Dennis, this is to continue our discussion - if you want to.
I worked for an EPA contractor for years and did my undergraduate thesis on remediation of groundwater contamination. I can assure you that even if MTBE was completely eliminated, we would have significant groundwater and waterway contamination. The problem of leaking underground storage tanks, spills when re-filling such tanks, and rainwater runoff from gas stations would ensure that.
There is no question that oxygenated gas has had significant air quality impacts - making it more breathable for all of us. In cities like Los Angeles it is critical for reducing summer smog. Without oxygenated gas we would most certainly have much higher rates of asthma and other lung/breath related issues within the population (especially among kids and older individuals).
The science behind oxygenated fuel is pretty clear about this. If you go to the EPA website you can read more about it
. If you want more about MTBE here is a link
The government does not mandate MTBE - it is just an industry chosen oxygenate. There is a push in some places to eliminate MTBE because it migrates further underground than fuel itself and it is highly soluable in water. So states like Massachusetts are requiring ethanol instead of MTBE.
If we want to have cleaner air and cleaner water then the only real solution is to move toward wind, solar, and geothermal energy. As long as we are reliant on petroleum based energy sources we will have air and soil/water contamination.