Urban Watershed Pollution and Green Infrastructure Efficacy: An Empirical Field Study

Thesis Abstract

Urban watershed pollution poses persistent threats to urban water quality, ecosystem services, and public health, with green infrastructure (GI) implemented across stormwater networks yet varying in efficacy due to design, maintenance, and contextual factors. This study aims to evaluate the efficacy of GI in reducing pollutant loads and controlling hydrological responses in an urban watershed, linking structural performance to water quality outcomes and community-scale resilience. The objectives are (1) to quantify changes in runoff volume and peak discharge associated with GI installations; (2) to assess reductions in key pollutants (nutrients, total suspended solids, and heavy metals) at multiple points upstream and downstream of GI facilities; (3) to analyze the influence of maintenance regimes, retrofit duration, and spatial configuration on GI performance; (4) to test the applicability of the Pollution-Drainage and Green Infrastructure Performance (PDGIP) framework in predicting pollutant attenuation; and (5) to provide evidence-based recommendations for optimizing GI design and management in urban watersheds. A mixed-methods approach combines quantitative hydrological and water quality monitoring with qualitative assessments of maintenance practices and stakeholder perceptions. The population comprises three urban sub-watersheds within a metropolitan region that have implemented green roofs, permeable pavements, and bioretention cells over a five-year period. A stratified random sampling method yields a longitudinal panel of 300 rainfall events recorded at four monitoring stations, with 60 events selected for detailed water-quality analysis. Instrumentation includes automated rain gauges, flow meters, and in-situ water-quality sondes measuring turbidity, dissolved nutrients (nitrate, phosphate), total suspended solids, and trace metals. Laboratory analyses follow standard methods (APHA) for nutrient nutrient/speciation and metal concentrations, complemented by stormwater mass load calculations. Data collection also incorporates maintenance records, GIS-based GI configuration metrics, and community survey data (n=400 respondents) to elucidate process-traceability factors. Analytical techniques encompass time-series regression and generalized additive models to relate GI performance to hydrological and water-quality responses, multivariate regression to attribute pollutant reductions to specific GI types, and ANOVA to compare performance across sub-watersheds. A difference-in-differences approach is applied to established pre- and post-implementation data to isolate GI effects from climatic variability. Structural equation modeling tests theoretical links between GI design attributes, maintenance intensity, hydrological attenuation, and pollutant reductions, while the PDGIP framework is adapted to assess the causal pathways of GI performance within urban hydrology. The qualitative component utilizes thematic analysis of maintenance logs and stakeholder interviews (n=30) to identify barriers and enablers of GI efficacy. Expected findings indicate that GI installations reduce peak discharge by approximately 18–28% and runoff volume by 12–22% on average, with pollutant load reductions of 25–40% for nutrients, 30–45% for TSS, and 15–35% for metals, contingent on maintenance frequency and retrofit maturity. Bioretention cells and permeable pavements are anticipated to yield the greatest pollutant attenuation when integrated with upwind source control and complemented by routine maintenance schedules exceeding twice yearly. The study anticipates that maintenance quality, spatial distribution, and hydraulic connectivity will significantly modulate GI performance, with the PDGIP model explaining a substantial portion of variance in observed outcomes (R2 > 0.60). Contribution to knowledge includes empirical substantiation of GI efficacy under real-world urban conditions, clarified linkages between design configurations and pollutant attenuation, and a validated analytical framework integrating hydrological, water-quality, and governance dimensions for urban watershed management. The study informs policymakers and practitioners on optimizing GI portfolios, maintenance regimes, and monitoring protocols to maximize pollutant control and resilience against climate-induced hydrological extremes. The main conclusion posits that well-designed, properly maintained GI networks—with explicit maintenance regimes, strategic spatial configuration, and integrated source control—meaningfully improve urban watershed water quality and flood resilience. Recommendations emphasize standardized maintenance schedules, performance-based design guidelines, adaptive management under climate variability, and the integration of GI performance metrics into urban water planning and budgeting processes.