If, as you indicate, there is a union contract, then if it has any terms or conditions relating to termination, suspension, discipine, etc. that apply to your situation, they must be fired. However other than that, you may be fired at any time, for any reason by your employer, which means you may be fired in the meantime for the DUI. Employees in the United States are "employees at will," who can be fired for any reason at any time, except to the extent there is an employment contract (including a union or collective bargaining agreement) to the contrary. So you need to check your union contract and with your union rep to see what rights you have in this situation. Good luck.