Taking the Pulse: Tracking Wastewater Nutrients Through a River Using Lagrangian Sampling and Isotopic Tracers

Effective management of river pollution is often limited by low-frequency monitoring approaches that fail to resolve the spatiotemporal dynamics of nutrient sources, hydrodynamic transport, and in-stream biogeochemical processing. To address this, we applied a high-resolution Lagrangian sampling framework to a well-characterised reach of the River Thames, enabling continuous tracking of water parcels downstream of key nutrient inputs. This approach combined nutrient concentration data, optical characterisation, and stable isotope tracers with detailed hydrodynamic measurements to resolve nutrient sources, mixing behaviour, and short-reach processing. Water samples were collected for conventional nutrient analysis, excitation–emission matrix (EEM) fluorescence, and isotopes of nitrate and phosphate. Field measurements were supported by drone-based infrared imaging to characterise surface flow structure and a remote-controlled survey vessel equipped with Acoustic Doppler Current Profiler, Single Beam Echo Sounder, and GPS to resolve hydrodynamics and channel morphology. In situ sondes and large-volume sampling further captured water-quality variability. Phosphate oxygen isotopes (δ¹⁸O_p) were used to directly trace wastewater-derived phosphorus downstream of a wastewater treatment works (WWTW) outfall. Nineteen river samples collected at ~20 m intervals were compared with upstream river and WWTW effluent end members. Effluent phosphate exhibited a distinctly lower δ¹⁸O_p value than background river phosphate, enabling a two-endmember isotope mixing model. Results indicate that WWTW-derived phosphate contributed approximately 20–55% of riverine phosphate across most of the reach, with localized zones of near-complete effluent dominance. A pronounced low-δ¹⁸O_p anomaly coincident with elevated phosphorus concentrations is interpreted as a localized hydrodynamic pulse of wastewater phosphate superimposed on progressive biological reprocessing. Together, these results demonstrate that wastewater phosphorus can exert strong, spatially heterogeneous control on riverine phosphate over very short distances, even under conditions of active mixing and biological cycling. More broadly, this integrated Lagrangian-hydrodynamic-isotopic framework provides a powerful new basis for quantifying nutrient sources, transport, and transformation in rivers, with direct implications for more effective nutrient management strategies.