I have an older truck that is not running ATM because it’s gas is contaminated with a bunch of water. Later today, I am going to drain the gas tank, but when I’m done I’ll have about 5 gallons of a water/gas mix. I can handle draining the tank alright, but I have no idea of how to dispose of this old gas. What is the proper way to get rid of this?
Mix? Won’t it separate out if left standing, allowing you to separate it by siphoning or titration?
I think local disposal laws might vary. As a painter/remodeler, I have to pay to dispose of certain hazardous wastes through the sanitation department.
One of your city bills should have a phone number. If not, check the Blue Pages.
Call your local dump. Mine is happy to take paints, solvents and chemicals rather than have people throw them into the drain.
Mine also didn’t charge me, since I’m only a homeowner and wasn’t getting rid of industrial quantities of the stuff. They were happy, though, to take about 30 half-filled paint cans left behind by the previous owner.
I remember an SDMB thread a couple years ago that dealt with this issue. Try searching for it. From the thread I seem to recall that simply burning it was one of the best solutions. Assuming, of course, that local ordinances will allow it…
Hmmm, autumn is the perfect time for a bonfire. 
Seriously, check with a quick lube place, they might take it off your hands for you. Or a recycling center. We have a place locally that will accept hazardous household waste, such as this. Definitely do not dump it.
Mangetout is correct - it should be reasonably easy to separate.
Get a largish glass (or at least transparent) container. Pour in some of the contaminated mixture and let it sit until it has separated. You’ll have fairly clean gas on top, and a mostly water mixture on the bottom. With a plastic syphon hose and some care, you can syphon the water into another container, leaving usable gas. You’ll be generating a lot less pollution than if you try to dispose of the whole 5 gallons.