Contextualizing Inflow and Infiltration Within the Streamflow Regime of Urban Watersheds

Defects in sanitary-sewer infrastructure enable exchange of large volumes of fluids to and from the environment. The intrusion of rainfall and groundwater into sanitary sewers is called inflow and infiltration (I&I). Though long recognized in the assessment of sewers, the impacts of I&I on streamflow within urban watersheds are unknown. We quantified rainfall-derived I&I (RDI&I), groundwater infiltration (GI), and total I&I using measured flows within sanitary-sewer pipes serving four watersheds near Atlanta, Georgia, USA. Flows were monitored in pipes that parallel local stream channels and compared with streamflow measured at nearby gauging sites. Freshwater diverted into the sewer system due to I&I ranged from 24% to 36% of the flow measured within individual pipes. The RDI&I was the smaller component of I&I, ranging from 4.2 to 9.8 mm per year among watersheds. The GI was typically an order of magnitude greater than RDI&I, ranging from 24 to 41 mm per year among watersheds with annual stream discharge of approximately 500 mm. The I&I occurring at specific moments in time commonly represented 0%-20% of the flow measured in the adjacent stream. The enhancement of low flows in streams that could be achievable if I&I were abated ranges from as much as 6%-36% across watersheds. Our discussion presents explanations for the seasonality of I&I and associated impacts on streamflow in urban watersheds, while identifying important sources of remaining uncertainty. Our results support the conclusion that I&I substantially reduces flows in urban streams, especially low flows during dry weather.

Automated summary

Defects in sanitary-sewer infrastructure allow for the exchange of fluids with the environment. The intrusion of rainwater and groundwater into sanitary sewers, known as inflow and infiltration (I&I), has unknown impacts on streamflow in urban watersheds. Our study quantified the effects of I&I on streamflow using measured flows in sanitary-sewer pipes in four watersheds near Atlanta, Georgia, USA. We found that I&I substantially reduces flows in urban streams, especially during dry weather.