Attorney General William French Smith said yesterday that U.S. authorities, in cooperation with the government of Colombia, have arrested more than 500 people and have seized nearly 6.5 million pounds of marijuana over the past 14 months.

The effort, nicknamed "Operation Tiburon" (Shark), was concentrated in southern Florida, and has resulted in 495 arrests in the United States and another 40 in Colombia and the seizure of 95 boats by U.S. authorities.

The marijuana seizures included 1.76 million pounds in the United States and 4.7 million pounds in Colombia, and represents an estimated 20 percent of the flow into this country during the 14-month period.

Assistant Treasury Secretary John M. Walker said that the last several months of the project involved the use by the Customs Service of airborne warning and control system (AWACS) aircraft.

It was unclear how many of the seizures were due to Operation Tiburon. "A great deal of what occurred would have occurred anyway," said Adm. John B. Hayes, commandant of the Coast Guard. But Smith said most of the seizures could be attributed to the special operation.

Customs, the Coast Guard and the Drug Enforcement Administration worked on the project with state and local law enforcement authorities in the Gulf states and along the Eastern Seaboard.

Smith said a decision has been made to add 20 DEA agents and 40 FBI agents to offices in south Florida to try to deal with widespread drug trafficking. Although the Coast Guard has been given increased responsibilities for intercepting drug traffic, that agency is faced with major cutbacks, including 3,500 personnel, 11 Coast Guard cutters and three air stations.

Colombian Ambassador Fernando Gabiria appeared at the news conference called to announce the operation and said the Colombian government is determined to fight "this traffic we consider lethal and abominable."

Asked whether the Colombian government would be willing to use the herbicide paraquat to eradicate the marijuana, a major cash crop in his country, he said the government might consider it, but he added that he would like to see the United States go first with a paraquat spraying program, "say in the state of California."

Government figures released yesterday indicate that the retail price of marijuana ranges from $240 to $3,200 per pound, depending on quality, and that annual marijuana consumption in the United States is between 23 million pounds and 34 million pounds per year, with an estimated 25 million Americans using the drug regularly.