Thelma Cavanaugh;82;of 412 Wood St.;Belle Vernon;died Monday;Aug.27;1990;at 2:30 p.m.;in the West Hills Health Center;Moon Township.  She was born in Fayette City Dec.14;1907;a daughter of the late Stephen and Mary E.Smith Riskey.  Mrs.Cavanaugh had lived in the Belle Vernon Area for the last 50 years.  She was a member of the Belle Vernon American Legion Auxiliary.  In addition to her parents;she was preceded in death by her husband;John Cavanaugh;on Sept.28;1982.  She is survived by a daughter;Mrs.Yvonne Koeper of Moon Township;Coraopolis;a foster son;Willian Hixenbaugh of Elyria;Ohio; a brother;John Riskey of Monroeville;a sister;Mrs.Margaret Bickerton of St.Petersburg;Fla.;10 grandchildren;and four great-grandchildren.  Friends will be received at the Ferguson Funeral Home Inc.;700 Broad Ave.;Belle Vernon;Wednesday from 2 to 4 and 7 to 9 p.m.  Funeral services will be held Thursday at 11 a.m.in the funeral home.  Interment will be in Belle Vernon Cemetery.  The Ferguson Funeral Home Inc.is in charge of arrangements.