How much water is stolen by sewers? Estimating watershed-level inflow and infiltration throughout a metropolitan area

Abstract

Human activities can have substantial impacts on watersheds, yet a major but understudied impact on many urban watersheds is the inflow and infiltration (I&I) of water into sewage infrastructure. I&I is important, because it is a major cause of sewer overflows, increases wastewater treatment plant costs, and reduces base flows of urban streams. Unfortunately, I&I information is deficient at the watershed scale. Therefore, this study uses a water-budget approach to quantify the magnitude of I&I for 90 watersheds in the Atlanta, Georgia USA region during 2013–2020. I&I for each watershed is calculated by subtracting outflows (i.e., stream discharge, water withdrawn by public water systems, and actual evapotranspiration (AET)) from inflows (i.e., precipitation, water-supply pipe leakage, and non-I&I effluent from wastewater treatment plants). Included in the AET estimates is irrigation water from public water systems and water withdrawals for agriculture. Results show that I&I is a major contributor to the total outflow in urban watersheds. The mean annual I&I total for the 15 most urbanized watersheds is 138 mm, constituting 25% of stream discharge. The mean annual I&I total from the watersheds with the five highest totals is 216 mm, which is 40% of stream discharge. These annual I&I totals align well with totals calculated for urban catchments in Europe, where most of the previous research has been conducted. Regression analyses show that the density of older housing, which is a proxy for deteriorating sewage infrastructure, is the most important predictor of I&I across the Atlanta region. Despite the uncertainties in estimating annual totals for all components, especially AET for urban watersheds, of the water budget, we conclude that estimating I&I using the water-budget approach is a useful initial approach to estimating I&I throughout a region.