You ask a great question but there is no easy answer. Climate, soil, tree type and even sun exposure all determine how much damage tree roots will do to a septic field. In my opinion your drain field will be safe if the new Katsura tree is planted 60 feet away. I would avoid birch and poplar trees any closer than 100 feet from the drain field. In general a tree can send roots out three times as far as the spread of the tree branches - and roots grow toward moist soil. Trees do not stop growing. This means in 40 or 50 years a tree that is considered a safe distance away in the present can cause damage in the future. The installer of your drainage field may be the best one to advise you on safe trees in your area. Keep growing, Marianne Binetti