How deep can I bury my septic or cistern tank? Posted on April 21, 2016 by Mark Peabody We recommend you bury your tank not more than 24″ deep. We have manhole extensions available in 15″ and 24″ lengths to allow for access to your tank buried below grade without having to dig a hole in the ground to get to it. Mark Peabody Are your above ground tanks & below ground cistern tanks NSF approved? Can I use my septic or cistern tank above ground?