U.S. Attorney Joseph E. diGenova's apparent decision to leave office next month comes on the heels of resignations by several other key prosecutors who have been directing investigations of the D.C. government, moves that are expected to cause some delays in those probes.

DiGenova, whose four-year term expired Dec. 3, is expected to join the D.C. firm of Bishop, Cook, Purcell & Reynolds in about six weeks, before any indictments will have been returned in the current probes of city contracting and alleged skimming of drugs and money by vice officers in the city's 4th Police District, sources said.

The probes have been central to diGenova's administration as U.S. attorney and some observers expressed surprise yesterday that he would leave before the probes are completed and suggested that his departure might signal the end of the investigations.

But several current and former prosecutors said yesterday that they believe the probes will suffer only brief delays because of the changes in personnel, and will not be impeded or stopped. "We have collected far too much evidence for these investigations to be ended," said a source familiar with the investigations.

"Of course there will be some delays. There always are when there are large-scale changes," a former prosecutor said yesterday. "These investigations are long-term, and the attorneys on the cases must rely on their institutional memories. But investigations go on."

Several prosecutors said it was not unusual for investigations to span the administrations of different U.S. attorneys and different presidents. For example, the investigation of Youth Pride Inc. and its head, Mary Treadwell, was begun in October 1979 when Charles Ruff was U.S. attorney. The indictment of Treadwell and others and her trial occurred while Stanley S. Harris was in the office.

D.C. Mayor Marion Barry, whose behavior and expenditures have come under scrutiny of the U.S. attorney's office during diGenova's term, said in an interview earlier this week, "It doesn't matter to me" whether or not diGenova remains.

"My agenda is not set by him," Barry said. "It's set by me."

He added, "I maintain there is no system of corruption in the District government."

Among those who have left the U.S. attorney's office in recent weeks is Stephen R. Spivack, who as head of the special prosecutions unit supervised all of the attorneys working on the city investigations. In addition, Spivack had been working on the probe of alleged payoffs to convicted cocaine dealer Karen K. Johnson in exchange for her silence 4 1/2 years ago before a federal grand jury investigating alleged drug use by Barry and other city officials.

Spivack has said that his decision to join the firm of Spriggs, Bode & Hollingsworth in Washington was prompted by an attractive offer from the law firm to specialize in white-collar defense work and was not connected to the city investigations.

Earlier this week, diGenova named veteran prosecutor Paul L. Knight to replace Spivack, a move that observers say signals an intention to continue the probes at the same intensity.

Knight has most recently served as deputy chief of special prosecutions and head of the organized crime drug task force and is known for his skillful trial work and his extraordinarily long work days. In addition, he is already familiar with the work being done on each of the investigations.

Knight has been directing the investigation of 4th District vice officers and may continue on the case, sources said. But, as with most major investigations in the U.S. attorney's office, two attorneys have been assigned to the police case, and sources said Charles Harkins, who has worked the investigation with Knight, will take over the bulk of the work now.

Roger M. Adelman, who served as senior litigation counsel, also left the office at the end of the year to join Kirkpatrick & Lockhart as a white-collar defense attorney. Although Adelman was not directly assigned to the city investigations, he had the most trial experience in the office and often advised other attorneys on their cases.

Mark Biros, the attorney who has been conducting the grand jury investigation of the District's bond program, will leave the U.S. attorneys' office at the end of the month to join the D.C. firm of Casson, Harkins & LaPallo, also as a white-collar defense lawyer.

Sources said one other attorney working on one of the part of the D.C. investigation will leave the office in the spring.