A sediment accumulating device for use as an accessory disposed below a groundwater receiving chamber engulfed in granular backfill material has a compartment bounded by sidewall structure elongated upon a vertical axis between upper and lower extremities. The upper extremity is open and has a perimeter disposed in a plane orthogonal to the axis. The bottom extremity is closed. Retaining means protrude radially outward from the sidewall structure for receiving downward gravity force from the granular material.