Quantifying the Contribution of On-Site Wastewater Treatment Systems to Stream Discharge Using the SWAT Model.

Area of Science:

• Environmental Hydrology
• Water Resource Management
• Wastewater Engineering

Background:

• On-site wastewater treatment systems (OWTSs) are prevalent in the southeastern US, but their impact on water resources, specifically consumptive use, remains debated in Georgia.
• Understanding the hydrological effects of OWTSs is crucial for effective water resource planning and management in regions reliant on these systems.

Purpose of the Study:

• To quantify the effect of OWTSs on streamflow within a Georgia watershed using a watershed-scale model.
• To estimate the consumptive water use associated with OWTSs and compare it with existing assumptions.

Main Methods:

• Utilized the Soil and Water Assessment Tool (SWAT) watershed-scale model, incorporating a novel OWTS algorithm, to simulate streamflow.
• Calibrated the model using data from 2003-2006 and validated it from 2007-2010 using SWAT-CUP 4, achieving satisfactory Nash-Sutcliffe efficiency coefficients.
• Compared water balance outputs from simulations with and without OWTSs to assess hydrological impacts.

Main Results:

• Simulations indicated a 3.1% increase in total water yield at the watershed scale and a 5.9% increase in high-density OWTS subbasins.
• The influence of OWTSs on water yield was most pronounced during dry years, suggesting a greater impact under drought conditions.
• Estimated mean OWTS consumptive water use was approximately 5.7%, which is lower than commonly assumed by Georgia water planning agencies.

Conclusions:

• OWTSs contribute to watershed water yield, with their impact amplified during periods of water scarcity.
• The actual consumptive water use of OWTSs is significantly lower than often presumed, necessitating a revision of water management strategies.
• Findings provide valuable data for OWTS users and watershed planners to better understand and manage water quantity in Georgia and similar regions.