Deciphering phosphorus dynamics in impacted river systems using phosphate oxygen and lead isotope analysis

Phosphorus (P) is a major driver of eutrophication in river systems, but its sources and cycling remain poorly understood. This study employs a novel combination of phosphate oxygen isotope (δ 18 O-PO 4 ) and lead isotope ( 206 Pb/ 207 Pb) analyses to trace P sources and biogeochemical processes in river waters, shallow sediments (<5 cm), and sediment cores (up to 60 cm) along a ∼100 km stretch of the River Nene, UK. δ 18 O-PO 4 signatures were used to identify phosphate origins and transformation pathways in river sediments, while 206 Pb/ 207 Pb ratios provided insights into sediment provenance and historical pollution. Results reveal significant spatial variability in P sources and sinks, with sewage inputs dominating in some parts, whereas and agricultural and urban contributions evident in others. Shallow (<5 cm) river sediment δ 18 O-PO 4 values are close to those in the overlying river water suggesting microbial cycling or some sorption to iron oxides at the river-sediment interface. Deeper sediments exhibit anthropogenic 206 Pb/ 207 Pb ratios throughout the core, with the δ 18 O-PO 4 shifts possibly influenced by organic matter decomposition and remineralization. Sedimentary deposition conditions and permeability appear critical in determining the extent of mixing and diffusion between sediments and overlying water. These findings highlight the complex and dynamic interactions among microbial cycling, organic matter breakdown, iron reduction and reversible phosphate sorption processes. Integrating δ 18 O-PO 4 and 206 Pb/ 207 Pb analyses offers a potential framework for disentangling sediment-associated pollution and improving management of nutrient-enriched river systems. • Dual δ 18 O-PO 4 and Pb isotopes effectively trace phosphorus sources and sediment pollution. • Phosphorus inputs vary spatially, with sewage, agricultural, and urban sources dominating different reaches. • Shallow sediments reflect river-water phosphate cycling; deeper sediments show anthropogenic lead and altered δ 18 O-PO 4 signatures. • Sediment conditions control P mobility through microbial cycling, organic matter breakdown, iron reduction, and sorption processes.