I have used ash from my fire pit. I seems to deter them. If I can get an old bag of cement (one that maybe got wet - most masons will give them away) and just roughly put it in the hole and wet it. That works too.

The residual odor draws them to the old holes. Change the odor in and surrounding the hole with any number of things (used cat litter works well so does coyote urine in a bottle) and then fill it in.

Dave

I don't have a fire pit and bottled urine seems temporary. Can I bury mothballs?

Our cat's "full" litter box worked, just pack tight it into the hole as best you can and top it off.

FWIW, once had a neighbor with that problem, his wife decided that after "pooper scooper" detail following the dog around the yard, she proceeded to fill the hole with that. Worked.

22 caliber will make good solutions of pest problems.

Most municipalities place restrictions on shooting with city ordinances. Also many regulations against hunting while it is dark, which complicates such control of nocturnal animals (which groundhogs are, actually, not). Violating those restrictions makes you the pest.

My groundhogs come out in late afternoon and early morning..no problem seeing them. I have shot them, trapped them with conibear traps, and used gopher smoke bombs. All 3 work,,depends on the circumstances what and when you use them. I prefer the smoke bombs because then I don't have to handle a corpse...you just have to seal their other entrances, light the stick, put it in the hole and cover that hole with a garbage can lid. They come in "herds" here...they are a constant problem and can wipe out a garden very quickly. I am always on watch for them and get them before they get my food.

Our county has a specific ordinance for GHs. If you trap it, you must put it down.