Modeling Non-point Source Phosphorus Load on a Rural Basin with Onsite Wastewater Treatment Systems

Abstract

This study was conducted in the Hazu River basin which encounters pollution from non-point and point sources. This has led to eutrophication in the downstream Sengari Reservoir due to increased nutrient load mainly phosphorus which is the limiting nutrient for the production of algae. The aim of this study was to apply Soil Water Assessment Tool (SWAT) model to simulate discharge and total phosphorus (TP) load, quantifying phosphorus load from different land-use types and assess the effect of Onsite Wastewater Treatment System (OWTS) such as septic tanks on TP generated in the basin. The simulation period ranged from 2009 to 2015. Sequential Uncertainty Fitting Algorithm (SUFI-2) was used as the optimization tool for model calibration and validation. The model yielded satisfactory performances based on evaluation criteria for both discharge and TP simulation. Paddy fields had the highest TP load per unit area followed by residential areas and forests. From the scenario analysis where all septic tanks are replaced to rural sewage treatment plants, it was shown that the TP load at the basin outlet could be reduced by 617 kg/year. The model can be significant in quantification and management of non-point source and point-source phosphorus load flowing into the Sengari Reservoir to help in the preservation of water quality.