We broke the story ... Arenas was pulled over by the California Highway Patrol on June 27 for speeding ... and during the bust, cops say they found more than 100 pounds of fireworks.
Arenas was arrested -- and now, we've learned he's been charged with misdemeanor possession of dangerous fireworks without a permit.
Law enforcement tells TMZ ... because he had more than 100 pounds of bang bang, he faces up to 1 year in county jail and up to $10,000 in fines.
TMZ cameras were rolling during the bust -- as the LAPD bomb squad (carefully) removed the explosives from Gilbert's Ford truck.
Arenas is due in court for the matter next month.