A Fairfax County judge yesterday sentenced Charles D. Stewart, 22, of Clinton, Md., to life imprisonment at hard labor for his part in the murder-for-hire slaying of a Springfield service station owner last January.

Circuit Court Judge William G. Plummer imposed the sentence after Fairfax Commonwealth's Attorney Robert F. Horan Jr. said Stewart had aided prosecutors. Horan originally had asked for the death penalty for Stewart.

Horan said Stewart had, "in my opinion, told the truth" in testimony at the murder trial of another defendant in the case, Jamie Scarborough, wife of the victim. Horan therefore said he could "unhesitatingly and unqualifiendly recommend life imprisonment" for Stewart.

Plummer went along, adding that though Stewart would be eligible for parole in 15 years, he would write the state parole board now to recommend against Stewart's release when that time comes.

Stewart, who wore a light green suit in court and smiled frequently at his parents, Paul W. and Margaret Stewart, and others in the courtroom, will serve his sentence at the maximum security prison in Mecklenburg, Va., or at the state's main penitentiary in Richmond, Horan said.

Stewart's sentence yesterday draws nearly to a close the prosecutions of the four persons charged in the death of 45-year-old George Scarborough, who was shot to death as he entered his Franconia town house.

Stewart and James T. Clark Jr., also of Clinton, were convicted of fatally shooting Scarborough. Clark, who pulled the trigger according to court testimony, has been sentenced to die in Virginia's electric chair.

The victim's estranged wife, Jamie, was acquitted in October of the charge that she hired both men through an intermediary to kill her husband.

The final defendant, Betty M. Holler, of Alexandria, who has said she arranged the murder, pled guilty last week to a reduced charge of first-degree murder and will be sentenced next month.