Assessment of Water Quality Impacts of On-Site Wastewater Disposal Systems in a Semiarid Watershed

On-site wastewater disposal systems (OWDS) play a crucial role in managing wastewater in rural and low-density areas, accounting for approximately 21% of the US wastewater infrastructure. They are especially useful where centralized sewer systems are impractical due to low population density and terrain constraints. Concerns persist regarding their potential contribution to nitrate loading in streams. This study assesses nitrate loads from OWDS in the Rapid Creek watershed, which supplies water to Rapid City, South Dakota. Using a calibrated Soil & Water Assessment Tool (SWAT) model, informed by data from three US Geological Survey (USGS) gauging stations, we examined the effects of population growth and OWDS hydraulic failure on nitrate loading. Key hydrological parameters identified through the SWAT Calibration and Uncertainty Program (SWAT-CUP) autocalibration included curve number, soil water holding capacity, soil depth, and surface biomass. Streamflow calibration and validation showed strong performance, with R2 values ranging from 0.73 to 0.85 (calibration) and 0.71 to 0.94 (validation), and Nash-Sutcliffe Efficiency (NSE) values ranging from 0.65 to 0.80 (calibration) and 0.68 to 0.89 (validation). Water quality predictions were assessed by comparing model-estimated average nitrate loads with observed data. Scenario analysis revealed that both population growth and hydraulic failure of OWDS increase nitrate pollution, with hydraulic failures causing sharp, temporary spikes until system function is restored. The findings underscore the need to expand long-term water quality monitoring and develop robust modeling frameworks to assess the impacts of OWDS more effectively. High-resolution data and advanced modeling approaches are essential for effective water quality management and planning in watersheds that rely on decentralized wastewater systems.