Jeremy
My raised bed garden swales are over a leach field. I dug out my paths only 4-6" deep, flipped the soil to create a bed and added municipal leaf
compost to get a nice fat 8" raised bed which puts me at least 16" above the drainage.
Two main things I keep in mind are using shallow rooted perennials and harvest fruits not leaves or roots. For example I grow blueberries and okra. I also utilize the beds to grow out nursery stock for grafting etc. Create beds there and I guarantee you will find plenty to fill them in with, edible and other.
If there are any metals in the field that a plant can reach it will hold them its mass, not the fruit. Deep roots could jam up the drainage tiles. Stick with small bush fruits like gooseberry, currants, blueberry, honey berry, raspberry, etc..