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Infiltration and inflow (I/I) is excess water that flows into sewer pipes from groundwater and stormwater.

Groundwater (infiltration) seeps into sewer pipes through holes, cracks, joint failures, and faulty connections.

Stormwater (inflow) rapidly flows into sewers via roof drain downspouts, foundation drains, storm drain cross-connections, and through holes in manhole covers.

Most I/I is caused by aging infrastructure that needs maintenance or replacement.

inflow and infiltration in a side sewer example image
Graphic also available as a PDF file


Infiltration is groundwater, or groundwater that is influenced by surface or sea water, that enters sewer pipes (interceptors, collectors, manholes (MH), or side sewers) through holes, breaks, joint failures, connection failures and other openings. Infiltration quantities often exhibit seasonal variation in response to groundwater levels.

Storm events can trigger a rise in groundwater levels and increase infiltration flows. The highest infiltration flows are observed following significant storm events or following prolonged periods of precipitation. Since infiltration is related to the total amount of piping in the ground and not to any specified water-use component, it is usually expressed either in terms of the total land area being served, or in terms of the lengths and diameters of sewer pipe. The unit quantity used is gallons per acre per day (GPAD).


Inflow is surface water that enters the wastewater system from yard, roof, and footing drains, from cross-connections with storm drains and downspouts, and through holes in manhole covers. Inflow occurs as a result of storm events such as rainfall, snowfall, springs or snow melt that contribute to excessive sewer flows.

Peak inflow can occur during heavy storm events when storm sewer systems are surcharged, resulting in hydraulic backups and pooling of water.

Contact your local sewer service provider for specific information about the sewer system in your neighborhood.

Nicole Smith
Capital Project Manager