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 (δ¹⁸O_PO4) 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 δ¹⁸O_PO4 along 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 (Eδ¹⁸O_PO4). The δ¹⁸O_PO4 in the tributaries was more variable (+17.1 to +18.8‰), but only deviated from Eδ¹⁸O_PO4 by between 0.4 and 0.9‰. Several potential phosphate sources within the catchment were sampled and most had a narrow range of δ¹⁸O_PO4 values similar to that of river Eδ¹⁸O_PO4. Discharge from two waste water treatment plants had different and distinct δ¹⁸O_PO4 from 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 Eδ¹⁸O_PO4. 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 Eδ¹⁸O_PO4. This study has added to the global inventory of phosphate source δ¹⁸O_PO4 values, but also demonstrated the limitations of this approach to identifying phosphate sources, especially at times when microbial cycling is high.

Keywords: Animal slurry; Fertilizers; Microbial cycling; Phosphorus; Stable isotopes; Tracing.