The seizure this week of 7,000 pounds of marijuana at an airport in pastoral Hanover County, Va., is one example, according to drug enforcement officials, that the wild anl treacherous mountains of the Texas-Mexico border are no longer as safe for drug smugglers as they once were.

The drug traffic along the border, according to John C. Bullard, assistant regional director of the Drug Enforcement Administration (DEA), is "still a lot more than we can handle, but with the number of arrests, the smugglers are looking for safer places."

The green fields and bucolic countenance of a place like Hanover, Bullard said, are becoming more appealing to drug smugglers who hope that the law enforcement officers there will be as unsophisticated as the lifestyle.

In addition, Bullard said, there is the sheer volume of the illegal drug trade to consider. "It's like a flood," he said. "The more water you've got, the farther it will go."

In this case, the rising tide consists of marijuana, with 55,000 pounds of the drug seized by law enforcement officials along the border last year. Now, Bullard said, DEA agents are finding the smugglers using larger planes and flying farther inland with their contraband in the hopes of minimizing the possibility of arrests.

Planes laden with drugs are so common in Florida, Bullard said, that "people immediately know what's going on. I guess they figure they won't look so obvious in places were drug arrests aren't so common."

Large planes like the DC-4 used in the Hanover case have been found flying into Tennessee with as much as 18,000 pounds of marijuana, while boats with similar quantities have been seized as far north as Delaware and Mystic, Conn.

The difference between smuggling along the border and farther north often involves more than distance and a dissimiliarity in landscape.

In El Paso, drug enforcement agents tell hair-raising tales of being fired at by AK-47 automatic rifles as the fly their planes along the Mexican border at tree top level. They talk of hundreds of planes illegally entering the weekly, of converted B-25s hauling done of drug-heavy Lear jets smashing into barren mountainsides in the night.

In Hanover County, they talk about how odd it looked to see a gray and white DC-4 lumbering up an airstrip built for small- single-engine private planes, and how if it were look for the fact that there was a pilot's class there that night, the marijuana would have disappeared into the darkness.

Along the Mexican border, the agents sit around the small airports that dot the landscape and have coffee with the smugglers, picking up information, fencing verbally, even arguing the merits of marijuana laws.

In Hanover, airport manager Julian Walker, the man who alerted the county police to the suspicious looking plane, had never see marijuana before. "But I heared about those dope smugglers," he said. "They're dangerous people."

Along the border, where agents complain that their planes are not half as good as those flown by the drug transporters, there still exists an impressive arsenal to use against the smuggler. Included is a small fleet of light planes equipped with oversized gas tanks, an elaborate computer center that stores fresh information about illegal border crossings all over the U.S., radar and electronic sensors that the customs men frequently set out in combat-style patrols into the wild terrain around the border.

Hanover county investigator Mitchell E. Hopkins, on the other hand, had a revolver, a couple of deputies and a dog when he went to check the plane that the pilot said was carrying antique furniture. "Of course I didn't believe him," Hopkins said. "We know that much."

Along with seizing the marijuana Tuesday night, Hopkins arrested three men on board the plane. Police are still looking for several others who were seen running away after abandoning a flatbed truck and forklift that police said were brought in to unload the contraband.

Charged with possesion of marijuana with intent to distribut were Franklin M. Phillips, 22 of Lousville, Ky., Dutch Robbins, 51, of West Palm Beach, Fla. and Robert G. Eby, 29 of Ft. Lauderdale, Fla.

In addition, police suspect that an unidentified man killed by a truck on Interstate 95 near the airport Tuesday night may have been one of the people seen fleeing from the airport. Police have not yet identified the man, however.