Proponents of the North American Free Trade Agreement (NAFTA) have been fretting over persistent allegations that U.S.-owned manufacturing plants in Mexico, known as "maquiladoras," have been illegally disposing hazardous wastes. These claims pose a stumbling block to approval of the accord.

The U.S. Environmental Protec-tion Agency (EPA) has been frustrated in trying to enforce the law because it lacks the means to collect reliable data and information on cross-border waste shipments.

Now, however, the EPA thinks that it is on the way to tackling the problem with a painstakingly assembled computer-based system that processes data and information on wastes from both the United States and Mexico.

Despite its limitations, the system represents a giant leap forward from an enforcement program that, until recently, largely depended on informants, citizens' complaints and chance encounters to catch illegal waste handlers.

Nevertheless, the system can't save NAFTA from mounting political opposition and won't cure other trans-boundary pollution problems. For one thing, a number of other waste streams are not being charted. For another, the U.S.-Mexico border area has few proper disposal facilities and virtually no enforcement program.

Yet, the system has scored some successes. EPA says that in June it proposed nearly $200,000 in civil penalties against four U.S.-based maquiladora owners for alleged waste manifest improprieties and for misnaming waste haulers. The agency claims that it has a number of investigations in the works and, more importantly, that maquiladora owners are starting to ask about training in how to correctly dispose waste.

Not long ago, Mexico and the United States did not share waste shipment documents and other related information. Thus, enforcement officials at EPA were uninformed about the chemicals ma-quiladoras were using and the wastes they were generating. At the same time, U.S.-owned plants were required to return hazardous waste derived from imported chemicals to the United States for disposal. EPA officials report that when they visited the offices of Mexican environmental authorities several years ago, they found piles of unsorted documents lying around in boxes. Meantime, border checks by U.S. Customs were spotty and inconsistent, and shipping manifests arrived at the EPA weeks late in daunting batches that went unreviewed.

After surveying about 1,000 maquiladoras in 1990, Mexican officials found that only about 30 percent filed waste shipment manifests. Only a small number bothered to register for transportation of the wastes through Texas. At the same time, the EPA was unable to track the origin of waste shipments in California.

Environmentalists cited illegal dumping of wastes as proof that NAFTA would produce an ecological disaster, besides rewarding firms that moved to countries without strong environmental laws. In June, a federal district court ordered the Clinton Administration to study the trade agreement's possible environmental impacts.

Mexico's EPA counterpart, the Secretariat of Social Development, and U.S. environmental officials increasingly have been exchanging documents on chemical imports, waste shipments, registered haul-ers and disposal sites. Since neither agency had a verified list of maquiladora owners, an EPA employee used information from old newspapers and waste trade magazines to find relationships between U.S. corporations and Mexican plants. He found illegible waste manifests that had crossed the border. Some manifests did not contain the name of the Mexican generator, and some transporters used fictitious EPA identification numbers.

Slowly but surely, both countries have assembled a data base that allows enforcement officials to tie specific waste streams to certain maquiladoras, to final improperly identified waste, haulers and facilities and to locate waste generators that generally operate without documenting waste shipments.

Still, some environmentalists are unhappy. They want the data base open to public access and they want authorities to address problems outside the border region.

EPA officials concede that the system does not track border-area Mexican-owned businesses nor does the data base include chemical shipments and waste disposal for the increasing number of maquiladoras that deal only with Mexican suppliers. Moreover, the two governments trade raw data only on a quarterly basis.

About six million tons of hazardous waste was shipped from Mexico into the United States in 1992, according to an EPA estimate. That volume represents a 17 percent increase from 1991. EPA believes that the figures also represent a growing compliance by the maquiladoras, but less than half the amount that manufacturing reports would justify. The agency suspects that many plants are simply storing wastes to avoid both the costs and the regulatory spotlight associated with proper disposal methods.