That is a good question. I also wonder if planting fruit trees, or a garden over a septic drain field would pose any health risks. I think planting over a septic tank, is that the tank will have to be emptied at some point, and the honey-dippers would wind up dragging their vacuum hose over your garden. I imagine it is not the cleanest of hoses. Also, I might speculate that any of the deeper rooting plants would try to get directly into the solid waste of the septic tank. Maybe this would pose a health risk. Anybody else have any ideas on these questions?