REVERE – City Councilor Mark Casella, 45, pleaded guilty Thursday in Chelsea District Court to a second offense for operating a motor vehicle under the influence of alcohol, in exchange for two years’ probation.Weathered documents from 1992 helped Casella dodge a third conviction for drunken driving and certain jail time.His guilty plea allowed Casella to avoid prosecution for a more severe third offense charge that he could have faced based on prior convictions.Casella, who’s not seeking re-election, must give up his driver’s license for two years and attend an alcohol abuse treatment program.In response to the plea, Casella said he “just wants to get rid of it and put it in the past.”Jake Wark, Spokesman for Suffolk County District Attorney Dan Conley, said one of Casella’s charges had to be dismissed due to indecipherable documents.”Given the tactical difficulties of proving an 18-year-old case and in light of (Casella’s) willingness to accept responsibility and treatment, we agreed to dismiss one of the earlier OUI charges,” he said.Attorney Gerry D’Ambrosio said paperwork on Casella’s 1992 OUI conviction in Malden Court was incomplete and falling apart.In April, Casella suffered a head injury when he crashed his car into a neighbor’s brand new vehicle while in a drunken stupor.When officers arrived at Casella’s home, they discovered a large pool of blood approximately one foot in circumference on the front doorstep.Casella reportedly had a large open wound with a bump on the right side his forehead. Officers said Casella was unsteady on his feet and had bloodshot eyes and slurred speech.According to police, the windshield of Casella’s vehicle was broken in the middle of the window, typically consistent with a person hitting their head on the windshield.The rear view mirror was reportedly hanging by the wires, and damage was located on the front, right side of the window. The parked vehicle that had been struck had damage to the left rear.The Ward 6 Councilor, who works as a U.S. Postal Service Supervisor, was also investigated after “improprieties” surfaced at the Belmont post office where he worked in September 2005.He was placed on administrative leave for two weeks as the result of the investigation, which revealed that employees were altering time cards and falsifying work hours.