I mostly preparing for 72 hour survival scenario. I am luck to live in colorado, so my water is fairly safe already.

My thought on the original question was if I was in new environment. Example One: Desert: Find water in tire ruts.
Second example: Water in farm field.

It sounds like avoiding the "floating" petrolium by aquiring water from the "middle" and use activated charcoal to help remove pesticides and fertilizer is a good option.