I am looking for a non dimensional parameter that will tell me if I can neglect the flow of one of the gases and just consider the mass and momentum conservation equations for the background species. This is not a homework question. For example: If there is a pipe dumping a contaminant into a river at a rate of a few kilograms per hour, it will not affect the overall mass or momentum of the overall flow. I am having a lot of trouble finding this parameter. All of my fluids textbooks just say something along the lines of "it will be assumed the flow of the contaminant is negligible and will not affect the overall dynamics". I have a feeling it will have something to do with the mass fraction, but I can not find any specific statement in literature. Thank you for your assistance.