Sewer leakage, which leads to the occurrence of infiltration or exfiltration, is a chronic problem in sewer systems for many cities worldwide. Exfiltration can result in deterioration of groundwater quality and serious public health risks if groundwater is subsequently used for potable supply. Throughout the world there is evidence of contaminated groundwater leading to outbreaks of waterborne diseases. The UK has witnessed about 70 officially documented sewer-related waterborne disease outbreaks. As a first step in the assessment of microbiological risks to urban groundwater from leaking sewers, this study reviewed the recent literature on, and summarised estimates of, exfiltration. Furthermore, an algorithm to estimate the spatial distribution of exfiltration was developed using Nottingham as the case study area. The results show that out of the total 34 km2 beneath Nottingham, around 23 km2 is susceptible to exfiltration. Some 67% and 4% of the exfiltration-susceptible area receives higher (>500 l/day) and lower (<50 l/day) rates of exfiltration, respectively. The higher rates are closely associated with older sewers. With the current replacement/renewal frequency of once every 1300 years, sewers of all ages are therefore a potential risk to urban groundwater.

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