Scholarly article on topic 'The oxygen isotopic composition of phosphate in river water and its potential sources in the Upper River Taw catchment, UK'

The oxygen isotopic composition of phosphate in river water and its potential sources in the Upper River Taw catchment, UK Academic research paper on "Environmental engineering"

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{Phosphorus / "Stable isotopes" / Tracing / Fertilizers / "Animal slurry" / "Microbial cycling"}

Abstract of research paper on Environmental engineering, author of scientific article — Steven J. Granger, Tim H.E. Heaton, Verena Pfahler, Martin S.A. Blackwell, Huimin Yuan, et al.

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 δ18OPO4 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 (Eδ18OPO4). The δ18OPO4 in the tributaries was more variable (+17.1 to +18.8‰), but only deviated from Eδ18OPO4 by between 0.4 and 0.9‰. Several potential phosphate sources within the catchment were sampled and most had a narrow range of δ18OPO4 values similar to that of river Eδ18OPO4. Discharge from two waste water treatment plants had different and distinct δ18OPO4 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 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 δ18OPO4 values, but also demonstrated the limitations of this approach to identifying phosphate sources, especially at times when microbial cycling is high.

Academic research paper on topic "The oxygen isotopic composition of phosphate in river water and its potential sources in the Upper River Taw catchment, UK"

The oxygen isotopic composition of phosphate in river water and its potential sources in the Upper River Taw catchment, UK

Steven J. Granger a,*< Tim H.E. Heaton b, Verena Pfahler a, Martin S.A. Blackwella, Huimin Yuanc, Adrian L. Collins a

a Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK b NERC Isotope Geoscience Laboratory, British Geological Survey, Nottingham NG12 5GG, UK

c College of Resources and Environmental Sciences, China Agricultural University, No. 2 Yuanmingyuan West Road, Haidian, Beijing 100193, China

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HIGHLIGHTS

GRAPHICAL ABSTRACT

1 Can sources of aquatic PO4 be traced using its stable oxygen isotope ratio? Various PO4 sources within a catchment were analysed for their S18OPO4. 1 River S18OPO4 indicated that rapid mi-crobial cycling of PO4 was occurring. This method appears inappropriate in systems where PO4 cycling is rapid.

ARTICLE INFO

Article history:

Received 11 July 2016

Received in revised form 23 August 2016

Accepted 1 September 2016

Available online 14 October 2016

Editor: Jay Gan

Keywords: Phosphorus Stable isotopes Tracing Fertilizers Animal slurry Microbial cycling

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 (S18OPO4) 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 S18OPO4 along the main channel varied little, ranging from +17.1 to + 18.8%».Thiswasno> 0.3%o different to that of the isotopic equilibrium with water (ES18OPO4). The S18OPO4 in the tributaries was more variable ( + 17.1 to + 18.8%), but only deviated from ES18OPO4 by between 0.4 and 0.9%. Several potential phosphate sources within the catchment were sampled and most had a narrow range of S18OPO4 values similar to that of river ES18OPO4. Discharge from two waste water treatment plants had different and distinct S18OPO4 from one another ranging between +16.4 and + 19.6%o 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 ES18OPO4. 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 ES18OPO4. This

* Corresponding author. E-mail address: steve.granger@rothamsted.ac.uk (S.J. Granger).

http://dx.doi.org/10.1016/j.scitotenv.2016.09.007

0048-9697/© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

study has added to the global inventory of phosphate source 618OPO4 values, but also demonstrated the limitations of this approach to identifying phosphate sources, especially at times when microbial cycling is high.

© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license

(http://creativecommons.org/licenses/by/4.0/).

1. Introduction

The need to reduce both point and diffuse source pollution to aquatic ecosystems has been widely recognised (Conley et al., 2009). The movement of potential pollutants such as sediment, faecal pathogens and nutrients into surface waters at above 'natural' levels has long been known to lead to their degradation. In particular, nutrients such as nitrogen and phosphorus (P) can lead to eutrophication through the proliferation of harmful algal blooms (Smith et al., 1999) as well as potentially direct toxic effects (Lewis and Morris, 1986). Surface waters are particularly sensitive to P because critical concentrations of only a few tens of |ag available P l-1 can cause eutrophication, but are an order of magnitude lower than soil available P concentrations required for crop growth (Heathwaite and Dils, 2000).

Identifying the different pollutant source pressures on an impacted water body is critical to understanding the causes of degraded ecosystem health and has implications for matters such as point source consents, targeted remediation and habitat restoration. In the UK, water quality, which had been improving from the 1960s, had stalled by the 1980s. This had been blamed on a combination of increased emissions from both point and diffuse sources resulting from both a lack of investment in sewage treatment and an increase in pollution stemming from intensive farming practices (Heathwaite et al., 1996). In actuality, apportioning pollution to any given source or sources is fraught with difficulty (Bowes et al., 2008). However in recent years, several techniques have been developed such as sediment fingerprinting (Collins et al., 1997; Walling et al., 1999), natural fluorescence (Baker, 2002; Old et al., 2012) and the use of natural abundance stable isotope ratios particularly in relation to nitrate (Amberger and Schmidt, 1987; Granger et al., 2008). More recently, this stable isotope approach has been applied to soluble reactive phosphate (PO4) as it is this form ofPthat is considered most biologically relevant and is a parameter that is routinely monitored to assess a water body's ecological status as defined by the EU Water Framework Directive (WFD) (EEC, 2000).

The PO4 source tracing approach uses the stable oxygen (O) isotope ratio of 18O/16O within the PO4 (S18OPO4). At normal surface water temperatures and pH, and in the absence of biological activity, the P—O bonds in PO4 are stable and resistant to oxygen isotope exchange. Therefore, bonds are only broken through biological mediation, and in these cases PO4 exchanges O with the ambient water in (Blake et al., 1997; Longinelli and Nuti, 1973; Paytan et al., 2002). The most influential biological process in the environment is considered to be that of inorganic pyrophosphatase the result of this enzymatic cycling is that the S18OPO4 moves towards a predictable equilibrium (E618OPO4) with the isotopic ratio of the O in the water (S18OH2O) depending on its temperature (Chang and Blake, 2015; Longinelli and Nuti, 1973). In aquatic systems, where biological activity is limited by PO4 concentrations (Bowes et al., 2014), it is assumed that rapid biological cycling of PO4 leads to an over-writing of any original PO4 source 618OPO4 signature (Tamburini et al., 2012) through a process described by (Cohn, 1958). However, where rates of biological uptake are low in comparison to PO4 supply, measured 618OPO4 values should reflect the 618OPO4 of the original PO4 source(s), or lie between source 618OPO4 values and the expected equilibrium value. In contrast, for any source tracing work to be successful, it is essential that a measurable and statistically robust isotopic difference must exist among the various PO4 sources that are to be traced.

Gross et al. (2013) examined the 618OPO4 in dust and found that different source signatures from agricultural and natural soils prior to deposition in an aquatic environment could be determined. McLaughlin et al. (2006a, 2006b) and Elsbury et al. (2009) found that the S18OPO4

in lacustrine and coastal waters was not in equilibrium with the S18OH2O suggesting that source ô18OPO4 signatures were, in part, being preserved. However, data on the ô18OPO4 of potential PO4 sources remains limited. Gruau et al. (2005) found only small differences between the ô18OPO4 of chemical P fertilizers and the PO4 discharged from wastewater treatment plants. Young et al. (2009) then went on to publish the first collection of ô18OPO4 source values reviewed from the literature and from their own studies. They found that there was a considerable range of ô18OPO4 values (from 8.4 to 24.9%») for different types of sources and, importantly, statistically significant differences were found between several of the individual source types. They concluded that the ô18OPO4 tracing approach could be used for identifying PO4 sources for aquatic systems. More recently reviews by Davies et al. (2014) and Tamburini et al. (2014) update the state of this embryonic application, the differing methodological approaches and set out research key new research areas. There remains, however, relatively little published information with regard to the ô18OPO4 of different sources of PO4 (Ayliffe et al., 1992; McLaughlin et al., 2006a), and although many sources have a high global ô18OPO4 variability, locally, with a limited number of sources the range of ô18OPO4 may be more restricted thereby increasing the opportunities for source discrimination.

In the above context, a study was therefore carried out within the River Taw catchment in south-west England within which elevated levels of PO4 are one of the main reasons for failure to achieve 'good ecological status' under the EU WFD ( EEC, 2000). The study was designed to answer three research questions: 1) What are the ô18OPO4 values of different PO4 sources within the study catchment? 2) Is the ô18OPO4 within the study catchment at equilibrium with the river water?, and 3) If not at equilibrium, can the ô18OPO4 of PO4 sources be used to determine the origin of PO4 within the river catchment?

2. Study catchment

The River Taw catchment is located in Devon, South West England, and is a predominantly rural catchment covering an area of 914 km2 (Fig. 1a). The headwaters rise in the south on the Dartmoor granite plateau ca. 550 m above sea level. The river then flows northwards 72 km to the Taw/Torridge estuary, and the Bristol Channel. The soils of the catchment are predominantly agriculturally managed, typically poorly draining clay rich gley soils and typical brown earths, within which a narrow belt of well-drained, gritty reddish loams occur. The soils on Dartmoor granite consist of peat and podzols, some of low permeability.

Typical annual (1992-2014) rainfall averages 1601 mm on Dartmoor (50.703 N, - 3.976 W), to <940 mm at the river mouth (51.089 N, - 4.147 W).1 The majority of the precipitation falls in the winter and the climate as a whole is typical of temperate Atlantic Britain (5-20 °C). River hydrology is dominated by surface water, reflecting the low permeability of the soils, sub-soils, and lithology and, as a result, river discharge (Q) responds rapidly to rainfall. Despite the low groundwater storage capacity of the underlying geology, water stored within rock fissures does continue to feed river Q and maintain base flow during extended dry periods.

This study focussed on the Upper River Taw catchment (Fig. 1a) which is 29.3 km in length and stretches from the source of the river to Taw Bridge. Aside from the unimproved semi-natural grass and heathlands of Dartmoor, the land-use is predominantly one of improved agricultural grassland. This supports beef, dairy and sheep production.

1 Data provided by the National Meteorological Archive.

Fig. 1. a) Location of the Upper Taw catchment within the United Kingdom, the catchment boundary and main settlements and point sources, b) the sample locations along the main river, and c) the sub-catchments sampled feeding the main river.

Cereals and fodder maize are also produced, particularly on the sandier, free-draining soils. Human settlement consists of scattered farmsteads and small rural towns and villages. The towns and villages tend to be served by small unmanned sewage treatment works while the more isolated rural dwellings have individual domestic septic systems.

At Taw Bridge, the river is subject to routine monthly monitoring for various water quality parameters by the Environment Agency (EA). The river is classified as a type 2n according to the UK Technical Advisory Group on the WFD (EEC, 2000) guidelines, with an altitude of >80 m and an alkalinity of < 50 mg l-1 CaCO3. The EA dataset reveals that un-filtered PO4 concentrations exhibit a marked seasonal variation and can exceed 2000 ^gPl-1 in the summer, with an annual mean concentration of 300 mg P l-1 (1990-2012). This is in contrast to the annual mean concentration for good ecological status being defined as <42 |ag l-1 reactive P (United Kingdom Advisory Group (WFD-UKTAG), 2014).

Within the Upper River Taw catchment, nine approximately equidistant sampling sites were selected along the main river from R1, near the headwaters on Dartmoor, to R9 at Taw Bridge (Fig. 1b) to examine longitudinal spatial variations in unfiltered total reactive PO4-P (TRP). Ten tributaries were also sampled at locations just prior to their confluence with the main river (Fig. 1c) to investigate how different areas of the catchment were influencing the river TRP variability.

3. Methodology

3.1. River water sample collection and preparation

A water sampling assay was carried out four times during 2013 (March, June, September and December). Sampling was timed to coincide with low flow for the time of year (i.e. non-storm flow) to minimise the variable impact of localised inputs due to rainwater runoff (Preedy et al., 2001; Granger et al., 2010) and to minimise spatial variation in catchment flow (Fig. 2). Conventional grab samples were collected mid-channel using a sampling boom and stored in acid washed 60 ml

HDPE bottles at 4 °C prior to analysis. The TRP analysis was performed within 24 h of sample collection, after which samples were refrigerated until determination of total P (TP).

During the September sampling assay, when historical EA data indicated RP concentrations might be highest, samples were collected at three locations along the river (R1, R4 and R9) and from three tributaries (T5, T6 and T7) for S18OPO4 determination. At these locations when the samples were collected the temperature of the water was measured using a handheld digital thermometer. Two additional water samples were collected; i) a 25 ml sample bottle was completely filled and sealed, for the analysis of S18OH2O, and ii) a sufficient volume to contain approximately 50 ^moles P, in acid washed HDPE barrels, typically between 25 and 50 l.

Fig. 2. The 2013 river discharge as monitored by the Environment Agency at Taw Bridge with sampling event times as indicated.

3.2. Phosphate source sampling

The main potential sources of PO4 in the Upper River Taw catchment were identified and sampled during the course of the study. These were; 1) waste water treatment plant (WWTP) final effluent, 2) a dairy factory final effluent, 3) mains tap water, 4) agricultural inorganic fertilizers and animal wastes, 5) domestic septic tanks, and 6) river channel bed sediment.

3.2.1. Wastewater treatment plants and dairy factory final effluent

Two WWTPs discharge their final effluents directly into the River Taw within the Upper Taw catchment (Fig. 1a). WWTP1 serves a resident population of approximately 1350 and discharges into the River Taw just below sample location R3, while WWTP2 has a resident population of approximately 2000 and discharges into the River Taw between sample locations R6 and R7. The dairy factory also discharges into the river between R6 and R7, upstream of WWTP2. Samples of the WWTP final effluent were collected in June, September and December, while samples of dairy factory final effluent were collected in September, November and December.

3.2.2. Mains tap water

Three samples of mains tap water were collected from within the Upper Taw catchment in December. The water originates from a reservoir on Dartmoor and, as part of its treatment before supply to customers, has PO4 added in the form of phosphoric acid. This is a standard treatment across a large part of the UK and relates to reducing the propensity of many waters to be plumbo-solvent. Samples were collected after the tap had been left to run for 1 min.

3.2.3. Agricultural inorganic fertilizers and animal wastes

In the UK phosphate fertilizers are applied to land to increase agricultural production and to support high stocking densities. Furthermore, high stocking densities can generate large quantities of animal waste (Chadwick and Chen, 2002). Which aside from direct returns to land during grazing, is directly applied to land in the spring after being stored over the winter period as 'slurry'. Samples ofinorganic P fertilizer and slurry were collected from five representative farms within the Upper Taw catchment. Samples of fertilizer were ground to a powder to homogenise them, while the animal wastes were frozen and freeze dried before being ground to a powder. Phosphate was extracted from each sample by adding 0.5 g of material to 100 ml of de-ionised water, and shaking overnight. The resultant solution was then filtered through GF/F filters.

3.2.4. Domestic septic systems

In rural areas, where direct connections to local sewerage and WWTP are not possible, residential properties are often served by domestic septic systems. These systems treat domestic waste which typically consists of excreta and various washing water with associated chemicals. They may discharge either directly or indirectly to water courses, and because they contain human excreta and P bearing chemicals such as detergents, domestic septic systems can represent key distributed 'point sources' across the landscape, especially if they are poorly maintained or managed (Withers et al., 2014). Samples were collected from the main tank from two septic systems on three occasions in March and April 2013. Samples were oven dried at 30 °C before PO4 was extracted by adding between 1 and 5 g of material to 100 ml of de-ionised water, and shaking overnight. The resultant solution was then filtered through GF/F filters.

3.2.5. River channel bed sediment

Bed sediment from the river channel was sampled at seven locations in August 2013 along the main river, just downstream of R1, R2, R4, just upstream of R6, at Yeobridge between R7 and R8, and R8 and R9 (see Fig. 1b and Table S1). Samples of surface and ingressed material were

collected using a modified version (Collins et al., 2012) of the method described by Lambert and Walling (1988) and Collins and Walling (2007). This resuspension procedure has been shown to generate reliable samples of bed sediment material (Duerdoth et al., 2015). Briefly, a large plastic open-ended drum was pressed into the river bed at three locations across the channel. The water and bed sediment within it was strongly agitated using a battery powered drill with stirrer attachment and, after allowing 5 s for the coarser material to settle out; a sample of the sediment rich water was collected. In total, approximately 5 l of water/sediment was collected across the channel at each location. Samples were then wet sieved through a 63 |jm sieve to remove sand-sized particles before the remaining sediment was allowed to settle overnight in a refrigerated environment. The majority of the water was then discarded and the residual sediment/water was centrifuged down to a volume of 250 ml, frozen and freeze dried.

Due to the low sample mass collected and previous experience of low concentrations of water extractable TRP, a 1 M HCl acid was used as the extracting media to maximise TRP concentration for isotopic analysis (Tamburini et al., 2010). Between 0.6 and 8.4 g of bed sediment was added to 100 ml of acid and shaken overnight. The resultant solution was then filtered through Whatman GF/F filters before being analysed for RP and converted to Ag3PO4. Water extractable RP was determined by adding sediment to de-ionised water at a ratio of 1:5 which was shaken for 30 min at 20 °C before filtration through a Whatman GF/ F filter paper.

3.3. Sample analysis

In water samples that were to have PO4 analysed for 618OPO4 the PO4-P was quantitatively removed from solution through a process of co-precipitation with magnesium hydroxide otherwise known as bru-cite (Karl and Tien, 1992) and is a process that has been used successfully in other freshwater studies (e.g. Elsbury et al. (2009)). About 20 ml of 3 M MgCl2 was added for every 1 l of sample and mixed, followed by 5 ml of 1 M NaOH per litre of sample to induce brucite formation. The precipitate was allowed to settle overnight in a refrigerated environment to minimise microbial activity, before the supernatant was discarded and the brucite collected and centrifuged. The resultant solids were then dissolved in a minimal quantity of 1 M HNO3. If the resultant solution was >200 ml in volume the process was repeated by increasing the pH to 10-11 using 1 M NaOH, and re-precipitating, centrifuging and dissolving the brucite. The solution was then filtered through Whatman GF/F filters.

Reactive P concentrations for all samples/extracts were determined colourimetrically on an Aquachem 250 analyser using a molybdenum blue reaction (Murphy and Riley, 1962), while TP in water samples was analysed on the same equipment following oxidation with acidified potassium persulphate. The value of unreactive P (UP) was taken as the difference between TP and TRP. When samples were in a 1 M HCl solution they were diluted by at least 1/10 to avoid acid interference with the molybdenum chemistry. Bed sediment samples were analysed for TP using an ICP-MS after an aqua-regia digestion.

Any PO4 bearing extractions that were to be analysed for 618OPO4 were converted to silver phosphate (Ag3PO4) using the purification protocol described by Tamburini et al. (2010), subject to minor modification. The process utilizes a series of dissolution and precipitation reactions to isolate and purify dissolved PO4. The PO4 is precipitated firstly as ammonium phospho-molybdate before it is dissolved and re-precipitated as magnesium ammonium phosphate. Excess magnesium and chloride is removed through the addition of a cation resin and a small dose of silver nitrate crystals respectively. The resultant PO4 in solution is then converted to Ag3PO4 though the addition of an Ag-am-mine solution and subsequent adjustment of the pH down to between 7 and 8. The solution is then placed in an oven for two days at 50 °C. Although the Tamburini protocol uses a DAX-8 resin early in the extraction its use is not necessary unless organic contamination is present in

the subsequent Ag3PO4 (Tamburini pers. Comm.). Where samples were extracted using 1 M HCl, additional duplicate samples were also extracted using 18O-labelled 1 M HCl to test that no RP was being released through the hydrolysis of organic matter as described in the Tamburini etal. (2010) protocol.

For analysis of 18O/16O ratios, about 400 |ag of Ag3PO4 wrapped in a silver capsule is dropped into a thermal conversion elemental analyser at 1400 °C, and the resultant carbon monoxide carried in a flow of helium through a GC column into a Delta + XL mass spectrometer (ThermoFinnigan, Germany). The 18O/16O values are calculated by comparing the abundance ratios of mass 30 (12C18O16O) over mass 28 (12C16O16O) to those of an internally run Ag3PO4 laboratory standard ('ALFA-1'). The 618O values versus SMOW are calculated by assigning a S18Osmow value of +14.2%» to ALFA-1. In the absence of an international Ag3PO4 reference material we derived this value for ALFA-1 by comparison to the Ag3PO4 standard 'B2207' (Elemental Microanalysis Ltd., England) which has been measured in an inter-laboratory comparison study to have a S18O value of +21.7% versus VSMOW. All samples were run in triplicate, with a typical precision a < 0.3%. Sample purity was assessed by determining the CO yield compared with the yield of Ag3PO4 standards, and rejecting samples where this differed by > 10%. Samples were also analysed for their %C by separate elemental analysis; this was usually below 1%C. Samples extracted in duplicate were found to have differences in 618OPO4 of between 0.3 and 0.4%.

Water 18O/16O ratios were determined on CO2 equilibrated with water samples in an Isoprime Aquaprep coupled to an Isoprime 100 dual-inlet mass spectrometer (Isoprime Ltd., Cheadle, England). The isotope ratios are reported as S18OH2O values versus VSMOW, based on comparison with laboratory standards calibrated against IAEA standards VSMOW and SLAP, with analytical precision typically a < 0.05%.

6450 ngPl-1 and overall comprised > 83% of the TP. From WWTP2, the corresponding concentrations were higher ranging between 6438 and 10,100 |ag P l-1 and again TRP was the dominant form of P, comprising > 89% of TP. The TRP in the dairy factory final effluent ranged from 1737 to 8100 |ag P l-1 and comprised between 57 and 96% of the TP.

The S18OPO4 of the final effluent from WWTP1 had a mean of +19.2% (a = 0.8) and decreased from +19.7% in June to + 18.2% in December. Values of 618OPO4 at WWTP2 were lower than those at WWTP1 and less variable with a mean of +16.7% (a =0.3) and ranged from + 16.4% in June to +16.9% in December. The calculated E618OPO4 for both WWTPs was similar and rose from +17% in June to +18.5% in December. This increase was predominantly due to a marked drop in the effluent temperature caused by the onset of cooler winter temperatures. The S18OPO4 of WWTP1 had the greatest difference from ES18OPO4 being 2.6% higher in June and 2.1% higher in September but dropping to near ES18OPO4 in December. At WWTP2 the measured S18OPO4 was lower than E618OPO4 by 0.5, 0.2, and 1.6% in June, September and December respectively. The 618OPO4 of the dairy factory final effluent ranged between +16.5% in September and +17.8% in November with a mean value of + 17.1% (a = 0.7). As with the 618OPO4 values from WWTP2, the dairy factory final effluent 618OPO4 were all lower than E618OPO4 by between 0.7 and 2.2% with a mean of 1.5% (a = 0.8).

4.1.2. Mains tap water

The TRP concentration of three mains tap water samples were similar, with a mean of 976 |agPl-1 (a = 19), which comprised between 95 and 97% of the TP. The measured 618OPO4 was found to range between + 17.8 and + 18.4%o (x = +18.2%, a = 0.3) while the ES18OPO4 ranged between +18.4 and +18.8% (x = +18.7%, a = 0.2) with measured S18OPO4 values being just between 0.4 and 0.6% lower than E618OPO4.

3.4. Data analysis

Data summaries and Hierarchical Cluster Analysis were undertaken using Genstat 16 software (VSN International, 2013).

The 618OPO4 values expected for temperature dependant isotopic equilibrium with the waters measured S18OH2O were calculated using the following equation defined by (Chang and Blake, 2015) based on a rigorous and controlled laboratory calibration of the temperature-dependence of equilibrium PO4 and water, catalyzed by inorganic pyrophosphatase, over typical environmental temperatures:

ES18OPO4 = - 0.18T + 26.3 + S18OH2Owhere ES18OPO4 is the stable oxygen isotope ratio of PO4 at equilibrium in %, T is the temperature in degrees Celsius and S18OH2O is the stable oxygen isotope ratio of H2O in %.

This equation updates the traditionally used formula defined by Longinelli and Nuti (1973) and gives values typically 0.9 to 2.5% higher.

4. Results

4.1. Phosphorus sources

4.1.1. WWTP and dairy factory final effluents

Data from these two sources are presented in Table 1. The TRP concentration in the final effluent from WWTP1 ranged between 3053 and

4.1.3. Agricultural inorganic fertilizers and animal wastes

All five inorganic fertilizers were compound fertilizers, containing P with differing combinations of N, K and S, although three of the five were from the same supplier referred to as A, B and C in (Table 2). The P content was expected to range between 1.7 and 10.5% on the basis of the formulation supplied by the manufacturer and values of 618OPO4 measured for the fertilizers ranged between + 13.3 and + 25.9% (x =+ 20.9%», a = 5.6) and were not related to the amount of P present nor the manufacturer.

The five dairy farm slurries contained water extractable TRP concentrations ranging from 961 to 3502 |ag P g-1 (x = 2364 mg P g-1, a = 1.1) while their 618OPO4 ranged between +12.0 and + 15.0% (x = + 13.5%», a = 1.2) and were also not related to TRP content.

4.1.4. Domestic septic systems

The TRP concentration in the septic system waste differed between the two tanks sampled although did not change greatly within the tank over the time period sampled (Table 3). The mean TRP concentration in Tank A was 289 |ag P g-1 (a = 0.08) while in Tank B it was 34 ngPg-1 (a = 0.03). Due to the low TRP concentration of the sample from Tank B it was not possible to extract sufficient Ag3PO4 to analyse for 618OPO4. A lack of sample from Tank A for the March assay also meant that no 618OPO4 value could be determined; however sufficient

Table 1

Data parameters measured in the waste water treatment plants (WWTP) and dairy factory final effluents.

WWTP1 WWTP2 Dairy factory final effluent

June Sept Dec June Sept Dec Sept Nov Dec

TRPdigPr1) 6298 6450 3053 10,100 9832 6438 8100 1737 477

TP (^g P l-1) 7581 6959 3697 11,351 10,021 6952 8862 1803 833

Effluent Temp (°C) 16.0 18.0 9.5 17.2 19.8 10.5 20.6 11.7 12.3

S18Oh2o (%) -6.35 - 5.57 - 6.11 - 6.31 - 5.62 -5.88 - 4.46 -5.70 - 5.00

E818Opo4 (%) + 17.1 + 17.5 +18.5 +16.9 + 17.1 + 18.5 + 18.1 + 18.5 + 19.1

Measured 81sOPO4 (%) + 19.7 +19.6 +18.2 +16.4 + 16.9 + 16.9 + 16.5 + 17.8 + 16.9

Deviation from E81sOPO4 (%) 2.6 2.1 - 0.3 - 0.5 - 0.2 -1.6 - 1.6 -0.7 - 2.2

Table 2

Properties of inorganic fertilizers sampled from farms in the Upper Taw catchment.

Manufacturer Compound 518Opo4 (%)

A N (27): P (5): K (5): S (7.5) + 13.3

A N (16): P (16): K(16) + 25.5

A N (0): P (24): K(24) + 16.9

B N (0): P (16): K(36) + 22.8

C N (23): P (4): K(13): S (7) + 25.9

PO4 was extracted from the April samples. The 618OPO4 value of the PO4 in Tank A was found to be consistent in April and ranged from + 20.5 to + 21.1%.

4.1.5. River channel bed sediment

The TP of the < 63 |im fraction of the river channel bed sediment generally increased along the river, from a minimum at R1 of 1001 mg TP kg-1 to a maximum at R8 of 2444 mg TP kg-1 (Table 4). Concentrations of 1 M HCl extractable TRP followed similar trends to that of the TP. The proportion of TP extracted using 1 M HCl was significantly (r5 = 0.9156; p < 0.01) related to the TP of the bed sediment and increased in a positive and non-linear trend, ranging from 40.1 and 61.8% of the TP. The 618OPO4 values of the 1 M HCl bed sediment extractions showed little variation along the length ofthe channel and ranged between +16.7 to +17.5%» (x = +17.3%», a = 0.3).

4.2. Upper River Taw

4.2.1. Seasonal variability in catchment discharge

Although samples were collected at 'low Q', i.e. between storm events (Fig. 2), Qwas higher in the winter than the summer. This difference is important to take into account when examining RP concentration data. Flow was gauged by the EA at R2 and R9 within the Upper River Taw catchment (Table 5) with the highest Qat both sites occurring in March, and the lowest in September. The Qat R9 was positively related (r2 = 0.9995; p < 0.001) to the Qat R2 at the time of sample collection and increased exponentially. As sampling occurred during low, stable flows, it is assumed that tributary Q reflects that of the main river (i.e. Qbeing lowest in September and highest in March).

42.2. Upper River Taw TRP concentrations

The TRP concentrations measured in the Upper River Taw ranged between 1 and 2286 |g P l-1 (Table 5). The TRP concentrations showed a seasonal trend at each sample location, increasing from March when they were at their lowest, through to September when they were highest, and declining again in December. This pattern mirrors that of the river Qand may well reflect changes in TRP dilution within the water column. During each sample period, it was notable also that the TRP concentrations showed a spatial variability, with lowest concentrations between R1-3, increasing downstream. This was especially pronounced during the June and September sampling assays. The TRP component of TP also increased below R3 and was generally > 50% of TP.

Regression analysis of the TRP concentrations in the main channel showed that they were not linear and could be divided into groups. Hierarchical cluster analysis of the TRP concentrations found that sample sites R1-3 and R4-6 were grouped at the 99% confidence level, while

Table 3

The TRP content and 818Opo4 of two domestic septic systems sampled in March and April 2013.

Sample event logTRPg-1 518Opo4 (%)

Tank A March 377 -

Tank A April a 250 + 20.5

Tank A April b 241 + 21.1

Tank B March 10 -

Tank B April a 58 -

sites R7-9 were grouped at the 93% level. For sites R1-3, TRP concentrations were low. Sample site R1 had the lowest recorded concentrations (x = 2.75|agPl-1, a = 1.0), while the highest concentrations typically occurred at R2 (x = 4.70 |agPl-1, a = 3.4). The predominant form of P in the river in the upper section was UP, with TRP comprising between 5 and 47%. Sites R4-6 had TRP concentrations an order of magnitude higher than R1-3, with RP comprising between 24 and 92% of TP. Finally, from R7-9, the TRP concentrations were higher again although this increase was very pronounced in June and September when they ranged from 398 to 529 ^gPl-1, and 1611 to 2286 ng P l-1, respectively. The predominant form of P between R7 and R9 was always TRP comprising between 79 and 100% TP.

4.2.3. Upper River Taw S18Opo4

Although samples of river water were collected from R1, the TRP concentration was too low for successful Ag3PO4 precipitation. At R4 and R9, during the September sampling assay, TRP had similar 618OPO4 values of + 17.7 and +17.8%», respectively. The calculated E618OPO4 values for R4 and R9 were also very similar at +17.4 and +17.7%, indicating that the river water 618OPO4 was 0.3 and 0.1% higher than ES18OPO4.

4.3. Tributary TRP concentrations

The TRP concentrations measured in the tributaries ranged between 2 and 154 |ag P l-1 (Table 5). Concentrations were, in general, lower than those seen along the main river. In most tributaries, RP comprised < 50% TP, with the exceptions of T6, T7, T9 and T10. The form of P was also variable between the tributaries with most having UP as the dominant form, while in T6, 7, 9 and 10 the reverse was true with RP predominating. Total RP concentrations did not display the same consistent seasonal trend that was observed in the main river. Hierarchical cluster analysis of the RP concentrations in the tributaries revealed that T1 -5 and T8 were similar and could be grouped at the 97% similarity level. Sample sites T7, 9 and 10 were grouped at the 87% similarity level whilst T6 joined this second group at the 73% level. On the basis of this analysis, the tributaries could be divided into two groups; Group 1, consisting of sites T1 -5 and 8, and Group 2 consisting of T6,7,9 and 10.

The TRP concentrations in Group 1 were always lower than those in Group 2 for any given sampling assay, often by an order of magnitude or more. They ranged between 2 and 26 |ag TRP l-1 which in most cases comprised < 50% TP. Some tributaries had slight increases in RP with decreasing Q (e.g. T3 and 8), others were unaffected by Q (e.g. T2), while again others actually showed a slight decreased in concentration with decreasing Q(e.g. T4 and 5).

Concentrations of TRP in Group 2 tributaries ranged between 21 and 154 |ag P l-1 with highest concentrations always occurring in T6. Unlike Group 1 tributaries, Group 2 displayed a pronounced seasonality in concentration being highest in June and September when Q was lowest. Unlike Group 1 tributaries TRP comprised > 50% TP in most cases. At both T6 and 10 TRP was always the predominant form of P ranging between 58 and 100% of TP while at T9 RP was only < 50% on one occasion. Sample site T7 was an exception to the rule where TRP only > 50% on one occasion.

4.3.1. Tributary S18Opo4

The 618OPO4 of the TRP in the Upper Taw catchment tributaries were + 18.8, +17.1 and +17.7% for sample sites T5, 6 and 7, respectively. The water temperature at these locations was consistent, ranging between 15.0 and 15.6 °C which was similar to that measured in the main river close to these locations (R4 = 15.1 °C). While the S18OH2O within the river appeared similar along its length, more variability was observed in the tributary S18OH2O. At T6 and T7, values were similar to the main river at - 5.58 and - 6.26%, at T5 the S18OH2O was > 1% lower at - 4.44% This value is unusual as meteoric waters in this area typically have values of between - 5.5 and - 6.3% and this range is consistent with those S18OH2O measured at T6 and 7 and within the

Table 4

The concentrations of different forms of extractable P in the channel bed sediment of the Upper River Taw, and its 818OPO4 when extracted using 1 M HCl. Extractant

Sample location

Aqua-regia (mgTPkg-1)

1 M HCl

De-ionised water

(mgTRPkg-1)

% of TP

(mgTRPkg-1)

% of 1 M HCl extractable

Sheepfold Skaigh (R2) Taw Green (R4) Newlands (R6) Yeobridge Bondleigh (R8) Taw Bridge (R9)

1001 1070 1631 1278 2328 2444 2155

40.1 50.6

61.2 58.0

+ 17.3 + 17.6 + 16.7 + 17.5 + 17.3 + 17.5

0.42 4.52 7.27 2.94

0.08 0.31 0.49 0.24

Ö Opo4 (&)

main river stem. These data gave E618OPO4 of +19.2, +18.0 and + 17.2%» respectively meaning that at T5 and 6 measured 618OPO4 were 0.4 and 0.9 lower than their respective E618OPO4, while at T7 S18OPO4 was 0.5 higher than ES18OPO4.

5. Discussion

5.1. Phosphorus sources

The 618OPO4 data collected from the various P sources within the Upper River Taw catchment are summarised in Fig. 3. From this it can be seen that considerable variation in 618OPO4 occurs both between sources, and within sources. The TRP of these sources and their S18OPO4 are discussed in greater detail within the following sections.

5.1.1. WWTP and dairy factory final effluents

The concentrations of TRP in the discharge from WWTP2 were always higher than those from WWTP1, however without information on discharge from the two WWTPs and the dairy factory final effluent it is difficult to interpret their impact on the TRP concentrations in the river. Discharge from WWTPs is not only affected by precipitation, but also by patterns of human behaviour and processes within the WWTP as well permitted discharge limits (Bowes et al., 2012).

Table 5

Main river and tributary TRP concentrations and % TP for the four sampling assays. Measured Qvalues for sample sites R2 and R9 are included for reference.

Sampling time

Location March June Sept Dec

R2 480 193 128 278

R9 2531 447 167 805

MgTRP % MgTRP % MgTRP 81sOPO4 % MgTRP %

l-1 TP l-1 TP l-1 (&) TP l-1 TP

R1 3 31 4 12 1 5 3 29

R2 4 47 4 13 1 6 10 28

R3 4 43 4 13 4 18 4 31

R4 8 24 55 86 68 + 17.7 74 30 85

R5 11 85 50 92 107 74 21 74

R6 8 43 53 90 66 68 24 73

R7 33 100 398 89 1611 89 62 93

R8 32 100 500 100 1659 82 91 95

R9 25 94 529 100 2286 + 17.8 79 55 91

T1 5 57 11 30 11 18 10 49

T2 4 26 4 12 3 6 4 31

T3 5 14 6 19 8 26 4 31

T4 8 25 8 15 2 2 5 28

T5 16 31 13 22 7 +18.8 12 8 41

T6 36 100 105 62 154 + 17.1 67 101 66

T7 24 45 38 65 85 + 17.7 16 32 34

T8 9 31 12 30 10 12 6 28

T9 24 49 73 81 70 59 27 76

T10 21 100 102 93 152 78 22 58

Where the TRP proportion of TP is >50% it has been highlighted in bold text.

The measured 618OPO4 values in the WWTP final effluents were generally out of equilibrium with their water and indicated that initial source 618OPO4 values could be, in part, preserved within the effluent. Given the high TRP concentrations present, it would seem unlikely that all of the PO4 would have been completely cycled thus removing any original source values. The two WWTPs also had distinct 618OPO4 signatures from each other (Fig. 3) although what caused this difference is unclear. In Fig. 4 it can be seen that the measured values from WWTP2 were similar to those of the river water into which it discharged, while the 618OPO4 of WWTP1 was greater than that of the river potentially enabling its signature to be identified.

The S18OPO4 reported here for WWTP final effluents ( + 16.4 to + 19.7%o) were similar to those reported elsewhere, although those values do vary greatly. Gruau et al. (2005) reported a range of + 16.6 to + 18%o for samples from three WWTPs in France which were at or very near E618OPO4. Young et al. (2009) examined many effluents, both within and at the outlets, from three geographically distinct WWTPs in the USA and found 618OPO4 values of between + 8.4 to +14.2%. Young et al. (2009) concluded that the S18OPO4 of WWTP effluent can vary considerably both within, and between, different WWTPs and sometimes may not be in isotopic equilibrium with the WWTP water. This was due to a combination of factors including variability in source 618OPO4, changes during treatment, or changes in S18OH2O, temperature, and residence time within the treatment plant. Therefore, the 618OPO4 value of WWTP effluent must be directly measured for each study area where 618OPO4 measurements are being used to track source inputs.

The measured 618OPO4 of the dairy effluent was always lower than the E618OPO4 for their water when sampled but without information on what P sources enter the dairy factory (e.g. milk, process chemicals, etc.) and what processes the effluent undergoes prior to discharge (e.g. temperatures) it is impossible to comment further on the 618OPO4 other than to confirm its values are indistinguishable from those

River Water River Channel Bed Sediment Domestic Septic Systems

Agricultural Inorganic Fertilizer Mains Tap Water Dairy Factory final Effluent WWTP1 WWTP 1

10 12 14 16 18 20 22 24 26 28 61SOp04 №)

Fig. 3. Summary of 818Opo4 values for various PO4 sources within the Upper Taw catchment and the values measured within the river itself. All values are for water soluble/extractable TRP except for the River Channel Bed Sediment which is a 1 M HCL extraction. Range of E818OPO4 for the river is indicated by the grey area.

Fig. 4. The Upper Taw catchment S18Opo4 riverine values and S18Opo4 values of other relevant TRP sources. The ES18OPO4 for the main river channel was +17.4 (R4) and + 17.7%o (R9), and the distance between WWTP1 and R4 was 2 km.

expected for PO4 in equilibrium with river water (Fig. 3). Thus, in this case, the 618OPO4 cannot be used to distinguish this specific catchment source.

5.1.2. Mains tap water

It is not unsurprising that the TRP concentrations of the tap water samples and their 618OPO4 are similar given they share a common source. The S18OH2O was also similar between the samples (x = - 5.89, a = 0.0), and again it was variation in temperature (8.7 to 11.2 °C) that caused any variation in the E618OPO4 for the mains tap water which ranged between +17.4 and +18.0% The 618OPO4 of tap water was similar to the E618OPO4 for both the tap water itself and the river water (Fig. 3) and similar to other reported standard PO4 sources (Young et al., 2009). Gooddy et al. (2015) report that while there may be some isotopic effects associated with biotic and abiotic cycling of PO4 which could affect 618OPO4, it is primarily the orthophosphoric dosing acid that determines the 618OPO4. Their study indicated that within the region of our study, dosing acid 'B' was used with a mean 618OPO4 of + 19.7%», and that the mains waters in the region had a slightly lower value of +18.8 and +19.3% (which were 0.7 and 1.8% higher than the mains tap water E618OPO4). The mains tap water samples in our study, although with a slightly lower S18OPO4 than reported in Gooddy et al. (2015), fit well with the national and regional picture they describe, and are higher than the S18OPO4 of the measured river water meaning mains tap water entering the river could potentially be distinguished as a source.

5.1.3. Agricultural inorganic fertilizers and animal wastes

Chemical fertilizers are derived from the mining and processing of phosphorite rocks which primarily originate from marine deposits. Only a small number of 618OPO4 fertilizer values are available in the published literature and have been summarised by Young etal. (2009). Both Gruau et al. (2005) and McLaughlin et al. (2006a) reported a narrow range ofvalues of between 19.4 and 23.1 % while Young et al. (2009) reported a more widely distributed set of 618OPO4 values of between 15.5 and 22.3%. In this study, the 618OPO4 of chemical fertilizers had a wider range of values ranging from +13.3 to + 25.9% (Fig. 3).

Previous work has reported P concentrations in European dairy slurries ranging from 28 to 910 mg TP l-1 and that the TP, in most cattle and pig slurries, is largely insoluble (Scotford et al., 1998). However, 83% of the water extractable TP in fresh dairy faeces has been shown to be PO4 (Chapuis-Lardy et al., 2004). A limited number of 618OPO4 values have been published for animal wastes, and to the author's knowledge, none on stored agricultural wastes. Ayliffe et al. (1992) reported values

of between +19.8 to + 23.1% for modern guano from several different species of seabirds, while Young et al. (2009) reported water extractable S18OPO4 values of + 15.7% and +18.3% for dog and goose faeces, respectively. Agricultural slurries tend to be variable in their composition (e.g. dry matter content) and very difficult to sample representatively (Chadwick and Chen, 2002). Given the variable nature of the material, the resulting 618OPO4 values were surprisingly consistent and notably lower than, not only the existing data for animal wastes, but also most other materials reported by Young et al. (2009). These source values are the lowest found in this study, and are below E618OPO4 values expected for river water (Fig. 3). The slurry PO4 is most likely to be mainly sourced from animal excreta, which contains approximately 70 to 80% of the P ingested by the animal (Parr et al., 1998). The slurry S18OPO4 will therefore be a function of the animals feed, but most probably, the animal's internal temperature, S18OH2O and metabolic processes that most strongly affect the 618OPO4 of the excreted PO4. While the mineralization of organic matter by phosphatases and other enzymes can lead to lower 618OPO4 it can be calculated that the animals internal E618OPO4 is also low. Assuming the animals' internal temperature is 38 °C, and that its body water is similar to local meteoric waters (- 5.5%) then E618OPO4 would be around +14%. It is also possible that further cycling of PO4 occurs within the slurry during storage.

5.1.4. Domestic septic systems

Little information exists on the P content and contribution of domestic septic systems to water quality (Beal et al., 2005; Withers et al., 2011). This is perhaps unsurprising given the number of systems operating and their variability in age, type of discharge, treatment process and distance to receiving waters (Withers et al., 2012).

To the best of our knowledge, there are no reported 618OPO4 values for such waste material to date. The mean value of + 20.8% (Tank A) is higher than the few 618OPO4 values reported for 'animal wastes'. The S18OPO4 value is, however, one of the few TRP sources that was isotopi-cally distinct from the calculated E618OPO4 for river water in the study catchment (Fig. 3). More data are needed if these point sources are to be better understood as potential sources of PO4 pollution on the basis of their 618OPO4 signal.

5.1.5. River channel bed sediment

The TP concentration in bed sediment generally increased downstream with concentrations of around 1000 mg P kg-1 in the upper reaches, with peaks at Taw Green (R4) and Yeobridge of 1631 and 2328 mg P kg-1, respectively. The reason for this increase is uncertain;

but likely to be related to the WWTPs and dairy factory which represent significant point sources of TRP directly upstream from them. This relationship between high TP concentrations in bed sediment and significant sources of TRP has been reported byJarvie et al. (2005). This could also explain the increased percentage of acid and water extract-able TRP from the bed sediment along the river.

At all sites, the acid extractable TRP from bed sediment samples had very similar 618OPO4 values with a mean of + 17.3% (ct = 0.3). If the increasing P content of the bed sediment reflects the increasing contribution of one of more TRP sources being adsorbed onto the deposited sediment, those sources must either have similar initial 618OPO4 values, as fractionation due to abiotic processes like precipitation of P minerals is rather small (around 1%) (Jaisi, 2013; Jaisi et al., 2010; Liang and Blake, 2007), or that any distinct 618OPO4 source values were lost through biogeochemical cycling of the PO4. The latter is the most likely explanation as the acid extractable 618OPO4 of the bed sediment samples is very similar to E618OPO4 of the river water (Fig. 3). It is also possible that the 1 M HCl extraction 618OPO4 values do not reflect water soluble TRP source values sufficiently given that the acid extraction integrates all forms of TRP from loosely bound PO4 through to P associated with Ca minerals and detrital apatite (Tamburini et al., 2010). The 618OPO4 of water soluble TRP contained within these bed sediment samples may also be over-ridden by the 618OPO4 of any mineral PO4 given that < 0.49% of the acid extractable TRP was water extractable. To establish whether different P forms in the channel bed sediment samples have different S18OPO4 signatures, further studies using sequential extraction media are required to separate potential adsorbed PO4 from catchment sources from truly mineral PO4 forms (Cross and Schlesinger, 1995). Joshi et al. (2015) extracted sediment samples from the Chesapeake Bay in the USA and found that the 618OPO4 values of the ferric iron-bound and authigenic P pools were in a rather narrow range. However, the authors do not report isotope values for the other extracted P pools, i.e. the exchangeable and detrital P pools. Furthermore, a method for the determination of the S18OPO4 of organic P extracted from sediments or soils is still lacking.

5.2. The Upper Taw catchment

5.2.1. Upper Taw tributaries

5.2.1.1. TRP concentrations. The fact that no tributaries had TRP concentrations as great as those in the lowest reach of the main river suggests that there were no major point sources located within their catchments. However, statistical analysis of the tributaries show they can be split into 2 groups based on the change in their seasonal TRP concentrations and the predominant form of P. From these temporal concentration dynamics, it is possible to infer what the main sources of P might be within each group. Group 1 tributaries were generally located within the upper and middle reaches of the catchment, had no distinct seasonal relationship with Q, and were dominated by UP.

This implies that Group 1 tributaries had no significant P point sources and are not strongly influenced by any major diffuse inputs. Where slight seasonal patterns are observed it is probably as a result of domestic septic systems (Jarvie et al., 2008) or unregistered sources (Edwards et al., 2008). Group 2 tributaries, generally were located in the lower reaches of the catchment, had higher P concentrations than Group 1 and were TRP dominated. These catchments had a strong sea-sonality in TRP concentration with higher concentrations measured during the lower Q of the summer period suggesting that point sources were strongly affecting their water quality (Edwards and Withers, 2007; Jarvie et al., 2006). However, the prevalence of domestic septic systems within these tributary catchments was no different to those comprising Group 1. This highlights the complexity of P transport and delivery in such catchments, making interpreting spatial and temporal TRP patterns extremely difficult (Haygarth et al., 2005).

Tributary T7 differs slightly from the other Group 2 tributaries in that generally UP was the dominant P form. This is noteworthy as this

tributary has no domestic septic systems registered within it, but it does however drain a series of lakes and ponds which are managed as fisheries, which might explain why organic forms of P are more prevalent and further highlights the complexity of these smaller point sources in affecting the P dynamics within some rural watersheds.

5.2.1.2. S18Opo4 isotopic characterisation of tributaries. Of the three tributaries sampled for S18OPO4, one was a Group 1 tributary (T5) and the others were Group 2 (T6 and T7). The seasonal TRP/Q dynamics ofT5 indicated it was not affected by point sources, and that in fact it might be subject to TRP turnover and net immobilisation by the time of sampling and analysis for 618OPO4. It had the highest value of + 18.8% measured in the Upper Taw catchment, and the E618OPO4 here was also the highest within the catchment and this was because of the higher than typical S18OH2O of — 4.44% when compared to other surface waters sampled at that time (ranging between — 6.26 to — 5.00%). This anomalous S18OH2O value is explained by the fact that at the time of sampling, the Q in T5 was extremely low and it is possible that the very slow moving water was subject to considerable evaporative fractionation leading to an enrichment in 18O. The fact that the 618OPO4 is elevated and similar to that of the elevated E618OPO4 indicates that within this tributary the PO4 was being biologically cycled having picked up the evaporative enrichment S18OH2O signature. Tributary T6 was considered to be strongly P impacted due to its seasonal variation and high %TRP; however, its S18OPO4 was just 0.9% lower than E618OPO4 although this was the greatest difference measured in the catchment. Although not a large difference it suggests that a source signal(s) may be present. The Group 2 tributary T7 had characteristics of both T5 and T6 (i.e. high seasonal TRP variability but low %TRP) and this unusual combination of variables was put down to the presence of a small fish farm discharging into the tributary further upstream, although this potential source was not examined further. The measured 618OPO4 in September was found to be just 0.5% higher than E618OPO4, so again no conclusions could be drawn on the potential sources of the PO4 in the tributary.

5.2.2. Main river

5.2.2.1. TRP concentrations. Statistical analysis of the main river TRP concentrations divided the river into three sections. The Upper section (R1-3) had low TRP concentrations (< 10 ^gPl-1) with little or no pronounced seasonal variation. This is typical of a drainage system in a low intensity agriculture and semi-natural grassland system with no major P point sources, and where the main form of P was UP sourced from the peat moorlands of Dartmoor and in-stream autochthonous sources (Edwards et al., 2000; Turner et al., 2003). The middle river section (R4-6) had TRP concentrations greater than upstream, was predominantly TRP, and demonstrated strong seasonal variation in TRP concentration and Q. This section of the river would therefore appear to be dominated by point sources (Jarvie et al., 2006) and most likely that of WWTP1. Downstream of R7, in the lower reach of the main river, TRP comprised nearly all the TP and concentrations were higher still again particularly in June and September when Qwas at its lowest. As with the middle reach of the river, these trends are most likely to be controlled by the major TRP point source inputs from WWTP2 and the dairy factory final effluent. Without Qdata from these two point sources it is not possible to assess the effect of their mixing with the river water, but what is clear is that the combined effect of these two significant point sources is probably leading to the large increases and seasonal patterns in TRP concentration in the lower reach. What is not clear is what is causing the large increase in TRP between R7 and R9 in June and September. This increase occurs without any further significant point sources of TRP. There are no further significant point sources between these sampling locations and the TRP concentrations in the tributaries feeding this lower reach are still much lower than those in the main river and should be causing dilution of TRP within the river. We speculate that this is most likely the result of some in-channel processes,

possibly mineralization of UP sourced within the final effluents from WWTP2 and dairy factory final effluent. However, if this were the case it might be expected that the %TRP in the main river would also be increasing and in September this is not the case. Furthermore, given that the vast majority of P in the two effluents is already in TRP form it doesn't seem possible that there is sufficient UP to cause such a large increase in TRP. Channel bed sediment TRP release may also explain this effect but it is unclear whether such sediment is a TRP source or sink. Jarvie et al. (2005) reported that in rivers with high SRP concentrations at low flows linked to WWTP discharges, bed sediments consistently acted as net sinks. Further work is needed to understand the TRP dynamics in this lower reach and what is driving the spatial increase in TRP concentrations.

5.2.2.2. Isotopic characterisation of river PO4. The two samples collected in September for 618OPO4 characterisation were taken from R4 and R9 (Fig. 4). Sample site R4 was downstream of WWTP1, but upstream of WWTP2 and the dairy factory, while R9 is at the catchment outlet downstream of all sources within the Upper River Taw catchment. The measured 618OPO4 values were virtually identical to E618OPO4 (only 0.3 and 0.1% higher, respectively). At R4 WWTP1 was most likely the main point source contributor for TRP in the river. This assumption is based on the observed jump in TRP concentration between R3 and R4, the very high concentration of TRP in WWTP1's effluent, that no other point sources are present between R3 and R4, and that T2 was also low in TRP. However, despite the 618OPO4 signature of the final effluent from WWTP1 being 2.2% higher than the river E618OPO4 it was not detectable some 2 km downstream where the measured 618OPO4 was just 0.3% higher than E618OPO4. Given this it would seem as though over this distance the WWTP1 effluent S18OPO4 signature had become completely cycled (e.g. Goldhammer et al., 2011; Stout et al., 2014).

At R9 the river S18OPO4 is similar to that at R4 (0.1% higher). At R9 the river TRP dynamics are far more complicated than at R4, with two other point sources contributing in addition to WWTP1 as well as a larger catchment area. The 618OPO4 of both the dairy final effluent and WWTP2 are similar, but lower than WWTP1. Their values are however, much closer to that of the river E618OPO4, meaning that their signals are indistinguishable from E618OPO4. Given these values it is perhaps unsurprising that at R9 the river 618OPO4 is again almost the same as E618OPO4 (0.1% higher). It seems likely that the high concentrations ofTRP in the river at R9 are derived from the main point sources, but as seen in the spatial TRP concentration variability of this lower river stretch, some dynamics are occurring that are as yet undefined.

6. Conclusions

This study increases the current published information on the S18OPO4 of various P sources and 618OPO4 values within a river catchment which had an E618OPO4 of between +17.2 and +19.2%. In answer to our first research question, our specific findings were:

• Two WWTPs appeared to have different and distinct 618OPO4 ranging between +16.4 and +19.6%» with, on average, a 2.4% difference between them

• A dairy factory final effluent had S18OPO4 values ranging between + 16.5 and +17.8%

• Inorganic fertilizers had a very wide range of 618OPO4 ranging between + 13.3 and +25.9%»

• Dairy farm animal waste slurries were consistently low, with a mean S18OPO4of + 13.5%

• Domestic septic systems may have elevated 618OPO4 (>20%) when compared to E618OPO4 (and WWTPs), although more data is needed

• River channel bed sediment were found to have acid extractable S18OPO4 very similar to E618OPO4 and along the channel network (+16.7 to +17.6%)

In general, sources were often found to have a narrow range of S18OPO4 values very similar to the E618OPO4 calculated for the river network, although a great deal of variability did occur within some sources. In response to our second research question, the 618OPO4 values measured within the river network show that:

• The 618OPO4 within the river network were all very similar ranging from + 17.1 to +18.8%

• The 618OPO4 within the main river stem was found to be no > 0.3% different to that of ES18OPO4

• The 618OPO4 in the tributaries showed more variability and deviation from E618OPO4, but only by between 0.4 and 0.9%

• Rapid microbial cycling would appear to be occurring within the river network as demonstrated by the loss of the WWTP1 618OPO4 signal over just a 2 km river stretch and through the transference of the enriched S18OH2O signal in T4 into the TRP.

From this work it was not possible to draw any definitive conclusions using the 618OPO4 signature of P sources. In the study river, through either, or a combination of, the observations, we conclude that a) the majority of sources did not have suitably distinct 618OPO4 signatures from each other or E618OPO4, or b) that within-river microbial cycling was rapidly removing any original source 618OPO4 values. While this tool may yet prove useful in examining riverine P sources and dynamics, more work is needed in the following critical areas:

1. Better source characterisation. It is fundamental that potential sources are characterised by higher between-, rather than within-source variation - and on the basis of this dataset, that poses some challenges as individual sources can be characterised by quite a range of tracer values. Further, many sources remain relatively uncharacterised, such as managed and unmanaged animal wastes, aquaculture, and channel bed sediment.

2. Better spatial characterisation. The dynamics of within-channel TRP are still poorly understood, let alone the 618OPO4 signals. If rapid cycling in rivers is occurring then it should be detectable, but higher spatial and temporal sampling resolutions are needed to resolve this uncertainty.

Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scitotenv.2016.09.007.

Acknowledgements

The authors would like to thank the Westcountry Rivers Trust for funding this research via the Taw River Improvement Project (TRIP) (CRF040). The work was further supported through a NERC grant; NERC (IP/1375/0513). The authors would like to thank Federica Tamburini for her technical advice and Dan Dhanoa for his statistical advice. Rothamsted Research is supported by the Biotechnology and Biological Sciences Research Council (BBSRC) (BB/J004286/1). The authors wish to thank the anonymous reviewers and editor for their time improving this manuscript.

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