Sewer leakage: first nationwide estimate of pollution leaking from urban systems, Germany

Sewer systems are a potentially major source of urban groundwater
pollution; water can leak through structural faults (or ‘exfiltration’ — i.e.
through fractures or intruding tree roots) and be absorbed by surrounding soil,
which can introduce nutrients, suspended solids and microbes into nearby water
bodies. Estimating sewer leakage and exfiltration is important for effective
management of urban waste-water systems, say the researchers of a new
study that explores sewer leakage at the national scale in Germany.

https://ec.europa.eu/environment/integration/research/newsalert/pdf/564na4-1303_en_pollution-leaking-from-urban-sewers-in-germany.pdf