Inter-Event Water Quality Variability and Intra-Event Pollutant Dynamics in Context of Effective Impervious Area

Abstract

Water quality modeling has been studied extensively to better manage urban runoff impacts on stream health and effectively address stormwater permitting requirements. Recent studies have indicated that previous modeling approaches using buildup and washoff models or regression equations are inadequate because they fail to capture important dimensions in urban runoff and pollutant transport processes. The effective impervious area (EIA) has been identified as one of the greatest determinants of urban stream health in many studies due to connection of these areas via stormwater infrastructure. Thus, accurate measures of EIA should offer additional explanatory power in assessing the variability in urban water quality. Recently refined estimates of EIA were used to assess water quality variability among storms and within storms to provide insight to pollutant dynamics and identify the role of EIA in these trends. Variability among storm events for E. coli, total suspended solids (TSS), copper, and zinc was assessed over multiple storm events on three urban watersheds in Knoxville, Tennessee. Inter-event variability supported findings from other studies but indicated differences among storm events associated with runoff expected to be confined to the EIA, and those with runoff generated by less closely connected impervious areas and pervious sources. Qualitative investigation of pollutant dynamics using hysteresis loops indicated complex patterns that differed by pollutants as well as by the most probable runoff source for the event. Transport trends were well-organized for some events but reflected harder to explain variability for others. The antecedent moisture conditions (AMC) are believed to play a role in this because wet conditions (higher soil moisture) promote runoff connection from areas outside of the EIA. Pollutant delivery along these more disconnected pathways is not well-predicted by typical water quality modeling. These results demonstrate the utility in including both the EIA and AMC in urban water quality modeling moving forward as well as the need for further research in variably sourced runoff effects on water quality loading in urban systems.