The main reason for including a Leakage Detection System (LDS) in a landfill is to capture and to laterally convey liquid entering due to leakage of a primary liner. Figure 1 presents a schematic of a leak detection system, indicating the primary source of leakage. There can be additional sources of leakage into the LDS including (i) construction and compression water already present in the LDS; (ii) consolidation water from the upper compacted clay liner (if a compacted clay liner is present); (iii) infiltration water from leaks in the lower geomembrane; and (iv) liquid flow from leakage of pipes penetrating the LDS. This manual considers leakage through the upper liner as the primary, and only source of liquid in the LDS.

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In general, leakage from a hole in a geomembrane depends on: (i) hydraulic head; (ii) the size and shape of the hole; and (iii) the nature of the media underlying and overlying the primary liner. For a composite liner system typically used in landfills, the underlying medium is a compacted clay liner or a geosynthetic clay liner. The material overlying a primary liner is of a permeable nature, such as waste or a drainage medium. For the case of composite primary liner with circular defect, the leakage rate can be calculated as [Giroud, 1997]: