veggies proximity to septic

I have a fenced off area, 82 feet per side, for my homes raised septic field.  The actual raised septic mound is 40 feet wide and orientated tight to one side.  I suppose, for whatever reason, if the field failed that another one could be built on the other half.  I'll cross that path when and if it ever occurs.

Last spring I planted 4 apple trees outside the fenced area opposite of the raised septic.  That puts the trees roughly 50-60 feet from the edge.  I may stand corrected, but I felt it was sufficient distance to feel safe about eating the apples.  I'd like to put in a small greenhouse in the far corner but have concerns about contamination.  How close is too close?

I live in Michigans eastern upper peninsula, zone 5a.  When my wife and I moved out here from California I had grand plans for a walapini, which is obviously out of the question for this location.  Unfortunately, the enclosed area is my only option for a greenhouse location on my property without encroaching on wildlife habitat, which I am unwilling to do.

The only danger from septic leach fields is if they fail and the leachate is at the surface.  It's perfectly safe to plant near functioning leach fields though trees should be avoided because their roots can clog the pipes.