There are a couple of ideas to ponder. Of course, pest control services work great. If it is really dry, they might be searching for water. If wet, they'll try to get away from it. The little bitty black ones are sugar ants, and they like, well, sweet things.
I've had great results on fire ants with a combination of bait insecticides and Orthene. Start with the bait, give it a couple of weeks, then follow any survivors with Orthene. The stuff stinks, but it hammers.
They really aren't that hard to eliminate.