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

Phosphorus Dynamics and Availability in the Nearshore of Eastern Lake Erie: Insights From Oxygen Isotope Ratios of Phosphate

“…The δ 18 O P we measured in the Grand River during April and early May (+9.4 ± 1.7 to +11.9 ± 0.7‰) are similar to values reported for Lake Erie tributaries in the central and western basins over summer and fall ), but lower than those reported for the River Taw, UK (Granger et al, 2017b). δ 18 O P at the river mouth site was near or overlapped δ 18 O P_EQ in late April and early May, but declined steadily from +9.4 ± 1.7‰ to a minimum of +5.7‰ through August, with the departure from δ 18 O P_EQ increasing to the end of August (Figure 5).…”

“…Such observations are not uncommon in P-rich systems (Paytan and McLaughlin, 2012), however, δ 18 O P moved further away from δ 18 O P_EQ as SRP concentrations declined from 6.9 µM in late May to a low of 0.08 µM in July, which is counter to expectation (McLaughlin et al, 2006b;Granger et al, 2017b). On one hand, this could indicate an input of an isotopically light source of P i such as waste water treatment plant (WWTP) effluent or P i liberated via the degradation of glyphosate (Li et al, 2016a).…”

“…Compared to marine systems (Colman et al, 2005;McLaughlin et al, 2006aMcLaughlin et al, ,b, 2013Goldhammer et al, 2011;Jaisi et al, 2011;Joshi et al, 2015;Li et al, 2017) and soils (Zohar et al, 2010;Angert et al, 2012;Tamburini et al, 2012;Granger et al, 2017a), freshwater environments are poorly characterized with respect to δ 18 O P [but see Granger et al, 2017b;Pistocchi et al, 2017)]. The δ 18 O P we measured in the Grand River during April and early May (+9.4 ± 1.7 to +11.9 ± 0.7‰) are similar to values reported for Lake Erie tributaries in the central and western basins over summer and fall ), but lower than those reported for the River Taw, UK (Granger et al, 2017b).…”

“…Consequently, knowledge of these reactions and pathways may provide a means to identify sources of P or the importance of specific P cycling pathways governing P i availability in aquatic ecosystems McLaughlin et al, 2013;Joshi et al, 2015;Granger et al, 2017b;Li et al, 2017).…”

The stable oxygen isotope ratio of resin extractable phosphate derived from fresh cattle faeces

“…One of the confounding issues surrounding this area of research is the narrow range of δ 18 O PO4 values that most PO 4 sources have and that they often overlap or they are similar to the Eδ 18 O PO4 value . A recent study by Granger et al, which characterised different sources within a river catchment found that farm slurry, a mix of fresh and aged animal urine, faeces, bedding materials and other farm washings, had a relatively consistent δ 18 O PO4 value for water‐extractable PO 4 despite its heterogenous composition. Furthermore, this study reported that its value was noticeably lower than the Eδ 18 O PO4 value in the rivers.…”