I'm assuming it's not a good idea to plant a vegetable garden over a septic field for sanitary reasons. Is this correct? We have a decently sized back yard, but there is a septic field, which makes a chunk of it unusable unless we build planters to set on top of it.
Personally I don't think contamination would be a problem since the soil would still act as a barrier. just rinse the produce well I'd say. the only thing that could be a concern would be heavy metals and the like, but i think most plants don't uptake too much . if you do plant over it, I'd make sure the entry (or whatever they call the access port) is left clear. I don't know how often they need to be checked, but you wouldn't want your tomatoes to get trampled every time. black water is used as fertilizers in some area, topically applied. heard that's how some e.coli happened.
Oh, I just remembered another issue could be if the roots started going too deep in the system. most times I've heard this happens in a dry year, because the roots seek moisture and end up clogging the drain field. any root crops would be out of the question too. if you know where the main pipes of your drainage field are, it might be prudent to put a walkway over them to keep the plants a little farther away.
Full disclosure, I've never had a septic field but I've never heard to not grow over it. my grandparents said thats where the best tomatoes come from