Read to pour a liter bottle of club soda down the hill, it kills them.
I am sorry to report to you @Lona , that the club soda remedy on Fire Ants has been de-bunked. Please see this link for more information. But basically fire ant homes can be up to 12 feet underground. One bottle of club soda is not going to drown them all and the carbon dioxide is too diluted to do much killing.
I use liquid laundry soap, it is basically Borax in a liquid form I mix it 50/50 with water and pour it directly on the mound seems to do the trick and doesn't kill the plants and goes inside the earth so I don't worry about birds eating it.
Here in Louisiana we have some mean fire ants. The best thing I have found is to use some Ortho Fire Ant powder on them. It comes in a black plastic container. I take my foot and brush the top of the mound. This stirs them up and then I sprinkle enough of the powder over the mound to cover it. In about 6 hours or so, your trouble with the mound will be history. You might have to treat the mound again in several weeks when the new eggs hatch again. But for the most part a single treatment should solve your problems.