Modeling Impacts of Residential and Agricultural Development and Beneficial Management Practice Scenarios on Phosphorus Dynamics in a Small Watershed

Abstract

<abstract> <b><sc>Abstract.</sc></b> Both agricultural land uses and residential on-site wastewater systems (OWS) have the potential to be nonpoint sources of phosphorus (P) in rural watersheds. Watershed-scale computer models are a commonly used tool for evaluating the impacts of increased human development on P dynamics and for evaluating how beneficial management practices (BMPs) can reduce P loading. Many agricultural field BMPs have been evaluated using watershed-scale models; however, no studies have compared their efficacy to OWS BMPs in a mixed land use watershed. The objectives of this research were to (1) use the P on-site wastewater simulator (POWSIM) and the Soil and Water Assessment Tool (SWAT) to simulate different agricultural and residential development scenarios in the Thomas Brook watershed in Nova Scotia, Canada, and evaluate their impacts on sediment and total P (TP) loads and TP concentrations and (2) simulate and evaluate different individual and combinations of select agricultural and OWS BMPs. A 50-year simulation period was used for both the development and BMP scenarios. The development scenarios evaluated included increasing the residential population and conversion of hay fields or pastures to corn-based cropping regimes. The agricultural BMPs evaluated were (1) corn and rotational crop replacement with no-till agricultural land uses and (2) no-till corn. Decreased failure rates, increased watercourse setbacks, disposal field replacement, and the use of high P sorption filter media were the OWS BMPs evaluated. The agricultural development scenarios increased cumulative sediment and P loads at the watershed outlet, while the residential scenarios increased the frequency of hyper-eutrophic conditions in the stream during the last 20 simulated years. Agricultural crop replacement BMPs produced the highest reductions in cumulative sediment (13% to 46%) and TP loads (26% to 34%) but had no observable change on the in-stream trophic status. The OWS BMPs produced small reductions in TP loading (2% to 15%), no sediment load reductions, but had a positive effect on the trophic status. The best ranked combination of agricultural and OWS BMPs resulted in a cumulative TP load reduction of 58%, and the dominant trophic state at the end of the 50-year period was mesotrophic, as compared to eutrophic for the reference simulation. The best combination BMP scenario included replacing corn and rotational crops with pastures, using high P sorption filter media (5000 mg P kg^-1 media), and lowering of the OWS failure rate to 5%. The results of these modeling scenarios showed that agricultural BMPs were most effective in reducing cumulative P loads, while OWS BMPs were effective in mitigating in-stream eutrophication impacts. This study highlights the importance of identifying specific water quality issues that need to be targeted prior to implementing a watershed BMP strategy. Further model refinement and testing are required with improved field measurements to improve model confidence before the model should be used for planning and design purposes.