The National Park Service hopes to get $20 million in Land and Water Conservation funds during the coming fiscal year to buy 4,000 acres of additional parkland in the greater Washington area, including $7.9 million to purchase 1,800 acres for the C&O Canal National Historical Park.

The 1,800 acres would complete the purchase of all remaining privately owned land within the boundaries of the 20,239-acre canal park created by Congress in 1971.

The parkland purchases would be the first significant expansion of federal open space here in several years and the largest acquisitions since the Land and Water Conservation Act was passed in 1965, according to Jack Fish, director of National Capital Region parks.

The act, which funnels revenue from leasing federal lands back into the purchase and development of parkland, will have $900 million to disburse next year. So far the program has provided $36 million for area federal parks, almost all in the 1960s. To date, state and local parks in Virginia have received $33 million, Maryland parks $30 million and District parks $8 million.

The Park Service also is proposing $12 million in restoration projects here under the 2-year-old Bicentennial Land Heritage program. Almost all the funds would be used to restore parks and park buildings, including $1 million in repairs on the C&O Canal - including all four locks in Georgetown - and at Fort Washington, built on a Potomac River bluff in Prince George's County. The single largest project would be $5 million in repairs on deteriorated bridges along the George Washington Memorial Parkway north of Rosslyn.

Congress must approve the allocation of both Bicentennial Land Heritage funds and Land and Water Conservation funds by the Park Service as well as any expansion of federal parks.

The largest park acquisition proposed, a 1,400-acre, $8.5 million expansion of Mansassas National Battlefield Park, apparently also is the least likely to receive congressional approval because of opposition from the Prime William County Board of Supervisors.

Other proposed parkland purchases here include the creation of Monocacy National Battlefield in Sharpsburg, Md., on 633 acres that would cost $3.5 million. The site of a major Confederate victory in 1864, Monocacy was declared a National Military Park in 1934. However, federal funds to buy the park site, still privately owned farmland, were not authorized until 1976 when it was made a national military park.

Farther afield in the Park Service's National Capital Region is the proposed $950,000, 108-acre expansion of Harper's Ferry National Historical Park. The Park Service plans to build an off-site parking lot, ban traffic from the historic section and start a shuttle bus system. With an additional $1.5 million in construction funds earmarked for Harper's Ferry, the Park Service also hopes to restore and open some of the many historic building now boarded up there.

Like the Park Service's national budget, the $12 million in proposed construction projects in the Washington area involves few new buildings. Instead, it emphasizes stabilization or restoration of exisitng buildings and parks.

The largest area construction job, $5 million for resurfacing five of the eight bridges on the George Washington Memorial Parkway north of Rosslyn, would consolidate projects originally not planned to be completed until 1983. The Park Service has resurfaced three of the bridges in the past three years, but instead of continuing to do one a year and impeding parkway traffic for the next five years "we're proposing to do it all at once and get it over with," said spokesman George Berklacy.

The resurfacing, which would take about a year, would begin after the start of the 1979 fiscal year Nov. 1, if approved by Congress. The five bridges, all about 15 years old, span Windy Run, Spout Run, Donaldson Run, Gulf Branch and Glebe Road, wher it ends near Chain Bridge.

In addition Bicentennial Land Heritage funds would be used to rehabilitate the existing Mansassas battlefield visitors center ($228,000) and Wolf Trap Farm's Filene Center ($393,000). Arlington House in Arlington National Cemetery would receive $151,000 for repair work and the largest section of George Washington's ill-fated Potowomack Canal at Great Falls, predecessor to the C&O Canal, would receive $393,000 worth of stabilizing work on its locks.

Fort Washington, considered by historians as one of the nation's most magnificent forts, is slated to receive $455,000 for needed repairs to its giant masonry walls and its many boarded-up, 150-year-old buildings. It would be the first significant repair work done on the fort in almost two decades. The Park Service also proposes to build additional comfort stations for the one-million picnickers a year who use the park grounds.

The Frederick Douglass Home in Anacostia, a federal park since 1962, is to get a $318,000 visitors center under the proposed Park Service construction budget.

The Expansion of the Manassas National Battlefield Park, commemorating the first battle of the Civil War in 1861 and a second major battle in 1862 - where Confederate Gen. Thomas J. Jackson acquired the nickname "Stonewall" - has been proposed for several years.

A bill authorizing the addition of 1,768 acres on the periphery of the present 3,000-acre park, including about 300 acres in scenic easements, has been sponsored by Rep. Herbert E. Harris II (D-Va.) and passed the House each of the past two years. It would include land around the historic Stone Bridge over Bull Run and groves of trees still pock-marked from the battles.

Virginia's two U. S. senators, Harry F. Byrd Jr. (I) and William L. Scott (R), while not themselves opposing the park's expansion, have held up passage of the park bill because of opposition from Prince William supervisors. County officials said Prince William needs development and have been against any federal, state or area park expansion within the county. Park Service officials say historical and environmental groups and the majority of land owners in the area favor the battlefield expansion.

An aide to Byrd said last week the stalemate continues and there apparently is little chance the park bill will pass this year.

The Park Service's National Visitors Center at Union Station, which would receive a $323,000 allocation in the proposed fiscal 1979 construction budget for repairs to its heating system, continues to be one of the major items in the Park Service's proposed $59.4 million operating budget. The Park Service is asking for $6.1 million to operate it next year.

The little-used center was to have a giant parking garage for tourists for the Bicentennial, but its construction was late and was finally halted by Congress in the summer of 1976 because of huge cost overruns. The garage, still incomplete, ultimately may cost $45 million.

More than $1 million of the annual operating costs go to staff the visitor center - it has a full-time staff of 28 - and another $1 million to heat, light and clean the giant Union Station building. The major budget item is the annual $3.5 million Park Service must spend for 25 years under the original $26 million lease-purchase agreement to buy the station from the railroads.

Under an agreement reached last fall by the secretaries of Interior and Transporation, Union Station, now used solely as a visitor center, will once again be used primarily as a train station. Plans to accomplish this, and to finish the garage, are to be presented to Congress when a study of the visitor center is completed.