construction period for Alignment D would be about two years. The main disadvantage of the open cut method is that it causes a considerable amount of above-ground disruption.

Beginning at the Southeast Transmission Main, Alignment D would be constructed as a tunnel until it reached MacArthur Boulevard and Foxhall Road NW. From that point, the main would be constructed by the open cut method along the old D.C. Transit trolley right-of-way until it reached Dalecarlia.

Shaft sites along Alignment D would be at 15th and N streets NW, 23rd and N streets NW and MacArthur Boulevard and Foxhall Road NW. CAPTION: Map, Crosstown Water Main, Alternate Routes, By Dave Cook - The Washington Post