The Texas Land Application Permit (TLAP), issued by the Texas Commission on Environmental Quality (TCEQ), allows treated wastewater to be applied to land rather than discharged directly into rivers or streams. Within the City of Austin, approximately 50 water treatment and reclamation facilities operate under this framework, releasing treated water back into the environment through methods such as spray irrigation and drip application. These land-application practices rely on the assumption that evapotranspiration removes a significant portion of the applied water, thereby reducing the amount that reaches nearby streams or groundwater. However, that assumption has not been field verified in Austin. Furthermore, those 50 sites consist of varying site conditions and treatment characteristics, which can lead to a variety of outcomes for stream health. Thus, assessing their impact with observational data and knowing which factors play a role in their effectiveness is a key Austin Watershed Protection Department (AWP) goal.
To that end, we have developed a modeling approach that considers both surface and subsurface water movement. In this study, three nested models were run to evaluate water quality and pollutant behavior: an evapotranspiration model, a hydraulic conductivity model, and a groundwater flow model based on the Boussinesq approximation. Together, these models provide a simplified but integrated way to examine how treated wastewater moves through the surrounding landscape. Through these models, we will be able to identify those facilities that are at risk of leaching treated wastewater to their receiving streams and prioritize for future water quality sampling
We estimated evapotranspiration using recent empirical data and water-quality information, such as pollutant concentrations, to estimate contaminants in the system. Next, we assessed subsurface movement by using hydraulic conductivity. Areas with higher hydraulic conductivity generally allow water and dissolved contaminants to move
more rapidly. Finally, we further examined groundwater flow by using the Boussinesq model, which represents the subsurface contribution of land application on the stream. By combining these modeling approaches, this study provides a practical framework for evaluating the hydrologic and water-quality implications of land-applied treated wastewater, supporting improved planning and regulatory decision-making for urban watersheds.