Exfiltration refers to the leakage of wastewater into soil and groundwater through faulty joints, damaged pipes, around manholes, etc.
The problem is difficult to discover, and there is a risk of contamination of the environment, such groundwater, lakes and ponds.
Advanced controls can sometimes measure a pressure loss in the pipes downstream from the leakage.
A demonstration can be found here: http://net.grundfos.com/doc/webnet/pumpingstations/int/
Select animation in the menu bar.