There are bombshells aplenty awaiting the Senate Environment and Public Works Committee today as it prepares to mark up a four-year authorization bill for federal highway projects, but perhaps none as remarkable as a last-ditch effort to create a detour around an environmental law for a 10-mile, $1 billion Hawaiian expressway.

Supporters and opponents have been flooding into Washington for days in preparation for the showdown, buttonholing members, telephoning reporters and floating enough fact sheets to set off the paper work alarms in the Office of Management and Budget.

The object of all this attention is Interstate H-3, which was planned more than 25 years ago as part of the state's federal highway system and is expected to be, mile for mile, the most expensive highway ever built.

H-3's price tag, however, isn't the issue, even in these days of fiscal austerity. The issue is whether H-3 should be granted an exemption from a 20-year-old federal law designed to protect parks and historic areas from highway intrusions.

As now designed, the six-lane expressway would carry traffic from the windward side of Oahu to the heavily populated leeward side, across the Koolau Mountains and along the boundary of a wilderness park called Ho'omaluhia, or "Place of Peace and Tranquility." The 9th Circuit Court of Appeals agreed with road opponents that the route skirting Ho'omaluhia would violate the law, and the U.S. Supreme Court declined to consider an appeal.

Under ordinary circumstances, that would have been the end of H-3. The state could either reroute the highway or use the money for other highway improvements or mass transit, as city officials in congested Honolulu have ardently urged.

But with the support of the state's entire congressional delegation, the Hawaii Department of Transportation has decided to try another tack: It wants Congress to exempt H-3 from the law.

As drafted last year by Sen. Daniel K. Inouye (D-Hawaii), the provision would suspend any and all federal laws that stand in the way of the road, much as Congress liberated the Tennessee Valley Authority's Tellico Dam from its headaches with the endangered snail darter. In 1979, Congress simply decreed that the Endangered Species Act didn't apply to the Tellico Dam.

Cheryl Soon, deputy director of the Hawaii transportation department, said the intent was to get the case back into court on more favorable terms for H-3. "It's not that the law is incorrect," she said. "It's the way it was applied that was incorrect."

But the proposal quickly widened the battle over H-3 from an essentially local skirmish to a national fracas, and may have drawn more attention to the details of H-3 than its supporters would have wished.

An economic analysis in 1982, for example, estimated that the highway would return about 22 cents worth of benefits for every dollar spent on construction. Put another way, the 32,000 cars that would use the six-lane road each day would be subsidized indefinitely to the tune of $3.30 for each 10-mile trip.

Moreover, Honolulu city planners contend that H-3 would create more problems than it would solve, partly because it would drop commuters in an already-congested area seven miles from downtown Honolulu.

Virtually every national conservation group has sided with the road's opponents, for economic and well as environmental reasons, and last week committee chairman Robert T. Stafford (R-Vt.) made his own position clear in a letter to committee colleagues.

"I am convinced that the H-3 project, as conceived almost 30 years ago, no longer addresses the changing transportation needs of the island," Stafford wrote, adding that he was "confident that if this exemption is granted, we will be flooded with similar requests that will engulf us in dozens of local controversies."

According to Soon, however, no one in Congress has suggested to the highway's supporters that it should not be built. "No one has ever told us that they meant the law to apply that way," she said.