my grandfather used his GOS pig (and rented her out) for this exact reason...
he would simply use a peice of rebar to make some holes in the ground as close to the sump as possible and will them with old fryer grease, fermented corn, realy smelly old cheese, even slightly curdled milk or just pour whey around the entire thing...her aboslute favorite though were applese burried aorund the trees, she LOVED apples!
this pig would root up trees for the sake of rooting up trees, but add some potential yumyness in the ground around the stumps you want gone and a tractor couldnt have done as good a job! she was a machine!, she loved to eat the roots too!
she even felled a couple of big live trees, my grandad warned the guy and told him to put electric strands a good 20ft away and around any trees he didnt want gone...
this guy insisted he knew better and pigs will only root up stumps...
she felled 3 huge live trees that he planned on keeping just because they had yummy roots!