The leak or overflow in your d-box may be because of the drain field's groundwater saturation, backing up of the water out into the d-box lid, and drain field flooding. When drainage blockage causes the d-box to overflow, simply sealing its lid or cover is not. A septic distribution box (D-box) is a concrete or plastic junction that evenly distributes wastewater from your septic tank to all drainfield lateral lines. When it fails, symptoms include uneven wet spots in the yard, slow indoor drains, and sewage odors. Regular inspection and. Diagnose problems at the septic system drop box: procedures for troubleshooting leaks, smells, or backups & flooding in the septic system D-box.