Answer: Can't give you specifics here, but in general terms, most sources indicate that trees should be planted far enough away to keep the roots from infiltrating the septic system. Most trees will develop root systems that venture out two to two and a half times the diameter of the canopy. So, if you're planting a tree with a mature canopy of 20' in diameter, and the root system has the potential to spread out 40' to 50', you'll want to plant that tree 50' away from the septic system. To be on the safe side, before planting I would research each tree to see what the average mature size is and then double that (for the root system) and then plant that far away from the septic system.
Best wishes with your landscape!
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