I/I is clean storm and/or groundwater that enters the sewer system through cracked pipes, leaky manholes, improperly connected storm drains, as well as illegally connected sump pumps, roof leaders and yard drains. Most inflow comes from storm water and most infiltration comes from groundwater.
There are many sourses of I/I that can occur on private property such as downspouts, driveway drains, and damaged laterals. Figure 2.01-1 shows several examples of I/I that may exist on private property and illustrates how these sources are connected to the public sewer.
Locating sources of inflow is a relatively straightforward process and the impact from these sources can be roughly quantified by measuring the contributing drainage area for each source. Identifying sources of infiltration, such as damaged laterals and estimating their impacts, is considerably more challenging. Variables such as groundwater elevation, antecedent dry period, magnitutde and duration of rainfall and local temperatures can affect infiltration rates and the ability to locate defects in a particular system.