If you have a flat outdoor concrete surface, pour some fluid on the surface and wait a few minutes until it evaporates. Lather rinse repeat.
In the summer around here I could probably dispose of a gallon an hour without ever having enough fumes in one place to be dangerous. So the job can be done over a couple weekends while otherwise hanging around the house.
As between polluting the air with VOCs versus polluting groundwater with dumping, I’d much rather use the atmosphere as my sink.
I’ll also point out that if burned in an engine 100% of those gasoline molecules, or their breakdown products, would have been released into the atmosphere anyhow.
Put one side of the gas can on a board or other support so as to create a specific low point. Allow wait a few minutes to allow the water to settle. (In an airplane I’ve heard one should wait ten minutes after refueling before testing for water during the preflight. But in that case there’s a lot of mixing going on.) Put the end of the siphon tube at the lowest point of the makeshift ‘sump’ and draw the water off into the glass container. When you have a layer of gasoline in the jar, the remaining liquid in the can should be gas.
I’ve bolded the important part - a well-tuned engine won’t be chucking out much in the way of unburnt fuel.
The breakdown products will be largely CO[sub]2[/sub] and water, which are a lot less noxious. (Not that I am suggesting that a few gallons’ worth of petrol vapour spread over a month or so will be the end of the world…)
Up until recently, burning yard waste was permitted where I live. One day I decided I needed a little accelerant to get the fire going, so I used just a little gasoline. By the time I’d rolled up some paper to use as a torch and lit it, the gas fumes had made their way to the other side of the fire ring and out the gaps of the blocks. When I lit the pile flames extended past the ring and onto the lawn, which slopes away from the house. The fumes burned off quickly and no damage was done; but I learned never to start a fire with gasoline.
Yeah, you wouldn’t want to use your fish tank siphon. Just about any hardware store should have a fuel siphon for not much money.
I should have mentioned that for proper siphoning the gas can needs to be higher than the jar. Undoubtedly you know this, but I’m putting it out there anyway.
Call your local fire department, and see if they’d use the gasoline for a training exercise. We commonly use gasoline fires in large metal trays for fire extinguisher and foam stream training.
Some will also use it for hazmat disposal training.
If it was a smaller amount (less than a half gallon), I’d use it to start a yard waste fire. Where I am, burning brush is legal under some circumstances.