The ATV Handbook [ATV95b] indicates that the amount of extraneous water content is between 33% and 66% of the dry weather flow, which corresponds to an external water surcharge of 50% to 200%. Significantly higher extraneous water surcharges of over 900% in studies by Decker [Decke1998] and Dohmann [Dohma99] and up to 1100% by Dohmann et. al [Dohma2004] and Hennerkes et al. [Henne2006] have been documented.

Groundwater infiltration contributes significantly to the entry of solids into the drainage system. This occurs in damaged or leaking sewer lines located within or below the groundwater table. The quantity of solids entry can vary greatly and is dependent on the leak size and shape, the type, particle size distribution and bulk density of the surrounding soil material, and the internal exposure (HP cleaning, leak test, changing water levels, backups) and external loading (dynamic loading, groundwater) of the sewer line.

Initial studies used to quantify the entry of solids through cracks, fragmentation and leaking pipe joints as well as the resulting consequences, such as the formation of cavities or cave ins, were conducted in a research project by Dohmann / Hennerkes. They concluded that a large entry of solids can be expected as a result of groundwater infiltration through cracks (longitudinal and transverse) beyond a certain size, this also included faulty pipe joints [Dohma2004a].

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