Urban and suburban streams increasingly receive treated wastewater effluent as a substantial component of baseflow, particularly in rapidly growing metropolitan areas. While individual wastewater treatment plant (WWTP) discharges are regulated through permitting frameworks such as the Texas Pollutant Discharge Elimination System (TPDES), the cumulative hydrologic and water-quality impacts of multiple discharges within a single watershed are often difficult to quantify.
We present a coupled modeling framework that integrates watershed-scale hydrology and constituent transport with receiving-stream water-quality processes to assess the magnitude and spatial extent of treated wastewater impacts in urban streams. The approach combines the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model to simulate watershed hydrology, runoff, and baseflow contributions, with the QUAL2K stream water-quality model to evaluate in-stream responses to wastewater effluent, including nutrients, dissolved oxygen dynamics, and related biogeochemical processes.
This framework will be applied to multiple urban watersheds in Austin, Texas, including systems with more than one permitted WWTP discharge. Model scenarios explicitly represent effluent flow volumes and concentrations under permitted conditions, allowing comparison of baseline (no-discharge) and discharge scenarios to isolate wastewater contributions relative to watershed-derived loads.