Urban streams in rapidly developing regions increasingly receive treated wastewater effluent as a significant component of baseflow, altering hydrology and water quality during dry-weather conditions. Although wastewater discharges operate in compliance with permitting frameworks, these permits may not include sufficiently protective limits for nutrients such as nitrogen and phosphorus, which can drive eutrophication, excessive algal growth, and dissolved oxygen depletion. As a result, evaluating how different treatment levels and effluent quality influence nutrient dynamics and benthic algae growth remains a challenge, particularly under variable hydrologic conditions that control dilution and transport.

We present a coupled modeling approach that integrates watershed-scale hydrology with in-stream water quality processes to evaluate wastewater impacts and management scenarios in an urban watershed. The Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model was used to simulate watershed hydrology and generate flow inputs to the receiving stream. These flows were then applied as boundary conditions in the QUAL2K model to simulate in-stream water quality responses, including nutrient dynamics, dissolved oxygen, and benthic algae growth.

The approach was applied to an urban watershed in Austin, Texas, where wastewater discharges influence stream conditions. QUAL2K was calibrated using observed data and subsequently used to evaluate alternative wastewater management scenarios, including variations in treatment performance and effluent quality. Scenario analyses quantify changes in nutrient concentrations and benthic algae growth along the stream network under different discharge conditions.

Results demonstrate that integrating watershed hydrology with stream water quality modeling provides a robust framework for isolating wastewater contributions and evaluating the effectiveness of treatment and management strategies. This approach supports decision-making by enabling comparison of regulatory and operational scenarios and identifying conditions under which wastewater discharges may contribute to water quality impairment or improvement.