This project proposed to develop new understanding of how potable and wastewater conveyance systems alter the water cycle within urbanized watersheds.  Collectively, we refer to these effects as infrastructure-mediated flows (IMFs).  One specific IMF we investigated is the inflow and infiltration of rainfall and groundwater into sanitary-sewer systems through various pipe defects.  Inflow and infiltration (I&I) are recognized as leading causes of sewer infrastructure degradation and sanitary sewer overflows.  I&I are also problematic for streams and aquatic ecosystems because I&I diverts fresh water away from the streams to which it should flow and into the sanitary sewer system.  A second IMF we examined is leakage of potable water.  It is common for the water source that supplies potable water to exist in a different watershed than the city and its residents.  The conveyance of water from that source to the receiving watershed is referred to as an inter-basin transfer (IBT) of water.  A significant fraction of that water leaks from the pipe systems and contributes to the urban water cycle, either evaporating, infiltrating into the subsurface, or flowing into streams.  Rarely has anyone quantified the comparative magnitudes of these IMFs and their impact on the water cycle within urban watersheds.

We used a wide array of publicly available data sources to address the lack of knowledge about how IMFs impact the urban water cycle.  The study was focused on the headwater tributaries of the South River, which emanates from the metropolitan area around Atlanta, GA U.S.A.  The data included flows of water and wastewater in pipes, flow in streams, rainfall and other meteorological data, groundwater levels from wells, public records of IBT amounts along with geographic data and human-population densities from the U.S. Census.  We also conducted interviews with governmental and non-governmental stakeholders to leverage their knowledge and characterize their perceptions of the impacts of these IMFs.

Our results showed that the leakage of potable water into the landscape ranged from approximately 17 – 45 millimeters (mm) of equivalent water depth per year across the years 2011 to 2020.  Average-annual rainfall depth in the same area is approximately 1240 mm, so the leakage represents a small relative addition of approximately 1 to 4 % per year.  Our results showed that, during calendar year 2019, total I&I among selected tributary watersheds of the South River ranged from approximately 30 to 50 mm equivalent water depth.  Flow in the streams amounts to approximately 500 mm per year, on average, so I&I causes approximately 6 to 10% reductions in streamflow annually.  On an annual basis, the equivalent depths of leaked potable water and I&I are similar, suggesting that one may nullify the other.  However, neither IMF is distributed uniformly across the landscape or across time within the year; both flows are episodic and spatially variable.  Our analyses showed that the impacts of I&I on streamflow are greater during seasonally dry weather, when flows of water in stream channels are already at their lowest.  During those conditions, I&I may cause diversions of freshwater into sewers that represent 6-36% of the flow in streams.   The spatial variability in potable water leakage is unknown, and the temporal variability can currently only be quantified at monthly time scales based on available data.  Our research has provided important new insights about the impacts of I&I on streamflow across multiple spatial and temporal scales, and potable water leakage for whole watersheds at monthly to annual time scales.  These insights fundamentally change the way we conceptualize the water cycle in urban watersheds and point to new imperatives for future research that will improve and refine this knowledge.

Major outcomes related to the intellectual merit of this NSF-funded research include (1) publication of five articles within high-ranking, peer-reviewed scientific journals; (2) completion of three Master of Science research theses by graduate students whose education was supported through the grant, (3) five public presentations of research by investigators during invited seminars, and (4) more than 10 public presentations of research by investigators and graduate students at state- to international-level scientific conferences.  Major outcomes related to the broader impacts of this NSF-funded research include (1) training of three graduate students belonging to underrepresented groups in the Geosciences, all of whom have secured employment in STEM-related professions including environmental consulting and math education, (2) integration of research concepts and results into new course offerings in Urban Hydrology at Georgia State University, and (3) support for Project WET training and certification for 31 K-12 educators from the Metropolitan Atlanta area.  The latter outcome, achieved in conjunction with Project WET coordinators from Georgia’s Department of Natural Resources, provided those participants with materials, curriculum guides, collegial networks, and practice aimed toward integrating water-focused curriculums into their K-12 classrooms—curriculum modules that are aligned with relevant state grade-level standards for STEM education.   

Last Modified: 06/04/2024
Modified by: Luke Pangle