Whether Lucio goes back to El Salvador next year as he plans, or is deported before then, he will leave behind about $4,000 as a contribution to the citizens of the United States.

After his nearly five years of work in a Georgetown restaurant, he said, that is how much has been taken from him by "La Fica." He was looking at the figure on his paycheck next to the letters FICA (Federal Insurance Contributions Act). The figure, showing the deduction for social security, is on the paycheck of millions of people, including illegal aliens.

Lucio is only one of millions of taxpaying illegal aliens in the United States who, according to a study prepared for the Social Security Administration in 1976, contribute at least $500 million a year to the social security trust funds. There is virtually no chance that they will ever draw benefits from their contributions.

For an agency that has been plagued by deficits- $96 billion was paid out of the principal trust funds in 1978, only $91.9 billion was taken in-half a billion dollars is not an inconsiderable amount, and that estimate may be low.

According to Wayne A. Cornelius, a professor at the Massachusetts Institute of Technology who has done extensive research on the subject, "One of the things that we can say with greatest authority on the basis of hard facts is that illegal aliens are subsidizing the social security system."

"You're talking about hundreds of millions of dollars a year," said Cornelius, possibly as much as $1 billion, "with literally minuscule amounts being paid out as benefits (to illegal immigrants). . . If you multiply that by 5, 10, 20 year, you're talking about billions of dollars that have been plowed into the social security trust fund."

Stanford Ross, commissioner of the Social Security Administration, said it is impossible to tell what percentage of the trust fund comes from illegal aliens, but minimized its significance.

"There are lots of people in this country who pay social security taxes who will not derive benefits," Ross said. He cited those who die before they reach retirement age, and others who do not make contributions for the required number of years to be eligible to collect benefits. "I don't really find anything very striking about the illegal alien category, I can't see the slant that this is a great enrichment of the social security system."

Ross said that most of the attention his administration has directed toward illegal aliens has been aimed at keeping them from receiving benefits.

Particular emphasis has been placed on keeping illegal aliens from acquiring legitimate social security cards."We've tightened up substantially in the last two years," said Ross. An applicant for a card must now present proof of legal permanent residence or U.S. citizenship.

But since there is no law against hiring illegal aliens in the District of Columbia or in most of the rest of the nation, few employers bother to question the legitimacy of the cards that are presented to them, and some will settle for any number the alien gives.

Giulio Santillo, owner of the tiberio restaurant on K Street NW suggested that foreign workers he has employed in the past "take a telephone number and add two digits."

On other occasions they may borrow cards from friends. "I demand to see the social security card," said one restaurant owner in Georgetown. "And I say 'Is this your name?' and of course he will say yes whether it is or not. And I say okay that is what I am going to call you as long as you are here."

One Salvadoran who was deported last week was named Neftali Garcias, according to sources at the U.S. Immigration and Naturalization Service district headquarters in Washington. But he had in his possession a card issued by the Social Security Administration with the name Salvador Batares. Neither immigration or social security authorities know how the card was obtained.

Some buy forged cards, which may cost in the neighborhood of $50, or easily available metal facsimiles, which are considerably cheaper.

Lucio bought a metal one with his real name and a fake number from the key shop at G. C. Murphy department store on G Street NW for $2.10. No one asked him for identification when he made the purchase, he said.

A Washington Post reporter bought a similar card there yesterday, without presenting identification. The number he gave was the Washington Post's main telephone number with two digits added at the end. One the shiny new card he got back, printed under the name and phony social security number was the legend "Name as registered with U.S. government."

Immigration investigators tell stories of numbers written on signposts and walls near the Mexican border that simply can be jotted down by illegal aliens passing nearby.

Ross said that fabricated numbers should eventually be rejected by the administration's computers, and illegal aliens may represent some part of the 1.4 percent of the system's "earnings items" that cannot be accounted for properly because of mixups with names and numbers.

"But I personally don't think it's there to assume that a substantial portion of these are attributable to illegal aliens," Ross said. "My guess is that with illegal aliens they're working for employers who demand legitimate cards" that are borrowed or fraudulently obtained.

It is impossible to trace how much money goes into the system through the use of such cards, Ross said.No indications would arise until the real owner of the acrd applied for a benefit and his record showed him working in several different places simultaneously.

The 1976 report, which Ross said he had not seen since he became commissioner last year, was prepared by consultant David North on the basis of data he obtained in 1975 while doing a study for the Labor Department on the impact of illegal aliens on the U.S. work force.

On the basis of interviews with 793 illegal aliens apprehended around the nation by the INS, North concluded that 77.3 percent had had social security taxes withheld from their pay-checks. They also tended to apy other taxes: 73.2 percent had federal income taxes withheld, while only 31.5 percent filed returns that would enable them to get refunds; 44 percent had hospitalization payments withheld.

Their use of direct public assistance, on the other hand, was slight: 27.4 percent used hospitals or clinics, only 3.9 percent collected unemployment insurance; 3.7 percent had children in U.S. schools; 1.4 percent participated in U.S.-funded job training programs; 1.3 percent got food stamps, and 0.5 percent got welfare payments.

North said that some employers may pocket the deductions, but the dollar figures he arrived at were based on very conservative estimates.

North assumed that only 1 million illegal aliens were working in the U.S. and were earning an average of $120.20 a week in 1975. By constrast, a Social Security Administration statistician has estimated that there were from 3.9 million to 5.7 million illegal aliens between the ages of 18 and 44 in the United States during the month of April 1973.

North then calculated that the three quarters of a million who had deductions withheld would contribute $565.3 million to the trust funds. Subsequent independent studies suggest that the percentage of illegal aliens who have social security taxes withheld is now closer to 90 percent.

An illegal alien could theoretically collect social security benefits if he were able to stay in the United States long enough and had a legitimate card, according to a spokesman for the Social Security Administration.

But, as Cornelius put it, "Even those who do have legitimate numbers are not likely to claim benefits beause there's too much risk" and fear involved when dealing with a government agency.

Without a legitimate number, the burden is on the illegal alien to prove how long he worked in the United States, how much he earned and how much was deducted, which is difficult even for a citizen to do.

North's report was originally submitted to John J. Carroll, the current acting director of the office of research and statistics.

"It was very interesting," Carroll said last week. "I sent it around to the various components to see if they would follow up on it." To his knowledge, Carroll said, no one did. CAPTION: Picture, A reporter bought official-looking card with phony number for $2.10.