Phosphate and nitrate isotope responses to lateral plume mixing and nutrient turnover downstream of a wastewater discharge

Wastewater treatment works (WwTW) are major point sources of nutrients to rivers, but interpretation of downstream isotope signals is complicated where hydrodynamic mixing and in-stream biogeochemical processing occur over similar spatial scales. Here, we combine phosphate oxygen isotopes (δ18O-PO4), nitrate nitrogen and oxygen isotopes (δ15N–NO3 and δ18O–NO3), and nutrient concentrations to investigate nutrient behaviour over a short River Thames, UK reach downstream of a WwTW discharge. The study resolves a high-resolution spatial pattern from a single sampling survey and is framed within a two-dimensional mixing context, in which effluent-influenced water and background river water are transported downstream in parallel rather than as a laterally well-mixed one-dimensional system. Phosphate concentrations and δ18O-PO4 values showed a strong response to the effluent input. Near the outfall, isotope compositions were broadly consistent with conservative mixing between effluent and river phosphate, with first-order estimates suggesting that effluent-derived phosphate contributed approximately 20–55% of the local dissolved inorganic phosphate pool at plume-influenced sampling points. Further downstream, several δ18O-PO4 values were close to the independently calculated temperature-dependent equilibrium composition of approximately 17.2‰, consistent with possible biological isotope resetting of phosphate within the reach. However, because travel times and turnover rates were not directly measured, this convergence is interpreted as evidence for short-reach isotope resetting rather than as a quantified rate of biological turnover. A pronounced low δ18O-PO4 value at approximately 220 m downstream fell outside the range predicted using the measured effluent end member, indicating end-member uncertainty or plume heterogeneity within the wastewater-influenced water mass. In contrast, nitrate concentrations and isotope compositions showed little spatial variation downstream of the discharge. δ15N–NO3 and δ18O–NO3 displayed no coupled downstream enrichment, indicating no detectable isotopic evidence for substantial denitrification over the sampled reach and hydrological conditions. However, the large background nitrate pool means that local nitrogen transformations, including nitrification, assimilation or localised denitrification, may have been difficult to resolve. These contrasting isotope responses show that phosphate was sensitive to both local plume structure and possible biological isotope resetting, whereas nitrate primarily reflected buffering by a large background nitrate pool. More broadly, the results demonstrate that interpretation of river nutrient isotope signals below point-source inputs requires explicit consideration of lateral hydrodynamic structure, end-member uncertainty and nutrient-specific reactivity.