Anyone know how long your inlet hose will need to be to reach the gas? Is there a minimum burial depth? I ask because if I remember correctly the height a hand-powered pump can draw up a liquid is somewhat limited. You might want to try it out in real life before basing your plans on this process. A gas-powered pump would work to much deeper depths but then you have other potential problems (boom).

I think I once saw a water-powered pump (don't laugh). You hooked a garden hose to it and this powered the main pump rotor. That might be an option.

-Blast