There is a fellow who I have seen exhibiting at some farm shows who sells a device which you connect to a gas-grill type propane cylinder, use it to run some propane into groundhog holes and then ignite the gas some how. Seems to work on the video. Just not a product you want across the aisle from you for three or four days! Might be safer than the chemical or anhydrous approaches. I'm not sure what the name is but he might have a website. Maybe someone else has seen the product and remembers the name.

Jim at Dawn