Considering the rain factor, that would be really hard. I've heard that Capsicum will keep ants away. You can get it powdered (cayenne pepper). Seems as it would “sink into” the ground it would still have an effect … but who wants to do something w/o knowing for sure! :-)
Just found this online:
Maybe something will pop out at you there. I know when we find something that feels right we're more successful with it.