title: The oxygen isotopic composition of phosphate in river water and its potential sources in the Upper River Taw catchment, UK.
abstract: The need to reduce both point and diffuse phosphorus pollution to aquatic ecosystems is widely recognised and in order to achieve this, identification of the different pollutant sources is essential. Recently, a stable isotope approach using oxygen isotopes within phosphate (δ18OPO4) has been used in phosphorus source tracing studies. This approach was applied in a one-off survey in September 2013 to the River Taw catchment in south-west England where elevated levels of phosphate have been reported. River water δ18OPO4along the main channel varied little, ranging from +17.1 to +18.8‰. This was no >0.3‰ different to that of the isotopic equilibrium with water (Eδ18OPO4). The δ18OPO4in the tributaries was more variable (+17.1 to +18.8‰), but only deviated from Eδ18OPO4by between 0.4 and 0.9‰. Several potential phosphate sources within the catchment were sampled and most had a narrow range of δ18OPO4values similar to that of river Eδ18OPO4. Discharge from two waste water treatment plants had different and distinct δ18OPO4from one another ranging between +16.4 and +19.6‰ and similar values to that of a dairy factory final effluent (+16.5 to +17.8‰), mains tap water (+17.8 to +18.4‰), and that of the phosphate extracted from river channel bed sediment (+16.7 to +17.6‰). Inorganic fertilizers had a wide range of values (+13.3 to +25.9‰) while stored animal wastes were consistently lower (+12.0 to +15.0‰) than most other sources and Eδ18OPO4. The distinct signals from the waste water treatment plants were lost within the river over a short distance suggesting that rapid microbial cycling of phosphate was occurring, because microbial cycling shifts the isotopic signal towards Eδ18OPO4. This study has added to the global inventory of phosphate source δ18OPO4values, but also demonstrated the limitations of this approach to identifying phosphate sources, especially at times when microbial cycling is high.
authors.affiliation: Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK. Electronic address: steve.granger@rothamsted.ac.uk.
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Granger
authors.affiliation: NERC Isotope Geoscience Laboratory, British Geological Survey, Nottingham NG12 5GG, UK.
authors.forename: Tim H E
authors.initials: THE
authors.lastname: Heaton
authors.affiliation: Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK.
authors.forename: Verena
authors.initials: V
authors.lastname: Pfahler
authors.affiliation: Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK.
authors.forename: Martin S A
authors.initials: MSA
authors.lastname: Blackwell
authors.affiliation: College of Resources and Environmental Sciences, China Agricultural University, No. 2 Yuanmingyuan West Road, Haidian, Beijing 100193, China.
authors.forename: Huimin
authors.initials: H
authors.lastname: Yuan
authors.affiliation: Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK.
authors.forename: Adrian L
authors.initials: AL
authors.lastname: Collins
keywords: Animal slurry; Fertilizers; Microbial cycling; Phosphorus; Stable isotopes; Tracing