Evaluating the stable isotopic composition of phosphate oxygen as a tracer of phosphorus from waste water treatment works

Eutrophication is a globally significant challenge facing freshwater ecosystems and is closely associated with anthropogenic enrichment of phosphorus (P) in the aquatic environment. Phosphorus inputs to rivers are usually dominated by diffuse sources related to farming activities and point sources such as waste water treatment works (WwTW). The limited availability of inherent labels for different P sources has constrained understanding of these triggers for eutrophication in natural systems. There have been substantial recent advances in the use of phosphate oxygen isotopes (δ18OPO4) as a way of understanding phosphate sources and processing. Results from all previous studies of the δ18OPO4 composition of WwTW effluent and septic tanks are combined together with significant new data from the UK to assess δ18OPO4 compositions in waste water sources. The overall average δ18OPO4 value is 13.9‰, ranging from 8.4 to 19.7‰. Values measured in the USA are much lower than those measured in Europe. A strong positive correlation exists between δ18OPO4 and δ18OH2O, suggesting biologically-mediated exchange between the water molecules and the phosphate ions. A comparison of δ18OPO4 and the offset from isotopic equilibrium showed a strong positive linear correlation (ρ = 0.94) for the data from Europe but no relationship for the historic USA data which may be due to recent advances in the extraction procedure or to a relative paucity of data. This offset is most strongly controlled by the δ18OH2O rather than temperature, with greater offsets occurring with lower δ18OH2O. Time series data collected over 8-24 hours for three sites showed that, although there were significant changes in the phosphate concentration, for a given WwTW the δ18OPO4 stayed relatively constant. Two new studies that considered instream processing of δ18OPO4 downstream of WwTWs showed mixing of the upstream source with effluent water but no evidence of biological cycling 3 km downstream. It is suggested that δ18OPO4 can be an effective tool to trace P from WwTWs provided the source of the effluent is known and samples are collected within a day.