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Phosphorus (P) loading from poorly designed or hydraulically failed residential on-site wastewater treatment systems (OWS) into neighbouring surface water systems is typically an unquantified process in many rural and suburban watersheds. The transport of P from OWS to surface waters is typically related to the subsoil conditions surrounding the OWS disposal field (DF), which impact lateral movement of the wastewater plume and P sorption capacity, and the level of OWS management by the homeowner (Gold and Sims, 2000). In Nova Scotia (NS), Canada there is a wide abundance of low permeability soils, shallow bedrock and high water tables that can cause failure and improper functioning of OWS DFs (Havard et al., 2008). McCray et al. (2005) identified a need for quantitative approaches, such as watershed computer models, to assess OWS pollutant loads because of the increased importance of total maximum daily load (TMDL) planning and watershed management. Current updates to the Soil and Water Assessment Tool (SWAT) watershed scale model (version 2009) include algorithms for simulating OWS using a biozone based treatment process (Jeong et al., 2011). However, there are no specific algorithms within SWAT (version 2009) to simulate P transport via lateral subsurface flow to surface water systems, demonstrating a need for an integrated approach to simulate P fate and transport from OWS at the watershed scale.