Maryland and Anne Arundel County this month were allocated $850,000 from the U.S. Department of Interior to help buy more than a mile of Chesapeake Bay beach for a major park. But the state and county may not accept the matching grant because, they say, they can't come up with the rest of the money needed to buy the park.

The proposed Beverly-Triton Beach Park, 341 acres of wetlands, woodlands and bay shore south of Annapolis, is expected to cost about $5 million. The site is at the tip of the densely developed peninsula between the South and Rhode rivers.

Developers are proposing to turn the former private beach club property into a large condominium and marina complex, although sewers would be needed and a lawsuit over the project is pending, according to county officials.

"The $850,000 doesn't do much for us and the state isn't offering very much either . . . only $500,000," said county director of administration Robert R. Strott. The county, which originally asked Interior for a $2.5 million matching grant and was promised $1.2 million from the state, simply doesn't have the $3.5 million it now needs to buy the park, he said. And, he added, development could cost several million more.

Although the state's generous Program Open Space has been providing the county about $1 million a year to build and develop parks, "that money is all committed for the next couple of years -- to develop the parks we've just bought," said Joe McCann, county parks and recreation director.

Anne Arundel is now developing its first county park, the 235-acre Downs Memorial, and recently bought a 290-acre site in the central part of the county, to be called Severna Park. "We have a small beach park, Mayo Beach, north of Beverly-Triton," McCann said, "but we don't have the money or manpower to open that." It is now used on a permit-only basis, primarily by senior citizen groups.

The $850,000 grant to Anne Arundel came just before the inauguration of President Ronald Reagan and was made by Interior Secretary Cecil D. Andrus from the department's Land and Water Conservation Fund, a $6 million emergency reserve. The matching grant is in addition to the $4.4 million Maryland is to receive this fiscal year under the 15-year-old federal open-space program, which distributes to the states much of the revenue from offshore oil leases and the federal tax on motorboat fuels.