A major drug bust in Birmingham.    Authorities say agents discovered 48 pounds of high-grade marijuana at a home in the 3000 block of Highland Drive.   The investigation involved several jurisdictions, and was led by the Shelby County Drug Enforcement Task Force.   Agents arrested 24-year old Joshua Christopher Burchfield, charging him with drug trafficking and unlawful possession of marijuana.   He was being held at the Jefferson County Jail.