Mayor Marion Barry is on "trial." That's the perception among many in the city regarding leaks about the mayor and a grand jury investigation of drug use by city employes. Ironically, he is being "tried" on a charge that may never go before a judge or jury.

All this becomes even more unfair when one considers the debate that has raged for years within legal circles about grand juries. What a grand jury is supposed to do is both simple and clear. That they often have failed has prompted criticism and resulted in calls for reform.

No person stands more alone than a witness before a grand jury. He faces an often hostile prosecutor and 23 strangers in a secret hearing with no judge present to guard his rights and often no lawyer to counsel him.

The federal grand jury, composed of people selected from voter registration or auto registration rolls, considers evidence and hears testimony presented in secret by a prosecutor. The prosecutor can ask the grand jury to subpoena witnesses and can offer immunity from prosecution. Some believe the jurors' only job is to bring criminal charges against possible wrongdoers. But they are also responsible for shielding people from being unjustly subjected to a criminal trial.

However, some critics charge grand juries too often act as rubber stamps for prosecutors. Judge William Campbell of the U.S. District Court of Chicago has described the grand jury as "the total captive of the prosecutor, who, if he is candid, will concede that he can indict anybody at anytime for almost anything, before any grand jury."

Abuses arise, say students of the grand jury system, due in part to the ability of prosecutors to lead the jurors in any direction they choose and to the fact that most jurors are lay persons who rely heavily on a prosecutor's expertise. Grand jury proceedings also lack procedural safeguards, such as those required at trials. Other critics say the secret nature of the proceedings provides an opportunity for misuse and prosecutorial excess.

In l977, the American Bar Association resisted opposition from the Justice Department and approved a set of principles to help reform the grand jury system. The next year U.S. News and World Report noted that this "centuries-old bulwark against government oppression . . . is coming under inceasing fire both in state capitals and in Congress. The result is a drive by critics to abolish or reform an institution that is misunderstood by millions of Americans."

Just last year, in an effort to limit grand juries, the U.S. Supreme Court ruled that federal prosecutors should no longer be allowed to share secret grand jury information routinely with interested federal enforcement agencies. And in an April l983 issue of the Legal Times, John F. Conroy, a local attorney, argued persuasively that "it is apparent that a reaffirmation of the secret nature of grand jury material is in order."

Some of these structural problems are behind Barry's charge that the U.S. attorney's office is out to "lynch" him with leaks. And many documented abuses of the system give a degree of credence to that charge.

To my mind, U.S. Attorney Joseph E. diGenova doesn't take enough responsibility for the leaks about the grand jury investigation. Beyond denying charges that he was the source of news reports about the grand jury investigation and stating that "ongoing investigations are never discussed, and they should not be," he has not detailed a whole range of judicial and other processes he could use to plug leaks. While witnesses who appear before a grand jury are permitted to discuss publicly their testimony, grand jurors and prosecutors and other court personnel are prohibited by court rule from discussing proceedings.

The leaks have placed Barry in a difficult position. First, he said it would be inappropriate for him to comment at all on the proceedings. But the leaks continued and Barry volunteered the information that the mayor had a personal relationship with Karen Johnson, a city worker convicted of dealing cocaine. Still, the leaks persisted, feeding rumor and innuendo, such as trouble within the Barry household.

"We've had six years of marriage and we're already planning our 10th anniversary," Barry said in a telephone denial. "I have a lot of social relationships, but I only have one sexual one -- that is with my wife."

The veracity of Barry's statement is as open to debate as is the ongoing controversy about the grand jury process. But one thing beyond debate is that when secrecy is as leaky as a sieve, it make the grand jury unable to carry out its job. In the end, that is a disservice not only to Barry but to us all.