An undercover investigation into alleged widespread drug trafficking at two Fairfax County shopping malls led to the predawn arrests yesterday of 15 adults and three juveniles on charges of distributing various controlled substances.

County narcotics investigators used informant tips and staged drug purchases over a five-month period at Springfield Mall and the Rosehill Shopping Center, the scene of "several undercover drug busts involving phencyclidine (PCP)" last year, according to police spokesman Warren Carmichael.

The suspects were awakened at their Fairfax County homes about 5 a.m. yesterday by 12 members of the vice and narcotics units, the criminal investigations division and regular patrol officers. All but one of the arrests were drug related and the adult suspects were taken to the county jail. aThe three 17-year-old juveniles were released in their parents' custody.

Narcotics investigators called it "the largest simultaneous arrest to occur in the county in at least a year." They said they found only negligible amounts of drugs at the homes, but the charges generally involved the distribution of PCP, marijuana, cocaine and barbiturates that allegedly occurred between last November and this month.

Carmichael described those arrested as suspected "street vendors" rather than high volume dealers.

No further arrests were anticipated, Carmichael said, adding that it was believed the raid eliminated "the maximum number of current drug dealers at both sites."

Charged with one count each of distributing cocaine and marijuana was Leroy F. Davis, 20, 6224 Edison St.

Charged with one count of marijuana distribution were Richard D. Leffell, 27, 6023 Amherst Ave., Springfield; Larry D. Spencer, 20, 9506 Yawl Ct., Burke; Billy Lee Clark, 20, 7127 Barry Rd.; Donald J. Walthall, 25, 5915 Ridgeview Dr.; David M. Knowles, 18, 8305 Whickam Rd., Springfield, and Walter J. Overkott, 21, 5008 Celtic Dr.

Deborah L. Boyles, 21, 4305 Brookside Dr.; Michael L. Kohler, 20, 3305 Spring Dr.; and Harry L. Frazier Jr., 20, 3305 Spring Dr. were charged with one count of diazepam sales.

George R. Berube, 20, 6105 Scotch Dr.; Lee E. Athey, 23, 6220 Edison Dr., and Sherry L. Frazier, 26, 3305 Spring Dr. were charged with single counts of PCP, cocaine, and methaqualone sales respectively.

Stephen B. Willis, 18, 7217 Hadlow Ct., Springfield, was charged with three counts of marijuana distribution and 19-year-old James W. Phillips, 6823 Ben Franklin Rd., Springfield was charged with possessing a sawed off shotgun.