The United States has mountains of poison chemical weapons, some leaking, that will cost $3.7 billion to dispose of safely, according to a paper prepared by President Reagan's transition team at the Pentagon.

The paper warns that "the obsolete and deteriorating stockpile requires a comprehensive plan" for detoxifying chemical weapons stored around the United States, on Johnston Island in the Pacific and in West Geermany.

Reagan has earmarked $20 million to gear up for producing an advanced artillery shell, which would release a deadly mist upon impact, and $4 million to start planning how to dispose of the stocks of chemical weapons.

Reagan stopped short of starting to "demilitarize" the stockpile.Such an effort, the transition team wrote, would require $331 million from fiscal 1982 through fiscal 1986 as a down payment on the $3.7 billion "total stockpile demilitarization" cost.

The transition team said some of the munitions "have been identified as leakers." Before releasing the once-secret document, the Pentagon censored how many weapons are leaking.

However, the Army has said there are "defective" nerve and blister gas munitions stored in Alabama, Arkansas, Colorado, Kentucky, Oregon and Utah -- some of which leaked, creating "mild" symptoms for some people, but no deaths or "known lasting effects."

In addition to those states, the Army has stored lethal chemical weapons in Indiana, Maryland, Germany and on Johnston Island. These stockpiles were not listed as defective, however. The Maryland munitions are at the Aberdeen Proving Ground.

The Pentagon has said it must keep chemical weapons to deter the Soviet Union from using them in a war. The chemical weapons in the U.S. arsenal include 105 mm, 155 mm and 8-inch artillery shells, rockets, bombs, mortars and mines. They are filled with deadly nerve or blistering agents.

The $20 million Reagan added to former president Carter's fiscal defense budget would go toward building a plant in Pine Bluff, Ark., for producing 155 mm shells with nerve agents. The chemicals would be separated in the binary shell while in storage, making the shell safe. When it is fired, the acceleration would mix the agents, producing a deadly vapor when detonated.

The Pentagon, when asked about its detoxification plans for the chemical arsenal, said "ultimate destruction will only occur if a comprehensive and verifiable treaty can be reached" with the Soviet Union.

"As a minimum," the Pentagon statement said, "a retaliatory stockpile must be retained for at least the next 10 years, the agreed time to destroy the existing stockpile. In the interim, only leaking and obsolete chemical munitions will be demilitarized."

The Defense Department "intends to reprogram sufficient funds in fiscal 1981," as well commit $4 million in fiscal 1982, to find safe ways to detoxify chemical munitions, the statement added.

The transition team's paper seemed to call for a faster, broader program than the Reagan administration has outlined. The paper said, "The inherent risk involved in allowing the deterioration of the stockpile to continue without regard to the public and environmental safety is unacceptable. Implementation of a total demilitarization program is required in the near term to begin elimination of the potential hazzard."