Colombia's drug empires have begun to route great quantities of cocaine through Mexico, taking advantage of its long and porous United States border.

In the past five months, Mexican authorities have seized almost eight tons of Colombian cocaine, including a 4.8 ton cache that was the biggest single haul of the illicit drug recorded anywhere in the world.The seizures represented a huge increase over the corresponding period the previous year and underscored that Colombia's cocaine conglomerates are building new supply lines in their assault on the rich U.S. market for illicit drugs.

Mexico produces no cocaine of its own and narcotics officials say that the shipments impounded here all came from Colombia, the world's biggest exporter of refined cocaine, and were destined for the United States.

"The Colombians have begun switching (their routes) away from Florida and their prime alternative is the Mexican-American border," said a narcotics expert. "This shift is likely to continue."

Sitting astride major air and sea trade routes from South America to the U.S. East Coast, Florida has long been the main point of entry for cocaine shipped into the United States. In 1987, Florida accounted for almost 80 percent of all the cocaine seized in the u.s.

According to the U.S. Drug Enforcement Administration, Florida remains the principal U.S. gateway for cocaine but its share in the overall traffic is declining.

Since the beginning of the year, DEA and other authorities seized more than 10 tons of cocaine in Florida and smashed several important distribution networks.

"Better law enforcement in south Florida has forced the South Americans to look for other routes," said Jack Hook of the DEA's field division in Miami. The assessment is shared by the DEA's headquarters in Washington.

Ironically, such successes show that the forces of law and order are winning some battles but not the war against the rich and ruthless Colombian trafficking rings who account for most cocaine consumed in the United States.

The new pipelines through Mexico to the American south and southwest - Louisiana, Texas, New Mexico, Arizona and California - make use of a border which is even more difficult to control than the hundreds of inlets and waterways which attracted drug runners to Florida.

Spanning 1,952 miles, the U.S.-Mexican border is the busiest in the world, with more than 130 million crossings a year. There is no way of making more than cursory checks of travelers and vehicles.

Smuggled goods cross the frontier as freely as illegal immigrants.

"It's the story of our lives," said a U.S. narcotics official based in Latin America. "One step forward, two steps back - or three, four, five, six . . . There are no real advances. We are not winning."

Latin Americans familiar with the multi-billion dollar cocaine trade say the loss rate in drug shipments through Mexico - despite the recent big seizures - is relatively small though overheads are higher than those of multi-ton consignments to U.S. seaports.

According to these sources, most of the cocaine piped into the United States through Mexico is flown here in consignments of several hundred pounds.

Offloaded at clandestine airstrips, the cargo is taken either to "warehouses" or broken up into smaller loads and ferried across the border in an infinite variety of ways.

In February, for example, Mexican authorities seized 330 pounds of U.S.-destined cocaine hidden in sacks of peanuts piled on a truck.

The bulk of the cocaine moved from Mexico to the United States goes by land, according to Latins familiar with the traffic, because drug pilots are increasingly reluctant to fly into U.S. airspace despite rich rewards for successful runs.

A 350-pound load - standard for the turbo commander 1000 favored by smugglers - can net its pilot $1.5 million for a single flight, sources close to the traffic say.

But the risks are high. "If you are caught, you are looking at a long stretch in jail," one knowledgeable Latin commented. "They get you for smuggling, illegal possession, conspiracy and a whole slew of other charges.

"If you run into trouble in Mexico, chances are you can pay your way out."

One reason for the preference for ground transport appears to be a new air surveillance network on the southwest border which involves huge balloons, each carrying 1.5 tons of radar equipment to detect low-flying aircraft.

The balloons, bigger than a Boeing 747, appear to have had considerable deterrent effect. But some sources say their surveillance has been beaten by particularly daring pilots using a technique called "mating."

This involves two aircraft flying in formation, one as close above the other as possible, to make the radar screen show a single blip.