"I think the point is that if dye shows up near the surface in the
leach
field, then it means the leach field is not working adequately. If the

water near the surface is from snow, rain, etc, it will not have dye. "

bingo. Though EXT is correct that drilling a test hole will also show
some dye if you drill in the right (wrong?) spot. The whole point is
that the leachate is not supposed to come up to the surface.