Conservation writer Ted Williams has it here.
First, no crop grown in the United States consumes and pollutes more water than corn. No method of agriculture uses more insecticides, more herbicides, more nitrogen fertilizer. Needed for the production of one gallon of ethanol are 1,700 gallons of water, mostly in the form of irrigation taken from streams either directly or by snatching the water table out from underneath them. And each gallon of ethanol produces 12 gallons of sewage-like effluent.
And then there is the question of whether we get a net gain or net loss in energy from ethanol. I'm still dubious.