In rural areas of the UK, properties are often served by septic tanks, which discharge sewage effluent to ground close to the property served. Dissolved phosphorus as orthophosphate (OP) is present in the raw sewage and the discharge. This study investigated whether OP from sewage effluent discharges to ground from septic tanks is a significant OP source within the Upper River Nar at the catchment scale. An approach involving multiple strands of evidence was used: first, a high-level source apportionment apportioned only 0.9–1.0% of the catchment phosphorus to septic tanks compared with 40% agricultural sources and 9.9% urban (Mileham village); second, a bulk phosphate dilution assessment, based on diluting the total OP from septic tank discharges by potential recharge (excluding attenuation) predicted diluted effluent concentrations that were insufficient to affect surface water; third, an individual septic tank assessment using a groundwater fate and transport model showed that septic tank OP for the majority of properties is attenuated to concentrations below 0.001 mg P l−1 before the effluent reaches surface water. Consideration is given to higher rural population densities, septic tanks clustered proximal to surface water and where greater fracture flow in the Chalk reduces the aquifer's ability to attenuate the OP.