Assessing the use of δ¹⁸O in phosphate as a tracer for catchment phosphorus sources

Procedures for source apportionment of nutrients transported in rivers and streams are often uncertain and difficult to validate. In this study, we investigated if the oxygen isotope ratios in stream dissolved phosphate (δ¹⁸O_P) can be used to distinguish between the phosphorus (P) originating from agricultural runoff and from rural sewage, at different periods of the year. Water was collected from septic tanks, drainage wells and the stream outlet in a small (7.3 km²) agricultural catchment to capture variations in the isotope signature at different flow events. The δ¹⁸O_P signature in septic tank effluent (+13.2 to +14.5‰) was significantly different from that in drainage water (+9.0 to +15.7‰). Four different septic tanks had surprisingly similar isotope signature when 24 h composite samples were collected. Most of the water samples were not in oxygen isotope equilibrium. In three drainage wells, the δ¹⁸O_P signature varied from +7.8 up to +15.7‰ with higher values in periods with a larger contribution of superficial streamflow generation. A rainfall soon after manure had been spread resulted in a δ¹⁸O_P of 15.7‰ in a drainage well, and was also reflected in a similar value at the catchment outlet. This implies that the source isotope signal may be conserved during moderate or high flows. A three end-member model including the geological background would be a useful start to quantify the P contribution from diverse sources. Temporal differences in biological activity and the predominant transport pathways through soil profiles must be considered along with information about fertilisation. In combination with hydrological modelling of water pathways, this may considerably improve our understanding of catchment P losses.