The Supreme Court, in its first signed decision of the 1984-85 term, ruled yesterday that the federal government is required to pay only the fair market value of $225,000 to Duncanville, Tex., in compensation for taking over the local garbage dump.

The unanimous decision, written by Justice John Paul Stevens, overturned an appeals court ruling last year that would have required a payment of $723,624.01 based on what it would cost the city, a suburb of Dallas, to replace the 50-acre facility.

The decision in U.S. v. 50 Acres of Land is likely to save the federal government -- and cost cities and states -- substantial sums whenever local property is condemned for federal projects. In this case, the federal Lakeview Lake flood-control project required taking land used by Duncanville since 1969 for the Ballwag landfill.

Duncanville, citing the Fifth Amendment's requirement that the government pay "just compensation" when it takes private property, argued that the federal government should pay the full price for a new facility because a municipality has an obligation to provide such a service.

But Stevens said a city is entitled only to the same "fair market value" compensation that is owed a private individual if the government condemns that person's property for public use.

Most private-property owners are not legally obligated to replace property taken by the government, Stevens acknowledged. But he noted that in some cases -- such as a condemnation that forces a homeowner to find a new place to live -- there are similarities.

The court, which began its term Oct. 1, is expected to issue about 150 signed opinions before it recesses in early July.