A watershed modeling framework for phosphorus loading from residential and agricultural sources

Area of Science:

• Environmental Science
• Water Resource Management
• Wastewater Engineering

Background:

• Phosphorus (P) loading from residential onsite wastewater systems (OWSs) into surface waters is poorly understood in rural watersheds, especially those with mixed agricultural land use.
• Accurate assessment of P loads from OWSs is crucial for managing water quality in these environments.

Purpose of the Study:

• To design a Phosphorus Onsite Wastewater SIMulator (POWSIM) for assessing P loads from residential OWSs.
• To simulate and compare P loading from agricultural and residential sources in a mixed agricultural watershed using POWSIM and the Soil and Water Assessment Tool (SWAT) model.

Main Methods:

• Development of POWSIM with three computational components: disposal field selection, P treatment dynamics in the disposal field, and subsurface plume P treatment.
• Integration of POWSIM with the SWAT model to simulate P loading in the Thomas Brook Watershed (TBW), Nova Scotia.
• Comparison of simulation results with and without POWSIM to evaluate model performance.

Main Results:

• The combined POWSIM-SWAT model provided a better simulation of baseflow total P (TP) loads in both residential and agricultural subcatchments compared to SWAT alone.
• OWSs contributed 48% of the simulated average annual TP load in the residential subcatchment and 39% in the agricultural subcatchment.
• Watershed-scale sensitivity analyses identified P loading rate, long-term P removal, soil P sorption capacity, and the mass of native soil as key sensitive parameters for OWS operation periods.

Conclusions:

• Residential OWSs are a significant source of phosphorus loading in mixed agricultural watersheds.
• The POWSIM-SWAT modeling approach enhances the accuracy of simulating P loads from OWSs.
• Understanding P dynamics and sensitive parameters is vital for effective management of OWS impacts on water quality.