A federal grand jury investigation has been opened here into allegations that a former chief aide to Sen. Herman E. Talmadge (D-Ga.) falsely obtained Senate funds and that the money was passed along through a secret bank account to the Georgia senator and his family.

Knowledgeable sources yesterday said the grand jury, which began meeting earlier this month, has begun gathering information about the charges, which have come from the aide himself, Daniel Minchew, Talmadge's former administrative assistant and campaign chairman, Minchew has denied any wrongdoing. So has Talmadge.

The grand jury has issued a subpoena to the Riggs National Bank here for the bank's complete files involving an account in which $12,894.67 was deposited by Minchew. The former Senate aide said he obtained the money by filing false expense vouchers for Talmadge to the Senate in 1973 and 1974.

On Friday Talmadge sent a personal check to Senate Secretary J. Stanley Kimmitt for $37,125. The money was repaid to the Senate by Talmadge for improper expense claims the Georgia senator made between 1972 and 1977. Auditors who examined Talmadge's files concluded the claims were made for expenses that were either nonexistent or not reimbursable under Senate rules. This money was separate from the roughly $13,000 involved in the Minchew allegations.

Investigators for the Senate Ethics Committee have also subponenaed the Riggs bank records on the account. The Ethics Committee is conducting a broad investigation into Talmadge's financial affairs.

Minchew, who left Talmadge's staff in October 1974, is now a member of the International Trade Commission here. Through his Atlanta attorneys, Robert G. Flerer and Bruce Morris, he has claimed that he acted on Talmadge's instructions in filing the false Senate claims and that money from the bank account ended up being paid to Talmadge.

Talmadge has denied either knowing about the Riggs bank account or benefiting from it, and last month made a memo from Minchew about the account available to Justice Department investigators.

Minchew has declined thus far to say whose name the bank account was in. Sources said yesterday that in addition to the Riggs account Minchew claims he maintained for Talmadge, additional accounts were opened at the Riggs bank for Minchew and his wife, Shirley.

It was also learned yesterday that the Riggs account Minchew says he operated for Talmadge contained additional funds beyond the nearly $13,000 that has been reported so far. A source familiar with the account said "a considerable amount" - more than Officials of the Senate committee said they asked that the records be turned over by the bank tomorrow.

Ethics Committee sources said it is likely Minchew will testify next month in closed session. The sources said the Justice Department is expected to request a 30-day delay from the committee before questioning Minchew in order to allow the grand jury to conclude its own criminal investigation.

[TEXT OMITTED FROM SOURCES] several thousand dollars above the reported figure - was in the account during the 1973-74 period.

It was not immediately clear yesterday what happened to the money in the account beyond the reported $13,000.

Mel Chrisman, senior vice president for the Riggs Bank, declined yesterday to comment on the subpoenas issued by the grand jury and the Ethics Committee. "I will comply with any subpoena issued to the bank at the time when the subpoena requires me to comply," he said.

Federal sources said they expected to get the information sought by the grand jury within the next two weeks.