Definition - What does Exfiltration mean?
Exfiltration is leakage from a watertight pipe system into the surrounding soil, and potentially, the water table. The US Environmental Protection Agency states that, "a combination of relatively low groundwater table and shallow sewer creates the potential for widespread exfiltration."
Trenchlesspedia explains Exfiltration
When it comes to exfiltration and the reasons for it, there's no difference in the physical process, regardless of whether the pipe is a water supply line, sanitary sewer, storm sewer or other underground facility, Eventually, all watertight pipes will discharge their contents into the ground. However, exfiltration is not a result of being old and brittle from years of service; its the result of an engineering or construction flaw that allowed the required shallow sewer placement, relative to the equally shallow water table.
The problem can often be repaired with a liner appropriate for the type service of the piping.