jewels,
my
:
The DE will eventually wash away from the rain. The grains are so small that they can easily wash down and percolate into the soil or through cracks in your pavement. But while it's there, it disables/kills the ants.
If you want to buy a smaller quantity than you mentioned above, go to a pet store that has aquarium supplies. The DE is used in special aquarium water filters and I've seen it sold in pretty small containers.