Abstract: A tracer-liner portion comprises an inner tube and an outer tube, each including at least one wall having an internal surface and an external surface, the walls of the inner tube and the outer tube being impermeable to fluid flow, the outer tube having an internal diameter which is larger than the outer diameter of the inner tube, the inner tube being arranged coaxially within the outer tube such that at least a portion of the external surface of the inner tube is covered by the outer tube; the internal diameter of the outer tube exceeds the outer diameter of the inner tube by an amount sufficient to form a space between the external surface of the inner tube and the internal surface of the outer tube, and a tracer material containing a tracer compound is present in the space.