They have some pretty elaborate filtration units so if they are functioning correctly I doubt it's sediment.
My bet would be water. Drain the fuel from the tank and collect it in a clear bottle. Let it sit for a few hours and see if it becomes stratified.
You would probably have better luck talking to chevron than to the station. Chevron probably does not own the station but if it is a franchise their name it on it and will probably stand behind it just to prevent bad
PR.