A city public works employee has been placed on paid administrative leave after being arrested on drug charges. David Caira, 50, was arrested by Newton police on charges of possession with intent to distribute heroin, possession of cocaine, and conspiring to violate drug law. He was arraigned in Newton District Court on Aug. 9, pleaded not guilty, and was released on personal recognizance, said MaryBeth Long, a spokeswoman for Middlesex District Attorney Marian Ryan. Waltham Mayor Jeannette McCarthy said she received a tip about Caira’s activity and reported it to Acting Police Chief Keith MacPherson. McCarthy said there are civil service proceedings in the works, and Consolidated Public Works Department director Michael Chiasson will determine what consequences to impose. The results could include taking away Caira’s pay, McCarthy said. Those proceedings could take a few weeks to get underway, she said. Caira’s job includes driving special public works vehicles, for which he has a specific license, McCarthy said.