An alleged longtime drug dealer suspected of trafficking heroin and fentanyl in Whitman was arrested during a traffic stop after an alleged transaction, police said.Whitman police said they arrested 31-year-old John Townsend of Berkley Friday evening after a two-month investigation into heroin and fentanyl activity in the community.An investigation into the trafficking operation began in February when Whitman police detectives developed information concerning the distribution of opioids on the west side of town, authorities said.