I tested positive on a random urine test at work alcohol level of 0.05%, I have been discharged and have a grievance hearing coming up tomorrow, how can I fight this and prove that I wasn't impaired? I live in CA and work for a cement plant

If it registered.. then all research and studies on the subject show you were in fact impaired, long before you felt the effects of it yourself. The thresholds for impairment weren't a random number...research proved all subject have measurable impairments at the lowest threshold.

I think your chances of winning are very, very low. WITH a fantastic lawyer. Zero if you defend yourself.

Does your employer have a written policy regarding alcohol consumption? A BAC of 0.05% is below the level of being legally impaired with respect to driving in all states. However, given that your employer does random checks I suspect that they probably have a more stringent policy, which should be in writing and which you should have agreed to as a condition of employment. For example many employers have a policy that you are considered to be "under the influence" if alcohol is found in your body to be "more than an identifiable trace," which 0.05% certainly is. Is that what your employer's policy states? If so, I think your only recourse is to ask that they produce the consent form that presumably you signed when you were hired in which you acknowledged the policy - perhaps if they have lost it, or you never signed one in the first place, you can plead ignorance of the policy.