Money that federal workers invest this month in their new tax-deferred savings plan will earn 8 1/2 percent interest. That is down from the 8 5/8 percent interest rate the fund was paying in June, and reflects the drop in the interest rate of long-term (four years or more) Treasury Department securities.

Federal and postal employes are now putting $5 million per day into the payroll deduction plan that is part of the new Federal Employees Retirement System. All investments this year are limited to a special G-fund made up of Treasury securities. Next year workers who are part of the FERS system can invest in two additional options, one offering a fixed-rate of interest and the other a higher risk/higher reward stock market fund. Employes can change their investment options, start, stop, increase or decrease their deductions twice each year.

The G-fund interest rate changes monthly. When the program started in April it was 7 5/8 percent, and in May it rose to 8 3/8 percent.

Workers covered by the FERS program -- that is nearly everybody hired since the start of 1984 -- can invest up to 10 percent of their salary this year (or a maximum of $7,000) into the fund. Anyone putting in 5 percent or more gets a matching 5 percent tax- deferred contribution from Uncle Sam.

Workers still covered by the old Civil Service Retirement System (those hired before 1984) can invest 5 percent of salary into the tax-deferred plan, but they will not get matching government contributions unless they move into the new FERS program. The open season for switching to the new FERS pension plan runs through Dec. 31.Union News The National Treasury Employees Union is urging the Senate Judiciary Committee to take a hard look at Supreme Court nominee Robert Bork. While conceding that Bork has been a "decent, intelligent" U.S. Court of Appeals judge, NTEU President Robert Tobias says Bork on the court would be "the decisive vote for rolling back hard-won rights and liberties already worn thin."Beth Moten has moved up to be legislative director for the National Federation of Federal Employees. She succeeds Tricia Thomas as NFFE's chief lobbyist.The Joint Bargaining Committee of the two major postal unions says last weekend's picketing exercise was a morale-booster, and sent a message to management. U.S. Postal Service contracts with the National Association of Letter Carriers and the American Postal Workers Union expire this month.

An estimated 6,000 union members participated in the show of support and put up an informational picket line around postal headquarters. They were joined by a contingent of white-collar federal workers from the American Federation of Government Employees union local in Baltimore. Job Mart

Agriculture in Hyattsville is looking for a writer-editor, Grade 5 or 7. Call 436-7776.

General Services Administration wants an executive director ($64,700 to $77,500) for its public buildings service. Call W.B. Snear at 566-1207. GSA also is looking for a GS 14 agency liaison manager. Call Karen Gough at 566-1805.

Office of Justice Programs wants a personnel actions clerk (typing), GS 4 through 6; a clerk-typist, GS 4; and a grants technical assistant, GS 5 through 7. Call 724-7725.

Consumer Product Safety Commission needs a GS 5/6 secretary (typing); a GS 2 through 4 clerk-typist and a GS 4/5 clerk (typing). Call 492-6500.