Herbert L. Tucker, director of the D.C. Department of Environmental Services, has resigned effective Sept. 30. "I'm just physically tired," Tucker said yesterday, "and I decided I'd sit down."

Tucker, 62, has been the department's director for three years. He said the resignation was timed to allow him to take advantage of an anticipated 6.9 percent cost-of-living benefit increase to be given to all persons who retire before Oct. 1.

Tucker said he notified City Administrator Elijah B. Rogers of his decision to resign late last month. He said he had not been forced out. Tucker was not on Mayor Marion Barry's list of allegedly incompetent administrators targeted for removal.

Tucker, who served 21 years in the U.S. Army and retired in 1961 as a lieutenant colonel, became director of the department in 1976 after the departure of William C. McKinney.

During Tucker's stewardship the city government was criticized because it repeatedly issued late and inaccurately calculated water bills. Tucker's department also once proposed a cutback in trash collection from twice to once a week. That proposal was never put into effect.

Tucker is the second major department head to leave office by retiring voluntarily since the Barry administration took office. Sam D. Starobin, another retired serviceman who was director of the D.C. Department of General Services resigned earlier this year.

Several other holdovers from the administration former Mayor Walter E. Washington have been forced out and replaced.

Rogers said yesterday that the administration regretted Tucker's departure. He said he did not yet know who Tucker's successor might be, but that interviews would begin soon.