I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
You could just plant the tree as it will not likely do any significant damage as long as the septic tank is properly built and thus well sealed. That tree is not so big anyway so its root development is correspondingly limited. If you wish to be extra careful you will need to set some type of vertical barrier into the soil near the septic system to form an underground wall about a foot or so deep. Thick pond liner plastic is one option for such a material.
Thanks for the question. Please stop in again soon!
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