Inflow and infiltration are terms used to describe the ways that clean groundwater and/or storm water flow into the wastewater system, due to cracked sewer lines, leaky manholes or through erroneously connected storm drains. Most inflow comes from storm water and most infiltration comes from groundwater.
Inflow is surface water that enters the wastewater system through improper connections, such as catch basins, yard, roof and footing drains, downspouts, groundwater sump pumps, and through holes in manhole covers. Inflow typically occurs as a result of storm events. Peak inflow occurs during heavy storm events when storm sewer systems are full, resulting in backups and ponding.
Infiltration is groundwater that enters the wastewater system through holes, breaks, joint failures, connection failures and other openings in the pipe. Infiltration amounts 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 when the ground is saturated with water.