WASHINGTON (AP) _ The Army has tentatively concluded it would be safer to dispose of obsolete chemical weapons at the eight depots where they are stored rather than transport them to one or two large incineration plants, a top official said Tuesday.

The ''tentative preferred alternative,'' which will become the subject of public hearings this summer, calls for the construction of small incineration plants at the existing depots to minimize the risk of transportation accidents and terrorist attack, said Amoreta M. Hoeber, deputy under secretary of the Army.

Those depots are located in Utah, Alabama, Maryland, Indiana, Kentucky, Oregon, Colorado and Arkansas.

''There is no alternative for taking care of the stockpile that doesn't have some risk, including doing nothing,'' Ms. Hoeber said.

''The risks involved with the demilitarization (destruction) plus the risks involved with transport are higher in the transportation alternatives than in the on-site alternatives,'' she said.

''There is no way that any simple solution will make everyone happy. We're doing the best we can to come up with a solution that makes sense ...,'' Ms. Hoeber added.

She appeared Tuesday at a special news briefing at the Pentagon to formally release a draft environmental impact study prepared by the Army. A Kentucky congressman who represents a district with one of the storage depots, Republican Rep. Larry Hopkins, had disclosed the Army's tentative findings last week.

The Army expects to produce a final impact statement by next December, clearing the way for a final decision on the matter by Army Secretary John O. Marsh in January.

The Army hopes to begin the process of destroying the old chemical stocks, which now average more than 25 years in age, by 1991. Congress has ordered the stocks destroyed by 1994 under a decision last year that authorized the Pentagon to begin producing a new type of weapon known as a binary chemical round.

The United States has not produced any chemical weapons since 1969. The Reagan administration fought for five years to win congressional approval for the binary weapon, arguing it was much safer and absolutely essential to revitalize America's deterrent capability in the face of continuing Soviet production of chemical weapons.

A binary bomb or rocket contains two chemicals that are harmless until mixed together after firing.

The Army flatly refuses to say how large the existing chemical stockpile is, citing secrecy classifications. Published estimates of the stockpile range from 25,000 to 40,000 tons, however.

In conducting the initial impact study, the Army examined the possibility of shipping all the munitions to a national disposal site at the Tooele Army Depot near Salt Lake City, or alternatively, to two disposal centers - one at Tooele and one at an the Anniston Army Depot in Alabama.

Tooele would be the logical site for a national disposal center because it already houses 42.3 percent of the nation's chemical stocks, Ms. Hoeber said.

The Anniston depot has 7.1 percent of the chemical agents, compared to 12 percent at the Pine Bluff Arsenal, Ark.; 11.6 percent at the Umatilla Army Depot in Oregon; 9.9 percent at the Pueblo Army Depot, Colo.; 5 percent at the Aberdeen Proving Ground, Md.; 3.9 percent at the Newport Army Ammunition Plant, Ind., and 1.6 percent at the Lexington-Blue Grass Army Depot, Ky.

The remaining 6.6 percent of the chemical stockpile is stored outside the United States.

Ms. Hoeber said Tuesday that each of the alternatives would cost the service about the same - roughly $2 billion. She also said the Army had proven the safety of the incineration process, leaving only the issue of where to build the plants.

The process of transporting the obsolete weapons by rail to central sites is sufficiently complicated that the possibility of something going awry increases, she continued.

Transportation of the chemical stocks would require convoys of trains, including guards, emergency personnel and special handling equipment, that would have to wend their way through 11 or more states at slow speed under constant surveillance, she said.

Hopkins, whose district includes the Lexington-Blue Grass depot, has launched an effort to block on-site incineration. He maintains the weapons stored in his district are too close to populated areas and schools for a safe disposal operation.

Several congressmen led by Sen. James Sasser, D-Tenn., have already lined up on the Army's side, however, arguing that transporting the weapons would increase the risk of terrorist attack.

The draft environmental impact statement reaches a similar conclusion, stating that threats to public health ''equal to or greater than those that result from accidents could result from sabotage or actions directed against storage magazines, disposal facilities or transport convoys.''

In a related development, the Army released a film on Tuesday of experiments conducted in the early 1970s to gauge the safety of M-55 chemical rockets transported in rail cars. The study concluded that rifle fire directed at a boxcar, if sufficiently accurate, could set off a rocket motor and produce a fire that would lead to release of the chemical agents.